Process and Techniques of Mold Polishing
Polishing is a crucial and final step in mold manufacturing. As plastic products are increasingly used in various industries, higher aesthetic standards are also demanded. Therefore, the surface polishing quality of plastic mold cavities must also improve accordingly. Molds requiring mirror finishes or high-gloss surfaces demand even finer surface roughness, which raises the standards for polishing work.
Polishing not only enhances the appearance of components but also improves corrosion and wear resistance. It facilitates subsequent injection molding processes by making parts easier to demold and reducing production cycle times.
Currently, commonly used polishing methods include the following:
1. Mechanical Polishing
Mechanical polishing removes surface protrusions by cutting or plastic deformation to achieve a smooth surface. It generally uses oil stones, wool wheels, sandpaper, etc., and is mainly done manually. Special-shaped parts like rotary bodies can be polished with auxiliary tools such as rotating platforms. For high-quality surface finishes, ultra-fine polishing techniques can be employed. These involve specialized grinding tools rotating at high speed in an abrasive slurry, achieving surface roughness down to Ra0.008μm - the highest among polishing methods. Optical mold polishing often uses this method.
(1) Basic Procedure of Mechanical Polishing
To achieve high-quality polishing results, the most important factors are high-quality tools such as oil stones, sandpaper, and diamond paste. The choice of polishing procedure depends on the prior machining condition - milling, EDM, grinding, etc.
Typical mechanical polishing steps:
① Rough Polishing:
After milling, EDM, or grinding, use a rotary surface polishing machine (35,000–40,000 rpm) or ultrasonic grinder. Commonly, a Φ3mm WA #400 wheel removes the white EDM layer. Follow with hand oil stone grinding, using kerosene as a lubricant or coolant. A typical sequence is: #180 → #240 → #320 → #400 → #600 → #800 → #1000. Some manufacturers start directly from #400 to save time.
② Semi-Finishing Polishing:
Use sandpaper and kerosene: #400 → #600 → #800 → #1000 → #1200 → #1500. Note: #1500 is suitable only for hardened steel (above 52HRC); using it on pre-hardened steel may cause surface burns.
③ Finishing Polishing:
Use diamond paste. A common order with polishing cloth wheels and diamond paste is: 9μm (#1800) → 6μm (#3000) → 3μm (#8000).
Then, use felt and diamond paste: 1μm (#14000) → 1/2μm (#60000) → 1/4μm (#100000).
Polishing with 1μm or finer abrasives should be done in a clean polishing room. For higher precision, a dust-free environment is essential, as particles like dust, dandruff, or saliva droplets can ruin hours of delicate work.
(2) Tips for Mechanical Polishing
Ⅰ. Sandpaper Polishing:
Use soft wood or bamboo sticks to hold the sandpaper. Softwood is better for curved surfaces, while harder woods like cherry are better for flat surfaces. Shape the stick end to match the workpiece to avoid scratches.
Change the polishing direction by 45°–90° between different grit sizes to make previous sanding marks visible and removable.
Clean the surface with 100% cotton and alcohol before changing grit or switching to diamond paste. Even a small abrasive particle can damage the next polishing stage.
When using #1200 or #1500 sandpaper, use light pressure and a two-step sanding method: polish in two directions per grit, turning 45°–90° each time.
Ⅱ. Diamond Polishing:
Use light pressure, especially for pre-hardened steels and finer pastes. Typical pressure for #8000 is around 100–200 g/cm². Use a soft stick with a thin handle or notched bamboo stick to control pressure.
Both work surface and hands must be clean.
Keep polishing time short to avoid defects like "orange peel" or "pitting."
Avoid heat-generating methods such as buffing wheels; heat can cause surface defects.
After polishing, clean the surface thoroughly and apply anti-rust coating.
Polishing is still largely manual, so the technician's skill remains the key factor. Additionally, material quality, pre-polishing surface conditions, and heat treatment all affect outcomes. Uniform hardness and composition help. Inclusions and pores in steel can hinder polishing.
(3) Effect of Hardness on Polishing
Higher hardness makes polishing more difficult, but results in lower roughness. It also reduces over-polishing risk but increases time required.
(4) Effect of Surface Condition
Improper machining (cutting stress, EDM) can damage the surface and affect polishability. EDM surfaces are harder to polish than milled or heat-treated surfaces. Finishing EDM operations must avoid forming a hardened layer. Poor EDM finishing can leave a hardened layer up to 0.4mm deep, which must be removed by rough grinding to prepare a consistent surface.
2. Chemical Polishing
This involves dissolving microscopic surface protrusions in a chemical solution to smooth the surface. Advantages: no complex equipment, effective for complex shapes and multiple parts simultaneously. Key: chemical solution formulation. Roughness is typically in the tens of microns.
3. Electrolytic Polishing
Similar to chemical polishing but uses electric current to eliminate cathodic reactions and improve results.
Two stages:
① Macroscopic leveling: Surface geometry improves (Ra > 1μm).
② Micro-smoothing: Brightness improves under anodic polarization (Ra < 1μm).
4. Ultrasonic Polishing
Workpieces are placed in abrasive suspension within an ultrasonic field. Oscillations cause abrasives to polish the surface. It causes minimal deformation but requires complex tooling. Can be combined with chemical or electrochemical methods for better results - ultrasound improves solution mixing and removes dissolved products.
5. Fluid Polishing
Uses high-speed liquid flow with abrasive particles to polish surfaces. Methods: abrasive jet machining, fluid jet polishing, fluid dynamic grinding. Abrasives are suspended in low-viscosity, pressure-driven fluids. Common abrasives: silicon carbide powders.
6. Magnetic Polishing
Magnetic abrasives form a "brush" under magnetic fields to polish surfaces. This method is efficient, delivers high-quality results, and is easily controlled. Can achieve surface roughness as fine as Ra 0.1μm.
Final Notes
In plastic mold processing, "polishing" often refers to mirror finishing, which differs from general surface polishing. It demands not only polish but also high flatness, smoothness, and geometrical accuracy.
Mirror finish standards (by roughness Ra):
AO = Ra 0.008μm
A1 = Ra 0.016μm
A3 = Ra 0.032μm
A4 = Ra 0.063μm
Because methods like electrolytic or fluid polishing can't precisely control shape, and methods like chemical, ultrasonic, and magnetic polishing can't meet ultra-fine surface quality needs, mechanical polishing remains the primary method for precision mold mirror finishing.
