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Plastic Part Design Fundamentals


1.Thickness 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.

“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.

2. Undercuts 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.

3. Draft Angle

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.

4. 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.

5. 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.

6. 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 centre. If the plastic is too thick, the centre 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.

7. 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.

8. 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.

9. 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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