Shrink rates for injection molded plastic parts vary depending on the materials used and wall thickness. Designing uniform wall thickness offers substantial shrink rate control; on the other hand, non-uniform walls can lead to large pressure drops during filling, significant differences in shrink rates, and internal stresses within the injection molded part that could cause warpage or similar defects.
For instance, thicker areas within the part can act as “runners” that alter the way the plastic fills the tool. 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 there's not adequate venting in these areas to allow the escape of any trapped air.
Major Drawbacks of Non-Uniform Wall Thicknesses
When gating an injection-molded 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 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 under-packed conditions in the thick area, resulting in sink and/or warp in the part.
Varying injection-molding wall thicknesses can also impact cooling rates. Thicker areas take longer to solidify; as a result, the entire part must stay in the tool until it's cooled sufficiently to be ejected. Although this isn't 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 degrees of 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 the flow, and 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 injection-molded 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.
The Importance of Design for Manufacturability (DfM)
Most non-uniform wall thicknesses are part of the original part design, especially those of smaller, complex, multi-functional parts. 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 difficulties created by non-uniform wall thickness, these thicknesses are typically required for proper functionality. Therefore, the designers expect the injection molder to make it happen. Conducting a Design for Manufacturability (DfM) analysis in the initial design phase is a good way to identify potential design modifications that will reduce the amount of variable wall thickness and the process challenges it creates.
To learn more about the importance of DfM and the advantages of partnering with an experienced injection molder, read our DfM Case Study: