Mold polishing tips

Mold polishing not only improves product quality but also increases aesthetics, enhances corrosion resistance, and reduces injection molding cycles.

This makes the polishing process extremely important in plastic mold manufacturing. In this article, we will share with you 6 mold polishing methods, factors that affect quality, and important notes during this process.


The mold has been polished
The mold has been polished

Here are 6 common methods for polishing plastic molds:

1. Mechanical polishing:

This method is the most commonly used. It involves using tools such as oil stones, ceramic stones, polishing cloths, sandpaper, and diamond cutting tools to remove dents and defects, creating a smooth and polished surface. Mechanical polishing is usually done manually. Supporting tools like rotary tables can be used for special or intricately shaped surfaces. For high-quality surface requirements, a super-finishing method using special tools can be applied.

Fine polishing and mirror polishing are often performed using specialized grinding tools such as air grinders or electric grinders fitted with polishing cloths and diamond polishing compounds, operated at high rotational speeds on the mold surface. This technique can achieve surface roughness as low as Ra 0.008μm.

Mechanical polishing does not require significant investment in tools and provides good results. It is suitable for various mold structures. However, due to manual polishing, it is time-consuming, labor-intensive, requires highly skilled craftsmen, and does not achieve high productivity.

2. Chemical mold polishing:

The second method is chemical polishing. This process involves dissolving and leveling the raised surface in a chemical environment, thereby creating a smooth surface.

The advantage of this method is that it does not require complex supporting equipment and can polish molds with complex shapes. Additionally, it allows for polishing multiple molds at the same time and achieves high efficiency.

However, an important issue with this method is the preparation of the polishing solution. The surface roughness achieved through chemical polishing is typically around 10μm.

The disadvantage of chemical polishing is that it can be hazardous to the operator. Furthermore, selecting the correct type of chemical is crucial because using the wrong chemical can damage the mold and make repairs difficult.

3. Electrolytic polishing:

The third method is electrolytic polishing. The basic principle is similar to chemical polishing, where the surface is smoothed by dissolving the raised portions on the mold surface in a specific electrolyte.

Compared to chemical polishing, electrolytic polishing is more effective as it can eliminate the influence of cathodic reactions.

The electrolytic polishing process is divided into two steps:

Rough polishing: The surface is dissolved and diffused into the electrolyte, reducing the surface roughness of the material, and achieving a value of Ra > 1μm.
Fine polishing: Using an anode electrode to improve the surface brightness, achieving a value of Ra < 1μm.

4. Ultrasonic mold polishing:

The next method is ultrasonic polishing. This is a technique that utilizes high-frequency vibrations to continuously agitate a polishing slurry in contact with the mold surface, resulting in a more uniformly polished surface and providing significant benefits for surface finishing.

This method is carried out by placing the mold and the polishing slurry in the same ultrasonic field. Due to the minimal applied force, it causes almost no deformation to the workpiece. However, due to the complexity of this process, manufacturing and installing the necessary tools can be challenging.

Ultrasonic mold surface treatment can be combined with chemical or electrochemical methods. Based on the process of solution corrosion and electrolysis, ultrasonic vibration is used to agitate the dissolving solution on the workpiece surface, creating uniformity in the corrosion or electrolysis process near the mold surface.

5. Liquid flow mold polishing:

The fifth method is polishing by liquid flow. This method relies on the erosion caused by the flow of a liquid containing abrasive particles on the mold surface to achieve polishing effectiveness.

There are three common methods for liquid flow polishing:

  • Spray abrasive compound.
  • Liquid jet.
  • Hydraulic polishing.

In the hydraulic polishing process, hydraulic pressure is used to propel the abrasive-laden liquid to flow over the mold surface at high velocity.

The abrasive slurry is typically composed of special substances (such as similar polymer materials) that are flexible, non-sticky, and easy to lubricate. Silicon carbide powder can also be used as an abrasive material.

6. Magnetic polishing:

The sixth method is magnetic polishing. This method utilizes magnetic abrasive particles to form a polishing brush that moves on the mold surface under the influence of magnetic force, achieving effective mold polishing.

This method offers high processing efficiency, good quality, and easy control. With suitable abrasive materials, the surface roughness can reach a value of Ra 0.1μm.

The basic process for mechanical polishing:

To achieve high-quality polishing results, the most important factor is to use high-quality polishing equipment, tools, and accessories such as grinding stones, sandpaper, and diamond paste.

The choice of polishing process depends on the surface condition after previous processing steps, such as milling, electrical discharge machining (EDM), grinding, and similar processes.

The general process of mechanical polishing includes the following steps:

1. Rough grinding:

After performing milling, turning, EDM, or other processing processes, the surface can be polished using a rotating grinding machine at a speed of about 35,000 to 40,000 revolutions per minute. To avoid electrical sparks during the grinding process, a grinding wheel with a diameter of 3mm and WA #400 material is commonly used.

Next is the manual grinding process using oil stones, combined with lubricants or coolant to dull any scratches. The usual sequence is from #180 to #240, #320, #400, #600, #800, and #1000. Many mold manufacturers start with #400 to save time.

2. Fine grinding:

In the fine grinding process, we primarily use sandpaper and honing oil to smooth the surface of the workpiece before proceeding to polish. The sequential levels of sandpaper used are #400, #600, #800, #1000, #1200, and #1500.

However, #1500 sandpaper is only suitable for hardened steel above 52 HRC and is not suitable for pre-treated steel as it can damage the surface of the workpiece.

3. Polishing:

In this final stage, we typically use diamond polishing paste. Various polishing methods are depending on the shape of the mold.

If we use a polishing cloth to evenly distribute diamond abrasive paste on the surface of the workpiece, the typical polishing process achieves high efficiency with grinding levels of 9μm (#1800), 6μm (#3000), and 3μm (#8000). Polishing cloth and 9μm diamond paste can also be used to remove small scratches from sandpaper #1200 and #1500.

Subsequently, to achieve a better, nearly mirror-like finish, we use adhesive cloth and diamond paste in the following order: 1μm (#14000), 0.5μm (#60000), and 0.25μm (#100000).

Polishing processes with precision higher than 1μm (including 1μm) need to be performed in a cleanroom environment. Any type of dust, moisture, and other factors can be detrimental to the mold surface polishing process, especially when high precision is required.

Mold polishing
Mold polishing

Points to note in the mold polishing process:

Pay attention to the following points after polishing with sandpaper and oil stones.

  • When polishing round or spherical surfaces, it is recommended to use flexible polishing bars such as rubber bars or bamboo bars. Hard bars like wood are more suitable for polishing flat surfaces.
  • The grinding tools must have a suitable and similar shape to the mold surface to avoid workpiece deformation during grinding and prevent deep scratches caused by the impact of the grinding tool. For example, sharp edges of the grinding tool can create deep scratches when polishing concave surfaces.
  • When changing the grinding stages or types of sandpaper, the polishing direction should be changed from 45° to 90° to differentiate from the polishing lines from the previous stages or sandpaper.
  • Before switching from one type of sandpaper to another, the polished surface should be thoroughly cleaned with a cleaning solution like 100% cotton alcohol-soaked paper. Any remaining dirt on the mold surface can disrupt the polishing process.
  • The cleaning process is also crucial when transitioning from sandpaper polishing to diamond polishing. All dirt particles and oil residue must be completely cleaned before continuing the polishing process.
  • To avoid scratching the workpiece surface, special attention should be given when using #1200 and #1500 sandpaper. Apply gentle force and polish the surface using a two-step polishing method. When polishing with each type of sandpaper, perform the polishing process twice in different directions, rotating the workpiece 45° to 90° each time.

Points to note after diamond grinding and polishing:

  • Polishing should be performed with the lightest pressure possible, especially when polishing pre-hardened steel parts using fine diamond powder.
  • When using #8000 diamond slurry for polishing, the typical load ranges from 100 to 200 g/cm2. However, maintaining the accuracy of this load can be challenging. To achieve better control, a grinding tool with a smaller and more delicate handle can be used to ensure that the grinding pressure is not excessively high.
  • When grinding and polishing with diamonds, not only should the working surface be cleaned but also the hands of the technician should be thoroughly cleaned.
  • Each polishing session should not be prolonged. The shorter the time, the better the efficiency. If the polishing process is extended for too long, it can cause the phenomenon of “orange peel” and “pitting”.
  • When finishing the polishing process, it is important to ensure that the mold surface is thoroughly cleaned and carefully remove any abrasive and lubricant residues. Furthermore, applying a layer of rust-resistant paint on the surface can provide additional protection.

Points to note after polishing plastic molds:

Polishing in plastic mold processing differs significantly from other industries. Specifically, polishing plastic molds requires achieving a level of finish resembling a mirror-like surface. It demands not only polishing but also high standards of flatness, smoothness, and geometric precision.

Surface polishing typically only requires a glossy finish. Mirror polishing standards are divided into four levels:

A0 = Ra 0.008μm
A1 = Ra 0.016μm
A2 = Ra 0.032μm
A3 = Ra 0.063μm

Electrolytic polishing and liquid flow polishing methods are difficult to control in terms of geometric precision. Methods such as chemical polishing, ultrasonic polishing, and magnetic polishing do not meet the requirements for surface quality. Therefore, mirror polishing of plastic mold surfaces primarily relies on mechanical polishing.

The influence of hardness difference on the mold polishing process:

The higher the hardness, the more challenging the grinding process, but the surface roughness will decrease during polishing.

As hardness increases, the time required to achieve low roughness also increases.

At the same time, as hardness increases, the excessive polishing capability decreases.

The influence of the initial surface condition on the mold polishing process:

During milling, the surface of the workpiece may be damaged due to heat, internal stress, or other factors. Improper cutting parameters will affect the effectiveness of the polishing process.

The surface after EDM processing is more difficult to polish compared to surfaces after cutting or conventional heat treatment processes.

Some issues when polishing plastic molds and their handling:

Over-polishing of molds:

Over-polishing is a common issue in plastic mold processing. When over-polished, the mold surface can become rough and uneven, resulting in a decrease in surface quality.

This problem often occurs during the mechanical polishing process when the polishing time or force exceeds the necessary level.

The main cause of the “orange peel” phenomenon on the mold surface:

The main cause of the “orange peel” phenomenon on the mold surface is the mold surface being too hot or the polishing process being prolonged. This usually happens when there is high polishing pressure and excessively long polishing time. Harder steels can withstand higher polishing pressures, while softer steels are more prone to over-polishing.

Remedies for eliminating the “orange peel” phenomenon on molds:

To address the “orange peel” phenomenon on the mold surface, it is recommended not to increase the polishing pressure or extend the mold polishing time. Instead, the following measures can be applied:

  1. Use slightly larger abrasive particle sizes to clean the defective surface. Then, perform the grinding process with lighter and more even pressure.
  2. Reduce the tension at a temperature 25°C lower than the steel annealing temperature, and then grind with the finest abrasive particles until the desired result is achieved. Finally, polish with lighter force.

Surface pits often occur due to metal impurities being pulled out from the steel surface during the mold polishing process, resulting in pitted marks. The main causes of this phenomenon include excessive polishing pressure, excessively long polishing time, impure workpieces with high impurity content, rusting of the mold surface, and unremoved burnt parts.

To eliminate surface pits, the following measures can be applied:

  1. Carefully re-grind the surface. Use slightly larger abrasive particle sizes than before and use soft and sharp oil stones for the final grinding step before polishing the mold again.
  2. Avoid using soft polishing tools with grit sizes smaller than 1mm.

Use minimal polishing force and the shortest possible polishing time.

Finishing out the mold
Finishing out the mold

On the mold surface, even after mirror polishing, there may still be tiny scratches present. Although they may not be visible when viewed head-on, they become apparent when illuminated at appropriate angles.

The main cause of this issue is the final polishing process not achieving the desired particle size, the use of low-quality polishing compounds containing impurities, or an inadequate working environment in terms of cleanliness.

To address this issue, the following measures can be taken:

  1. Use high-quality diamond polishing compounds from reputable brands.
  2. Use smaller abrasive particle sizes to eliminate the small scratches. The particle size can be reduced to 1μm or 0.5μm.
  3. Carry out the polishing process in a cleanroom environment.

In mold production, precision is crucial, especially for products that require high surface quality and accurate dimensions. Therefore, the use of highly accurate measuring devices such as FARO 3D laser scanning measurement machines is necessary to ensure the quality of the molds.

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