When injection molded parts have defects, it’s often the molder that takes the blame. However, the root cause of many defects is a poorly designed part. In fact, it is the part designer that has the greatest potential to improve part quality and reduce costs.
1. Non-uniform wall thicknesses
The first rule of injection molded part design is to have a constant wall thickness throughout the part because thick wall sections take longer to cool than thin ones.
2. Improper wall thickness
Suitable wall thicknesses can vary anywhere from 0.020” to over 1” depending on the material being used. The ease of which a part will fill during the molding process and the time it takes for the part to cool are both directly related to the wall thickness.
Consequence: Walls that are too thin can cause difficulty in filling the mold cavity, leading to short shots or increased injection pressure requirements. Walls that are too thick will lead to long cycle times, increased part cost, and possible sink marks.
Solution: Check with the polymer manufacturer for the recommended wall thickness for the material being used. Thinner walls not only reduce material costs, but they also cool faster resulting in lower processing costs. Run a mold flow analysis to ensure the part will fill correctly at the chosen wall thickness.
3. Not adding draft
In order for a part to release from the mold as it opens, all side walls including ribs and bosses must have draft (i.e. be tapered in the direction the mold opens).
Consequence: A part with insufficient draft will not release from the mold.
Solution: Add draft to the part before adding fillets. The amount of draft required depends primarily on the type of polymer, the depth of the part (the deeper the draw, the higher the required draft angle), and the surface finish. Check with the polymer manufacturer for detailed draft angle requirements for the specific material being used. As a general rule, use at least 1 degree of draft on vertical faces and add 1 degree for every 0.001” depth of surface texture. Run a draft analysis on complex parts to ensure all surfaces are drafted properly.
4. Not using rounded corners
During the injection process, the polymer can more easily flow around rounded corners resulting in lower required injection pressures and less molded-in stress. Rounded corners are also stronger as stresses get distributed over a larger area.
Consequence: Not rounding corners can increase injection pressure requirements and lead to areas of stress concentration that weaken the part.
Solution: Internal corners should have a radius of at least 0.5 times the adjacent wall thickness. External corners should have a radius equal to the internal radius plus the wall thickness to keep wall thicknesses uniform. At the base of a boss or rib, the radius should be 0.25 times the adjacent wall thickness. (Larger radii in these areas can increase wall thickness and cause sink marks.)
Undercuts are areas of the part that require movement in the mold to release the area before the part can be ejected. While sometimes unavoidable, good design practices can be used to minimize the number of undercuts.
Consequence: Undercuts require motion in the mold to release the part, making the mold more complex and more expensive. The motion in the mold can also decrease tool life and increase the risk of flash on the part.
Solution: Allow access to the undercut through an adjacent wall when possible. For internal threads, use less than a full thread revolution when possible.