Mold polishing
Mold polishing is the process of removing tiny surface bumps on the mold through mechanical, chemical, or electrochemical methods, reducing surface roughness and improving surface finish. It is the final finishing step in mold manufacturing. Polishing quality directly impacts the mold’s service life and the surface quality of the finished product. Especially for plastic and die-cast molds, a high-gloss mold surface reduces mold release resistance and improves surface replication accuracy, enabling products to meet desired appearance requirements without secondary processing. Mold polishing should follow the principle of “coarse to fine, step-by-step” application, using abrasives of varying grit sizes to gradually refine the surface until the desired finish is achieved.
A wide variety of tools and abrasives are available for mold polishing. Polishing can be categorized into rough polishing, medium polishing, and fine polishing, depending on the polishing stage. Rough polishing uses grinding wheels or sandpaper with a grit of 80-240 grit to remove machining marks left by previous processes (such as milling and grinding), reducing surface roughness from Ra3.2μm to Ra1.6μm. Medium polishing uses abrasives with a grit of 240-800 grit to further refine the surface, reducing roughness to Ra0.4-0.8μm. Fine polishing uses diamond paste or chromium oxide abrasives with a grit of 1000-8000 grit, achieving a mirror finish of Ra0.025-0.1μm. Polishing tools include handheld polishers, ultrasonic polishers, and electrolytic polishing equipment. Handheld polishers are suitable for flat and simply curved surfaces. Ultrasonic polishers use high-frequency vibrations (20-40kHz) to drive the abrasive to impact the surface, making them suitable for complex cavities and narrow gaps. Electrolytic polishing uses electrochemical action to dissolve surface protrusions, making it suitable for mirror polishing materials such as stainless steel.
The mold polishing process is tailored to the mold material and surface requirements. For mold steels (such as Cr12MoV and P20), the typical approach is “rough grinding with a grinding wheel → sandpaper polishing → fine polishing with abrasive paste.” For rough grinding, use an alumina grinding wheel at a speed of 1500-2000 r/min, employing a cross-grinding method to remove machining marks. For sandpaper polishing, use 240#, 400#, and 600# water-repellent sandpaper, lubricated with kerosene or water. Polish manually or mechanically, ensuring that the previous sandpaper marks are completely covered. For fine polishing, apply diamond paste (1-3μm grit) to a wool wheel or suede, rotate at a speed of 800-1500 r/min, and polish at a pressure of 0.1-0.3 MPa until the surface achieves a mirror-like gloss. For example, the polishing route of the plastic model cavity is: 80# grinding wheel rough polishing → 400# sandpaper medium polishing → 1000# grinding paste fine polishing → 5000# grinding paste mirror polishing, and the final surface roughness is Ra0.05μm.
Polishing methods for different mold areas require tailored adjustments. A flat polisher with a magnetic worktable for workpiece absorption ensures uniform polishing for the cavity bottom and flat surfaces. Handheld polishing tools are used for cavity sides and curved surfaces, adjusting the angle to accommodate the curve to avoid localized dents. Specialized polishing rods (0.5-5mm diameter) are used for manual or mechanical rotary polishing of small gaps and deep holes to ensure no machining marks within the gaps. For EDM surfaces, the white layer (the hardened layer produced by EDM) must first be removed. This can be achieved by sanding with 600# sandpaper or specialized electrolytic tools. Otherwise, the polished surface may appear spotty. For example, deep cavities in die-casting molds are first polished with an ultrasonic polisher, followed by fine polishing with a 3mm diameter wool polishing head to ensure a consistent finish on the bottom and sides of the cavity.
Mold polishing quality control requires a combination of surface roughness testing and visual inspection. The roughness tester can accurately measure the Ra value (accuracy 0.001μm) to ensure compliance with design requirements (e.g., Ra ≤ 0.025μm for mirror molds). Visual inspection should be conducted under natural light or specialized lighting, observing the surface for defects such as scratches, pitting, and haze. A qualified polished surface should be free of visible defects and exhibit uniform reflectivity. Careful cleaning is essential during the polishing process. Each time the abrasive is changed, the surface should be cleaned with alcohol or acetone to prevent coarse abrasive residue from affecting subsequent polishing. For molds produced in large quantities, rust prevention treatment is required after polishing. Application of a specialized anti-rust oil or chrome plating (coating thickness 0.01-0.03mm) is recommended to improve wear resistance and facilitate demolding.
Polishing molds made of special materials requires special measures. Aluminum alloy molds are relatively soft, so the polishing pressure should be reduced (0.05-0.1 MPa) to prevent the abrasive from embedding into the surface. Chromium oxide abrasives can be used for fine polishing. Stainless steel molds are prone to forming oxide layers, so specialized coolants (such as stainless steel polishing fluid) should be used during polishing to prevent surface rust. Carbide molds are relatively hard (HRC 65-70), requiring the use of diamond abrasives and an increased polishing speed of 2000-3000 rpm to ensure efficient material removal. With technological advancements, automated polishing equipment (such as robotic polishing systems) is increasingly being used in mold manufacturing. By programmable control of the polishing trajectory and pressure, automated polishing of complex cavities is achieved, increasing polishing efficiency by 5-10 times compared to manual methods and providing greater quality consistency. Through scientific polishing processes and strict quality control, mold surface quality and performance can be significantly improved, ensuring the production of high-quality products.
