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.
Varying wall thicknesses can also have an impact on the cooling rate. Thicker areas take longer to solidify—as a result, the entire part must stay in the mold until it is cooled sufficiently to be ejected. Although this is not exactly a quality issue, it does extend the cycle time; it would be more efficient to have the entire part cool in the same amount of time.
Shear stress in the flowing plastic can also be affected by non-uniform wall thickness. At a constant fill rate, thin areas force the flow to move faster, increasing shear stress in the process. Different shear stress across a part will promote warpage. This same shear stress also aligns fiber reinforcements. Fibers are much stiffer in the direction of flow as compared to 90 degrees to flow; variable stiffness can also lead to warp.
Perhaps one of the biggest impacts of varying wall thickness is how it affects the appearance of the part. Varying wall thickness can result in undesirable sinks and cosmetic issues like flow lines. It can also be difficult to maintain cavity contact for cooling and picking up the gloss or texture of the cavity surface.
Most non-uniform wall thicknesses are part of the original product design, especially for smaller, complex, multi-functional products. For example, there may be insufficient space for a mating component in the assembly, so the plastic wall gets thinned out. This solves the design issue, but makes it more difficult to mold the part.
Although most designers are aware of the molding challenges created by non-uniform wall thickness, these thicknesses are typically required for the proper function of the product and they expect the injection molder to make it happen. Design-for-manufacturability meetings early on can sometimes modify these designs to reduce the amount of variable wall thickness and the process challenges they create.