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

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Plastic Material With Low Viscosity

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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Uneven Cooling

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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

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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.

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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

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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

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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

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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

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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

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Uneven Wall Thickness

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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

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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?

Refer to https://www.improve-your-injection-molding.com/

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

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

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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

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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

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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

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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

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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.

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

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

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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

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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

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Hard Chrome

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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

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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

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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)

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

3. Coatings

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Dry Film Teflon Spray

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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

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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

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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.

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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

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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

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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

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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

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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

What is injection mold cost?

It is the price an injection molder will pay to purchase a mold. But that is not the full story.

If you are an injection molder then you might be interested in reading more so that you can save yourself a ton of money and avoid some serious headaches.

There is a second cost which I call residual cost. These are costs which start once a new mold is in production and occur over the life of a mold. Residual costs relate to any inefficiencies in a mold such as molding rejects or slow cycle time.

Residual costs will increase your cost per part.

The benefit of taking both costs into consideration is to get a realistic picture of your mold makers ability to improve your injection molding business.

Stage 1 – The Initial Purchase Price

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This is the quoted price of a mold plus any freight costs. If a molder buys from a local injection mold manufacturer then freight costs are negligible. If a mold is bought overseas and transported by ship then costs are still relatively low.

The initial purchase price is influenced by the following factors:

• Part size – large parts require a large mold so mold material cost is higher than with smaller parts.
• Part design – complicated part designs will require complicated mold designs so this will make the price high. Whereas, simple part designs require simple mold designs and lower prices (although this is not always the case).
• Part material – if the plastic material is corrosive such as PVC then stainless steels will be required and they are expensive.
• Part quality requirement – will require quality mold steels, quality machining techniques and quality injection mold design – the purchase price will be a premium. Molders with low part quality requirement often buy cheap molds but be careful about doing this as this can cost more in the end due to slow cycle times and the cost of producing large quantities of rejects.
• Annual quantity requirement – high volumes will require high cavitation and therefore a bigger mould. High volume requires a mold with good internal strength so high quality materials are required which increases the injection mold cost.
• Cycle time requirement – fast cycle times will require well designed and built molds which will be a premium cost.
• Molding machine size – mold should be big enough to support high tonnage machines. Mold should cover most of platen area to ensure quality parts are produced over the long term. Small molds in large machines will give quality problems because small molds damage the machine platens. Damaged machine platens will increase the reject rate in all molds.
• Country of manufacture – low cost countries will be a lot cheaper than western countries.

These costs are easy to see and understand. However, what is not so easy to see and understand are the costs associated with poor mould quality –called residual costs.

Stage 2 – Residual Costs

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Residual costs occur over a period of time and start from first mold trial.

A well designed and built injection mold will have a low residual cost because it will be easy and quick to setup, easy to start, have a low reject rate, have fast cycle time and the mold will consistently perform well beyond its required life expectancy.

Whereas, a poorly designed and built mold will have a high residual cost because it will have long setup times, be difficult to start, have high reject rates and a slow cycle time. And you can be sure that if a mold is delivered in this condition it will only get worse day by day.

These types of molds cannot be fixed.

These molds are usually cheap and if bought overseas there is the cost of travel – the molder must pay an airfare to meet the mold maker and sign a contract or to give final mold quality approval.

What’s more, there are other high residual costs and the effects, of which, are difficult to measure. These include the effect on employee morale (due to time pressures and unsafe work practices) and the cost of damage to molding machines (poorly made moulds will usually not be flat and this will slowly destroy tie bars, platens and toggle clamping systems).

And that’s not all, damaged molding machines will gradually damage all other moulds which will increase part reject rate day by day.

Additional Comments

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Injection mold cost is really about taking into consideration long term performance not just the initial purchase price. Having this information will give the best chance of making the right decision in future mold projects.

Remember, cheap molds make expensive parts and expensive molds make cheap parts.

Which one do you prefer?

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There are several benefits to using a hot runner mold as opposed to a cold runner mold. When designed and built correctly, hot runners are easy to use and allow an injection molding company to save costs and increase productivity.

Eliminates Cold Runner

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Not having to deal with a cold runner saves money in a number of ways:

• no wasted plastic material
• no need to pay someone to make regrind
• no sprue picker robot required
• faster cycle time for increased productivity levels

Thinner Walled Parts

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A hot runner makes it easier for a molding machine to inject plastic into a mould cavity. A hot runner increases the capability of a molding machine. It reduces the plastic flow length so a molder can save material by making thinner and lighter parts.

Shorter Cycle Time

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A shorter opening stroke can be used as there is no runner to eject saving cycle time.

Injection fill times are shorter because the plastic flow length is shorter which also saves cycle time.

What’s more, for high cavitation moulds runners need to be thick and cold runners will need a long cooling time before being solid enough to eject from a mold.

Hot Runner Manufacturers

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There are more than 20 hot runner manufacturers making high quality hot runners around the world today. Each one can custom make a hot runner to fit a new mold design.

The high number of manufacturers means competition keeps prices under control. Good support is also available from most of them.

Mold Makers Can Make Hot Runners

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Some injection mold makers are capable of making their own hot runners. The advantage is that one company can take full responsibility of the entire mold and it reduces the hot runner price.

To make a hot runner requires standard tool making equipment such as a radial drill, cnc machining center, a surface grinder, a cylindrical grinder and a cnc lathe. All heater elements can be bought and a person qualified in electrical wiring can connect them to the temperature controller.

The materials required to make a hot tip hot runner are P20 for the manifold and H13 for the nozzles and insulators. The nozzle tips are usually made from a copper alloy but can also be made from H13 and thru hardened for long life.

Other Benefits:

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Other benefits of hot runners:

• hot runners available for both single cavity and multi cavity moulds
• can be used with most plastic materials and part designs
• are easy to maintain
• reduce the number of moving plates in a mold
• gives good gate vestige
• can use with close pitch mold designs

Challenges With Hot Runners

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Hot runner molds involve the use of electricity so some training will be required for die setters and process technicians so that safety procedures are followed.

A hot runner mold will have a bigger mold height compared to a cold runner mold so this may limit which molding machines can be used due to the larger physical size of the mold.

Additional Comments

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The decision to use a hot runner or cold runner must be taken on a case by case basis but the fact is that a molder having a medium to high annual quantity requirement for any particular part will benefit from a hot runner mold.

The extra cost of a hot runner will be recovered through faster cycle times, better quality parts and material savings.

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3 plate mold designs are used in multi cavity cold runner mold tooling when a 2 plate mold design does not permit a suitable gate location.

3 Plate Mold Example.

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Figure 1 shows an assembly section of a three plate mold producing a 47mm cap.  This design allows the gate to be placed on the top of each cap.  The caps and cold runner are ejected by their own individual stripper plates during mold opening.

Right click here to download Figure 1 in PDF format.

You will need Adobe Reader (the latest version is recommended) installed on your computer in order to open and read this file. You can download Adobe Reader here (a new window will open so you can download it without leaving this page).

If you want to open the file in your browser window, just click on the link (not all browsers have this feature). However, if you want to download the file to view later, then right-click on the link and choose “Save Target As” or “Save File As.” Then select where you want to save the file on your hard drive.

Once you have saved the file, locate where you saved it, and double click to open it.

In order to print, open the downloaded file, and select the “Print” option from the e-book

During the opening sequence the first split is between the cavity plate and the stripper plate (for runner) due to spring pressure (see spring in figure 3) for the distance defined by limit screws (A) in figure 2.  At this stage the runner is exposed but held firmly to the mold by the sucker pins (the sucker pins are labelled in Figure 1) .

Right click here to download Figure 2 in PDF format.

As the mold continues to open, the 2nd split is between the cavity plate and the stripper plate (for cap) for a distance defined by limit screws B in figure 3.

Right click here to download Figure 3 in PDF format.

The 3rd stage opening is between the back plate on the fixed side and the stripper plate (for runner) for a distance defined by limit screw (C) in Figure 4 which ejects the runner off the sucker pins. The runner falls to the ground.

At this stage the mould is fully opened, so the KO bar must be initiated by the machine ejector to remove the caps from the mold tool which is the 4th.

Should I Use a 3 Plate or 2 Plate Design For My Part?

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When designing a cold runner mold tool a 2 plate mold design should be considered first because it is easier and cheaper to make.

One potential limitation of a 2 plate design is that the gate must come from the side of the part which has the potential to cause the following quality issues in some parts: weld lines, jetting, unfavourable shrinkage rates and wall thickness variation due to core shift in tall parts. Longer flow paths may also be required which can place higher demands on the injection molding machine and consume a lot more energy.

Having said that, the 2 plate mold is suitable for many types of plastic parts.

The advantage of a three plate mold design is that it permits the gates to be located on top or bottom of the part at any point on the surface.  Well placed gates will produce quality parts every cycle.

Figures 5 & 6 are an example of a 2 plate & a 3 plate mold design respectively for the same 47mm alcohol cap. Notice the different gate locations.

Right click here to download Figure 5 in PDF format.

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Heat treating tool steel is a very complicated task. Choosing the right type of heat treatment in plastic injection mold making can also be very difficult if you don’t have the knowledge.

The bottom line is that the heat treatment selected for an injection mold must keep the mold in good working condition for the life of the mould.

So how to choose the right treatment for your mould? I am glad you asked.

The type of heat treatment depends upon the following factors:

• Choice of tool steel. The choice of steel largely dictates the type of heat treatment.
• Mould price. This should take into account the type of heat treatment. If the price is too low then there is probably not going to be any heat treatment at all.
• Part design. If part has deep undercuts or threads then mould design will need some moving components in order to release the part from the mould. Moving components require special attention so that the surfaces don’t wear out quickly or get damaged.
• Mold design. This is heavily influenced by the part design.
• Annual production quantities. Small quantities can use softer, less expensive materials but high volume moulds require long lasting materials and heat treatments.
• Environmental factors. Does the molding take place in a corrosive environment?
• Mould maintenance issues such as corrosion.

Types Of Heat Treatments For Tool Steels

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1. Surface treatment – the surface of the work piece becomes harder than the inside. Treatments include case hardening, nitriding, flame hardening, hard chromium plating, nickel plating, titanium nitride and titanium carbide.
2. Through hardening treatment – gives uniform hardness throughout the entire work piece.

Below are 4 of the most common steels used to build injection molds and their recommended treatments:

P20 Heat Treating Tool Steel

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P20 – (DIN No. 1.2312) – supplied in the thru hardened and tempered condition at a hardness of 310 HB (34HRC). It has good polishability, photo-etching properties (for surface texturing).

This steel is mainly used for the mold bolster (it is a holding steel) but can be used for core, cavity, gate inserts, sprues and sliding inserts in moulds with shot production quantities less than 500,000 per year such as in automotive and home ware products.

P20 can be hard chromium plated which is useful for mould reconditioning purposes.

P20 can also be electroless nickel plated for extra corrosion resistance. Because electroless nickel plating takes place in a bath, the plating will also cover internal water cooling channels, which makes mould maintenance easier. The plating will add 0.005mm per side or 0.01mm to plate thickness.

Sliding inserts made from P20 should be nitrided for wear resistance and guard against possible damage when using with a P20 bolster.

It can be welded which is good for repairs. It can be flame hardened or nitrided for extra resistance to wear and erosion. A nitride surface also increases the corrosion resistance.

H13 Heat Treating Tool Steel

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H13 – (DIN No. 1.2344) – is a through hardening tool steel which has excellent hot tensile properties, high hot wear resistance , adequate toughness and resists tempering at high operating temperatures.

These properties makes this steel an excellent choice for cores, cavities, stripper rings, sliding parts or rotating cores in moulds designed to produce millions of parts per year at fast cycle times. Thin wall molding is an example of this type of application (containers and cutlery).

H13 can be nitrided. Nitriding is required if an H13 sliding component moves within another H13 component to prevent damage. For example, if an H13 moving centre core is fitted to an H13 core block then the normal procedure is to nitride the centre core. Without nitriding, the centre core will, in effect, weld itself to the H13 core and all movement will stop.

With great difficulty, the centre core will then have to be separated from the core and all damage (also called “pick up”) will have to be machined out. There is a good chance that the centre core will not be recoverable and a new one will have to be made.

H13 can be hard chromium plated which is useful for mould reconditioning purposes. It can be used to rebuild worn interlocking surfaces between a core and cavity. But be careful about using it around shut off edges because it is prone to chipping at corners during machining.

H13 is more expensive than P20 but the extra cost is more than offset by the outstanding performance of this steel.

Recommend through hardness 48-52 HRC. A hardness of 52 HRC will give longer mould life but it is harder to perform finish machining operations at this hardness compared with 48 HRC.

Welding is possible but proper precautions must be taken (elevated working temperature, joint preparation, choice of consumables and welding procedure).

Ramax Heat Treating Tool Steel

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Ramax – (DIN No. 1.2085) – is a through hardened stainless steel that offers good corrosion resistance which prevents clogging of water cooling channels that could otherwise affect cycle time consistency and mould maintenance.

It is a holding steel and is supplied with a uniform hardness of 340HB(38HRC) (which is more than P20 ) so it is a more durable steel for mould bolsters and gives a longer life time.

Welding is possible but proper precautions must be taken (elevated working temperature, joint preparation, choice of consumables and welding procedure).

Polishable, but only recommended for parts requiring low to medium polishing demands.

Stavax Heat Treating Tool Steel

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Stavax – (DIN No. 1.2083) – is a through hardened premium stainless steel with good corrosion resistance, good polishability and good wear resistance.

The combination of these properties gives a steel with outstanding production performance. The practical benefits of good corrosion resistance in a plastics mould can be summarized as follows:

• Lower mould maintenance costs. The surface of the cavities maintain their original finish over extended running periods. Molds stored or operated in humid conditions require no special protection.
• Lower production costs. Since water cooling channels are unaffected by corrosion (unlike P20 steel)heat transfer characteristics and therefore cooling efficiency are constant throughout the mould life, ensuring consistent cycle times.

These properties makes this steel an excellent choice for cores, cavities and stripper rings in moulds designed to produce millions of parts per year at fast cycle times (containers and cutlery).

Its suitable for molding corrosive materials such as PVC and abrasive filled materials.

Its good polishability makes it suitable for optical parts such as camera and sunglass lenses.

It can be photo-etched and welded.

It is more expensive than H13 steel.

Recommended through hardnes is 45 – 54 HRC

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

The most important first step in plastic parts design and material selection is to list the environmental conditions that the part will be exposed to.

For example, will it be subject to elevated temperatures? If so, for how long and how often? Plastic behaves differently at different temperatures.

This is just 2 questions of several that need to be answered before one is in a position to select the most suitable plastic material.

Plastic part failure in the field is often a result of poor research.

Delivery lead times are becoming so short  that often plastic parts design and the material selection process are not given the attention and time needed to make proper decisions.

This is where the process of Concurrent Engineering can be of great benefit to a new product project. Concurrent Engineering brings together the mold maker, molder, part designer, plastic supplier, marketer and end user so that the proposed plastic parts design can be analysed for economic and technical viability. That is, whether or not it can be made at a particular cost so that all parties involved can make a profit.

Part Shape

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The next step is to decide on the shape of the part and the wall thickness. This can be done by people who have the specialized knowledge such as molders, mould makers and industrial designers.

The use of Finite Element Analysis (FEA) can also be of great benefit. FEA will identify any potential weak areas in the part design. Keep in mind however, the computer generated results are only as accurate as the information that is fed into it. So get accurate information.

Mold Design

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When you are confident that the part will work in the field, an injection mold maker must check that the part can actually be made. A mould needs to be designed around the part. Where is the gate to be located? Is more than one gate required? Where are the parting line surfaces? How will the part be ejected? For complicated part shapes, computer simulation techniques must be used.

Once these mold design points are answered, make sure the plastic material selected is capable of filling the mould cavity. Some materials such as polycarbonate are very viscous so a thick wall section is required to be able to make full parts.

In addition to this, in order to have long term reliable production, there are a number of other things to consider. After all, we do want a project to work don’t we?

Draft

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Draft can be the enemy for part designers. Too much draft on a cosmetic part can detract from its appearance. But the fact is, draft is required to make quality parts at fast cycle times.

Draft angle should be at least 0.5 degree per side but 3 degrees is better. The greater the draft the fewer scratches and marks will be present on the parts.

What’s more, draft is required for stacking purposes. Parts with large draft angles can be easily stacked and separated. They also reduce the combined stack height of the parts so that  transportation costs are kept as low as possible.

Uniform Wall Thickness

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The easiest parts to mould have uniform wall thickness. If, for some reason, the part has to be thicker in a particular section, then consider locating the gate in this section so that sink marks can be avoided.

There are some other circumstances when uniform wall thickness does not apply. Ribs are often made thinner than the nominal wall thickness to reduce the sink mark which detracts from its appearance.

Radii

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Any sharp corners on the part will not only make it more difficult to eject from the mould, it will also create stress concentrations and likely lead to part failure.

There should be at least 0.5mm radii everywhere. Bigger radii will make the part stronger and increase its lifespan. Bigger radii will also help with processing by improving material flow around corners.

Dimple

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Gate brittleness ( which can cause part failure) is the result of stress set up in the gate area. In practise, these stresses are usually caused by the application of incorrect moulding conditions, such as too high or too long hold pressure. Risk of gate brittleness is substantially reduced by having a dimple opposite the gate by locally increasing the wall section by 40%.

Standard Rib Design

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The main function of ribs is to improve the rigidity and strength of a molded part. They may also be used to help flow during processing and decrease part warpage.

Ribs must be carefully designed in order to minimize sink marks and stress concentrations.

Minimizing sink marks is especially important for parts that have a perfectly clean matt or glossy finish such as housings for electrical appliances. The width of the rib can be as low as 50% of the adjacent wall in order to reduce the effect of sink.

Keep in mind however, that with such a thin rib, the part will be harder to process so good mould design and molding machine capabilities are critical. The machine must be capable of applying adequate hold pressure in the rib area during processing.

Ribs have the potential to create venting issues so mold design must take this into consideration.

Alternate Rib Design

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The standard rib design is very effective in creating strength in a particular area of a part. However, this type of rib can cause quality issues when venting is not taken into consideration. It can also create a high nest height which means fewer parts will fit on a standard transportation pallet which increases freight costs.

An alternative rib design is often used in the food packaging industry. To minimize warping, an angular panel is designed into a lid which acts just like a standard rib but will not cause quality issues. The lids will also maintain a low nest height.

A lid also has another built in rib which is the skirt wall. The skirt wall will also aid in minimizing warp.

Conclusion

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The most critical aspects of plastic parts design is to know where it will be used and what conditions it will be exposed to. Without this knowledge there is no chance of a successful moulding project.

At the start of a new part project, here is a list of questions to answer before selecting the plastic material:

1. What temperature range will the part be exposed to and for how long?

2. What are the mechanical loads?

3. What are the nature of loads – cyclic or constant?

4. Direction of loads – does the load act in one or more axis?

5. Environmental factors such as sunlight, water or chemicals exposure?

6. Electrical requirements?

7. Tolerance requirements? Does the part fit with other parts?

8. What are the long term effects on the chosen material? Is there any information available?

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