How do manufacturers prevent sink marks in high-gloss polypropylene trash cans?

Manufacturers employ several sophisticated techniques to prevent sink marks in high-gloss polypropylene trash cans, ensuring a flawless surface finish. Sink marks are surface depressions that occur when thicker sections cool and shrink more slowly than adjacent thinner areas, pulling the surface inward. To combat this, the process begins with material selection. Using high-crystallinity polypropylene with specific additives and nucleating agents promotes more uniform crystallization and reduces overall shrinkage.

The core strategy lies in meticulous mold design. Engineers implement conformal cooling channels that follow the contour of the mold cavity. This allows for precise and even temperature control, ensuring the entire part cools at a consistent rate. The gate location is strategically placed to optimize material flow, and the mold surface is highly polished to a mirror finish, which is essential for achieving the desired high-gloss effect.

Process parameters during injection molding are critically controlled. Manufacturers use higher injection pressures and packing pressure profiles to continually push additional material into the mold cavity as the plastic cools and contracts, compensating for shrinkage. Holding pressure is meticulously maintained for a specific duration to pack the mold effectively. The cooling time is also extended to allow the part to solidify completely and uniformly before ejection, preventing any deformation.

Furthermore, part design is optimized. While the aesthetic demands a glossy finish, wall thickness is kept as consistent as possible. Where variations are unavoidable, gradual transitions are designed to minimize stress concentration. For areas that must be thick, such as ribs or bosses, they are designed to be significantly thinner than the main wall (typically 50-60% of its thickness) to prevent the formation of sink marks. Through this combination of advanced material science, precision engineering in mold design, and strict control over the manufacturing process, manufacturers successfully produce high-gloss trash cans completely free of sink marks.