Injection Molding Design Guide
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Injection Molding Design Guide

At TEAM MFG, we work with our customers to develop the most robust and complex plastic injection molded products.There are four major elements to developing the best possible product:

Injection Molding Design Guide

At TEAM MFG, we work with our customers to develop the most robust and complex plastic injection molded products.There are four major elements to developing the best possible product:

● Part Design

● Material Selection

● Processing

● Mold Design

This guide serves as a basic step-by-step manual that our customers can use to get started designing their own plastic parts.

Injection Molding Design Guide

The parting line location depends on the shape and function of the part. It can also be minimized by its draft allowance and part geometry.The parting line is usually located on the other side of the mold. It can be seen from the bottom half of the part.


One of the most important factors to consider when designing a plastic part is that it should not have a straight pull design since it will prevent the plastic from going into the mold in the first place and forming a cavity. This design eliminates the need for an injection mold if the two halves are separated by a metal bar. A straight pull mold can also be made with slides and lifters.Creating a straight pull mold is also key to keeping the cost of an injection mold down. When the two halves are separated by a metal bar, there is no path of the plastic that can block the way of the pull. This eliminates the need for an injection mold if the part has an undercut.This design also prevents the part from ejecting from the injection mold if it has an undercut. This procedure is done using slides and lifters.Slides can be either mechanical or hydraulic and are expensive features to add to any mold. Lifters are typically smaller and are always mechanical.

Draft Allowance

The draft angle is a vital part of the design process to ensure that the parts are ejected from the mold properly. It should be set at 1 degree on the ribs and 1 degree on the plate.When a part is designed to accommodate a snap feature, the minimum draft angle should be 5 degrees.

Wall Thickness and Uniformity

Another important rule to consider when designing a plastic part is the uniform wall thickness.The uniform wall thickness helps in the flow of material in the injection mold. It also reduces the risk of sink marks and mold stress.Wall thickness is also important to consider when designing a plastic part since it affects the cooling cycle times of the components. Generally, the wall thickness of injection molds should be .060” to .180”.

Having uniform wall thickness allows the cavity to expand without restricting the part's flow. Also, it prevents the plastic from freezing in the first place.Since the thinner sections are already solidified, they do not tend to yield. This means that they can cause cracks if they are excessively thick.

Coring is a method where plastic is removed from a thick area to help maintain uniformity.To maintain a uniform wall thickness, the corner should have a minimum fillet radius of 50% and an outer radius of 150%.


Bosses are usually designed to be attached to plastic parts. They should be mounted on the ribs or the walls using a template or plates.Ideally, the internal diameter of the boss should be at least 2.5 times that of the insert or screw.This design feature can be easily overcome if the part has multiple ribs. However, bosses should not be mounted on polished surfaces since they can cause ghosting or show sink marks.The draft thickness should not exceed 60% of the wall thickness.Bosses should be isolated from the corner to avoid thick sections and aid in the elimination of sink marks.

Gussets and Ribs

In addition to improving the strength and appearance of a part, ribs and gussets can also be used to add structural support. Although the rib base thickness should not exceed 80% of the adjoining wall thickness, it should not go beyond 60% if the outer surface is excessively rough.For appearance-related parts, the rib thickness should not exceed 60% of the outer wall thickness if the outer surface is not rough and has sink marks.Ideally, the ribs should measure at least 3 times bigger than the wall thickness to provide the strongest possible support. The draft should be 12 degrees per side nominal.Multiple ribs should be placed at least 2 times the wall thickness apart to help reduce molded in stress as well as any problems cooling.

Radii and Corners

Sharp corners are often the cause of a part's failure. They should also be avoided since they can increase the wear of the part and restrict the flow of plastic parts.

Material Shrinkage

The material used for making the part shrink when it's molded. This process causes the part to shrink in size for up to 24 hours after the injection molding.The exact amount of shrinkage that should be used for a part design and construction is not always known by the vendor. Unfortunately, this information is not always available.In some cases, the material used may vary. It is important that the grade of the polymer be selected prior to the injection to ensure that the proper shrinkage is achieved.Material shrinkage can vary depending on the part's design and the mold's flow direction. For instance, a thick wall will shrink more than a thin one.The temperature and the cavity pressures are also affected by the material's shrinkage. As a result, cooler plastic injection machines will shrink while the hot melt temperatures will increase it.


Gate design and type can vary depending on the material selection, component design, and the dimensional requirements of the finished product.The gate should be designed to connect the part to the runner system. It should also be selected according to the part's geometry and the chosen polymer.

● Gates should be located away from high stress or impact areas.

● Gate designs should eliminate secondary de-gating operations as possible.

● The gate should be located in area to best fill the part and typically placed in the thickest area

● In some cases, multiple gates will be necessary dependent upon part size, geometry and polymer selection.


Conducting proper venting is also important to prevent gases from escaping through the injection mold. Doing so can prevent the parts from getting damaged and causing issues such as short shots and sink marks.Aside from venting, there are also various ways to get rid of a mold. Some of these include using ejector pins, running rails, and vents.A vent should be placed in the part's geometry to allow the gases to escape as it's filled. It should also be placed in the same location when venting is required.

Knit/Weld Lines

Weld lines, also known as knit lines, are areas in a molded part where the two or more flow fronts meet. They should be designed to be placed away from high stress areas. Mold flow software can also help identify areas of the cavity where plastic filling can be made.The design of a part should also take into account the locations of its knit lines to avoid them from flowing in the opposite direction.This software can also simulate the filling of a cavity using a template. It can then identify areas of the molded part that could cause mechanical or cosmetic issues.

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TEAM MFG is a rapid manufacturing company who specializes in ODM and OEM starts in 2015.

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