Plastic Injection Molding Machine Selection – Why It Is Important

Selecting the right plastic injection molding machine is one of the most important criteria in making quality parts consistently and profitably. The right machine will help keep costs low and make you more competitive. This will allow you to sell more parts, earn more money and at the same time establish a reputation as a quality manufacturer.

Even better, you will have the security of long term customers.

How To Keep Your Costs Low:

  • Minimize reject rate (set a target <1%) Click here to read about how to eliminate warpage quality issues.
  • Reduce energy consumption of all injection molding machines
  • Have faster cycle times
  • Eliminate unscheduled machine downtime –(no breakdowns)

A properly selected plastic injection molding machine will give you all of the above.

Plastic Injection Molding Machine

Common Machine Selection Mistakes And Their Consequences

The best way to appreciate the importance of correct plastic injection molding machine selection is to list the most common mistakes made when selecting machines and the effect on part quality and productivity.

Mistake #1 
Buying Used Injection Molding Machines That Don’t Work

The key to buying a reliable used machine is to find one that has been fully inspected and tested before delivery so that you won’t suffer loss of production due to multiple breakdowns.

Click here to see a case study of how incorrect machine selection can effect part quality Example #2).(opens in a new window)

Mistake #2
Selecting Large Screw Diameters.

Large screw diameters can cause quality problems due to material degradation. When small shot sizes are used with large screw diameters, the plastic material spends more time being heated by the barrel heaters compared to large shot sizes. Material degradation is more likely to occur if the shot size is small compared to the injections units maximum shot capacity especially for heat sensitive materials.

Another problem with large screw diameters are the large shearing forces generated during screw rotation, this can also degrade the material and is likely to lead to reject parts.

Mistake #3
General Purpose Screws

The advantage of a general purpose screw is that they can be used with most plastic materials such as PP, PE, Nylon, PET and PC so they are very flexible and good for moulding companies that mould a variety of different materials.

The disadvantage is that, for some materials, part quality and productivity rates will be lower compared to more advanced injection molding screw designs such as the barrier screw. Click here to learn more about screw selection.

Mistake #4
Injection Pressure Limited.

To consistently make quality parts the molding process must not be limited by the injection pressure. It is advisable to have at least 10% injection pressure in reserve so that the injection molding machine can automatically adjust to normal variation in the plastic material viscosity.

Insufficient injection pressure will produce short moldings.

The injection units screw diameter governs the available maximum injection pressure so it is critical to choose the correct diameter when buying a plastic injection molding machine.

Mistake #5
Inadequate Clamp Tonnage

If clamp tonnage is too low then it will be difficult to produce quality parts. Low clamp tonnage means inconsistent weights, flash, short shots, wall section variation, poor surface finish and size variation.

Machine and mold wear will be excessive.

Mistake #6
High Energy Consumption

Have you ever noticed how there is almost no difference in your cars fuel consumption with 2 people in it compared to just 1 person? A moulding machine is the same. A small mold will require almost as much energy to open and close the platen compared with a medium size mold. A properly selected plastic injection molding machine will use power very efficiently.Click here to learn about how to reduce your energy bill on your existing machinery by changing parameters in the plastic injection molding process.

The type of machine design also plays a significant role in power consumption. There are 3 types of machine designs: fully hydraulic, fully electric and hybrid machines. Hybrid machines are partly electric and partly hydraulically operated. Selection largely depends upon the type and quantity of parts to be molded.

Plastic Injection Molding Machine Selection Process

1. Know the plastic parts you intend to mold
2. Select machine type: Hydraulic, hybrid or electric
3. Calculate clamp tonnage requirements
4. Calculate the injection unit size.

1.Know The Plastic Parts You Intend To Mold

The process of selecting the right machine starts with knowing the particular plastic parts that will be moulded by the machine. Molding parts that are not suited to the machine will result in frustration with on-going quality problems, slow cycle times and machine and mold damage.

If you are planning to buy Cap or Closure Molds then click here to get a quick mold quote from a reputable mold maker. (Opens in a new window)

You should know the part:

  • Plastic material
  • Weight
  • Length x width x height
  • Average wall section
  • Gate location
  • Maximum flow length from the gate
  • Estimated cycle time. Click here to estimate cycle time.
  • Quality requirements
  • Annual quantity requirements. Click here to get to the production calculators used for scheduling.

In addition you should also know the mould size and weight.The correct part information will then allow you to find the injection unit size, clamp tonnage requirements and machine type.

2.Select Machine Type

Types of plastic injection molding machine designs available:

  • Fully Hydraulic
  • Fully Electric
  • Hybrid – combination of hydraulic and electric drive

Fully Hydraulic Machines

The types of hydraulic machines available are defined by the type of clamp design, type of hydraulic pump design and the presence or absence of an accumulator.

Clamp design is either a toggle clamp or by hydraulic ram.

Hydraulic pump design can be a constant displacement pump (also called fixed displacement pump), a variable displacement pump (also called a piston pump) or a servo pump (also called frequency control or RPM control) .

If an accumulator is present then some machine manufacturers use it to drive the entire machine, while others use it for the injection stage only. Accumulators are required for molding of parts which have wall thicknesses in the range of 0.3mm to 0.8mm – this is known as thin wall molding.

Accumulators might also be required for moulding thicker parts but this must be checked on a case by case basis.

Which one is suitable for you?

It depends on your priorities.

Here are some criteria to consider before selecting a hydraulic moulding machine:

  • Buy a new or used machine
  • Purchase price
  • Energy efficiency
  • Plastic material to be processed
  • Part design
  • Mould design
  • Clean room requirement
  • Cycle time requirement
  • Hold time requirement
  • Local service agent capability

If low power consumption is a priority then choose a hydraulic machine with a servo pump as this is the most energy efficient of the fully hydraulic machines. A servo pump will only operate when the machine requires movement, the rest of the time it is using minimal power. However, the purchase price is 10%-15% higher than a hydraulic machine with a constant displacement pump or even a variable displacement pump.

Keep in mind a servo pump control will only save you power if you have long cooling times or machine inactive times such as long take out time for robot.

Fully Electric Plastic Injection Molding Machines

Every movement of the machine is done with direct drive electric servo motors only drawing power when movement is required so they are very energy efficient. Electric machines have excellent repeatability which virtually guarantees consistent part quality.  This makes them suitable for molding medical devices. Click here to learn more about medical injection molding.

Electric machines are suited to clean room operations because there is no hydraulic oil that can leak on to the floor. Click here to learn more about electric molding machines.

Hybrid Plastic Injection Molding Machines

A hybrid plastic injection molding machine uses a combination of hydraulic drive and electric direct drive. The hydraulic drive is used to generate fast injection speeds when moulding thin wall parts. The rest of the machine uses electric drive servo motors and each axis of the machine has its own dedicated motor. For example, the mould opening/closing stroke has its own motor.

These injection molding machines are very energy efficient

3.Calculate Clamp Tonnage Requirements

Clamp tonnage requirements for your plastic injection molding machine can be calculated by:

  • By experience – injection molders and machine manufactures have this information
  • Computer simulation software (Click here to learn more about the benefits of using simulation software.)
  • As a rough guide, a simple calculation Projected Area X Average Cavity Pressure X Number of Cavities. (Click here to find the machine clamp tonnage for plastic parts made from polypropylene).

In order to find the tonnage using the simple calculation the following part information is required:

  • Plastic material
  • Length x width x height
  • Projected area (length X width)
  • Average wall section
  • Gate position
  • Maximum flow length from the gate
  • Number of cavities in the mold

The average cavity pressure is also required and this information is available in some plastic injection molding machine manufacturers hand book for certain materials.

Most injection molded parts average cavity pressures lie in the range of 300-800 bar. Parts with long flow paths and thin walls will have pressures in the upper range while parts with short flow lengths and thicker wall sections will have much lower pressures.

Plastic material selection also influences the cavity pressure.

In addition, you need to know the mold weight and size so that you can check that the mold will physically fit into the machine and that the machine can carry the mould weight. Click here to learn about our mold design services.

4.Calculate The Injection Unit Size

To select the right unit for your plastic injection molding machine you must know part:

  • Plastic material
  • Cycle time
  • Cooling time
  • Shot weight (part weight, cold runner weight and number of cavities)
  • Peak injection pressure requirement
  • Plasticizing rate
  • Injection rate
  • Hold time and pressure

For a specific tonnage machine, manufacturers usually offer 2 injection units to choose from. Both units usually have 3 different screw and barrel assemblies on offer. In order to select the correct assembly, the shot size must be calculated as a percentage of the injection capacity and must lie between 25% and 65% so that good quality parts can be made.

Most screw and barrel assemblies are rated in grams of general purpose polystyrene (GPPS). To calculate the shot capacity for materials other than for GPPS the melt density needs to be known.

Lets assume we have a screw and barrel assembly with a screw diameter of 56mm and the capacity is 510 grams of GPPS. What is the injection capacity for polypropylene (PP)?

Assume melt density of GPPS = 0.945 grams per cubic centremeter

and melt density of Polypropylene  = 0.74 grams per cubic centremeter

(Be sure to use melt density & not room temperature density for this calculation)

Calculation to find injection capacity for PP:

(Density of PP/Density of GPPS)xBarrel capacity of GPPS grams

=(0.74/0.945)x510 grams=399 grams PP

So maximum injection capacity of PP for this 56mm screw diameter is 399 grams.

Next we need to make sure the shot size is within the maximum and minimum limits of between 25% and 65%.

If the shot size for the injection mould is 110 grams (includes cold runner) then the shot size as a proportion of the total injection capacity is:

=(110/399)x100= 28%

which is within limits.

Next, the following 3 criteria must be confirmed to be within limits of the injection unit with the 56mm diameter screw.

  • Injection pressure
  • Plasticizing rate
  • Injection rate

These can be found in one of 2 ways:

The first is by experience – perhaps you have a similar part in production in which case you can read the pressure directly off the screen. Be sure to check the screw diameter is the same.

The second is by computer simulation but keep in mind the results are only as good as the information that is entered. It is advisable to check these results against some real life examples.

Just remember, the screw diameter selected is crucial to long term part quality and productivity.

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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

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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

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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

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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

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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.

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

When to Consider Plastic Injection Molding

Products - Freeman Marine

When to Consider Plastic Injection Molding

Plastic Injection Molding is a diverse process with many possibilities. Often, the precision and repeatability of plastic injection molding makes it a viable alternative to machining.

As a rule of thumb, if you are using 1,000 or more parts per year, you may want to consider plastic injection molding as a way to save money and increase margins. While the initial investment required to produce the tooling can be significant, the reduced part price quickly enables you to start making that money back.

As a rule of thumb, if you are using 1,000 or more parts per year, you may want to consider plastic injection molding as a way to save money and increase margins.

Plastic Injection Molding Saves Money

Case in point: A customer approached us who had been machining 1,500 grommets per year out of Delrin acetal material. Using this method, their cost per part was approximately $2, costing them $3,000 per year.

We were able to build them an injection mold for approximately $2,500 and produce the parts for $.90 each. This allowed our customer to get a return on their investment in just over a year, and save over $1,600 every year after that using plastic injection molding.

Plastic Injection Molding as a Metal Replacement

In addition to the possibilities available to replace plastic parts, there are many plastics that can replace metal.  While plastic injection molding grades of plastic do not yet exist to match the tensile strength of stainless steel, there are many commercially available grades that are actually higher than some grades of aluminum.

While plastic injection molding grades of plastic do not yet exist to match the tensile strength of stainless steel, there are many commercially available grades of plastic injection molding that are actually higher than some grades of aluminum.

Also, keep in mind there are many products on the market today produced in steel that do not require the strength that steel provides, and steel inserts can be over-molded with plastic when only a portion of the product has strength requirements, such as threads.

With these and other facts in hand, many have product manufacturers have found plastic injection molding to be a cost effective alternative to machining metal.  Is yours one of them?

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Fundamentals Of Plastic Part Design Part 4 – Ribs And Bosses

Plastic Part Design Fundamentals

Ribs and Bosses

Rib Design

Ribs are a great way to add strength and stiffness to a part while keeping material consumption to a minimum. However, a word of caution:  ribs can cause sink marks to develop where they intersect the main wall. A rule of thumb to follow is the rib thickness should not exceed 60% the nominal wall thickness. Also keep in mind that thin, deep ribs can be very expensive to add to a mold.  Use the table at the bottom of this article as a guideline for rib design. Still, keep in mind that deeper ribs may require an electrical discharge machining (EDM) process.


Rib Width:  50%-60% of nominal thickness

Boss Design

Bosses are one of the most common features seen in plastic parts. However, similar to ribs if bosses get too thick relative to the nominal wall thickness, sink can occur. A boss-rib combination can eliminate sink marks. By using ribs to connect the boss to a side wall, this method of part design will provide the strength necessary to support screws, inserts, etc. Also, adding small radii to break the sharp corners will also greatly reduce stress concentrations.


In the part above, notice the poorly designed boss on the left compared to the well designed boss on the right.

Rib Depth

Avoid thin, deep ribs if possible. Thin, deep ribs are made using an EDM process which will add cost to the mold. It is also worth noting that thin, deep ribs tend to increase the difficulty of hand polishing. Below is a general guideline of rib width to depth ratios.


Pilot Holes

Screws are a common way to fasten two plastic pieces. Self-tapping screws eliminate the need for molded threads. The table below lists common pilot hole sizes for various self-tapping screws.


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Plastic Part Design Fundamentals Sink and Warp

Plastic Part Design – What is Sink?

Part manufacturers should always give careful consideration to the materials used in plastic part design, as this decision will have significant consequence in the sink & warp of plastic injection molded parts. Why is this? During the cooling stage of a plastic injection molded part, plastic first solidifies at the mold surface and moves inward toward the center. If the plastic is too thick, the center will stay molten for an increased period of time. This causes an inward pulling stress to develop which leads to sink marks on the outer surfaces of the part.


In the image above, notice the sink marks on the part shown to the right.

Preventing Sink in Plastic Parts

Ribs may provide stiffness for plastic injection mold parts, but also can result in sink marks on the outside of the wall. To prevent sink in plastic molded parts, the thickness of the rib should be about 60% of the thickness of the wall. This rule-of-thumb guideline should help keep sink from occurring as the part cools.


Plastic Part Design – What is Warp?

If uneven wall thicknesses exist in any plastic injection molded part, thinner sections will freeze faster than thicker sections, which will introduce stresses in the part between the thick and thin areas. If the stresses become excessive, the part will warp, illustrated below:


The part above on the right-hand side has a warped thinner section.

Preventing Warp in Plastic Parts

Plastic injection molded parts may experience ‘warp’ due to stresses in step transitions between wall thicknesses.  To combat sink, plastic part design can be improved through the use of a ramp. Additionally, the use of gussets can also provide support in corners of plastic parts to help avoid warping.


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Fundamentals Of Plastic Part Design Part 2 – Draft

In this edition of our Product Development series, we’ll briefly overview a fundamental concept for plastic part design, ‘draft angle’. Draft angle, or ‘draft’ is essential for all plastic injection molded products. It facilitates the removal of the part from the mold.  Adding draft angles will also improve cosmetic appearances of plastic molded products by reducing drag marks. The amount of draft needed is part-specific. Draft angles should be added to any face that is parallel to the mold opening/closing.  The images below offer examples of undrafted vs. drafted plastic molded parts:

The undrafted part (shown above) includes straight lines for all faces.  In the image below, the vertical faces of the plastic part have a slight draft angle applied to the design.



Pass Core Shutoff


A pass core shutoff is when metal from both sides of the mold slide together and create a seal off. Because the mold is constantly wearing every cycle, a minimum of 3 degrees of draft is required on all sliding faces. This higher draft angle increases tooling life and prevents galling.







Remember, for mold shutoff design apply a minimum 3 degrees of draft.

Draft Guidelines


Here are some rough guidelines to follow:


The guidelines associated with the amount of draft required will vary with geometry and other part characteristics (i.e. surface texture requirements), but in general, the more the better.

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