What are the Key Steps for Polishing Molds?

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Polishing molds, commonly used in plastic and die-casting industries, plays a crucial role in achieving high-quality mold surfaces. The traditional mold polishing process involved manual techniques such as filing and oil stone polishing to remove imperfections and refine the surface. However, with advancements in technology, the introduction of ultrasonic polishing machines, polishing compounds, and abrasives has revolutionized the industry. This article explores the key steps involved in mold polishing and highlights the significance of achieving mirror-like finishes.

The Polishing Process for Molds:

1. Semi-Polishing:

The semi-polishing stage primarily utilizes sandpaper and kerosene. The sandpaper grit sequence typically includes #400, #600, #800, #1000, #1200, and #1500. It's important to note that #1500 grit sandpaper is suitable for hardened mold steel (above 52HRC) but not for pre-hardened steel, as it may lead to surface burns.

2. Precision Polishing:

Precision polishing primarily involves diamond polishing compounds. The usual grinding sequence includes 9μm (#1800), 6μm (#3000), and 3μm (#8000). The 9μm diamond polishing compound and polishing cloth are used to remove the scratches left by #1200 and #1500 grit sandpaper. Subsequently, polishing is performed using adhesive felt and diamond polishing compounds in the sequence of 1μm (#14000), 0.5μm (#60000), and 0.25μm (#100000). For polishing processes demanding precision above 1μm (including 1μm), an extremely clean environment is required. Dust, smoke, scalp flakes, and saliva can all compromise the high-precision polished surface obtained after hours of work.

3. Mechanical Polishing:

To achieve high-quality polishing results, it is essential to use top-quality polishing tools and accessories such as oil stones, sandpaper, and polishing compounds. Equally important is the working environment, which should be a dust-free workshop. The selection of the polishing process depends on the surface condition resulting from previous processes such as machining, electrical discharge machining (EDM), and grinding.

4. General Procedure for Mechanical Polishing:

The general process for mechanical polishing is as follows:

- Rough polishing, precision milling, EDM, and grinding are followed by using a rotating surface polishing machine or an ultrasonic polishing machine with speeds ranging from 35,000 to 40,000 rpm. A common technique is to employ a 3mm diameter wheel with WA#400 to remove the white EDM layer.

- Next is manual oil stone polishing, using rectangular-shaped oil stones with coal oil as a lubricant or coolant. The recommended sequence includes #180, #240, #400, #600, and #1000 grit oil stones. Many mold manufacturers opt to start from #400 to save time.

Polishing molds, or rather a mirror-like machining, is significantly different from surface polishing in other industries. It demands high standards not only for the polishing itself but also for surface flatness, smoothness, and geometric precision. While various methods like electrolytic polishing and fluid polishing lack precise control over geometric accuracy, and chemical polishing, ultrasonic polishing, and magnetic abrasive polishing fail to meet the required surface quality, mechanical polishing remains the primary choice for precision mold finishing. By following the key steps outlined in this article and utilizing modern polishing techniques and equipment, professionals can achieve exceptional mold polishing results, improving efficiency and surface quality.