The Incredible Shrinking Wall (and other problems)
In theory, injection molding is a simple process. You create a mold with a cavity the size and shape of the part you want. You inject molten resin, which fills the available space. The resin cools and hardens. You open the mold and remove the finished part.
Unfortunately, nothing is ever that simple. First, materials expand when heated and shrink when cooled, and plastic resins are no exception. The shrink of the resin must be taken into account when designing the mold. The cavity that forms the part is designed slightly oversize to allow for shrinkage. How much oversize? That depends primarily on the resin. Some resins shrink uniformly in all directions and some resins, e.g. glass filled resins, have different shrink rates in the direction that the resin flows in the mold vs. the direction of cross flow.
Second, certain shapes seem to invite mischief. One of these is the overly thick wall, which is subject to several types of problems.
- Because shrinkage is proportional to resin depth, and because molded parts cool from the outside in, thick walls are susceptible to sink marks – low areas that can be both structurally and cosmetically problematic.
- Resin pellets are heated in a barrel and injected into the mold with a screw at pressures of up to twenty thousand psi. Air and gas can be dissolved in the resin melt, and when the parts cool, this gas comes out of solution. The result is either bubbles hidden within the part or voids visible at the surface. Both cause structural weakness. Voids at the surface create cosmetic problems as well. This problem is more pronounced in thick areas of the part.
- In some cases, particularly where wall thicknesses vary significantly, uneven shrinkage can cause parts to warp as they cool.
In short, excess wall thickness creates problems, adds little value to a design, increases weight, and adds to material cost. Fortunately, designers have come up with elegant ways of eliminating unnecessary thickness without compromising performance. The accompanying diagrams demonstrate some of these techniques.
Figure 1: Excess wall thickness
Figure 1 shows a part that, due to excess wall thickness, is susceptible to all the risks mentioned above. The right side is surrounded by thick walls, and the left side is one thick wall. (Note the six screw holes, presumably for attachment to some assembly.)
Figure 2: Eliminating excessive wall thickness
Figure 2 shows the same part redesigned, demonstrating a variety of techniques for eliminating excessive wall thickness. The three walls at the far right have simply been reduced in thickness. Each of the two angled walls at the center have been cored out, leaving two thin walls in place of one thick one. The “slab” on the left side of the part has been cored out in several sections, leaving a network of ribs to maintain strength. The diagonal rib in the section at the lower left provides torsional stiffness to help prevent twisting of the part. (The triangle, as you probably know, is an inherently stiff form.)
The revised design also shows several ways of reconfiguring the screw holes in the original design. At the right, two screw holes have been configured as bosses attached to the wall. Note that the wall thickness of the bosses is consistent with that of the part walls. In the center, two screw bosses have been made part of the rib formed by the coring of the “slab.” At the left edge of the part, the screw bosses have been separated from the wall, but are tied to it by ribs to add support for the bosses.
While part walls should not be unnecessarily thick, they need to be thick enough to allow the molten resin to flow and fill the part. They should also be thick enough to provide the strength necessary for your application. A chart showing the acceptable range of wall thickness for various resins can be found in our Design Guidelines section on our website.
Finally, uniform wall thickness is basic to good injection molded part design. Minimizing variations in wall thickness helps to reduce variations in shrinkage. This results in less warpage and better dimensional tolerances.
When you submit a design, ProtoQuote, Protomold’s online quoting and analysis software can provide a useful check on wall thickness from the standpoint of moldability. Areas that are too thick are marked in blue; those that are too thin are marked in yellow. But since ProtoQuote doesn’t know your application, it cannot take into account the use to which the part will be put or the forces to which it will be subjected. That is for the user to determine, which, of course, is why you do prototyping in the first place. The good news is that Protomold gives you fast turnaround on real, affordable, molded prototype parts so you can do the appropriate testing.
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