The surface smoothness of high-density polyethylene medicinal ointment boxes directly affects the risk of drug residue, ease of cleaning, and user experience. Mold polishing is a core factor determining surface quality. During injection molding of HDPE materials, the melt flow characteristics and mold surface condition work together. If machining marks or micro-defects exist in the mold cavity, these textures will be replicated after the melt cools, leading to increased surface roughness of the box. Therefore, optimizing the mold polishing process can significantly improve the surface smoothness of HDPE ointment boxes, ensuring drug safety and user experience.
The primary goal of mold polishing is to eliminate machining marks on the cavity surface. In traditional mold manufacturing, milling, EDM, and other processes leave tool marks or EDM pits on the cavity surface. These micro-defects become resistance points for melt flow, causing uneven shrinkage lines to form when the HDPE melt cools. Preliminary polishing using an oilstone or sandpaper during the rough polishing stage removes obvious machining marks, laying the foundation for subsequent fine polishing. This stage requires controlling the grinding pressure and direction to avoid excessive wear that could lead to dimensional deviations in the cavity. Regular abrasive replacement is also necessary to prevent impurities from embedding into the surface.
The fine polishing stage is crucial for improving surface smoothness, requiring the use of finer abrasives and polishing tools. Diamond polishing paste or alumina polishing liquid, combined with soft polishing wheels (such as cloth wheels or wool wheels), can create a mirror-like effect on the cavity surface. During polishing, the abrasive particles, under the combined action of mechanical friction and chemical corrosion, gradually remove scratches left from the rough polishing, significantly reducing surface roughness. This stage requires strict control of polishing time and speed. Excessive polishing time may lead to localized overheating and material phase transformation, while excessively high speeds may produce polishing haze, thus reducing surface quality.
The surface hardness of the mold has a decisive impact on the polishing effect. HDPE material has relatively low hardness; if the mold cavity hardness is insufficient, surface scratches can easily occur during polishing due to abrasive embedding or tool wear. Therefore, mold steel must be made of high-hardness, high-wear-resistance materials (such as H13 steel or S136 steel), and its appropriate hardness (typically 48-52 HRC) should be obtained through quenching and tempering. Increased hardness allows the mold surface to resist mechanical erosion during polishing, ensuring uniform action of abrasive particles and resulting in a smoother surface.
Controlling the polishing direction is equally important. HDPE melt has directionality when filling the cavity. If the polishing direction is inconsistent with the melt flow direction, it may lead to "flow marks" or "orange peel" texture on the surface. Therefore, the optimal polishing path must be determined based on the mold runner design and melt flow analysis. A combination of unidirectional polishing or cross-polishing is usually used. First, rough polishing is performed along the main melt flow direction, followed by fine polishing at a 45° angle to eliminate directional texture and achieve a uniform gloss.
Mold cleaning and protection are the final steps in the polishing process. If polishing fluid or abrasive particles remain on the polished mold surface, they will transfer to the HDPE mold surface during injection molding, forming contamination points. Therefore, the mold must be thoroughly cleaned using ultrasonic cleaning or high-pressure air gun purging, and a special rust-preventive oil should be applied to prevent oxidation. Furthermore, the mold should be stored away from hard objects to prevent surface scratches, ensuring a consistent surface condition before each production run.
Improved surface smoothness has multidimensional effects on the performance of high-density polyethylene medicinal ointment boxes. A smooth surface reduces the contact area between the drug and the box, lowering the risk of residue, especially crucial for high-viscosity or easily absorbed ointments. Simultaneously, a smooth surface facilitates cleaning and disinfection, meeting hygiene standards for pharmaceutical packaging. Additionally, reduced friction when opening the lid results in a smoother operating experience. These advantages collectively constitute the market competitiveness of HDPE ointment boxes, while optimized mold polishing processes are the technological cornerstone for achieving these goals.
