Designing wall thicknesses that are as uniform as possible helps control the shrink rate for that part or product. Shrink rates for different materials vary according to wall thickness of the part. Non-uniform walls can lead to large pressure drops during filling, causing significant differences in shrink rates. In turn, varying shrink rates cause internal stresses within the part, leading to warpage.
For example, thicker areas in the part can act as “runners” within the part that alter the way the plastic fills the mold. The molten plastic prefers to follow the easiest path, so its flow will always favor the thicker wall section first. As a result, molten material may race ahead in some locations, and then “backfill” the remaining space. This can be troublesome, especially if adequate venting has not provided in these areas to allow the escape of any trapped air.
When gating a part, it is important to gate into the thickest section and then flow into thinner areas. This is necessary to properly pack the part out after filling. The flow path of molten material must remain open so the plastic material can continue to flow into the part details during the cooling phase. Gating into a thin wall, or flowing through a thin area to get material to a thicker area, may create flow irregularities. The thinner area may freeze and solidify, preventing the additional material in the pack phase from reaching the thick section of the part. This can cause higher shrinkage due to the underpacked conditions in the thick area, resulting in sink and/or warp in the part.