What factors can affect the surface quality of plastic injection molded products?

Release time：2025/1/6   Click ：

1、 Mold factor
Mold accuracy: The dimensional accuracy of the mold directly affects the dimensional accuracy and appearance integrity of the product. If the cavity size of the mold is not accurate, the product will have dimensional deviations. For example, dimensional errors in the mold cavity may lead to uneven product thickness, resulting in shrinkage marks or deformation during the injection molding process. In addition, high-precision molds can ensure the flatness and smoothness of the product surface. When the roughness of the mold surface reaches Ra0.8 μ m or less, it is conducive to producing products with high surface quality.
Mold surface treatment: The surface treatment method of the mold has a significant impact on the surface quality of the product. For example, polishing the mold can give the product surface a high glossiness. Different polishing levels, such as mirror polishing (roughness up to Ra0.025 μ m or less) and regular polishing, will produce products with different gloss effects. In addition, texture treatment on the surface of the mold (such as fire patterns, leather patterns, etc.) can give the product a corresponding surface texture, like some product shells that imitate leather texture, achieving realistic effects by making leather patterns on the surface of the mold.
Gate and runner design: The position, size, and shape of the gate will affect the way and speed of plastic melt entering the mold cavity. If the gate position is not reasonable, it may cause uneven flow of plastic melt in the mold cavity, resulting in weld marks and affecting the integrity of the product surface. For example, for transparent plastic products with high appearance requirements, improper gate positions may result in visible weld lines. The design of the flow channel is also crucial. A reasonable flow channel can ensure that the pressure loss of the plastic melt is uniform during the flow process, so that all parts of the product are fully filled and the surface quality is consistent.


2、 Raw material factors
Raw material quality: The purity, particle size, and uniformity of plastic raw materials have a significant impact on the surface quality of products. Raw materials with high purity and low impurities can reduce the probability of defects such as black spots and impurities appearing on the surface of the product. For example, high-quality polycarbonate (PC) raw materials can ensure the transparency and smoothness of the lens surface when used in the production of optical lenses. Uniform particle size of raw materials helps to promote uniform flow of plastic melt during injection molding, avoiding unstable melt flow caused by differences in particle size, and thus preventing surface flow marks.
The flowability and shrinkage rate of raw materials: The flowability of raw materials determines their filling situation in the mold cavity. Raw materials with good fluidity can better fill the subtle parts of the mold cavity, reducing the occurrence of dents or incomplete filling on the product surface. Different plastic materials have different shrinkage rates, and excessive shrinkage rates may cause shrinkage marks on the surface of the product. For example, the shrinkage rate of polypropylene (PP) is generally higher than that of polystyrene (PS), and more precise control of process parameters is required during injection molding of PP products to prevent shrinkage marks.
Color matching and additives of raw materials: When adding color masterbatch or other additives to raw materials for color matching or performance improvement, uneven dispersion of color masterbatch can cause uneven surface color of the product. The type and quality of additives can also affect the surface quality of products, such as adding inappropriate lubricants that may cause oil stains or abnormal gloss on the product surface.


3、 Injection molding process factors
Injection molding temperature: Temperature has a significant impact on the flowability and molding quality of plastic melt. If the injection temperature is too high, the plastic melt may decompose and produce gas, resulting in bubbles or burnt marks on the surface of the product. For example, when injecting ABS plastic, if the temperature exceeds its decomposition temperature (about 270 ℃), this problem is prone to occur. On the contrary, too low a temperature will result in poor fluidity of the melt, leading to defects such as flow marks and insufficient filling on the surface of the product.
Injection pressure and speed: Injection pressure and speed control the flow state of plastic melt in the mold cavity. Excessive pressure and speed may cause burrs and burrs, as the melt may overflow the parting surface of the mold. At the same time, this may also cause the air in the mold cavity to not be discharged in a timely manner, resulting in trapped air and the appearance of bubbles or air marks on the surface of the product. If the pressure is too low or the speed is too slow, it will cause insufficient filling and affect the integrity of the product surface.
Holding time and pressure: The holding stage is crucial for the dimensional accuracy and surface quality of the product. If the holding time is too short or the holding pressure is insufficient, the product will not receive enough plastic replenishment during the cooling shrinkage process, resulting in shrinkage marks. For example, for plastic products with thicker walls, extending the holding time and increasing the holding pressure can effectively reduce shrinkage marks and ensure the flatness of the product surface.