07/25/17 9:57 am

plastic part part design

Injection molded plastics are used to produce a range of parts, from small electronic components to larger pieces of equipment.

The plastic part design process offers many versatile solutions and unique benefits, but it’s a practice that depends on the strength of its design.

A bad design for your plastic mold injection parts can mean losing time and money correcting mistakes, while the right design can result in durable, high-quality parts.

In this article, we’ll look at some of the considerations that go into designing a part for plastic injection molding, including wall thickness transitions, ribs for stability, bosses for attachments and assembly, and draft angles to assist in removing the part from the mold.

Wall Thickness

Many defects in injection molding part design can be reduced or even eliminated by taking steps to create the right wall-thickness design. This means ensuring the thickness of each wall is as uniform as possible. In its liquid form, plastic will follow the path of least resistance and gravitate to larger wall areas, meaning smaller sections can go unfilled.

Rib Design

Ribs increase the moment of inertia in a design, which in turn will increase the bending stiffness of a part without adding thickness.

Rib thickness needs to be less than wall thickness – between 50 and 70 percent — to minimize sinking. Professional mold designers pay attention to the location and height of the ribs and their degree of draft to make injection easier.

Weld Lines

Also known as meld or knit lines, these occur at the meeting of two plastic flows. Any part made from injection molded plastics will contain weld lines. Your design should make sure they do not happen in places that can compromise the integrity of your part. It’s important the vents in your mold are positioned properly. These vents allow air to escape and can minimize weld lines.


Bosses simplify the registration of mating parts for attaching screws, pins or other fasteners, or for accepting threaded inserts.

These components have similar height and width recommendations as ribs, and having the right boss/rib pairing – particularly in corners – can give you a stronger part and reduce the risk of sinking.

In bosses, wall thickness should be lower than 60 percent of the nominal wall to prevent sinking. If the boss is not part of a visible area, thickness can be increased to handle increased stresses from self-tapping screws.

Draft Angles

Mold drafts allow you to remove the part from a mold. The draft needs to be in an offset angle, parallel to the opening and closing of the mold.

When you allow for as much draft as possible, you make it easier to release parts from the mold. One or two degrees of drafts, along with 1.5 degrees per .25 millimeters depth of texture should suffice.

The mold part line should be positioned so that it splits the draft to minimize it. If design considerations conclude that no draft will work, a side action mold may be needed.

Corner Transitions

As we said earlier, molten plastic takes the path of least resistance. This can present a problem when working with parts that have extreme geometric features. A harsh angle can stop the movement of molten plastic. A smoother corner transition can mean a smoother plastic flow.

Working With Your Manufacturer

Most OEMs and product designers are not plastic injection molding experts. That is why it is important to find a manufacturing partner who understand the process and how to maximize the part design for manufacturability.

At Mars International, we not only provide high-quality manufacturing resources but on-staff design and manufacturing engineers to help you get the most from your part design and save money in the process. Our in-house product testing and prototyping teams ensure your products will perform as expected.

When you are looking for a manufacturer with injection molded plastics expertise, contact Mars International. We have the equipment and the experience to produce plastic parts of all colors, shapes and sizes, supporting a wide range of injection molding part design types and resin materials for many industries.

Learn how Mars can create the custom products you need.

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