The surface gloss of plastic packaging injection cups is a key indicator of their appearance quality. The mold polishing process, a critical step before molding, has a decisive influence on the final gloss. This correlation is reflected in the dynamic interaction between the mold surface condition and the plastic melt's flow, cooling, and molding process, requiring a comprehensive analysis of microstructure, molding conditions, and material properties.
The core goal of mold polishing is to reduce cavity surface roughness, providing an ideal replication template for the plastic melt. When scratches, micropores, or machining marks are present on the mold surface, the melt will experience turbulence during high-pressure injection due to differential flow resistance, resulting in uneven structures on the part surface corresponding to the mold defects. These microscopic undulations disrupt the regular reflection of light on the plastic surface, causing diffuse reflection of light that would otherwise appear mirror-like, ultimately resulting in a decrease in gloss. For example, the surface roughness of an unpolished mold cavity may exceed Ra3.2μm, while a high-gloss mold must achieve Ra0.05μm or less. This order of magnitude difference directly determines whether a plastic part can achieve a mirror-like finish.
The polishing process is also crucial for improving the chemical stability of the mold surface. High-quality polishing not only removes the deteriorated layer caused by machining but also eliminates residual stress within the steel. If the mold surface has defects such as uneven hardness or porosity during injection molding, micro-deformation can easily occur under the high pressure of injection molding, resulting in periodic ripples on the surface of the plastic part. This deformation, caused by mold stress release, accumulates over production batches, gradually degrading the gloss of the plastic part. Precision polishing combined with vacuum heat treatment can improve mold surface hardness uniformity to HRC50±1, fundamentally eliminating the risk of stress deformation.
Mold cleanliness, as an extension of the polishing process, directly affects the durability of the plastic part's surface gloss. Abrasives and degreasing agents used during polishing, if not thoroughly removed, will decompose at high temperatures during injection molding, producing residual gases. These gases form tiny bubbles or silver streaks during the melt cooling phase, disrupting the optical uniformity of the plastic surface. For transparent injection-molded cups in particular, even 0.1μm of residual oil on the mold surface can cause a gloss reduction of over 30%. Therefore, modern high-gloss molds require multiple purification steps after polishing, including ultrasonic cleaning and ion bombardment.
The coordinated design of the polishing process and the mold venting system determines the surface integrity of the plastic part. During high-speed injection, if air in the mold cavity cannot be promptly evacuated, high-pressure air pockets will form at the melt front, resulting in surface defects such as scorching and flow marks. Precision polishing can optimize the surface quality of the venting grooves, reducing gas exhaust resistance by over 50%. For complex injection cup structures, using stepped venting combined with mirror polishing ensures exhaust efficiency while preventing the venting grooves from becoming a source of gloss defects.
The mold polishing grade should be matched to the properties of the plastic material. For crystalline resins such as PP and PE, due to their regular molecular chain arrangement, higher mold replication accuracy is required, requiring diamond grinding and polishing to an A1 mirror finish. Amorphous resins such as PC and ABS, however, due to their disordered molecular chain arrangement, can be polished to an A3 finish using an oilstone. If reinforcing agents such as glass fiber are added to the material, mold polishing must be enhanced to ultra-precision levels to prevent uneven gloss caused by fiber orientation variations.
Modern polishing technology has expanded beyond the scope of traditional mechanical processing. The application of chemical polishing and electrolytic polishing further expands the scope of gloss control. Chemical polishing selectively dissolves micro-protrusions on the mold surface, achieving a more uniform surface than mechanical polishing. Electrolytic polishing eliminates the work-hardening layer caused by mechanical processing, forming a dense oxide film on the mold surface and improving corrosion resistance. These advanced processes enable mold surface roughness to reach Ra below 0.01μm, providing the technical basis for producing high-gloss injection molded cups.
The connection between mold polishing and the gloss of plastic packaging injection cups is essentially a manifestation of the manufacturing principle that "template accuracy determines replication quality." From microscopic surface topography control to macroscopic molding condition optimization, from single process parameter adjustment to multi-system collaborative design, polishing processes are integrated throughout the entire injection molding lifecycle. With the emergence of new technologies such as nano-polishing and laser micro-molding, mold surface precision is advancing from the micron level to the nanometer level, pushing gloss control of plastic packaging injection cups into a new dimension.
