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.

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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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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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Fundamentals Of Plastic Part Design Part 1 – Injection Molding

Plastic Part Design Fundamentals

Injection Molding

Plastic design is the building block of plastics manufacturing, and can make or break not only the initial investment required to get a product launched, but the long term profitability of that product. In this series we will discuss the fundamentals of this very important component of product development.

The phrase “Design For Manufacturing (DFM)” describes the affect a product’s design has on its ability to be mass produced. If a product is too thick, has undercuts or insufficient draft, or poorly designed features, it will increase cost, slow production, and create other delays in getting your product to market.

If a plastic part is too thick or has an uneven wall thickness, excess shrink, sink, internal bubbles and voids, and poor cosmetics are possible. This is a result of the plastic cooling from a molten state to a solid, room temperature state.

Too Thick

Cored Out

“Coring out” a part will eliminate the issues listed above. The idea behind coring out is to simply remove material from a plastic part, leaving a distinct rib structure behind. Ribs not only provide structural integrity to the part but also provide mating surfaces for other parts in the assembly if needed. Other advantages of coring out a part are reduced weight and production time and therefore cost. How a part is cored out will affect the strength. Leaving ribs in the right location or direction, particularly in bending, will maintain strength throughout the part and improve cosmetics.

Undercuts can be thought of as any indentation, protrusion, or hole that prohibits the injection mold from opening and ejecting the part in a linear motion. Undercuts often add significant cost to the mold as well as increased lead times. Many times undercuts can be eliminated with clever plastic design. This will provide a more efficient mold design and save tooling costs.

Below are a couple examples of undercuts. If a product can be modified to eliminate these features, it can save significant time and money.

Plastic Design

Plastic Design

Internal Threads Requires Collapsible Core/Unscrewing Mold

As you can see, and as explained in our previous post The Value Of Plastic Part Design, having the proper plastic design is critical to product development, and having it optimized before tooling is made can save exponential costs later in the product cycle.

In our next article we will discuss draft, sink and warp, and surface finishes.

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The Five Things You Need Before Asking For A Plastic Injection Molding Quote

Plastic Injection Molding – Before You Quote

injection molding ejector pin lines“I have the CAD drawing, what else could they possibly need?”  Does this sound familiar?  Preparing for a plastic injection molding quote may not be as simple as you think.

“…I have the CAD drawing, what else could they possibly need?”

While it is true that a custom plastic injection molder could prepare a quote from just a CAD drawing, or in some cases a hand-drawn two dimensional drawing, you are most likely adding unnecessary expense by not preparing a few essential items beforehand to provide more information for your molder.

5 Essentials For Your Best Plastic Injection Molding Quote

When any plastic injection molder quotes a project, there are a few essentials they look for from a customer:  a drawing, the application or use case, desired material, quantities, and finishing touches.

  1. A Drawing
    Seems pretty basic, right? Still, it’s important to point out that only having a description of the part available can leave a lot to the imagination, and words can be interpreted very differently from person-to-person. The small details you may consider insignificant could lead to significant differences in the tooling/mold cost. That’s why it’s crucial to have at leaset a well-dimensioned hand drawing – many molders will require much more.
  2. Application
    It is important for a plastic injection molding company to understand how your product will be used. Will it be subject to impact? Will it be outside? A good molder will be willing to recommend a material based on the information you provide if you have not already selected one.
  3. Material
    Did you know there are over 80,000 grades of plastic available on the market today? The commonly used grades typically cost $1.00-$5.00/lb., but highly engineered grades can exceed $40/lb.  As mentioned above, it’s important to select a material that has the right properties for your specific product, without over-engineering and adding cost.
  4. Estimated Annual Quantities
    This information is very important, and should be a key part of your marketing plan. Having an estimate of how many parts you expect to order annually will help the molder determine the number of cavities to build into the mold (cavity refers to the number of parts the mold produces each cycle). If your quantities are high, the molder may also recommend using hardened tool steel to prolong the life of the mold.
  5. Any Secondary Operations
    If you’re looking for a one-stop-shop, let them know in advance. Do you want any printing on the part? Does the part need to be assembled? Do you want the molder to package it? Knowing these things up-front may lower your overall costs if the plastic injection molding company views your project as a larger source of income for them.
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