Heat and pressure most famously make diamonds. But they do more than that: Nearly every plastic product in your house was made with the same two forces.
From the 1950s until now, injection molding has dominated consumer manufacturing, giving us everything from action figures to denture containers. Despite its incredible versatility, injection molding does have some design constraints.
The basic injection molding process is to heat and pressure plastic pellets until they flow into the mold cavity; cool the mold; open the mold; eject the part; and close the mold. Repeat and repeat, typically ten thousand times for a single plastic manufacturing run and a million times over the life of the mold. Producing hundreds of thousands of parts isn’t easy, but there are plastic part design alterations that help—among the simplest is paying attention to the wall thickness of your design.
Injection Molding Wall Thickness Constraints
If you take apart any of the plastic appliances around your home (as most engineers probably did as children) you’ll notice that the walls for most parts are about 1 mm to 4 mm thick (the optimal thickness for molding), and uniform for the entire piece. Why? Two reasons.
First of all, thinner walls cool faster, shortening the cycle time of the mold, the amount of time it takes to make each part. If a plastic part can cool faster after the mold is filled, then it can safely be ejected sooner without warping, and because time on the injection machine costs money, the part is less expensive to produce.
The second reason is uniformity: In the cooling cycle, the outer surface of a plastic part cools first. Cooling causes contraction; if the part is of uniform thickness, then the entire part will shrink away from the mold uniformly as it cools, and the part comes out smoothly.
However, if the part has thick and thin sections next to each other, then the molten center of the thicker area will continue to cool and contract after the thin areas and surfaces have already solidified. As this thick area continues cooling, it keeps contracting, and it can only pull material from the surface. The result is a little dimple on the surface of the part called a sink mark.
Sink marks merely show poor engineering on hidden areas, but on cosmetic surfaces, they can require thousands in retooling costs. How do you know whether or not your part has these “thick wall” problems with injection molding? Luckily, Fictiv has a tool for that.
Solutions for Thick Walls
Happily, thick walls have some simple solutions. The first thing to do is to notice the areas which are a problem. In the part below, you can see two common issues: thickness around screw holes, and thickness where strength is needed in the part.
For screw holes in an injection molded part, the solution is to use “screw bosses”: a small cylinder of material directly around the screw hole, tied to the rest of the housing using a rib or a flange of material. This allows for more uniform wall thickness and fewer sink marks.
When an area of the part needs to be especially strong, but the wall is too thick, the solution is similarly simple: ribbing. Instead of making the entire part thick and difficult to cool, thin the exterior face into a shell, then add vertical ribs of material to the interior for strength and rigidity. In addition to easier molding, this reduces the amount of required material, reducing costs.
Once you’ve made these changes, you can check with the DFM tool again, to see if the changes resolved the issues, by clicking on your part and then clicking on the “Upload revision” option to upload the updated part. And, of course, when everything is resolved, prototype the parts in a 3D printer to test them out before moving forward with manufacturing.