The road to a plastic part that functions as designed is paved with good intentions. Best-laid plans can be disrupted by failure to understand unique problems presented by the injection molding process. Factors to consider include draft angle, wall thickness, gate location, anisotropy, shrinkage, and warpage. You should also consider design elements that can significantly reduce costs to build and run a tool.
Consider first one of the most commonly observed problems: draft angle. "Having too tight of a tolerance on a part limits how much draft you can put into the tool," says Greg Warkoski, process technology manager for Solvay Advanced Polymers, Alpharetta, GA. "You have to be able to get the part out of the tool." For a simple illustration, look at an ice cube tray. Angled boxes in the tray make the cubes pop out easily. The same principle applies for an injection mold.
Draft lets the part break free from the mold surface by creating a clearance as soon as the mold opens. As a general rule of thumb, plan on a bare minimum of one-half degree of draft angle per side. One degree is better. That's equivalent to 0.017-inch taper per inch per side. Parts with texturing, such as leather graining, require additional draft.
"If you just can't allow for that much draft, there are tricks you can use," says Warkoski. "You can split the core by putting half of it on each side of the tool. That lets you cut the draft requirement in half." Another idea: design the tool to pull the core before it opens. That requires hydraulically activated action, however.
Why Wall Thickness Matters
Another issue sometimes overlooked is wall thickness. Thicker and non-uniform wall sections can result in sink marks due to the physics of how plastics cool. As plastic solidifies, it hardens from the outside (near the mold surface) toward the inside. In thick sections, this results in inward pulling stresses (due to shrinkage) that may cause sink marks in the outer surfaces of the part. Because thin sections solidify faster than thick sections, stresses may build up between the two sections and cause warpage. If you can't avoid variations in wall thickness, design ramps to smooth the transitions. Modeling with computer-aided engineering is a good way to predict these types of problems.
Gate into the axis of symmetry and fill from thick to thin.
Non-uniform wall thickness can also lead to filling problems. It's good practice to fill from thick to thin since thinner sections can freeze off and prevent proper densification of thicker sections.
Consider Gate Location
Pay particular attention to gate location and knit lines when designing parts with reinforced materials. "Knit line" or "weld line" refers to the spot where plastic flow fronts meet within a mold. Reinforcing fibers don't cross knit lines, creating a structurally weak spot in the part. A good rule of thumb: if your part has an axis of symmetry, you want to gate along that axis (as shown in the diagram). You have an axis of symmetry if you bisect a part and the areas on both sides of the line are mirror images of each other.
One final important point to consider is shrinkage. Metal is "isotropic" while reinforced plastics are "anisotropic". Isotropic refers to identical properties in all directions. The strength, shrinkage, and thermal expansion of reinforced plastics are affected by fiber orientation. Shrinkage is a function of flow and transverse directions. Materials suppliers provide those numbers based on test plaques gated along one edge. That provides data on 100 percent flow and 100 percent transverse in those two directions. In a real part, that is almost never the case. "When designing a tool, use a number that's half way between the two numbers for mold shrinkage," says Warkoski. "Then cut the tool steel safe, which means bigger cores and smaller cavities. Measure the part after you mold it, then make any needed changes to the tool. It's easier to cut away a little more steel than to replace it."
Also keep in mind that the effects of shrinkage grow as part dimensions grow. Holding a tolerance to ±0.002 inch is okay for a small dimension, but could be a real problem for bigger dimensions. That's because shrinkage is calculated on an inch per inch basis. Also take into account that there may be slight variations because of the difference in shrinkage between the flow and transverse directions.