Surface roughness of injection mold parts
The surface roughness of injection mold parts is a key indicator of mold processing quality, directly impacting product surface quality, mold life, and molding efficiency. Surface roughness refers to the degree of microscopic irregularities on a part’s surface and is typically expressed as the Ra value (arithmetic mean deviation of the profile), measured in micrometers (μm). Surface roughness requirements vary significantly among mold parts due to their varying functions. Properly controlling roughness ensures product precision while reducing processing costs, making it a critical consideration in mold design and manufacturing.
The cavity and core, as parts in direct contact with the melt, have the most stringent surface roughness requirements, typically requiring a range of Ra 0.025–0.8 μm. High-gloss products require a cavity surface roughness of Ra 0.025–0.1 μm, achieved through mirror grinding and polishing to ensure a consistent gloss without scratches or orange peel. For products with general appearance requirements, the cavity roughness can be relaxed to Ra 0.2–0.8 μm, using conventional grinding and fine polishing. For example, the cavity surface of a mobile phone case mold must be polished to Ra 0.05 μm, achieved through diamond grinding and fine polishing with chromium oxide polishing paste to achieve a mirror finish. For industrial component molds, a cavity roughness of Ra 0.8 μm is sufficient. Cavity surface roughness also affects melt flow. A smooth surface reduces flow resistance and lowers filling pressure, making it particularly suitable for high-viscosity plastics.
The surface roughness requirement for guide mechanism components (such as guide pins and guide sleeves) is Ra0.1-0.8μm to reduce friction and wear during relative motion. The roughness of the outer diameter of the guide pin and the inner bore of the guide sleeve must be controlled within Ra0.1-0.4μm. The smaller the clearance, the higher the roughness requirement. For example, the roughness requirement for guide pins and sleeves in precision molds is Ra0.1μm to ensure smooth movement without binding. Standard molds can be relaxed to Ra0.8μm, but regular lubrication is required to reduce wear. The surface roughness of the guide mechanism also affects the positioning accuracy of the mold. Excessive roughness will increase the clearance and affect the dimensional stability of the product. Therefore, precision grinding is required to ensure surface quality.
The surface roughness of ejector mechanism components (such as ejector pins and ejector plates) generally ranges from Ra0.4 to 3.2μm. The mating surface between the ejector pin and the ejector hole must reach Ra0.4 to 0.8μm to prevent sticking or leakage caused by excessive roughness. The working surface roughness of the ejector plate should be Ra1.6 to 3.2μm, which is sufficient to meet flatness requirements. For ejector parts that require frequent movement, the surface roughness should be appropriately reduced. For example, the ejector pin in an automotive bumper mold has a roughness of Ra0.4μm. The mating surface is nitrided to a hardness of HRC55 or higher, significantly improving wear resistance and service life. The roughness of the ejector mechanism must also be matched to the lubricant. A smooth surface combined with a specialized high-temperature grease can further reduce the coefficient of friction.
The surface roughness requirements for template components (such as fixed and movable templates) are differentiated by function. Working surfaces in contact with the mold cavity should have a roughness of Ra 0.8 to 3.2 μm to ensure a tight mold seal. Non-working surfaces (such as the sides and back) should have a roughness of Ra 3.2 to 12.5 μm, and only need to meet installation and strength requirements. The roughness of the template’s guide post holes, screw holes, and other mating holes should be Ra 1.6 to 6.3 μm. The hole walls must be smooth to ensure assembly accuracy. For example, the roughness of the guide post holes should be Ra 1.6 μm, achieved through reaming or boring, to ensure that the guide post does not loosen after being pressed in. Template roughness also affects the overall rigidity of the mold. Excessively rough surfaces can cause stress concentration, reducing the mold’s service life. Therefore, important templates require surface shot peening to reduce roughness and eliminate internal stress.
Controlling the surface roughness of mold parts requires a combination of processing technology and cost-effectiveness. High-precision roughness (Ra ≤ 0.1 μm) requires ultra-precision grinding, polishing, or electrochemical machining, which are costly and suitable for high-demand products. Medium roughness (Ra 0.2-1.6 μm) can be achieved through conventional grinding and milling, which is moderately cost-effective and widely applicable. Low-precision roughness (Ra ≥ 3.2 μm) can be achieved through rough milling and planing and is suitable for non-matching parts. In actual production, reasonable roughness standards must be established based on product requirements. For example, key parts in molds for medical-grade products must meet a roughness of Ra 0.05 μm, while molds for consumer goods can be relaxed to Ra 1.6 μm. At the same time, regular surface roughness testing of parts should be performed, using a roughness meter and multiple measurements at different locations to ensure compliance with design requirements and avoid product defects or mold failures caused by excessive roughness.
