Injection molding

In injection molding production, warping in thick-walled parts is a very common problem. To understand this phenomenon, Haina will start with the fundamental nature of plastic cooling and shrinkage.

As plastic cools from a molten state to a solid state, its volume shrinks. This shrinkage itself is normal. However, if different areas of the part shrink inconsistently, internal stress is generated, which in turn causes the part to warp.

The problem with thick-walled parts lies precisely in their greater wall thickness, which triggers a series of chain reactions:

First, the cooling rate difference between the outer and inner layers is significant. In thick-walled parts, the cooling rates of the surface layer and the core are completely different. The surface layer contacts the mold, cools quickly, and solidifies and shrinks first. The core dissipates heat slowly and remains molten while the surface has already hardened. When the core eventually cools and shrinks, it can no longer shrink freely — it is "locked" by the already-solidified surface layer, creating internal stress within the part. This internal stress is released after ejection, causing the part to warp.

Second, thick-walled areas inherently shrink more. The shrinkage rate of plastic is related to wall thickness. The greater the wall thickness, the more heat is stored per unit volume, the longer the cooling time, and the greater the final total shrinkage. If the wall thickness is uneven, thicker areas shrink more while thinner areas shrink less, creating a pulling effect between them that causes the part to warp toward the thicker side.

Third, packing pressure struggles to reach the core. During injection molding, the role of packing pressure is to compensate for shrinkage as the plastic cools. However, in thick-walled parts, once the surface layer solidifies, it becomes very difficult for the packing pressure to effectively reach the still-molten core. The core does not receive sufficient material compensation, its shrinkage is not offset, and internal stress and deformation are further aggravated.

Fourth, there is a conflict between cooling time and production efficiency. Thick-walled parts require longer cooling times to fully solidify. If the mold is opened prematurely to shorten the cycle, the core has not fully solidified, and the part continues to cool and shrink under residual heat after ejection, making the deformation even worse.

In summary, the root causes of warping in thick-walled injection-molded parts are uneven cooling between the outer and inner layers, uneven shrinkage, and difficulty in packing compensation — all stemming from excessive wall thickness. Together, these three factors create complex internal stress within the part, which ultimately manifests as visible bending or warping.