Plastic injection mold designers must consider the resin chosen for the part manufacturing when designing the mold as this plays a very important role in meeting part specifications. One characteristic of the plastic material that requires careful consideration is the cooling rate compared to the geometry of the part.
What is Sink
This plays an important role in the likelihood of parts developing sinks or warps, and can have significant consequences on successfully molding parts with consistent results. The cooling rate is extremely important because as the part begins to cool it will always begin at the mold surface and move inward, toward the center. If the geometry of the part is such that the part is too thick, the center area of the part will not solidify quickly enough. This center stays molten long enough for it to cause stress on the area already solidified, pulling downward towards the center and leaving a sink mark on the outer surface.
Avoid Sink in Plastic Parts with This Rule of Thumb
Ribs are used to add strength to plastic parts in thin wall or critical areas, but can also cause sink marks on the exterior side of the part. When designing plastic parts to avoid sink marks, remember the basic rule to keep the thickness of the rib somewhere around 60% the thickness of the wall. Following this rule of thumb should help to avoid sink marks during the cooling process.
What is Warp
If a plastic part design includes areas of uneven wall thickness, stress can develop during the cooling and solidifying stage. If this stress is excessive, warps will occur where thinner sections solidify faster than thicker sections.
Avoid Warp in Plastic Part Design
When undue stresses due to transitions in wall thickness occur, plastic part designers can combat this by using a ramp. Designers also use gussets in corners of some geometric areas to help prevent warping.