Causes of and Solutions for Over-polishing while Mold polishing

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Mold polishing is the process of using polishing tools to induce plastic deformation on the surface of a material, thereby removing protrusions and achieving a smooth surface. Commonly used tools for polishing include oil stones, wool wheels, and sandpaper, with manual operation being the predominant method. For workpieces with high surface quality requirements, specialized abrasive tools can be used for ultra-precision grinding and polishing.

Ultra-precision grinding and polishing involves pressing a specially designed abrasive tool against the processed surface of a workpiece while it is immersed in a polishing fluid containing abrasives. The tool undergoes high-speed rotational motion, resulting in a surface roughness as low as 0.008μm. During the polishing process, excessive polishing can sometimes occur, leading to a deterioration in the surface quality of the tool. This phenomenon is more common in mechanical polishing. Specifically, there are two different manifestations of excessive polishing: "orange peel" and "pitting." Let's explore the causes and methods to eliminate each.

Causes of "orange peel":

"Orange peel" refers to a high and irregular surface roughness on the polished workpiece. The causes of "orange peel" include excessive polishing pressure, prolonged polishing time, overheating of the mold surface, and excessive carburization of the workpiece. For example, when using a polishing wheel, improper control of the polishing time can easily generate heat on the work-piece surface, leading to the appearance of "orange peel." The degree of susceptibility to "orange peel" varies for different materials. Materials with higher hardness can withstand greater polishing pressure and are less prone to "orange peel," while softer work-pieces are more susceptible to excessive polishing and the occurrence of "orange peel."

Measures to eliminate "orange peel":

In practical operations, there are some erroneous approaches when attempting to improve surface quality upon encountering poor results. Many people opt to increase polishing pressure and extend polishing time. However, this practice may not only fail to improve surface quality but can also have counterproductive effects. The correct method to remove "orange peel" involves removing the defective surface first and then using a slightly coarser abrasive tool compared to the previous one. Polishing should be performed with a lower polishing force. Alternatively, the workpiece can undergo stress relief at a temperature 25 degrees Celsius lower than the tempering temperature. Subsequently, the finest grit abrasive tool should be used for grinding until satisfactory results are achieved, followed by polishing with a lighter force.

Causes of "pitting":

"Pitting" refers to the appearance of tiny pits on the surface of the workpiece after polishing. These pits result from the presence of non-metallic impurities, typically hard and brittle oxides, mixed within the metal workpiece. If excessive polishing pressure or prolonged polishing time is applied, these impurities can be pulled out from the steel surface, leading to the formation of tiny pits. This phenomenon is more likely to occur when the steel lacks sufficient purity and contains higher levels of hard impurities. Additionally, it is more prone to occur when the surface of the steel is rusted or when the scale material has not been properly removed.

Measures to eliminate "pitting":

To eliminate "pitting," the first step is to re-grind the surface of the workpiece using an abrasive tool with slightly coarser grit compared to the previous one. The grinding force should be minimized. Subsequently, the final step involves using a soft and sharp oil stone for the polishing process, once satisfactory results have been achieved through grinding. During polishing, if the size of the abrasive grains is smaller than 1 millimeter, it is advisable to avoid using the softest polishing tool. The polishing force should be kept as minimal as possible, and the duration of polishing should be kept short.

In summary, mold polishing is a process that involves removing surface imperfections through the use of polishing tools. The choice of tools, such as oil stones, wool wheels, and sandpaper, depends on the specific requirements of the polishing task. For workpieces that demand exceptionally high surface quality, ultra-precision grinding and polishing techniques can be employed.

Ultra-precision grinding and polishing utilizes specialized abrasive tools and polishing fluids to achieve surface smoothness with roughness as low as 0.008μm. However, during the polishing process, it is crucial to avoid two common issues: "orange peel" and "pitting."

"Orange peel" refers to an irregular and high surface roughness on the polished workpiece. It can be caused by factors such as excessive polishing pressure, prolonged polishing time, overheating of the mold surface, or excessive carburization of the workpiece. It is important to note that different materials exhibit varying susceptibility to "orange peel." Harder materials can tolerate higher polishing pressures and are less prone to this issue, while softer materials are more susceptible.

To eliminate "orange peel," it is essential to follow the correct approach. Instead of increasing pressure and extending polishing time, the focus should be on removing the defective surface and using a slightly coarser abrasive tool. Polishing should be performed with lower force to achieve the desired surface quality. Additionally, stress relief can be applied to the workpiece at a temperature of 25 degrees Celsius below the tempering temperature before the polishing process.

"Pitting" refers to the formation of tiny pits on the workpiece surface after polishing. This occurs due to the presence of non-metallic impurities, usually hard and brittle oxides, within the metal. Factors such as excessive polishing pressure, prolonged polishing time, and inadequate removal of rust or scale material can contribute to "pitting." The presence of impurities and the purity of the steel significantly influence the occurrence of this issue.

To address "pitting", the surface of the workpiece should be re-ground using a slightly coarser abrasive tool before proceeding to the final polishing step. It is crucial to apply minimal grinding force during this stage. Subsequently, a soft and sharp oil stone can be used for the final polishing, ensuring that the desired surface quality is achieved. When polishing, it is recommended to avoid using the softest polishing tools for abrasive grains smaller than 1 millimeter in size. The polishing force and duration should be minimized to prevent the formation of pits.

Mold polishing is a precise and meticulous process that requires attention to detail and adherence to proper techniques. By understanding the causes of issues such as "orange peel" and "pitting," and implementing appropriate measures, it is possible to achieve the desired surface quality in the polishing of molds.