Surface roughness of injection molded parts
The surface roughness of injection-molded parts is a key indicator of mold quality and plastic product performance. It directly impacts the appearance, performance, and lifespan of plastic products. Surface roughness refers to the microscopic irregularities on the mold cavity and core surfaces. Its magnitude is determined by a variety of factors, including mold processing technology, material properties, and application requirements. Properly controlling surface roughness is crucial for improving injection molding efficiency and product quality.
First, molded surface roughness has a direct impact on the appearance quality of plastic products. For plastic products with high appearance requirements, such as automotive interiors and household appliance housings, the molded surface must have a low roughness to ensure a smooth, delicate surface, free of scratches, blemishes, and other defects. Generally speaking, the molded surface roughness for such products should be controlled between Ra0.02-0.8μm. For high-gloss products, the molded surface roughness should even reach below Ra0.01μm to achieve a mirror-like effect. The lower the molded surface roughness, the higher the gloss and reflectivity of the plastic product, and the better the visual effect. Conversely, if the molded surface roughness is too high, the plastic product surface will replicate the mold’s microscopic unevenness, resulting in defects such as orange peel texture and scratches, which will affect the product’s market competitiveness. Therefore, during mold design and processing, the molded surface roughness value must be appropriately determined based on the appearance requirements of the plastic product.
Secondly, the roughness of the molded surface also has a significant impact on the performance of plastic products. For plastic products that need to be assembled, their surface roughness will affect the assembly accuracy and fitting performance. For example, if the surface roughness of plastic parts with sliding fit is high, the friction coefficient will increase, resulting in increased wear and reducing the service life of the product; while a lower surface roughness can reduce friction resistance, improve assembly accuracy and movement flexibility. For plastic products with sealing requirements, such as pipe joints and valves, the roughness of the molded surface will affect the sealing performance. A lower roughness can ensure that the sealing surface fits tightly and reduce the risk of leakage. In addition, surface roughness will also affect the corrosion resistance and coating performance of plastic products. Rough surfaces are prone to accumulate dust, moisture and corrosive media, accelerating product aging; while smooth surfaces are conducive to the adhesion of coatings, improving the uniformity and durability of the coating layer.
Mold material properties and processing technology are key factors in determining the surface roughness of the molded part. Different mold materials have different processing properties and thus have varying effects on surface roughness. For example, alloy tool steels (such as Cr12MoV) have high hardness and wear resistance, and can achieve low surface roughness after precision grinding and polishing. Cast iron, on the other hand, is softer and more susceptible to scratches, resulting in a generally higher surface roughness. The mold processing technology has a more significant impact on surface roughness. Common processing methods include milling, grinding, polishing, and electrical discharge machining. The surface roughness after milling is generally between Ra1.6-6.3μm, which is suitable for structural parts molds with low surface quality requirements; grinding can control the surface roughness to Ra0.4-1.6μm, which is suitable for molds with medium precision requirements; polishing can further reduce the surface roughness. Through mechanical polishing, chemical polishing and other methods, the surface roughness can reach Ra0.01-0.1μm, meeting the requirements of high-gloss product molds; EDM will form discharge pits on the surface, so its surface roughness is relatively high, generally between Ra0.8-3.2μm, but lower roughness can also be achieved by optimizing discharge parameters and performing subsequent polishing treatment.
Surface roughness also significantly impacts mold life and production efficiency. Lower surface roughness reduces friction between the mold and the plastic melt, lowering energy loss during melt flow and increasing mold filling speed and production efficiency. Furthermore, a smooth mold surface resists the adhesion of plastic debris, reducing the frequency and difficulty of mold cleaning and extending the mold’s continuous operating time. Furthermore, lower surface roughness improves the mold’s wear and corrosion resistance, reducing mold failure due to wear and corrosion and extending its service life. Conversely, higher surface roughness increases friction between the mold and the melt, leading to localized overheating and melt degradation, which not only affects the quality of the plastic product but also accelerates mold wear. Therefore, in mold design, it is important to consider both mold life and production costs while ensuring that the performance requirements of the plastic product are met. Appropriate selection of surface roughness is crucial.
In actual production, determining the molded surface roughness requires comprehensive consideration of multiple factors. First, determine an appropriate roughness range based on the intended use case and appearance requirements of the plastic product. For exterior parts, a lower roughness should be selected; for structural parts, the roughness requirement can be relaxed appropriately. Second, evaluate the feasibility and cost of achieving the target roughness, taking into account the mold material and processing technology. For example, using high-end mold steel and precision polishing processes can achieve lower roughness but at a higher cost; whereas using standard materials and conventional processing techniques is less expensive but results in higher roughness. Furthermore, the material properties of the plastic product must be considered. For plastics with poor flowability (such as PC and PMMA), a lower mold surface roughness helps improve melt flow and mold filling. For plastics with better flowability (such as PE and PP), the mold surface roughness requirement can be appropriately reduced. Finally, through trial mold verification, the surface roughness value can be adjusted based on the actual molding results to ensure optimal plastic product quality and production efficiency. With the continuous advancement of mold processing technology, such as the application of new technologies such as nano-polishing and laser surface treatment, the control accuracy of molded surface roughness will continue to improve, providing a stronger guarantee for the production of high-quality plastic products.
