Don’t Worry, It’s Cool – The Power of Conformal Cooling

In today’s world of plastic injection, there are several factors driving long cycle times. Thermal control of the mold is the most influential aspect. With the inability to evenly cool an injected part, the cycle times run long, warpage is increased and the end product is plagued with tension and stress which often ends with parts failing quality control.

Conformal cooling technology provides the industry greater thermal control over injection molds. The increased control is accomplished through the manufacture of conformal cooled inserts custom tailored to fit each and every need. Using cooling channels in places and shapes that conform to the geometry of the part being manufactured ensures greater control over these specific, hard to reach areas. Traditional or conventional tooling simply cannot achieve the shapes, paths, and channel geometries possible with conformal cooling.

Having control of both the hot and now the cold halves you have full process parameter control.  Your design solution places cooling (or heating) channels at the optimal distance from the part surface, allowing the mold to maintain a targeted, consistent temperature for complete thermal control.

Conditions sometimes dictate that heating is required within the mold and conformal heating channels operate in tandem with conformal cooling channels as well. Conformal heating can assist in maintaining a molten flow front of resin as it enters a hard to fill area of the tool. This takes place much like a hot manifold operates. The heating element, in this case high-temperature oil, will run through channels that follow the shape of the part to increase surface temperature of the mold. As the resin passes through, it still maintains the melt temperature needed to properly flow and fill tight areas, such as an automotive speaker grille.

Shown here (on the left) is an example of an insert that originally contained conventional cooling channels. Beneath the image showing the circuits you can see the thermal effect the resin has on the insert in question. To the right you will see the counterpart with conformal cooling channels to address the key warm areas of the mold insert. Beneath the channel image again you can see the thermal effect on the insert. Side by side comparison shows the conformal channels provide much greater thermal control and reduced delta when used with injection molding.

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

The Importance of Precise Hot Runner Temperature Control

Hot runners are known to offer many processing advantages. However, to maximize your productivity and production quality, maintaining precision temperature control is a critical component for these systems to work as designed. Maintaining precise temperature control is known to affect part quality and processing as temperature directly impacts processing variables associated with the Resin Characteristics and Hot Runner System.


All thermoplastics degrade under elevated temperatures. A well designed hot runner system eliminates hot spots from its design and precise temperature control will help to ensure temperature uniformity of the system during processing. Temperature uniformity is important because it allows you to bring the overall system temperature closer to the lower processing temperature limit. The greater the heat history of the polymer, the more the properties of the polymer will have been weakened.

Additionally, as the plastic melt is forced along the hot runner channel under pressure, it is subject to shear which also generates additional heat. Precise temperature control becomes especially challenging when processing shear sensitive or highly viscous polymers and additives. Otherwise polymer properties will deteriorate and part quality will be affected.


In a multi-cavity system, natural flow balance means the same flow length, same channel diameter and the same number of turns from the machine nozzle to each mold cavity. In conjunction with temperature uniformity across the entire system, this leads to uniform filling and back pressure conditions in each cavity, uniform part quality and the widest processing window. Where changes in material or injection parameters exist (like temperature) these changes will cause quality problems such as poor weight consistency.

The hot runner and mold cut-out are dimensioned so that all hot runner components are perfectly aligned at the specified operating temperature. To allow for heat expansion, sliding or rotational pressure seals are used between hot runner components. If excessive temperature variations exist in the system, it can cause leakage or even component fatigue and/or failure. The nozzle center axis must remain in an absolutely fixed position, in exact alignment with the gate at processing temperature.

Fast Response is Crucial

Injection molding is a cyclical process. The hot runner must maintain accurate and uniform temperature conditions through both the heating (injection) and cooling (hold) phases of the molding cycle. Since the gate is the most critical areas of the hot runner, it is essential for the TC to accurately measure gate temperature. For quick and accurate response to temperature over or under swinging, closed-loop temperature control is required for each and every nozzle. The quality of the molded part is directly related to temperature control response efficiency.

Unlock your Operations Full Potential

Mold-Masters offers precision temperature control through our comprehensive line of advanced TempMaster Hot Runner Temperature Controllers. They have the ability to optimize the performance of any hot runner system.

All TempMaster hot runner temperature controllers feature APS Technology which maintains set point for precision temperature control of 1 to over 500 zones. APS is a proprietary Auto-Tuning algorithm that continuously monitors, learns, predicts and automatically adapts to process variables every 20mS.


Benefits include:

  • Precise 1°F Control Accuracy
  • Superior System Reliability
  • Enhanced Part Quality & Gate Vestige
  • Improved Part Consistency
  • Reduced Scrap
  • Lower Power Consumption
  • Maximized Profit Margins
From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

Top Things to Consider When Assessing Ejector Pins for Breakage

Ejector pins are a critical component to the injection molding process. An ejector pin is driven into the rear of a mold cavity to force the finished piece out and is also often referred to as a knockout pin. Having pins you can count on can make the difference between manufacturing a product efficiently or creating future unwanted costs.

If you’re experiencing ejector pin breakage here are some things to consider when assessing your pins:

  1. What is the application? Are the pins supporting or ejecting?
  2. Is the mold design balanced (symmetrical)?
  3. How much of the working pin length is being used for ejection stroke vs. overall ejection stroke designed for the mold?
  4. How much of the ejector pin ‘working’ area is supported?
  5. Is guided ejection being employed within the ejector assembly?
  6. Is the ejector plate assembly experiencing deflection?
  7. Is the ejector pin experiencing deflection during injection or ejection?
  8. How is the ejector assembly returned? Springs, early ejector returns, return pins, press pull-backs, homemade method?
  9. What is the clearance around the ‘fit’ area of the ejector pin?
  10. Is the bore finish approximately 0.4 micron?
  11. Were the bores for the ejector pin jig bored or CNC to assure proper alignment and concentricity?
  12. Is the ejector pin a vented location?
  13. What is the polymer being molded? Is it ‘sticky’?
  14. Is the ejector pin being used in an area such as a ‘gate pad’ where packing pressures may be high and long in duration, creating deflection of the pin?
  15. Did just one pin break or a series of pins? Are they within the same quadrant of the mold if applicable?
  16. Consider a longer supported ‘working’ pin length and consider a longer shoulder
  17. Always test the pin hardness on a flat surface area of the pin and test with the appropriate hardness testing load on a scale, such as 60 kg. Do not use a heavy 150 kg load on the scale as it will provide an inaccurate reading.
From Website
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

Correct Plastic Injection Mold Design for Interlocking

In plastic injection mold design, interlocking refers to the way the fixed half and moving half of a mold are locked together.

Interlocking makes use of angled surfaces on both halves of the mold which engage when the mold is closed.

The purpose of interlocking is to prevent any small sideways movement of the moving side relative to the fixed side so that part wall section and weight remains even for every cycle.

The fact is, when one side moves relative to the other side, part quality problems will get worse everyday.

Interlocking is one of the most overlooked areas of plastic injection mold design. Plastic parts design is another overlooked area. Click here to learn about plastic parts design.

Interlocking must rigidly lock the fixed and moving sides of a mold together every cycle. Correct interlock design will ensure quality parts are produced for the expected life of the mold.

Correct interlock design also depends on using the right tool steels and heat treatments. Click here to learn more about heat treating tool steel.

Guide Pins And Plastic Injection Mold Design

In general, guide pins (sometimes called leader pins) do not accurately align the fixed and moving sides together so if they are used in place of proper interlocks expect some part quality problems such as wall thickness variation. There is a small amount of clearance between a leader pin and bush so this will allow movement between the fixed and moving sides of the mold.

Guide pins are used during assembly and disassembly of the mold so that the mold maker does not damage the core or cavity during mold maintenance.

Once the mold is in the molding machine the guide pins could, in theory be removed as they serve no purpose after installation – in fact some food packaging injection molders remove them so they don’t contaminate the parts with grease during ejection.

Guide pins have clearance on them that’s why they slide into the bush easily. Proper interlocking design will not allow sideways movement between the 2 halves of the mold.

Interlocking Case Study

Figures 1 and 2 show two different interlocking design for a standard square food container part.

food container mold
food container mold

Both designs have a stripper plate which is used to eject the container off the core. The only difference between the 2 designs is the interlocking design on the stripper plate. Click here to learn more about stripper plates.

Which one would you use?

Which design is more likely to make poor quality parts and which one will give long term quality results?

In theory, both plastic injection mold designs should work. The problem is however, for both designs to work everything must be perfect. That is, all mould plates must be flat and free of damage. The machine platens must also be flat and free of damage. The machine tie bar stretch must be equal in all 4 tie bars – and this is usually not the case especially when machines have not been maintained properly.

Uneven tie bar stretch or any damage on the mould or platens will put the mould on a slight angle when clamp tonnage is applied creating sideways forces when the cavity is filled with plastic. If the interlock design does not allow for this, then the stripper plate will be forced open on one side creating a GAP and the parts will flash.

See figure 3 where interlock A forces interlock B to open allowing flash to occur.

food container mold
food container mold

However, figure 4 shows a design which will not allow flash even if the there is some sideways force because the gap is created on the other side at interlock C where there is no molding. In this tool design, any sideways force will act towards the centre of the mold so there is no way the stripper plate can be forced open. The force on interlock C is transferred to interlock D holding it closed on that side of the interlock.

This is a plastic injection mold design that will last for millions of cycles provided it is made correctly and properly maintained.

Additional Comments

The sideways movement I am writing about is so small that one cannot see it with the naked eye. It is a vibration. Placing a dial indicator on the mold might show the vibration. In any case, with time, the hard mark wear pattern on the shutoff faces of the core and cavity will show evidence of this movement.

The fact is, both of these designs are easy to make with standard toolmaking equipment but they give a profoundly different result for the molder. So it is critical that correct plastic injection mold design is used so that quality parts can be manufactured for the life of a mold.

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

Injection Mold Design – A Little Known But Costly Mistake

Correct injection mold design is crucial to having a profitable business. A proper design will ensure quality plastic parts will be produced for the intended life of the mould at the expected cycle time.

Click here to see our video example of a well designed mould producing food containers at a 4.0 second cycle & to read about our Mold Design Services.

Correct injection mold design has the following benefits:

  • Quick mold setups
  • Faster cycle times
  • Quality parts
  • Low reject rate
  • High productivity
  • Long mold life
  • Long molding machine life
  • Higher employee morale

Unfortunately, many mold designs have some fundamental flaws which prevent the injection molder from achieving a higher level of productivity.

By learning about one of the most common injection mold design mistakes you can avoid waste and help make your company grow and be more profitable. Click here to learn about another common design mistake.

One of the Most Common Mistakes

Thin Back Plates

Most molds have 2 back plates with one on the fixed side and one on the moving side. Figure 1 shows the back plates in between the molding machines platens.

Figure 1Figure 1

The back plates have 3 main functions:

  • To hold the mold in the moulding machine using clamps
  • To form part of the runner system
  • Support the entire mould against excessive platen deflection

If the back plates are too thin then the resultant repetitive deflection during each cycle eventually causes the following part quality problems:

  • Flashing
  • Shorting
  • Weight variation
  • Voids
  • Sink
  • Balancing issues

These quality problems occur because the platens do not provide enough support to stabilize the mould against cavity injection pressure and clamp tonnage.

What’s more, repetitive deflection causes wear inside the mould. One of the things mould wear does is reduce the effectiveness of the venting. Poor venting will also result in reject parts – especially in multi cavity moulds.

It’s important to keep in mind that during the injection molding process most machines will have some amount of platen deflection. Platen deflection is not necessarily a bad thing, however, if the deflection is extreme then part quality will deteriorate over time. This can happen over a period of years, months or weeks depending upon the injection mold design and selection and condition of the moulding machine.

Platen deflection is usually worse on the fixed side platen because of the weakness created by the location ring diameter and the larger recess behind it. The diameter is quite large in most machines as it is clearance to allow the machine injection unit nozzle to connect to the mould (see figure 1).

On top of that, if the fixed half of a mold is a cavity (as opposed to a core) then this adds to the weakness of the system. A cavity is naturally weak because of its shape.

The combination of these 2 weaknesses require an extra thick and wider back plate on the fixed side to strengthen a mold.

Case Study

Thin Wall Rectangualr LidFigure 2

Figure 2 is a rectangular lid made from a 2 cavity mold. The lid started to severely flash after only 8 weeks from mould commissioning. An investigation found that there was too much platen deflection so the fixed side back plate was replaced on the mold.

The original plate was 90mm (see figure 5) while the new plate was 150mm. The thicker back plate bought the deflection within a manageable level so that quality problems were eliminated.

The main clue to the problem was the pattern of radial brown hard marks found on the back plate – see figure 3. This pattern was a clear indication of too much platen deflection causing the mould to wear on the shut off faces of the cores and cavities resulting in part flash. Figure 4 gives a closer view of the hard marks.

Mold Backplate Deflection Hard MarksFigure 3

Mold Backplate Deflection Hard MarksFigure 4

90mm Thick Back PlateFigure 5

Additional Comments

Example of Mold Deflection PatternFigure 6

Relating part quality problems to excessive platen deflection is not that difficult once you know where to look and what to look for.

This type of issue happens in all types of injection moulds from fast cycling thin wall moulds to slow cycling single cavity moulds and the sign is the same for all of them.

Look for radial lines of hard marks on back plates and also on shut off surfaces of cores and cavities.

Figures 6, 7 and 8 are more examples of the hard mark pattern produced when large platen deflection is present.

Example of Mold deflection PatternFigure 7

Example of Mold deflection PatternFigure 8

Example of Good Injection Mold Design

Good Injection Mold Design - Thick backplateFigure 9

Figures 9, 10 and 11 show a 4 cavity lid mold designed with an extra thick back plate on the fixed side 150mm thick.

This mould has produced millions of quality parts without any problems. Note the hard mark pattern on the back plate in figure 9, it is completely different to the pattern in the 2 cavity (figure 3).

This is how the hard marks on a properly designed mould should be.

Click here to learn how to improve mold design using moldmax copper alloys.

150mm Thick Backplate to Minimize DeflectionFigure 10

Figure 11Figure 11

Click here to learn more about another aspect of mold design called 3 plate mold design.

Have a Question About Injection Mold Design?

Are you having difficulties with mold design? Need help selecting the right steel? Not sure how to design a reliable ejector system?

This is the place where you can ask your question about mould design..

Ask a question and get your free 5 page troubleshooting pdf guide to eliminating short shots plus an 8 page pdf guide to eliminating flash. In addition to this, get my mold design checklist which will ensure you will never forget the important aspects of design.

The pdf files will be available after submission for immediate download. No email address required.

What Other Visitors Have Asked

Click below to see contributions from other visitors to this page…

Support pillars
Is there any specific calculation for using a particular size of support pillars and it’s placing?

Gas vent
How to improve gas vent for part having mirror face?

Life of a wear plate
How long is the maximum life of a wear plate? Answer: The purpose of a wear plate in an injection mould is to act as a sacrificial component in preference …

Gate Type & Mold Steels
Question by Mohamed: How to choose the correct gate type and the correct mold steel? Answer: The type of gate depends upon a number of factors …

Nox-Rust Paper
Question: What problems other than a visual indicator could be caused by using nox-rust paper on the back plates of injection molds? My answer: …

Two Plate Mould Design
Question by Satyajit: How to design a two plate injection mould? My answer: The 2 plate plate mould has one parting line opening as opposed to …

Details about core cavity seating
Question by Rajesh: How to make a core and cavity to accurate seating,details about guide pillar bushes assembly. My Answer: To get accurate seating …

Dimensional Stability
What guidance do you have for achievable limits on DST injection urea machines regarding the number of cavities that can be reliably balanced to give dimensional …

What do you think of diamond coatings used on molds?
I am wondering what your experience has been using diamond coatings on molds. Some coating enhance ejection so help with galling. Do you have experience …

Location rings
Can this be eliminated on a mold? Why do some groups use or not use? -Sam Answer: A location ring can be eliminated on a mold BUT it is …

Family molds
Whats your thoughts on family molds? Answer: You must know what you are doing when designing family moulds. The runner channel should give balanced …

Gate Location
Where is the best position to put the gate? Answer: Design guidelines. Locate the gate in: 1. the thickest wall section so that hold pressure …

Stripper plate ejection: Hydraulic or Pneumatic?
Should I use hydraulics or pneumatics for stripper plate movement? Answer: The easiest way to move a stripper plate in an injection mold is by using …

In what applications would nitrding surface treatment be used? Nitriding is useful for moving components in a mould. Typical applications include …

Where is the best location for venting in a mould? Answer: The vents should be located at the last areas to fill in the mould cavity. In this …

Mold material
What material is best to build a mold with? Answer: The mold material mainly depends upon the annual shot quantity requirement. But it also depends …

Hot runner or Cold runner
Which is better, cold runner or hot runner? Barry, It really depends upon your annual quantity requirements. If producing more than one million a …

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

Mold Design Services For Thin Wall Packaging Parts

Let Improve-Your-Injection-Molding mold design services take your business to the next level – its the easy way to get what you want.

Specializing in:

  • 4 and 6 cavity straight face mould design for Thin Wall parts
  • 2+2 and 4+4 stack mould design for Thin Wall parts

Video Example 1

Scroll down to see Stack Molds

 Thin wall 500ml Food Container 4 cavity hot runner mold tool.

Food container details: (see Figure 1)

Weight: 13.0 grams

Wall thickness: 0.45mm (0.0177 inch)

Material:  PP homo polymer

Cycle time: 4.0 seconds (free drop)

Ejection type: 4 individual stripper plates

Annual number of cycles capability: 7 million

Annual production capability:  28 million (4 cavities)

This mold was designed  by Improve Your Injection Molding & built to the Society of Plastics Industry (SPI) Class 101 mold classification.

500ml Food ContainerFigure 1

Example 2.
Thin wall Disposable Rectangular 750ml Tub 2+2 Stackmold

Part weight:  22.5 grams

Wall thickness: 0.55mm (0.022 inch)

Material:  PP homopolymer

Cycle time: 6.6 seconds (robot takeout)

Hot tip hot runner

Annual production capability:  17.5 million (4 cavities)

Machine clamp requirement: 300T


Example 3.
Thin wall Disposable Rectangular Lid 2+2 Stackmold

Part weight:  12.4 grams

Wall thickness: 0.51mm (0.02 inch)

Material: PP homopolymer

Cycle time: 3.7 seconds (free drop)

Hot tip hot runner

Annual production capability:  30 million (4 cavities)

Machine clamp requirement: 250T


Example 4.
Thin wall Disposable Rectangular Lid 4+4 Stackmold

Part weight:  12.4 grams

Wall thickness: 0.51mm (0.02 inch)

Material: PP homopolymer

Cycle time: 5.0 seconds (robot takeout)

Hot tip hot runner

Annual production capability:  45 million (8 cavities)

Machine clamp requirement: 500T


Mold Design For Packaging Products Including Thin Wall

Thin wall products  – Food containers, cups, plates, buckets & lids.

Polycarbonate Drinkware

Caps & Closures

Cutlery – fork, spoon & Knife

Storage Containers 5, 9, 12, 18, 20, 30, 40 litre

2 Shot Mold Design

Mold design outsourcing is becoming more common as the skills gap widens in the plastic injection molding industry.

If you are like many injection moulding & tool making companies you may be searching for the perfect blend of price, delivery time, quality and support.

If you are becoming frustrated in your search for the ideal mould design service then we might have the answer for you.

Get It Right The First Time

If we had found precision mold design services specializing in high volume products back when we did mold making for thin wall packaging, we believe it would have saved us 5 years of frustrations.

We are not exaggerating. It takes a lot of trial and error to work this stuff out and we suggest, if at all possible, you take the shortcut past that stuff and start doing what works.

To be able to injection mold tens of millions of quality parts consistently and at a profit not only requires knowledge of mold design but also knowledge in the other key areas of part design, machine selection & of course processing.

We can advise you in all of these areas.

If you are ready to take your business to the next level & get the mould designs that will help you achieve the results you want from your business then we invite you to scroll down to the contact form.

Mold Design Services for Molders & Toolmakers

This service is perfect for those injection molders who currently make their own molds & plan to light weight their products by manufacturing injection molds for thin wall packaging products and need help with mold design & machine selection.

This service is also ideal for toolmakers (mold builders) who plan on building moulds for thin wall packaging products (with or without IML) or just need extra design capacity.

Existing Quality Issue

If you have an existing quality issue such as wall thickness variation & want to avoid this issue on your next mold design then we can help you!

Risk Free Approach

What do we mean by risk free?

If you get stuck during mold building, mold trialling or even during a production run, we will keep providing the necessary advice required for you to achieve the expected part quality and productivity levels at no extra charge.

Privacy Policy

We know how important confidentiality is to our customers so your information is safe with us.

Example 5

Working Together as a Team

In order to get a design that will be compatible with your business it is critical that we get your input into the mould design.

Such things as expected annual quantity requirement, In-Mold Labeling requirement (IML), mold ejection method & water fitting size need to be discussed before mould design begins.

Mold Design ServicesExample of mold assembly

Example 6.
WuLiangYe Chinese Liquor 3 Piece Cap
16 cavity collapsing core mold

Part weight:  4.8 grams

Material:  K-Resin

Cycle time: 13.7 seconds

Hot tip hot runner

Annual production capability:   33 million

Machine clamp requirement : 150T

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

Stripper Plate In Injection Molding

What Is a Stripper Plate?

A stripper plate is simply a plate that is used to push a part off an injection mold core. In other words, it removes the part off a core, preparing the mold for the next shot. It makes full contact with the outer edges of a part and this makes it a reliable method of ejection in injection molding.

When To Use.

A stripper plate is used when ejector pins or pressurized air will not be enough to eject a part off a core. Examples of parts using a stripper for ejection are caps, containers and lids.

Stripper plates are very common in thin wall injection molding because by their nature these parts are weak so the ejection method requires full contact with the outer edge of the part to remove it off the core.

Parts with thicker walls (greater than 1mm) can usually be ejected with ejector pins and pressurized air but often this is unreliable, as parts do not eject properly 100% of the time – sometimes they can hang up on the core. Moreover, cycle time is usually longer especially with tall parts even when they do eject well.  A stripper plate will eject parts quickly 100% of the time. Stripper plates can be used for both single cavity and multi-cavity injection molds.

P20 stripper plate for injection moldingStripper Plate for 10 Litre Tub Mould

How To Make A Stripper Plate.

It can be made from a number of different types of tool steels – H13, P20, D2 and stainless steels can all be used. The choice depends upon cycle time, annual production quantity requirement and the type of plastic material to be moulded. For example, a stripper plate made from P20 pre-hardened tool steel is capable of achieving well over 1 million cycles before reconditioning is required to correct any quality issues. Thru hardened steels such as H13 are capable of achieving tens of millions of cycles.

A stripper plate can be made with standard machine tools but care must be taken by the machinist to work with close tolerances of 0.02mm (0.001 inch).

It is easier to make a stripper plate for a round part than for a square part.  A round part needs a round stripper ring so a cylindrical grinder can be used. It is easier to hold tight tolerances with a grinder than with a milling machine which is required for square parts.

Making stripper plates for square parts usually requires some final fitting to the core by a hand grinding process called bedding. The bedding process will ensure the exact fitting tolerances required so that quality parts are produced for the life of the mould.

You will need Adobe Reader (the latest version is recommended) installed on your computer in order to open and read this e-book. Yo 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 menu.

How To Move.

There are 4 ways:

  1. with the machines mechanical ejector
  2. with hydraulic pistons within the mould
  3. with pnuematic (air) pistons within the mould
  4. with pull bars

Using a moulding machines mechanical ejectors is one of the most common ways of moving a stripper plate. It is cheap and easy to make and is very reliable. The machine ejectors can connect directly to the stripper plate or to a subset of ejector plates within the injection mold if they do not conveniently align with the mould stripper plate.

The disadvantage of this method is that it can only be used in a limited number of molding machines because machine ejector hole positions are different in different machines. However, this will not be a problem if this fact is taken into consideration during the mold design stage.

Hydraulic pistons within the mold is another common method.  This method is used when the moulding machine ejectors are not in correct position to easily connect to the stripper plate. Some moulds are large and require large distances between the machine ejectors so the stripper plate moves smoothly during every cycle and doesn’t jam or get stuck.  During the design stage, hydraulic pistons are placed close to the 4 corners of the mould so that proper ejection is achieved.

The 2 disadvantages of using hydraulic pistons is that they can leak oil and contaminate the parts and  the machine must have a core pulling system which is used to control the hydraulic pistons.

Pneumatic pistons within a mould is just as easy to make as hydraulic pistons the only difference is the seal  used on the piston.  One is for pneumatic and the other is designed to be used with hydraulics although it is possible to get a seal to be used with both designs.

The advantage of using pneumatics is that any air leak will not contaminate the parts but there is less flexibility on pressure and speed control compared with hydraulics.

The use of pull bars is another cheap and easy way to move a stripper plate but the disadvantage is that they normally limit access to the mould in the machine so when a part gets stuck it is difficult to remove it by hand. Also, a die setter must take care in setting the mold opening stroke or the pull bars will break if the mould is opened past its limit. This is an expensive and time consuming repair.

The Disadvantages Of A Stripper Plate.

Making a mould with a stripper plate is a lot more difficult than making a mould with ejector pins.  If it is not designed and made right there will be constant part quality issues such as flashing.  Cycle time will also suffer.

Moulds with stripper plates require more mould maintenance than moulds without.  There is always a waxy residue which builds up over time behind a stripper plate and this must be cleaned on a regular basis – usually every 48 hours of production.  If cleaning is not done part quality issues will result sooner rather than later.

Additional Comments

Although an injection mold made with a stripper plate will cost more than a mold made with ejector pins, the productivity improvement is significant. As an example, a 20 litre container mould with ejector pin ejection had a cycle time of 45 seconds. When it was converted to stripper ring ejection the cycle time reduced to 35 seconds. That is a 22% increase in productivity.

Refer to

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

Fundamentals Of Plastic Part Design Part 6 – Ejection And Surface Finish

Plastic Part Design Fundamentals

Ejection and Surface Finish



When it comes to successful plastic part design, ejection and surface finish considerations are critical factors to consider.  The most common way to remove a part from the mold is through ejector pins. They apply a force to eject a part from the mold, and in some cases can leave marks, sometimes called ‘witness lines’.

At Rex Plastics, we make a point to design and position ejector pins that minimize their effect on your parts, moving any undesired witness lines to a location where they may not be an issue.

Pins are located in the B-side mold half, the side in which the part will stay when the mold opens. Once the mold is opened, the pins extend into the mold cavity, push the part out, and then retract, allowing the mold to close and be refilled.  The image below shows an example of what the witness lines may look like after pins push the part out of the mold:

injection molding ejector pin lines

Let us know if there are critical surfaces in the plastic part design where witness lines left by the mold ejection could potentially be an issue. At Rex Plastics, we can propose an ejection layout for approval before spending money to build the mold.


Types of Ejection

While pins are most commonly used for ejecting a part from a mold, they are not the only option. The table below shows a short list of other commonly used ejector systems:

Pins Round
Blades Rectangular
Sleeves Tubular
Stripper Plate Moving Plate

Pins are common on most parts however they are not ideal for every situation. Ejector blades are rectangular in shape and can be utilized on thin walls that cannot support a circular pin.

Sleeves are ideal for bosses. They provide a 360 degree bearing surface around the boss which facilitates easy part removal. Stripper plates provide ejection around the entire perimeter of the part. Its goal is to “strip” the part off of the core.


Surface Finish

When it comes to plastic part design and surface finish, there are two main considerations to keep in mind – polishing and texturing.

Polishing:  Polishing is a manual process that removes texture, machining marks, etc. on the mold surface and provides a uniform finish. Polishing can be very time consuming, especially if there are deep ribs in the mold. The picture at the bottom of this post shows a variety of SPI finishes.

Texturing:  There are several ways to texture a mold surface such as bead blasting, EDM, or etching. A surface with heavy texture will require increased draft angles in order to release from the mold.

SPI Surface Finishes

Below, you’ll find some standard mold finish call-outs (priced low-to-high). While these standard SPI call-outs are very common, a wide variety of textures are available. Some textures that can be applied include:

  • Natural\Exotic
  • Micro Surface Finishes
  • Multi-Gloss Patterns
  • Graphics
  • Leather Grains/Hides
  • Woodgrain, Slate and Cobblestone
  • Geometric and Linens
  • Images or Logos incorporated into the pattern

SPI Surface Finish Examples

You will find a few examples of various SPI Surface Finishes in the image, below:

SPI surface finish examples

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

Fundamentals Of Plastic Part Design Part 5 – Gates And Parting Lines

Plastic Part Design Fundamentals

Gates and Parting Lines



Each plastic part design must have a ‘gate’, or an opening that allows the molten plastic to be injected into the cavity of the mold. There are several styles of gates that are commonly used in molding.

Care and consideration should be taken when selecting a gate when designing your plastic part.

Gate type, design and location can have effects on the part such as part packing, gate removal or vestige, cosmetic appearance of the part, and part dimensions & warping. A list of commonly utilized gate shapes is offered below:


commonly utilized gate shapes


Gate Locations

To avoid problems in plastic part design from your gate location, below are some guidelines for choosing the proper gate location(s):

  • Place gates at the heaviest cross section to allow for part packing, also minimize voids and sink
  • Be sure to allow for easy manual, or automatic de-gating
  • Gate should minimize flow path length to avoid unwanted cosmetic flow marks
  • In some cases, it may be necessary to add a second gate to properly fill the parts
  • If filling problems occur with thin walled parts add flow channels, or make wall thickness adjustments to correct the flow

Rex Plastics will analyze each part individually and recommend a best gate design based on the product requirements. If gate appearance is critical, Rex Plastics will propose the optimum location for customer approval.

Parting Lines

A ‘parting line’ is the line of separation on the plastic part where the two halves of the plastic injection mold meet. The line actually indicates the parting ‘plane’ that passes through the part. Within more basic plastic part design plans this plane can be a simple, flat surface, but it is often a complex form that traces the perimeter of the part around the various features that make up the part’s outer ‘silhouette’.

Keep in mind (that) the melt will always flow toward the parting line because it is the easiest place for the displaced air to escape, or vent.

Part lines can also occur where any two pieces of a mold meet. This can include side action pins, tool inserts and shutoffs. Parting lines cannot be avoided; every part has them. Keep in mind when considering plastic part design that the melt will always flow towards the parting line because it is the easiest place for the displaced air to escape, or ‘vent’.

Parting Lines can be split into two broad categories:  Straight/Flat and Stepped/Curved.  Examples of each are shown below:




plastic part design fundamentals stepped curved


Mouse Holes

Mouse holes can be a great way to get a cut-out in the side of a part without requiring a side action. Below is a typical example of a mouse hole.

Keep in mind when designing a mouse hole that there is ample draft on the side walls. In order for the metal to seal off, a minimum of 3° is recommended to increase tool life.

plastic part design mouse hole

a typical mouse hole

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

Plastic Injection Molding As An Alternative To Fabrication

Plastic Injection Molding Costs

Products---Over-molded-TipOne of the most difficult obstacles preventing product developers from having their product injection molded is the initial tooling investment. At first glance, many product developers form the conclusion plastic injection molding is too expensive. These folks are placing more importance on short term needs, most likely during a prototyping phase, when true return on investment is realized once a new product has been brought to market and begins to gain momentum.

Plastic Fabrication Explained

Plastic fabrication is a general term for manually producing plastic products. Plastic fabrication may include the machining, saw cutting, laser cutting, forming and fastening of plastic parts. It is a versatile and effective way to prototype and market test a new product with very little initial investment, but it does not scale well in higher volume scenarios.

Limitations of Plastic Fabrication

The limitations of plastic fabrication come into play when a product actually gains market traction, terrible timing as this should be the goal for any new product. When this happens, the part manufacturer will experience difficulty meeting demand due to the labor intensive nature of fabrication. With so much part-by-part labor involved, capacity to output parts remains low. It’s at that point that people often begin to investigate plastic injection molding.

Transitioning to Plastic Injection Molding

At Rex Plastics, we are proud to have helped many product developers save money by switching their product from plastic manufacturing to plastic injection molding. Our customers are consistently able to achieve a return on their tooling investment – if their sales volume is there to support it. Consider the following scenario:

If a product is fabricated for $2 each, and has a sales volume of 500 units per month, the annual cost of production would be $12,000. If a single cavity plastic injection mold (one that produces one part each machine cycle) can be built to produce that same product for $5,000, and be produced for $.70 each, that product owner can recoup their tooling costs in less than eight months ($5,000/$1.30 savings on each part, which is just short of 4,000 parts).

At that point, the product owner’s margin increases by $1.30 per part, allowing for higher profits or the option of lowering prices to gain market share.

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