Mastering the Complete Guide to Stainless Steel Mirror Polishing Techniques

 

Common Tools and Specifications for Stainless Steel Mirror Polishing

 

 

1. Common tools used for mirror polishing of stainless steel include: sandpaper, oil stones, felt wheels, polishing compounds, alloy files, diamond grinding pins, bamboo sticks, fiber oil stones, and rotary polishing machines.

2. Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1000#, 1200#, 1500#

3. Oil stones: 120#, 220#, 400#, 600#

4. Felt wheels: cylindrical, conical, square pointed

5. Polishing compounds: 1# white, 3# yellow, 6# orange, 9# green, 15# blue, 25# brown, 35# red, 60# purple

6. Files: square, round, flat, triangular, and other shapes

7. Diamond grinding pins: typically with 3/32 or 1/8 shank, available in round, cylindrical, long straight cylindrical, and long conical shapes

8. Bamboo sticks: various shapes suitable for operators and mold shapes, used to press sandpaper against the workpiece for achieving the required surface roughness

9. Fiber oil stones: 200# black, 400# blue, 600# white, 800# red

 

 

Process Steps for Stainless Steel Mirror Polishing

 

(1) Coarse Polishing

 

After processes like precision milling, electrical discharge machining (EDM), or grinding, the surface can be polished using a rotating surface polishing machine with speeds ranging from 35,000 to 40,000 rpm. Followed by manual oilstone grinding, using rectangular oilstones with kerosene as a lubricant or coolant. The sequence of grits used is as follows:

180#, 240#, 320#, 400#, 600#, 800#, 1000#.

 

 

(2) Semi-Fine Polishing

 

Semi-fine polishing primarily utilizes sandpaper and kerosene. The grits of sandpaper used are as follows:

400#, 600#, 800#, 1000#, 1200#, 1500#.

In practice, #1500 sandpaper is only suitable for hardened mold steel (52HRC and above) and not for pre-hardened steel, as it may lead to surface damage in pre-hardened steel parts and fail to achieve the expected polishing results.

 

 

(3) Fine Polishing

 

Fine polishing mainly employs diamond polishing compounds. If polishing is carried out using a polishing cloth wheel with a mixture of diamond abrasive powder or paste, the typical sequence is as follows:

9μm (1800#), 6μm (3000#), 3μm (8000#).

The 9μm diamond polishing compound and the polishing cloth wheel are used to remove the scratch marks left by 1200# and 1500# sandpaper. Subsequently, polishing is performed using a felt pad and diamond polishing compounds in the following sequence:

1μm (14000#), 1/2μm (60000#), 1/4μm (100000#).

 

 

(4) Polishing Environment

 

The polishing process should be carried out in two separate work areas: one for coarse grinding and the other for fine polishing. It is important to ensure that any abrasive residue from the previous step is cleaned from the workpiece surface.

 

Generally, after coarse polishing is completed up to 1200# sandpaper, the workpiece should be transferred to a dust-free environment for polishing, ensuring that there are no dust particles adhering to the mold's surface. Polishing processes with precision requirements of 1μm and above (including 1μm) can be conducted in a clean polishing room. For more precise polishing, it is necessary to have an absolutely clean space since dust, smoke, dandruff, and saliva can potentially ruin highly precise polished surfaces.

 

After completing the polishing process, the workpiece's surface should be protected against dust. When the polishing process stops, all abrasive and lubricants should be carefully removed to ensure a clean workpiece surface. Subsequently, a layer of mold rust prevention coating should be sprayed onto the workpiece's surface.

 

 

 

Factors Affecting Stainless Steel Surface Mirror Polishing

 

1.Workpiece Surface Condition

 

During the mechanical processing of materials, the surface can be damaged due to heat, internal stresses, or other factors. Incorrect cutting parameters can affect the polishing outcome. Surfaces that have undergone electrical discharge machining (EDM) are more challenging to polish compared to those from mechanical machining or heat treatment. Therefore, EDM surfaces should undergo electrical discharge finishing (EDF) before final polishing to prevent the formation of a hardened layer on the surface. If the EDF standards are selected improperly, the maximum depth of the heat-affected zone can reach 0.4mm. The hardened layer has higher hardness than the base material and must be removed. Thus, it is advisable to include a rough grinding step to provide a solid foundation for the polishing process.

 

2.Quality of the Steel Material

 

High-quality steel is a prerequisite for achieving good polishing quality. Various inclusions and pores in the steel can affect the polishing result. To achieve excellent polishing results, it is essential to specify the surface roughness for polishing at the beginning of the machining process. When a workpiece requires a mirror finish, it is crucial to select steel materials with good polishing properties and ensure they have undergone heat treatment; otherwise, the desired results will not be achieved.

 

3.Heat Treatment Process

 

Improper heat treatment can result in uneven surface hardness or differences in material characteristics, making polishing more challenging.

 

4.Polishing Technique

 

Since polishing is primarily a manual process, human skill is currently the most significant factor influencing polishing quality. It is generally believed that polishing technique affects surface roughness. In reality, achieving satisfactory polishing results requires not only excellent polishing skills but also high-quality steel materials and the correct heat treatment process. Conversely, even with excellent steel materials, poor polishing techniques cannot achieve a mirror finish.

 

 

 

 

Considerations for Mirror Polishing Different Types of Stainless Steel

 

(1) Considerations for Mold Sandpaper Polishing and Oilstone Grinding

 

1. Only clean and soft oilstone polishing tools should be used on mold surfaces with higher hardness.

2. When switching between grit levels during grinding, both the workpiece and the operator's hands must be thoroughly cleaned to avoid transferring coarse abrasive particles to the next, finer grinding operation.

3. During each grinding step, sandpaper should be applied from different 45° angles until the previous grit marks are eliminated. After removing the previous grit marks, an additional 25% of the grinding time should be added before moving on to the next, finer grit.

4. Changing directions during grinding helps prevent the workpiece from developing waves or uneven surfaces.

 

 

(2) Considerations for Diamond Grinding and Polishing

 

Diamond grinding and polishing should be conducted with as little pressure as possible, especially when polishing pre-hardened steel components and when using fine abrasive pastes. When polishing with an 8000# abrasive paste, a commonly used load is 100-200g/cm2, but maintaining the precision of this load can be challenging. To facilitate this, a thin and narrow handle can be created on a wooden bar or a portion of a bamboo stick can be cut away to make it more flexible. This aids in controlling the polishing pressure, ensuring that excessive pressure is not applied to the mold surface. When using diamond grinding and polishing, not only is a clean working surface required, but the operator's hands must also be meticulously clean.

 

(3) Considerations for Polishing Plastic Molds

 

Polishing plastic molds differs significantly from the surface polishing requirements in other industries. Strictly speaking, the polishing of plastic molds should be referred to as "mirror finishing." It not only demands high-quality polishing but also places stringent standards on surface flatness, smoothness, and geometric precision. The standards for mirror polishing are divided into four levels:

 

A0 = Ra 0.008 μm
A1 = Ra 0.016 μm
A3 = Ra 0.032 μm
A4 = Ra 0.063 μm

 

Due to the challenges of precisely controlling the geometric accuracy of parts using methods like electro-polishing and fluid polishing, and because surface quality from methods such as chemical polishing, ultrasonic polishing, and magnetic abrasive polishing may not meet the requirements, precision mold mirror finishing primarily relies on mechanical polishing.

 

 

 

 

Considerations for Stainless Steel Mirror Polishing:

 

 

1. When starting to process a new mold cavity, inspect the workpiece surface and clean it with kerosene to ensure that the oilstone surface does not become contaminated, which could lead to a loss of cutting functionality.

2. When grinding rough marks, follow the sequence of starting with difficult-to-reach areas, especially tight corners, deeper bottoms should be ground first, followed by the side surfaces and large flat areas.

3. Some workpieces may consist of multiple components assembled together for polishing. It is advisable to first individually grind the rough marks or spark marks on each component and then polish them all together until smooth.

4. For workpieces with large flat or side surfaces, after grinding away the rough marks with oilstone, use a straight steel plate for backlight inspection to check for any irregularities or inverted surfaces. In case of inverted surfaces, it can make demolding difficult or lead to damage to the parts.

5. To prevent the occurrence of inverted surfaces or to protect certain mating surfaces during mold component polishing, adhesive saw blades or sandpaper can be used on the edges for effective protection.

6. When grinding mold flat surfaces, move back and forth, and try to keep the oilstone handle as level as possible, not exceeding 25°. Excessive slope can exert downward pressure, leading to the formation of many rough marks on the workpiece.

7. If using copper or bamboo strips to press sandpaper for polishing flat surfaces, the sandpaper should not be larger than the tool's surface area, as this could lead to unwanted grinding in some areas.

8. It is best to avoid using grinding machines to finish sub-mold surfaces, as grinding wheel heads can leave a relatively rough and uneven surface. If necessary, when using them, it is essential to balance the concentricity of the grinding wheel head.

9. The shape of the grinding tools should closely match the surface shape of the mold to ensure that the workpiece is not deformed during polishing.

 

 

 

Addressing Common Issues in Stainless Steel Mirror Polishing

 

1.Over-Polishing

 

The most common issue encountered during daily polishing is "over-polishing," which means that the longer the polishing time, the lower the quality of the mold surface. Over-polishing manifests in two ways: "orange peel" and "pitting." Over-polishing is more likely to occur during mechanical polishing.

 

 

2.Causes of "Orange Peel" on Workpieces

 

An irregular, rough surface is referred to as "orange peel." There are many reasons for its occurrence, with the most common being excessive heat or carburization of the mold surface, excessive polishing pressure, and prolonged polishing time. For example, when using polishing wheels, the heat generated can easily lead to "orange peel." Harder steel can withstand more polishing pressure, while relatively softer steel is more prone to over-polishing. Studies have shown that the time required to induce over-polishing varies with the hardness of the steel.

 

 

3.Measures to Eliminate "Orange Peel" on Workpieces

 

When the surface quality is found to be poor, many people tend to increase the polishing pressure and extend the polishing time, which often worsens the surface quality. Remedies may include:

• Remove the defective surface, grinding with a slightly coarser grit than previously used, and then polish with less force.
• Stress relieve at a temperature 25°C below the tempering temperature, grind with the finest grit before polishing until a satisfactory result is achieved, and finish with lighter pressure.

 

4.Causes of "Pitting" on Workpiece Surfaces

 

"Pitting" on workpiece surfaces is caused by the extraction of hard and brittle non-metallic impurities, typically oxide particles, from the steel surface during the polishing process. The primary factors leading to "pitting" are:

• Excessive polishing pressure and prolonged polishing time.
• Insufficient purity of the steel with a high content of hard impurities.
• Rust on the mold surface.
• Residues of scale are not properly removed.

 

5.Measures to Eliminate "Pitting" on Workpieces

• Carefully regrind the surface with a slightly coarser grit than previously used, using a soft and sharp oilstone for the final step of grinding before proceeding with the polishing procedure.
• Avoid using the softest polishing tools when the grit size is smaller than 1 mm.
• Use the shortest possible polishing time and minimal polishing pressure.

 

 

In summary, selecting the appropriate polishing tools and compounds can result in a cleaner final product with no visible polishing marks.