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Injection Molded Plastic Design for Manufacturability

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Last updated Dec 21, 2023 | Published on Aug 4, 2016

One of the most important phases of creating a new plastic component is assessing the design for optimal manufacturability.

Plastic Design for Manufacturing (DFM)

A critical aspect of project planning for any part is assessing the design and providing considerations for part manufacturability. During this the design for manufacturing (DFM) process for injection molded plastic components, Boyd's program managers and engineers are searching for anything that might cause problems in the molding process, including both cosmetic and dimensional issues. Typical issues in injection molding include wall thickness, wall-to-rib ratio, draft angles, boss diameters, undercuts, weld line locations, and texture choices. Boyd solves these problems with years of experience and state-of-the-art software simulators that allow engineers to get a closer look at the part by dissecting it into smaller pieces. These tools can help identify these issues so that they can be solved prior to production.

Wall Thickness

One of the main issues faced during project development is wall thickness. The wall thickness depends on many factors and there is no density that works universally for all projects, so it must be customized per part. Wall thickness is important as it affects part processing which can affect both cosmetics and dimensions depending on how far the material needs to flow. If the walls are too thin, the melted plastic moves slowly through the tool, making it difficult to fill. On the other hand, if the walls are too thick, then it takes longer for the plastic to cool and risk part shrinkage. Areas of plastic on the outside that cool more quickly than the inside can cave in. Plastic designs should strive for even wall thickness and smooth transitions for the material to flow through correctly.

Wall to Rib Ratios

Wall to rib ratios are another important consideration during part design. Ribs are commonly used in plastics manufacturing as a way to increase the structural strength of the part. It's key to note that rib thickness must be 60 % or less than the wall thickness of the part or the ribs will sink and be seen through the other side of the part.

Draft Angles

One of the main considerations during injection molding manufacturing is the part draft angles. A part needs to be removed from the tool so there cannot be a 0 degree or exactly perpendicular draft. A 0 degree draft angle would cause the part to get stuck inside the tool. A part with heavy texture will need to increase the draft angles more because the plastic is more likely to stick to the tool. Proper design and correct draft angles will make the part look good without getting stuck.

Boss Diameters

Boss diameters, the holes where screws are inserted, are important considerations as well in injection molding. The boss diameter typically holds a metal insert, which must be the right size because to fit in correctly. If the boss wall is too thick, there will be sinking on the other side of the part. If the diameter is too small, the insert will not fit in the hole. There are industry specific standards based on each manufacturer.

Undercuts

An undercut can be defined as a recessed area of the part and in terms of molding this means that the undercut area makes the part unable to release out of the tool. The plastic is injected around the undercut feature and the part cannot be ejected because the shape curves inward. The part is stuck because the plastic has formed around the tool which causes problems during production. Injection molded part designs must be evaluated prior to tooling to prevent unnecessary costs to remedy the issue.

Weld Lines

Weld lines are a cosmetic issue, which is an essential consideration for customer-facing components. Weld lines occur when the material wraps around a feature and comes back together around an obstruction while filling the tool. When this happens, a small line is formed called a weld, or knit, line. This needs to be considered during part design because the weld line is tricky to hide. When determining the location of a weld line, it is important to look at the plastic temperature, gate location, speed of flowing plastic, gate thickness, gate location, and gate height.

Surface Texture

Texture can be used to hide molding flaws. For example, when the part design will give you sink, a heavier texture can help to hide this. Using texture in this manner has a tradeoffs in design which must be thoroughly be explored in the design process. During the design stage of project planning, all of these factors are critical to build a successful plastic part that meets customer specifications. Boyd’s expertise in DFM can help ensure your plastic component’s success in your product. Contact us today for assistance with your plastic manufacturing needs.

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