Injection Mold Polishing Guide: Process And Best Practice

Table of Contents

Polishing is one of the most critical finishing steps in injection mold manufacturing. A well-polished mold not only enhances the appearance of molded parts but also improves demolding performance, extends mold lifespan, and ensures precision. Whether the goal is to achieve a mirror finish for transparent components, reduce surface roughness for high-gloss housings, or improve sealing accuracy in functional parts, mold polishing plays a decisive role.

In this guide, we will walk you through the essentials of injection mold polishing—including the most commonly used tools and materials, step-by-step polishing processes, expert techniques, and best practices that guarantee consistent, high-quality results.

1. What Is Mold Polishing?

Mold polishing is the process of refining the surface of an injection mold using oil stones, sandpaper, polishing compounds, and felt wheels to achieve a smooth or mirror finish. It removes machining marks and defects, improves surface quality, and ensures better part release, accuracy, and mold durability.

2. The Purpose of Mold Polishing

Enhance Surface Quality

Polishing effectively removes cutter marks, machining lines, and EDM textures, resulting in high-gloss mold surfaces. This ensures the final plastic parts have smooth, aesthetic surfaces, which is especially important for home appliance housings, consumer electronics, and transparent parts.

Improve Demolding Performance

Polished mold cavities have lower friction coefficients, which makes molded parts easier to release. This prevents sticking, scratches, and ejector marks, while improving molding stability and efficiency.

Extend Mold Lifespan

By removing micro-cracks, burrs, and stress concentrations, polishing delays fatigue crack initiation and helps molds maintain durability during long production runs.

Ensure Accuracy and Sealing

Precision polishing can correct minor geometric deviations, improve parting line sealing, and ensure injection-molded products meet strict dimensional accuracy and assembly requirements.

3. Common Tools and Materials

At Alpine Mold, our polishing experts rely on a variety of specialized tools and materials to achieve high-gloss and mirror finishes for injection molds. These include:

Sandpaper

Oil Stones

Felt Wheels

Polishing Compounds

3.1 Sandpaper

Function: Mainly used during semi-finishing to eliminate oil stone marks, refine surface texture, and prepare for precision polishing.

Grits: 150# → 180# → 320# → 400# → 600# → 800# → 1000# → 1200# → 1500#.

Notes: Use bamboo or hardwood sticks as backing to ensure even pressure. Change polishing direction (45°–90°) when switching grits to remove previous scratches. For high-gloss or transparent parts, 1200#–1500# water sandpaper is essential.

3.2 Oil Stones

Function: Used in rough polishing to quickly remove machining marks and EDM white layers, forming a flat foundation.

Grits: 120# → 220# → 400# → 600#.

Notes: Always pair with kerosene as a lubricant to prevent overheating. Shape the stone’s tip to match the workpiece surface to avoid deep scratches.

3.3 Felt Wheels

Function: Essential for fine and final polishing, serving as carriers for polishing paste.

Types: Cylindrical, conical, flat-head, mushroom-shaped—adaptable to various mold geometries.

Notes: Each grit of polishing paste requires a dedicated felt wheel to prevent contamination. Avoid overheating, which can cause orange peel or haze.

3.4 Polishing Compounds

Function: Used for precision and mirror finishes by eliminating microscopic peaks and valleys.

Grades: 1# (white), 3# (yellow), 6# (orange), 9# (green), or by micron sizes (9 μm → 6 μm → 3 μm → 1 μm → 0.5 μm → 0.25 μm).

Notes: Apply in small amounts, with light pressure, and clean thoroughly between grit changes to avoid scratches.

3.5 Other auxiliary tools

Our other tools include diamond files, diamond burrs, bamboo sticks, fiber stones, and rotary polishers. Bamboo sticks are excellent for pressing sandpaper against curved surfaces, while fiber stones are ideal for narrow slots. Rotary polishers help speed up material removal during rough polishing.

4. The Injection Mold Polishing Process

At Alpine Mold, we understand that polishing is not just about achieving brightness—it is critical for product quality, demolding, and mold life. That’s why we follow a strict, standardized polishing workflow:

Rough Polishing

Semi-Finishing

Precision Polishing

4.1 Rough Polishing

Steps: After CNC machining and EDM, we use high-speed rotary polishers (35,000–40,000 rpm) or ultrasonic grinders to remove EDM layers.

Tools: WA#400 wheels, followed by oil stones (#180 → #240 → #320 → #400 → #600 → #800 → #1000).

Method: Apply kerosene as lubricant, keep a 70° cross-angle, and stroke evenly (40–70 mm).

Advantage: Alpine Mold’s technicians ensure EDM marks are thoroughly removed, laying a solid foundation for semi-finishing.

4.2 Semi-Finishing

Steps: Use sandpaper (320#–1500#) with bamboo or hardwood sticks.

Method: Cross-polish at 70°, replace sandpaper every 10–15 strokes to maintain sharpness, and gradually transition up to 1200#–1500#.

Advantage: Our strict control over sanding direction and frequency ensures superior flatness and avoids orange peel defects.

4.3 Precision Polishing

Steps: Diamond paste polishing with cloth or felt wheels.

Sequence:

Cloth wheel + paste: 9 μm → 6 μm → 3 μm.

Felt wheel + paste: 1 μm → 0.5 μm → 0.25 μm.

Method: Light pressure, dedicated wheels per grit, strict cleaning between steps.

Advantage: We achieve Ra ≤ 0.1 μm, meeting the most demanding requirements for transparent parts and mirror-finish surfaces.

5. Tips and Best Practices

Sandpaper Polishing

• Use softwood or bamboo sticks for curved surfaces, and harder wood for flat surfaces.

• Change sanding direction (45°–90°) when switching grits to fully remove previous marks.

• With #1200–1500 sandpaper, use light pressure and two-step polishing (two passes per direction).

Diamond Polishing

• Apply light pressure, especially on pre-hardened steel or with fine compounds.

• Modify sticks or handles for better control of force and angle.

• Avoid excessive polishing time to prevent orange peel or pits.

• Keep hands and surfaces clean; avoid high-heat tools.

Final Protection

After polishing, ensure the mold surface is perfectly clean, remove all abrasives and lubricants, and apply a uniform anti-rust coating to protect during storage or shipping.

6. Factors Affecting Mold Polishing

Polishing Technique: Since polishing is mostly manual, operator skill is crucial. Poor technique leads to uneven surfaces or defects.

Workpiece Surface Condition: Improper machining parameters or EDM surfaces increase polishing difficulty and require special treatment.

Steel Quality: Impurities or porosity in the steel can cause pits during polishing. High-quality tool steel ensures better results.

Heat Treatment: Incorrect heat treatment may result in inconsistent hardness, making polishing more difficult. Proper treatment stabilizes the structure and improves results.

7. Injection Mold Polishing Case Studies

Many customers choose Alpine Mold not only for our mold-making precision but also for our ability to deliver mirror-finish polishing results. Here are some examples:

switch panel injection mold polish

switch panel injection mold polish

Bathroom show head injection mold polish

Bathroom show head injection mold polish

Conclusion

Injection Mold polishing is a crucial step that determines the surface quality, part release, and durability of injection molds. At Alpine Mold, we combine decades of experience, strict process control, and professional craftsmanship to deliver mirror-polished molds that exceed customer expectations in industries such as automotive, electronics, home appliances, and medical devices.

If you are looking for a reliable partner for custom plastic injection molding and mold manufacturing with superior polishing quality, contact Alpine Mold today.