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Bruce Wold, GMN
By Bruce Wold | Aug 4, 2016
Designing for manufacturability

A very important aspect of project planning is assessing the design and providing design considerations for part manufacturability. During this phase, GMN Plastics program managers and engineers are looking for anything that might cause problems in the molding process including both cosmetic and dimensional issues. Typical issues include wall thickness, wall to rib ratio, draft angles, boss diameters, undercuts, weld line locations, and texture choices. GMN Plastics solves this problem 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.

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 because it affects processing of the part and depending on how far the material needs to flow, this can affect both cosmetics and dimensions. If the walls are too thin than the melted plastic moves slowly through the tool, which makes it difficult to fill. On the other hand, if the walls are too thick than it takes longer for the plastic to cool, which can cause part shrinkage. This happens because the areas of plastic on the outside cool more quickly than the inside, which can cave in. Where possible, there needs to be even wall thickness and smooth transitions for the material to flow through correctly.

Wall to rib ratios are another important consideration during part design.  Ribs are commonly used in plastics manufacturing as a way to increase the strength of the part structurally. It’s important 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. 

One of the main considerations during injection molding manufacturing is the part draft angles. This is because the part needs to be able to get out of the tool and to do this 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 even more because the plastic is more likely to stick to the tool. The key takeaway here is that correct draft angles will make the part look good without getting stuck.

Boss diameters, the holes where screws are inserted, are important to consider too. The boss diameter needs to be the right size because it typically holds a metal insert which needs 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.

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.

Weld lines are a cosmetic issue for consideration. A weld line occurs 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.

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 like this has a lot of tradeoffs in design and many customer negotiations occur.

During the stage of project planning when part design occurs, all of these factors are critical to build a successful part that meets customer specifications. The main issues to consider in regards to part design include wall thickness, wall to rib ratio, draft angles, boss diameters, undercuts, weld lines, and texture.