Production Calculators

The production calculators below will calculate the answers to 3 of the most common scheduling questions in an injection molding plant:

  1. What is the production rate?
  2. How long will it take to complete each production run?
  3. How much plastic material will be required for each production run?

Production Rate


Use this calculator to find the number of parts produced per hour given cycle time & number of cavities in the mould.

Reduce the cycle time by just 1 second to see how many more parts can be made per hour. The more parts made per hour the lower the part cost.

Calculation For Part Count Per hour
Cycle time (seconds):
Mold cavitation:
 
Parts per hour:

Production Time


Use this calculator to find the time required to complete a production run given the part quantity, cycle time & number of cavities in the injection mold.

Reduce the cycle time by a small amount to see the overall reduction in the production time.  For example, if a single cavity mould is required to produce 20,000 parts with a 19 second cycle time then the number of hours required is 105.6hours. However, if the cycle time is reduced to 18 seconds then the time is reduced to 100.0 hour. That’s more than 5 hours less which not only makes the production run cheaper per part but also makes the moulding machine available much sooner for the next production run.

Calculation For Number of Hours Required to Make x Parts
Quantity of parts required:
Cycle time (seconds):
Mold cavitation:
 
Hours required:

 

Mold Design Services

Click here to see a video example


Production Calculators for
Plastic Material Requirement


One of the most useful production calculators is a material requirement calculator.

Use this calculator to not only find out quantity of material required to produce a given number of parts  but also to work out material savings when comparing a hot runner mould to a cold runner mould.  For example, a 2 cavity cold runner mould making handles for buckets – the handle weight is 20 grams and the runner weight is 8 grams per shot. So for a 50,000 production run 1200kgs of material  is required.  However, if the same 2 cavity mold is converted to a hot runner then the amount of material required is 1000kg which is 200kgs less resulting in a saving of hundreds of dollars.

Although the cost of a hot runner mould is more than a cold runner mould this cost difference must be compared against the cost of managing the regrind (paying employees to feed cold runners into a grinding machine then blending it with virgin material). On top of this, the cost of a grinding machine itself along with the electricity cost must be included.

If the quantity of parts required are large enough then the extra cost of a hot runner will be justified. Even better, hot runners will generally have shorter cycle times than cold runner moulds meaning your production time will be lower.

Calculation For Plastic Material Requirement
Preferred unit of measurement:
Quantity of parts required:
Part weight (grams):
Mold cavitation:
Runner weight per shot (zero for hotrunner):
 
Material requirement (kg) :

 

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Estimating Cycle Time

Estimating cycle time in injection molding can be difficult because every injection molding company has a different way of doing things. Different levels of operator training and machine capability play a significant role in cycle time.

Material selection and part design also have an influence.

 

Part Design


The wall thickness of a part will directly affect the cycle time; thicker walls create longer cycle times while thinner walls create shorter cycles that’s why thin wall molding has cycle times in the 2 to 5 seconds range.  Click here to learn more about thin wall injection molding. The fact is, the part designer needs to make the wall thickness as thin and uniform as possible while still maintaining adequate strength in line with the application.

The height of a part also effects cycle time. The higher the part the further the machine platen has to open and the more time required to eject it off the mould core.

Mold Design


The cooling system within a mould is a heat exchange system.  There must be a sufficient number of cooling channels and sufficient flow rate of cooling fluid (usually water) to be able to extract the heat energy out of the plastic part within a specified time – this is called the cooling time of the moulding process and this directly effects cycle time. Different injection mold designers will design cooling systems that have different capabilities. This means cycle times will be different when using different designers.

Another aspect of mould design that will directly effect cycle time is the ejector system.  The ejector system should be rigid enough to remove the part off the mould core without delay.  Slow moving ejectors will only add to cycle time unnecessarily.  What’s more, it is important to know when to use ejector pins and when to use a stripper plate for ejection as there is a big difference in the cost of the mould.

The cycle time calculator does not take into consideration hydraulically or pneumatically operated moving slides and cores. This method will add considerable time to mould ejection.

Also, the use of compressed air in many applications is vital to achieving quick cycle times because this allows the vacuum between the part and the core to be removed before the ejector pins (or stripper plate) take over.  Without air, ejection is slower and will likely cause quality issues such as part distortion.

Machine Capability


A moulding machine will affect the cycle time in a number of ways:

  • New technology machines are faster than old technology machines.
  • Machines that have been well maintained will be faster and more reliable than machines that have not been maintained.
  • Smaller machines generally cycle faster than larger machines.
  • Machines with toggle clamps will give shorter cycle times than machines with hydraulic clamping units because platen speed will be faster.
  • All-Electric machines have the fastest platen speeds so cycle time will be the shortest (everything else being equal)

Temperature Controller


The temperature controller (chiller) needs to be able to extract the heat energy from the cooling fluid (usually water) fast enough to maintain mould temperature for the required cycle time.

Adequate flow rate is also essential so the pump must have sufficient power. Centralized systems can be inadequate especially when additional moulding machines are installed without regard to temperature controller capacity.

Technician Capability


Ask 5 different technicians to set the same mould and you will probably get five different cycle times.Different technicians have been trained differently and have different experiences and understandings so the chance of variation is huge.

Runner System


When estimating cycle time, the type of runner system will play a role. Hot runners and insulated runners (thick runners) are faster than cold runners (used in 2 plate and 3 plate moulds). This is because there isn’t a runner to eject between each cycle and the injection time is less since the flow length is shorter.

Whats more, hot runners and insulated runners are not affected by cooling time whereas cold runners, when too large, require more cooling time than the part needs.

Cycle Time Calculator


Use the following cycle time calculator for general purpose molding to get an estimation keeping in mind that the points above can have considerable influence. This calculator is to be used as a guide and the results are not guaranteed.

It accounts for parts with wall thicknesses between 0.8mm and 5.0mm  (0.032 and 0.196 inches).

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Plastic Dryers, Moisture Measurement And Part Quality

Plastic dryers are used to remove moisture (water) that has been absorbed either onto the surface of plastic pallets or into the plastic pallets internal structure before they go into the injection molding machine to make parts.

There are 2 groups of plastics. The first group will only hold water on its surface (such as polypropylene) while the second group will absorb it into its internal structure (such as PET polyester).

Drying is required to ensure good quality parts are made consistently. Plastics with excess moisture levels will react when processed in the molding machine barrel and will produce by-products that will effect such things as impact strength in the finished molded part.

Plastics That Need To Be Dried Before Processing:

1.       SAN

2.       PVC

3.       ABS

4.       PPO

5.       ACRYLIC

6.       ACETAL

7.       PPS

8.       POLYCARBONATE

9.       PET POLYESTER

10.   PEI

11.   POLYURETHANE

12.   NYLON

13.   PBT POLYESTER

Plastics  1 to 7 need to be dried for cosmetic reasons only. Excess moisture will cause bubbles, flow lines or surface defects in the molded part. However, their mechanical properties are not affected by moisture.

Excess moisture in the plastics 8 to 13 will affect the mechanical properties in the molded part. The part will have reduced impact and tensile strength but will not show any cosmetic defects.  This fact is very important to know so that moulders do not rely upon visual checks to ensure the quality of the molded part.

 Why It Is Important To Measure Moisture Level After Drying.


Processing plastic pallets in an injection molding machine that have not been dried to the required level can lead to disastrous consequences in the field.  Parts that look good are not necessarily strong enough to function properly. This means you are not just wasting time producing rejects but more importantly are likely to damage your company’s reputation as a quality supplier.

Just because the plastic material is dried at the recommended temperature and length of time doesn’t mean the material is dry enough to process. If the desiccant dryer has not been maintained properly then the plastic material may still have too much moisture and may need to be dried for a longer period of time.

That’s why it is critical that the moisture content is measured before processing. This should be done on a daily basis so that moisture can be eliminated as a reason for any part quality issues.

Moisture Measurement Methods.


There are 2 different systems of moisture measurement – massed based and sensor based instruments.

Sensor based (usually Karl Fischer) are the most accurate because they will only measure the moisture level in the pallets. However, the massed based systems will measure the moisture level along with other volatiles produced during the analysing process giving a false reading.

The appeal of massed based systems are their lower purchase price compared with sensor based and are easier to use. But these are not reasons to use this type of instruments because they do not give accurate and repeatable results.

Additional Comments


As an injection moulder you spend millions of dollars in equipment such as  molding machines, moulds and chillers so there is no excuse not to spend a bit more on quality plastic dryers and moisture measuring equipment.

What’s more, material cost is an injection moulders largest ongoing expense so it is critical that waste is eliminated.

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Thin Wall Injection Molding Basics

Thin Wall Injection Molding Basics

Thin wall injection molding is increasing in popularity as material costs continue to rise and delivery lead times are squeezed. Thin wall molding offers the opportunity to reduce material part cost and to increase production output through faster cycle times.

This trend is taking place in all types of injection molding industries such as automotive, packaging, electronic, medical, optical, mobile communications and computing equipment.

But the fact is, thin wall molding is very difficult to master.

It is littered with traps to the unaware.

 

Definition


Thin wall parts have a wall thickness of less than 1mm. The size of the part also plays a role in the definition since the bigger the part, the more injection pressure is needed to fill the mould cavity. Click here to scroll down to an example of process parameters for a thin wall part.

A thin wall part is defined by comparing the maximum flow length of the material from the gate to the wall thickness. Thin wall starts at 200:1. Consider a flat round part with a radius of 250mm and a wall thickness of 0.8mm which is gated in the centre. The ratio is 312:1 (250/0.8=312))

Typical food packaging parts are in the range of 250:1 The higher the ratio the more difficult it is to mold.

Thin Wall Mould DesignThin Wall Mould Core Block

Benefits of Thin wall Parts.


  • Low material cost per part
  • Low electricity cost per part due to fast cycle time with hydraulic machines
  • Higher production output due to fast cycle times

Minimizing wall thickness is the key to making lighter parts and improving production outputs. A thinner wall section allows shorter cooling time and cycle time.

Challenges of Thin wall Injection Molding


High Capital Investment.

Thin wall molding requires injection molding machines that have the ability to generate high injection speeds, high injection pressures, are repeatable and are rigid enough to be reliable for the long term. These machines are expensive.

Similarly, the injection mold needs to be strong enough to withstand high cavity pressures, clamp tonnages and production rates.

A poorly designed mould will break very quickly.

The mold cooling system design must be optimized so that heat is extracted quickly. Just having thin walls on a part is not enough to get fast cycle times.

Expect to spend substantially more on both machines and moulds compared with general purpose molding. You simply have to spend the money to get long term reliability and quality. If you skimp on price then you will get ongoing quality issues and machine breakdowns.

Steep Learning Curve

Thin wall injection molding requires new technology machines that have a multitude of control functions. Learning about the machine and all of its functions will allow you to get the most out of it.

What’s more, process parameters play a vital role in successful thin wall molding. To produce quality parts the process needs to be fine-tuned as the operating window for parameter settings is very narrow. Some parts require injection times of 0.1 second and any variation in this time will cause part quality problems such as flashing or short shots. Parts with thicker wall sections have a much bigger operating window and are easier to mold.

Often robots are used to remove and stack parts in thin wall molding. This is especially common in food packaging. Learning how to use the robot effectively is another area that must be mastered.

Because moulds for thin wall parts are made with a very tight tolerance, mould maintenance needs to be done daily. Any build up of residue across the venting surfaces will result in part quality issues especially on multi cavity moulds.

Finally, learning how to trouble shoot effectively is necessary. Even if you have high quality equipment and have well trained people there will still be quality issues. Thin wall molding magnifies the smallest faults in a mould, a machine or in wrong process parameter settings.

Additional Comments


If you are currently injection molding thick walled parts and are considering a reduction in thickness, be aware that any slight change will effect the process. Higher clamp tonnages and injection pressures will be required and there will be more stress on the machine,mold and process technician. So make sure your machine is not operating on the limit before deciding to reduce wall thickness.

The key to being successful in thin wall injection molding is to grow the business slowly. This gives time to develop markets, save for new technology equipment and to properly train process technicians and management in the exact science of thin wall molding.

Example Of Process Parameters For A Thin Wall Part


Process Parameter
Part Description: 500 ml Tub
Wall Thickness (mm) 0.45
Material (Homopolymer) PP 40mfi
Mould: 2 cavity
Valve Gate Husky
Hot runner Husky
Injection Time (sec) 0.2
Injection Pressure (bar) 1400
Hold Time (sec) 0.5
Hold Pressure (bar) 700
Cushion (mm) 3
Cooling Time (sec) 0.8
Opening Time 1.6
Closing Time (sec) 0.9
Plastizing Time (sec) 1
Clamp Tonnage 230
Cycle Time (sec) 4.0
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Troubleshooting Part Quality – Warpage

Plastic Injection Molding Troubleshooting Warpage Improves Production Performance

In plastic injection molding troubleshooting, warpage is one of the most common quality issues.

So what is warpage?

Warpage is excessive change of shape of the part after it has been ejected from the injection mould. Also known as deformation, a warped part will twist or bend due to internal stresses in the part caused by uneven shrinkage rates or by some mechanically applied forced such as during the ejection phase.

Eliminating part warp can have huge cost savings and should be part of a lean manufacturing program.

But how to fix warp?

The root causes of warp can be grouped into 5 different areas:

1. Incorrect processing parameters

2. Mould issue

3. Injection moulding machine issue

4. Part design issue

5. Poor plastic material selection

Isolating which area is causing the warp can be quite difficult so it must be done in a step by step procedure eliminating each possible cause one at a time.

When doing plastic injection molding troubleshooting, keep an open mind because the cause might be different to what you think it is.

1. Incorrect Processing Parameters


When using this plastic injection molding troubleshooting guide below change one processing parameter at a time by a small amount (say 5%) until you get a result.

Low Injection Rate


Explanation: Injecting with a lower rate will leave higher residual stresses in the moulded part. A lower injection rate causes a lower viscosity in the material which requires higher injection pressure to push the material into the mould cavity. A short shot might also result.

Action: Increase the injection rate by increasing the injection speed. Take note of the fill time parameter before and after increasing the injection speed. If it doesn’t reduce then increase the injection pressure until the fill time reduces to the target time & part warp is eliminated.

High Injection Rate


Explanation: A high injection rate can result in a high shear rate of the material at the gate. If the shear rate is too high it will add to the internal stress in the part causing it to warp.

Action: Reduce injection rate.

Short Hold Time


Explanation: Short hold times don’t allow the machine to compensate for the natural shrinkage that occurs as the part cools in the mould cavity. The machine needs to apply hold pressure to the part after first stage fill to compensate for most of the shrinkage.

Action: Increase hold time.

Short Cooling Time


Explanation: Short cooling times don’t allow sufficient heat to be extracted from the part while it is still held in the mould cavity. A part that is ejected hot is more likely to warp than a part that is ejected cold.

Action: Increase cooling time but also consider reducing barrel temperatures or screw back pressure during the plastizing stage.

Low Melt Temperature


Explanation: Low melt temperature will cause higher internal stresses in the part.

Action: Increase barrel temperature or screw back pressure during the plasticizing stage.

2. Mould Issue


When plastic injection molding troubleshooting, the mould is usually the first area to be checked.


Uneven Cooling


Explanation: Uneven cooling will cause different shrinkage rates within the part. This causes stresses in the part.

3 common reasons for uneven cooling are:

1. Poor mould design: For example: water channels might be spaced too far apart.

2. Poor mould setup resulting in low water flow rate: For example: looping too many water circuits externally or having small diameter hoses or fittings connected to the mould will restrict the flow.

3. Lack of mold maintenance.

Action: Check the moulding surfaces of the core and cavity for temperature difference. Large differences can be noticed by hand touch  while smaller differences will need the aid of a temperature sensor. When checking make sure it is done immediately after the mould stops cycling. Any delay will give a false reading.  Then modify mould design.

Alternatively, check that the water flow meters have a similar flow rate and measure the difference between the inlet and outlet water temperatures by placing a temperature sensor on each individual water fitting connected to the mould  The temperature difference should be less than 4 degrees Celsius. Also,don’t loop any water circuits; use individual water circuits. This might not fix the problem but it at least eliminates it as a cause.

Also consider descaling (cleaning) cooling channels.

Ejection


Explanation: The ejection stage can warp products for the following reasons:

1. Poor design of ejector system: If the ejection system doesn’t provide even force to the part then there is a possibility of distortion. For example , having too few ejector pins.

2. Sharp corners make the part hold tighter in the mould.

3. Polish: a high gloss finish will make it more difficult to eject the part especially on side walls with little draft. Also scratches or machining marks that are not in the direction of part ejection will cause the part to stick tighter in the mould.

Action: Radius sharp corners (R0.2mm minimum), repolish if necessary or modify ejector system.

Wrong Gate Location


Explanation: In most cases, the plastic material should be moving away from the gate at all times inside the mould cavity during the filling stage. If any portion of the flow front starts to move back in the direction of the gate then weld lines or voids due to air entrapment will setup extra stresses in the part.

In other words, the flow front should be as uniform as possible and gate position plays a critical role in this.

Action: Find new gate position. Seek the services of an engineer experienced in both plastic mold flow simulation software and the injection molding process.

Gate Too Small


Explanation: A small gate size can generate large shear rate in the material as it passes thru the gate. A high shear rate will stress the material resulting in warpage.

Action: Increase gate size but consider first changing to an easier flow material or trying another moulding machine with better capability. Increasing the gate size will also increase the gate mark on the part which can detract from the appearance.

Poor Mold Making


Explanation: For multi-cavity moulds, if there is a slight difference in the physical size of the steel cavity or core moulding surface dimensions between cavities due to poor machining practices, then this will cause different cavity pressures.

Action: Measure all cavity and core moulding dimensions.

3. Injection Molding Machine Issue


Poor Injection Response


Explanation: Slow acceleration of the injection screw during the filling stage will increase the internal stresses in the part.

Action: Increase injection pressure or move mould to a better machine.

4. Part Design Issue


Sudden or Large Differences in Wall Thickness


Explanation: Changes in wall thickness creates localised stresses due to different shrinkage rates. This can distort the part.

Action: Redesign part so that wall thickness change is gradual or make wall thickness even across the entire part then modify mould to suit.

Part Too Thin


Explanation #1 : If the wall thickness is very thin then the part will be naturally weak. A weak part can be distorted during ejection from the mould especially during short cycle times.

Explanation #2 :If the wall thickness is very thin compared to the flow length then high injection pressures and speeds are required to fill the mould cavity (this is required in thin wall injection molding). If injection times are too long then high internal stresses will be in the part causing warpage.

Action: Make wall section thicker or modify mould so that the ejector system is more reliable.

Lack of strengthening Ribs


Explanation: Strengthening ribs improve rigidity of the part when placed in the right areas such as in corners.

Action: Modify part design so that ribs are included. The wall thickness of the ribs don’t need to be as thick as the rest of the wall section to provide strength. 50%-80% of nominal wall thickness is adequate.

Part Shape


Explanation: Flat rectangular parts will warp more than round tall parts. An example of a flat rectangular part is some kind of lid for food packaging and an example of of a round tall part is a drinking cup.

Did you ever see a warped plastic drinking cup? I havent.

Action: Part designers should keep this fact in mind when designing.

5. Poor Plastic Material Selection


Low MFI Material


Explanation: A low mfi material will leave more internal stress inside the moulded part.

This stress can cause the part to warp.

Action: Change to a higher mfi material.

Wrong Choice of Material


Explanation: Crystalline materials such as polypropylene, polyethylene, PET & Nylon tend to warp more than amorphous materials such as polycarbonate, polystyrene, SAN and PVC.

Action: #1 Change to an amorphous material however, this is easier said than done because these materials have different shrinkage rates, properties, costs and may require a modified mold design in order to meet the end user part requirements.

Changing the material in most cases is unlikely to be the right solution.

Action: #2 Add fiber reinforcement. Fiber reinforced materials warp less than non-reinforced materials provided the part shape is symmetrical and gate location is correct.

Additional Comments


Plastic injection molding troubleshooting can have a huge affect on cost savings and your production performance.

Although it can be costly to get the root causes of part warp repaired the question you have to ask yourself is:

What is the long term cost of not fixing warpage quality issues?

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Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

Troubleshooting Part Quality – Shorting

Troubleshooting Short Shots In Injection Molding  Increases Production Performance

Short quality issue on 34 litre tub

Shorts or short shots are one of the most common quality issues.

A plastic part is said to be short when the part is smaller than its normal shape.

Shorts occur when plastic material does not completely fill the mould cavity.

Eliminating shorts can have huge cost savings and should be part of a lean manufacturing program.

But how to fix shorts shots?

The root causes can be grouped into 5 different areas:

  1. Poor plastic material selection
  2. Incorrect processing parameters
  3. Mould issue Click here to see a case study (example#4) and to get help with mould issues. (Opens in a new window)
  4. Moulding machine issue
  5. Part design issue

Isolating which area is causing the problem can be quite difficult so it must be done in a step by step procedure eliminating each possible cause one at a time.

When doing injection molding troubleshooting, keep an open mind because the actual cause might be different to what you think it is.

1. Poor Plastic Material Selection


Plastic Material With High Viscosity


Explanation: A material with high viscosity is a lot more difficult to inject into a mould cavity than a material with a lower viscosity. Many things affect the viscosity of a material such as barrel temperature but the grade of plastic material used also plays a role.  Melt flow index (MFI) is often used as a guide to measure the ease of flow but this does not give the full story. MFI tests are performed under low shear rates but injection moulding uses higher shear rates during the injection fill stage. Some materials behave differently under different shear rates so only use the MFI as a guide.

Sometimes a high viscosity material (low MFI) can be beyond the capability of the machine -that is, the injection unit cannot generate enough injection pressure to fill the mould cavity.

Action: Experiment with different grades of plastic materials with higher MFI which might make it easier for the molding machine to fill the mould cavity.

Keep in mind some additives can also change the MFI so of you are mixing before feeding into the machine then be aware the flow properties might change.


Issue With New Batch of Material


Explanation: If short shots coincide with a new batch of material then it is likely that it is not exactly the same as the previous batch even though your sales representative told you it was.

Action: a sample of the new batch needs to be checked against the previous batch of plastic material. Click here to learn how to do this.(opens in a new window).

2. Incorrect Processing Parameters


The first thing to do when shorting is an issue is to compare the current process parameters with past process parameters that were documented when quality parts were first produced.

When using this injection molding troubleshooting guide below change one parameter at a time by a small amount (say 5%) until you get a result.

Using Non-Optimized Fill Time.


Explanation: The correct fill time is critical to producing quality parts. If the viscosity of the virgin plastic material changes (which is common) then the fill time will change resulting in reject parts if the optimized fill time has not been established

Action: Do a viscosity curve experiment as done in Scientific Molding by John Bozzelli.

Injection Pressure Too Low


Explanation: A low injection pressure might be limiting the injection speed required to fill the mould cavity.   If the injection speed is too low then the flow front of the plastic material in the mould cavity will become cool and solidify before it is full and create a short shot.

Action: Increase injection pressure by 5%. If the part has improved then increase it another 5% until it is full.

Low Injection Speed


Explanation: If increasing the injection pressure does not make a full part or 95% of a full part then more injection speed is required. If the injection speed is too low then the flow front of the plastic material in the mould cavity will cool and become solid before it is full and create a short shot.

 Action: Increase injection speed by 5%.  If the part has improved then increase it another 5% until it is full.

Low Hold Pressure


Explanation: If the part is already 95% full  then more hold pressure is required to fill the part

Action: Increase hold pressure.

Plastic Material Temperature Too Low


Explanation: Low temperatures will allow the plastic material to solidify easier in the mould cavity.

Action: There are 2 ways to increase the plastic material temperature. The first is to increase the barrel temperatures(check plastic manufacturers temperature processing range and increase to upper limit).If this does not fix the problem then increase the back pressure setting during the plastizing stage.

Clamp Tonnage Too High


Explanation: Excessive clamp tonnage will limit air escape (venting) during the injection  stage.

Action: Optimize clamp tonnage. In other words, reduce clamp tonnage until part quality is compromised.

3. Mould Issue


When troubleshooting a mould keep the following in mind:

Single cavity moulds are easy to troubleshoot because there is only one cavity to investigate and the runner is short so it is a simple system to work with.

However,multi-cavity moulds are more difficult because they are extremely sensitive to any type of faulty mould design or poor mold building such as small differences in wall thickness from cavity to cavity due to machining error.  On top of this, multi-cavity moulds have a longer runner system than in single cavity moulds and shorting is often due to a fault in the runner system.


4. Injection Molding Machine


Shot Size Too Small For Molding Machine.


Explanation: If the shot size is less than 25% of the injection units maximum shot capacity then it will be difficult to maintain a stable process.An injection screw needs a reasonable distance to move to be able to maintain control over speeds and pressures.

Action: Move mold to a machine with a smaller shot size capacity.

Uneven Tie Bar Stretch


Explanation: Uneven tie bar stretch will produce uneven clamp pressure on the mould. Assuming a symmetrical part is being molded such as a square lid, the short will occur on the side of the tie bar providing the most clamp.

Action: Get the machine service agent to check or check yourself by using 4 dial indicators. Attach the magnetic base of each dial indicator to a stand which is sitting on the floor at the end of the machine and put the stylist of the dial indicator on the end of each tie bar.

Set each dial indicator to zero.

Clamp the mould and read the tie bar stretch directly off each dial indicator.

They should be within 0.05mm (0.002 inch) of each other

Note: it is important that the mould, plate or ring that is being clamped in the machine has been checked to be flat. If it is not it will give a false tie bar stretch.

Damaged Molding Machine Platens


Explanation: During mould installation and removal, platens can be knocked about by the mould causing some impressions. These impressions have high spots around them which will cause the mould to clamp up unevenly. This uneven pressure can cause short shots.

Action: Always clean the platens with a suitable stone to remove any high spots before installing the mould.

5. Part Design Issue


Explanation: Uneven wall thickness is a common reason for shorts due to part design. Uneven wall thickness puts uneven forces inside the mold cavity potentially forcing one side of the mold to open during the filling and hold phases of the molding process.

Action: Increase clamp tonnage. If this is not possible then make the wall thickness even but this can be very costly and time consuming because both the part design and mould will need to be modified.

Additional Comments


Injection molding troubleshooting can have a huge affect on cost savings and your production performance.

Although it can be costly to get the root causes of short shots repaired the question you have to ask yourself is:

What is the long term cost of not fixing them?

From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

Troubleshooting Part Quality – Flashing

Using Injection Molding Troubleshooting To Improve Production Performance

Fix Your Flash.

In injection molding troubleshooting, flashing is one of the most common quality issues.

A plastic part is said to have flash when plastic extends beyond its normal shape.

Flashing occurs when plastic material flows across the shut-off surfaces of the injection mold.

Eliminating part flash can have huge cost savings and should be part of a lean manufacturing program.

But how to Fix Flash?

The root causes of flash can be grouped into 5 different areas:

  1. Poor plastic material selection
  2. Incorrect processing parameters
  3. Mould issue
  4. Moulding machine issue
  5. Part design issue

Isolating which area is causing the flash can be quite difficult so it must be done in a step by step procedure eliminating each possible cause one at a time.

When doing injection molding troubleshooting, keep an open mind because the cause might be different to what you think it is.

1. Poor Plastic Material Selection


Plastic Material With Low Viscosity


Explanation: A material with low viscosity flows easier into a mould cavity than a material with higher viscosity. Many things effect the viscosity of a material such as barrel temperature but the grade of plastic material used also plays a role.  Melt flow index (MFI) is often used as a guide to measure the ease of flow but this does not give the full story. MFI tests are performed under low shear rates but injection moulding uses higher shear rates during the injection fill stage. Some materials behave differently under different shear rates so only use the MFI as a guide.

Action: Change to a lower MFI plastic material. A lower MFI will be less likely to flow across shut-off surfaces as it is more viscous but injection pressure will probably increase.

Keep in mind some additives can also change the MFI so of you are mixing before feeding into the machine then be aware the flow properties might change.

2. Incorrect Processing Parameters


The first thing to do when flash is an issue is to compare the current process parameters with past process parameters that were documented when quality parts were first produced.

When using this injection molding troubleshooting guide below change one parameter at a time by a small amount (say 5%) until you get a result.

Shot Size Too Big


Explanation: Injecting more plastic material than required will overcome the clamp tonnage and open the mould allowing flash formation.

Action: Reduce screw start position. For example from 80mm to 75mm. This will reduce the amount of plastic going into the mould

High Injection Speed


Explanation: A high injection speed reduces the viscosity of the material and allows it to flow easier.

Action: Reduce injection speed by a small amount, say, 5%.

High Hold Pressure


Explanation: High hold pressure can force the mould to open slightly and allow plastic to flow across the shut-off surfaces. To check if this is the cause mount a dial indicator between the fixed and moving platens as close as possible to the mould. If the mould opens during the hold phase then reduce the pressure until the flash disappears. 0.02mm opening is enough to give a slight flash around the part.

Action: Reduce hold pressure.

Incorrect Change Over Position


Explanation: Change Over position tells the machine when to change from injection pressure to hold pressure. If the cavity is already full when the change over position is reached then the hold pressure can force the mold to open and allow plastic to flow across the shut-off surfaces

Action: Increase change over position. For example from 20mm to 25mm.

High Barrel Temperatures


Explanation: High barrel temperatures allow the plastic to flow much easier into the mold by reducing its viscosity.

Action: Reduce barrel temperatures in 10 degree increments. Will need at least 30 minutes to get a result as the barrel will take time to stabilize.

High Back Pressure


Explanation: High back pressure raises the plastic material temperature by adding more shear heat to the material during plasticizing which causes it to flow easier into the mold cavity.

Action: Reduce back pressure slightly. Effect should be immediate.

Low Clamp Tonnage


Explanation: Although this could have been tried first and might have fixed the flashing problem, its not a substitute for excessive injection speed or hold pressure.

Action: Increase clamp tonnage by 5 ton at a time.

3. Mould Issue

When injection molding troubleshooting, the mould is usually the first area to checked. Injection moulds can cause flash through mould deflection, poor mould building quality, inadequate mould design, lack of maintenance and poor die setting procedures.


Uneven Cooling


Explanation: If the temperature through out a mold cavity is not even then the plastic flow will give preference to the warmer areas during the filling phase. In other words, the warmer sides of the mold cavity will fill first creating a pressure imbalance which can open the mold resulting in flash formation.

Action: Cycle the mould a dozen times then immediately measure the cavity surface temperature in 4 different places using a pirometer. The difference should not be more than 5 Degrees Celsius. Do the same for the core.

If there is a difference do everything you can externally to improve the water flow – for example, make sure all of the water hoses into the mould are big enough to get good water flow and are not restricted by small water fittings. Also, do not loop any water circuits; make them all individual if possible.

Also, if a mold has not been maintained properly the water channels probably need cleaning. Calcium build-up is common on the inside of water channels and reduces the cooling ability of the mold. Build up can be removed chemically or physically by hand with a drill or some kind of sharp object. Click here to see an example of this (example #3) and to get help with other mould issues (opens in a new window).

4. Injection Molding Machine


When injection molding troubleshooting the molding machine is usually the last area to be checked. After all, if the machine has been producing good parts for years then why should this suddenly be the cause of flash?

The reason is because the condition of a machine will slowly change over a period of time which affects part quality in a very small way at first. Gradually part quality will get worse.

Shot Size Too Small For Molding Machine.


Explanation: If the shot size is less than 25% of the injection units maximum shot capacity then it will be difficult to maintain a stable process.An injection screw needs a reasonable distance to move to be able to maintain control over speeds and pressures.

Action: Move mold to a machine with a smaller shot size capacity.

Uneven Tie Bar Stretch


Explanation: Uneven tie bar stretch will give uneven clamp pressure on a mould. For example, if we assume a symmetrical part is being molded such as a square lid, the flash will occur on the side of the tie bar providing the least amount of clamp.

Action: Get the machine service agent to check or check yourself by using 4 dial indicators. Attach the magnetic base of each dial indicator to a stand which is sitting on the floor at the end of the machine and put the stylist of the dial indicator on the end of each tie bar.

Set each dial indicator to zero.

Clamp the mould and read the tie bar stretch directly off each dial indicator.

They should be within 0.05mm (0.002 inch) of each other

Note: it is important that the mould, plate or ring that is being clamped has been checked to be flat. If it is not it will give a false tie bar stretch.

Worn Molding Machine Platens


Explanation: Often any particular molding machine is used to produce parts from several different sized moulds. A common mistake moulders make is putting a small mould into a big machine and running it for long periods of time.

What eventually happens is the small mould hobbs into the platen leaving it with an impression. And when the small mould is replaced with a slightly larger mold the parts have flash. Click here to see an example of this (example #1) (opens in a new window).

Action: Do a visual check of both platens without the mould in the machine. A hobbed impression is easy to see and you can also feel the edge of it with your fingers. Make sure the platens have been cleaned first.

Damaged Molding Machine Platens


Explanation: During mould installation platens can be damaged by the corners of a mould causing some impressions. These impressions have high spots around them which will cause the mould to clamp up unevenly. This uneven pressure can cause flash.

Action: Always clean the platens with a suitable stone to remove any high spots before installing the mould.

Injection Unit


Explanation: Sudden flashing problems can mean an issue with the injection unit.

Action: Check consistency of cushion position and screw start position. Make sure they are within normal limits.

5. Part Design Issue


Uneven Wall Thickness


Explanation: Uneven wall thickness is the main reason for flash formation due to part design. Uneven wall thickness puts uneven forces inside the mold cavity potentially forcing one side of the mold to open during the filling and hold phases of the molding process.

Action: Increase clamp tonnage. If this is not possible then make the wall thickness even but this can be very costly and time consuming because both the part design and mould design will need to be modified.

Additional Comments


Injection molding troubleshooting can have a huge affect on cost savings and your production performance.

Although it can be costly to get the root causes of part flash repaired the question you have to ask yourself is:

What is the long term cost of not fixing the flash?

From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

A Nucleating Agent Can Reduce Cycle Time And Improve Part Quality

When it comes to trends in the injection molding industry, plastic raw material technology plays a critical role.  Improved part quality at shorter lead times and lower unit costs are 3 of the most important current trends. Material technology contributes to all three.

Polypropylene


One of the most commonly used plastic raw materials is polypropylene resin. Polypropylene has a wide variety of applications and packaging is one of the largest.  A number of major plastic manufactures have developed materials for thin wall applications that claim to reduce cycle time and machine energy consumption while increasing part quality.

For example, one manufacturer claims that their high flow polymers (up to 110 MFI) allow a broader processing window, lower machine energy consumption and give a shorter cycle time. The shorter cycle time and lower energy consumption is because lower barrel temperatures can be used.

Similarly, a plastic injection molding company claim they have achieved nearly 8% energy saving by switching to a different random PP copolymer for their houseware products.  The melt temperature was reduced by 35 degrees celsius while cycle times were cut by 15.5% due to improved flow. That resulted in a 7.7% reduction in energy use. Click here to get help with finding a suitable material for your next project.

Polycarbonate


A polycarbonate is another resin that has been developed for easier processing purposes.  Bayer Material Science have developed a grade that provides ease of colouring as well as high flow characteristics for production of complex part designs.

Other polycarbonate grades have been developed for easier demoulding. These grades are useful when parts are difficult to eject from a mold core. They are available in the basic grade, medical grade and the easy flowing grades.

Polyamides


In regards to polyamides,  one manufacturer claims that their material not only provides a higher flow –which should give easier processing- but also minimizes machine corrosion which can be a problem with flame retardant polyamides.

Polymer Additives


Another important field is that of polymer additives.  They have the potential to increase part quality and productivity. Commonly used additives are slip agents for easier demoulding, process stabilizers for melt and colour stability and nucleating agents for faster cycle times.

Nucleating agent additives effectively reduce the time required for a plastic part to cool from a melt to a solid inside a mould tool. This means the cooling time component of a cycle can be reduced.  Nucleating agents can be added to polypropylene, polyethylene, nylons, PBT and PVC plastic raw materials. Click here to read about a case study that improved productivity by 13%.

Additional Comments


Although there are a number of ways to make processing techniques easier and to improve part quality or reduce energy consumption, few techniques are more immediate than the appropriate change in plastic raw material or the addition of an additive resin.

In any case, they should not be a substitute for poor research and planning at the beginning of a new molding project.  Proper part and mold design, material and machine selection will avoid any need for change in materials or additives at a later time. This will only eat into your profit margins.

From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

Corrosion Resistant Technology For Plastic Injection Molds

Having corrosion resistant injection molds is vital to achieving optimum production rates.

Maintaining corrosion free molds should be an important part of any mold maintenance program.

There are several commercially available corrosion resistant processes used on injection molds. They can significantly reduce the corrosive effects of water and plastics.

Which technique should be used?

I am glad you asked.

The following factors should be taken into consideration when deciding:

  • Process cost
  • Mold component material and size
  • Environmental conditions such as high humidity
  • Plastic material being moulded
  • Future mould maintenance requirements such as welding due to damage or anticipated part design changes

The different types of corrosion resistant processes can be grouped into 3.

1. Heat Treatment



Polished Stainless Steel Cavity

When it comes to corrosion resistant tool steels Stavax stainless steel  (DIN No. 1.2083) provides the best result. When through hardened, Stavax is corrosion resistant and  is designed to be used for core and cavity inserts in injection molding tools.

Ramax (DIN No. 1.2085) is another grade of stainless steel which provides excellent corrosion resistance and is designed to be used for mould bolster plates.

But what can be done with existing injection molds made from other tool steels?

There are a number of options.

Nitriding


Nitriding is another option but can only be applied to some tool steels. These include the most commonly used steels P20, H13, D2, S7, 4130 and 4140. These steels have the correct alloying elements to allow the nitriding process to form a hard case providing good corrosion resistance.

An additional benefit of a hard case is that it provides excellent wear resistance for sliding mold components.

Another advantage of nitriding is that it can be applied to finished mold components without the need for post nitriding machining.

The disadvantage of nitriding is that welding (if not done properly) can reduce the quality of the steel properties and result in mould failure.

Another disadvantage is in identifying nitriding steels in second hand moulds. Attempting to nitride a non-nitridable steel will not damage it but it will probably reduce its hardness level making it weaker and reducing its life.

However, if nitriding doesn’t work there are plating and coating process options available.

2. Plating


Hard Chrome


A thin coating of hard chrome plating will give excellent corrosion protection and can be applied to most tool steels including, P20, H13 and 4140.

Electroless Nickel


A thin coating of hard nickel plating will give excellent corrosion protection and can be applied to most tool steels including, P20, H13 and 4140.

One of the main advantages of electroless nickel plating is that it takes place in a bath so the plating will cover the entire surface area of the mold component including internal water cooling channels. This makes mould maintenance so much easier because it slows down the calcium build up inside the channel and prolongs cooling efficiency and cycle time.

The plating can be as thin as 0.005mm (5 microns) and will be of uniform thickness across the entire mold component. Such a thin coating means that the size of the mold component changes by just a small amount and in most cases will not require machining to reinstate size tolerances.

When machining is required the grinding process will easily do the job.

Electroless nickel plate is commonly used on P20 bolster plates in injection moulds as it is a cheaper alternative to Ramax stainless steel.

Electroless Nickel Teflon


This technique will provide a reasonable level of corrosion resistance but shouldn’t be used in place of electroless nickel unless a low coefficient of friction is also required. A low coefficient of friction can aid in improving plastic part ejection off a mould core.

Hard Anodising (aluminium)


Some grades of aluminium can be hard anodized which will provide an excellent degree of corrosion resistance.

3. Coatings


Dry Film Teflon Spray


Comes in a self spray aerosol can. It can be sprayed onto outside surfaces of a mold bolster to improve corrosion protection. It is non-toxic and easy to do.

Laser Cladding


This technique involves applying a metal or an alloy in its powder form to a mold component.

As well as providing good corrosion protection, this technique also gives a high level of wear resistance.

To find out more go to http://www.hardchrome.com.au/technologies/laser-cladding/

Additional Comments


Planning for corrosion protection during the mold design stage is the best way to guard against corrosion in the long term. Use of stainless steels and having good mould maintenance habits will ensure good quality plastic parts will be produced consistently over the life of the mould.

When Stavax stainless steel is not an option due to price restrictions, there are cheaper options available, which, in some cases can be just as effective in preventing corrosion in injection molding tools.

From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

How to Source Mold Manufacturers in China

Low cost mold manufacturers in Asia have been very popular with western plastic injection molding companies during the past 20 years.

This behaviour has been primarily driven by the opportunity for OEM’s and injection molders to save up to 70% of mold price compared with locally made molds.

As an injection molding company there are 3 ways to approach it:

  • Through your local mold maker
  • Face to face
  • Over the internet

Finding Mold manufacturers Through Your Local Mold Maker


In order to stay in business many western mold making companies have some form of cooperation with Chinese mold manufacturers. This has allowed them to offer lower mould prices to their local injection molding customers.

Although the prices may not be as cheap as going directly to china, there are plenty of other advantages to using your local manufacturer.

Your local mould maker takes all the risk and does all of the travelling. There is no requirement for you to spend time establishing a new relationship with people in a foreign country.

That’s time you can spend running your business.

Most importantly, you have the security to know that your local mold maker will repair the mold if anything goes wrong.

Face To Face


Another way to source molds from China is to go and meet the Chinese manufacturers. If you make the effort to travel thousands of kilometres just to spend time with them in their factory and have dinner and do karaoke then this goes a long way in demonstrating to them that you are serious about business.

Chinese like to become friends first before doing business. A genuine friendship will give you the highest possible chance of getting your mould project contractual requirements satisfied.

The down side of this approach is that it requires a lot of travelling by one person in your company in order to maintain the relationship and to keep an eye on quality.

Because you are dealing with a foreign company, it might be difficult to communicate all of your quality requirements at first, so you will need to keep talking to them to keep them on track. This is most effectively done face to face until you have done several projects together and you understand each other.

Over The Internet


This is the quickest and easiest way.

Do a Google search on mold manufacturers and you will find many Chinese sites. The sites will allow you to upload your plastic part cad drawings so they can provide you with a quote. You can get a dozen quotes from a dozen different mold manufacturers with very little effort on your part with the option of choosing the lowest price. In a matter of weeks, a mold will be on your factory floor ready to go.

Sounds good doesn’t it?

This approach is highly risky however.

The most obvious is quality control.

How do you know the correct grade of steel has been used? After all, most steels look the same from the outside. The wrong grade of steel can quickly crack through water channels causing part rejects.

To make matters worse you try fixing the crack by welding. At best, this method has a 50/50 chance of stopping the leak even if the welder uses the same steel as the mold steel. If he welds with different steel then the chance of sealing the crack and stopping the leak are extremely low.

In fact, it will probably make the crack worse and now you have a mold that can only be used as a boat anchor.

Getting the wrong grade of steel is the worst thing that can happen because it cannot be fixed.

Now you know the consequences of buying the cheapest mold.

Additional Comments


Which approach is best? It depends upon your requirements. If plastic part quantity and quality is low then it makes sense to buy a cheap mould.

However, if part quality and quantity requirement is high then you need a mould that will produce quality parts around the clock without any unscheduled down time.

What ever you do, don’t let price be the driving force because you will pay for it one way or another.

From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold