Selection of surface roughness values for injection molding
The surface roughness of injection-molded products is a key indicator of surface quality. It not only impacts the product’s aesthetics but also significantly influences its performance, assembly accuracy, and wear resistance. Choosing the optimal surface roughness value requires comprehensive consideration of factors such as the product’s application scenario, functional requirements, material properties, and production costs. Scientific analysis and practical verification are then used to determine the optimal parameters.
First, clarifying the product’s application and appearance requirements is the primary basis for selecting a surface roughness value. For exterior parts, such as automotive interiors and appliance housings, a high surface finish is typically required to create an aesthetically pleasing and refined visual effect. The surface roughness (Ra) of these products is generally controlled between 0.02-0.8μm, with high-finish products achieving Ra values as low as 0.02-0.1μm. This is achieved through high-precision mold polishing and optimized injection molding processes. For structural or concealed components, such as gears and brackets within machinery, appearance requirements are lower, prioritizing functionality and assembly. Surface roughness values can be relaxed to 1.6-6.3μm to reduce production costs. Furthermore, certain specialized applications, such as non-slip handles and knobs, require a higher surface roughness (Ra 6.3-25μm) to increase surface friction and meet operational requirements.
Secondly, the functional requirements of the product play a decisive role in the selection of surface roughness values. In applications where sealing performance is critical, such as pipe joints and valve sealing surfaces, a low surface roughness is required to ensure a tight fit and reduce the risk of leakage. Typically, the Ra value is controlled between 0.1-0.8μm. For sliding friction components, such as bearings and guide rails, the surface roughness value must be selected based on both wear resistance and the coefficient of friction. Typically, the Ra value is between 0.4-1.6μm. Too low a roughness may lead to poor lubrication, while too high a roughness may increase friction and wear rate. In applications where optical performance is critical, such as lenses and prisms, the surface roughness must be kept to an extremely low level (Ra <0.02μm) to minimize light scattering and reflection losses and ensure optical performance. Furthermore, for products requiring post-processing such as coating and electroplating, the surface roughness should be moderate (Ra 0.8-3.2μm). Excessively smooth surfaces can impair coating adhesion, while excessively rough surfaces can result in uneven coating thickness.
Material properties are crucial factors when selecting a surface roughness value. Different plastic materials vary in properties such as fluidity, shrinkage, and hardness, resulting in varying degrees of ability to replicate mold surfaces, which in turn affects the surface roughness of the finished product. Materials with good fluidity, such as PE and PP, can better replicate mold surface details, allowing them to produce products with a high finish even with low mold surface roughness. Therefore, a lower Ra value can be selected. Materials with poor fluidity, such as PC and PMMA, on the other hand, have a poorer ability to replicate mold surfaces. Pursuing an excessively low surface roughness requires higher-precision mold processing and stricter process control, increasing production costs. Furthermore, for brittle materials like PS, excessively rough surfaces can easily lead to stress concentration and cracking, so a relatively low surface roughness value is desirable. Tough materials, such as ABS, are more adaptable to varying surface roughness levels and can be flexibly selected based on functional requirements.
Mold processing accuracy and cost factors also affect the determination of surface roughness values. The surface roughness of the mold directly determines the surface quality of the product. To obtain low-roughness products, the mold cavity must be polished with high precision, such as mirror polishing, which significantly increases the manufacturing cost and processing cycle of the mold. Therefore, when selecting the surface roughness value, it is necessary to strike a balance between quality requirements and cost. For large-volume, high-value-added products, a higher cost can be invested in the production of high-precision molds to achieve ideal surface quality; for small-volume, low-cost products, the surface roughness requirements can be appropriately relaxed and conventional mold processing can be adopted. In addition, the injection molding process parameters will also affect the surface roughness of the product. By optimizing parameters such as injection speed, temperature, and pressure, the surface quality of the product can be improved to a certain extent, reducing the dependence on high-precision mold processing.
Finally, referring to relevant standards and practical experience is an effective way to select surface roughness values. Different industries have clear standards and specifications for the surface roughness of injection molded products, such as the QS9000 standard in the automotive industry and the IPC standard in the electronics industry. Appropriate values can be selected based on specific industry standards. At the same time, learning from the successful cases of similar products and making adjustments based on one’s own production conditions can improve the accuracy of the selection. In actual production, mold trials can be conducted to produce samples with different surface roughness levels. Appearance inspection, performance testing, and cost accounting can be carried out to ultimately determine the most appropriate value. In addition, the impact of subsequent processing technology on surface roughness, such as friction during assembly and corrosion in the operating environment, must also be considered. A certain margin must be reserved to ensure that the product meets the requirements throughout its entire life cycle.
