Injection molded plastic parts are made by melting plastic granules and injecting them into a mold. The design process must consider the part’s purpose, including the nominal wall, protrusions, holes, and indentations. Varying wall thicknesses can cause defects, and bosses must not be thicker than the nominal wall. The location of holes and indentations affects the part’s strength, and rounded corners help the plastic flow better. A low-stress design with smooth transitions results in a stronger part.
Most plastic components on the market today are injection molded. An injection molded plastic part is created using an injection molding machine, which works by melting plastic granules and injecting them into a mold.
The design of plastic parts is a complex process and its production purpose must be taken into account during the design phase. There are three elements of an injection molded plastic part, the nominal wall, the protrusions, the holes and the indentations.
Nominal wall refers to the thickness of the wall. The wall thickness throughout the part should not vary by more than 10%. Varying wall thicknesses in injection molded plastic part are a major cause of product defects. When designing your part, you want to avoid excessively thick walls, simply because thick walls use more plastic and are more expensive to produce. However, there is a limit to how thin even the part can be. The strength of the part is a function of thickness and the end use of the product will determine the desired strength. Some special injection molded plastic parts require reinforcement with glass or carbon fibers.
A boss is an element of the part that protrudes from the nominal wall, such as stiffening ribs. The ribs must not be thicker than the nominal wall, otherwise sink marks may occur.
Indentations and holes are also an important design element. The location of any holes or indentations can significantly affect the overall strength of the part. Trying to create a hole in the side of a part is particularly challenging and the need for side holes should be minimized in the initial design.
Another design factor is the angles of the part. How the plastic enters and fills the mold is of great concern. Since the molten plastic is a fluid, the fluid dynamics will also dictate the design, to a large extent. As it flows into the mold, the plastic may have to go around corners. The plastic will flow better if the corners are rounded. The plastic will pull at a sharp angle during the cooling process and could result in a bad part. Creating a low-stress design with smooth transitions will result in a stronger part that is less likely to crack or break.
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