How to Improve Precision of Mold Polishing Quality?

Time：2019-05-22 09:12:40 / Popularity： / Source：

In mold manufacturing process, forming portion of mold often requires surface polishing. Mastering polishing technology can improve quality and service life of mold, thus improve quality of product.

Mold polishing method and working principle

Mold polishing usually uses oil stone strips, wool wheels, sandpaper, etc., so that surface of material is plastically deformed and convex portion of workpiece surface is removed to obtain a smooth surface, which is generally performed by hand. Method of super-fine grinding and polishing is required for high surface quality. Super-fine grinding and polishing is made of a special grinding tool. In polishing liquid containing abrasive, it is pressed against machined surface to perform high-speed rotary motion. Polishing can achieve a surface roughness of Ra0.008 μm.

Common tools and specifications for mold polishing

1) Common tools for mold polishing include: sandpaper, oilstone, felt wheel, abrasive paste, alloy trowel, diamond grinding needle, bamboo, fiber whetstone, round rotary grinding machine.
2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1 000#, 1 200#, 1 500#;
3) Oil stone: 120#, 220#, 400#, 600#;
4) Felt wheel: cylindrical, rounded, square tip;
5) Grinding paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple);
6) Sickles: square, round, flat, triangular and other shapes;
7) Diamond grinding needle: generally 3/32 shank or 1/8 shank, with round shape, cylindrical shape, long straight column shape and long round cone shape;
8) Bamboo: Each shape is suitable for operator and shape of mold. Function is to press sandpaper and grind it on workpiece to achieve required surface roughness.
9) Fiber whetstone: 200# (black) 400# (blue) 600# (white) 800# (red)

Polishing process

(1) Rough polishing

Surfaces after finishing, EDM, grinding, etc. can be polished with a rotating surface polisher with a rotational speed of 35 000 to 40 000 r/min. Then there is a manual oil stone grinding, strip of oil stone plus kerosene as a lubricant or coolant. Order of use is 180#→240#→320#→400#→600#→800#→1 000#.

(2) Semi-fine polishing

Semi-finishing mainly uses sandpaper and kerosene. Number of sandpaper is: 400#→600#→800#→1000#→1200#→1500#. In fact, #1500 sandpaper is only suitable for hardened die steel (above 52HRC), not for pre-hardened steel, because it may cause damage to surface of pre-hardened steel and cannot achieve desired polishing effect.

(3) Fine polishing

Fine polishing mainly uses diamond abrasive paste. If grinding with a polishing cloth wheel to mix diamond abrasive powder or abrasive paste, usual grinding order is 9 μm (1 800 #) → 6 μm (3 000 #) → 3 μm (8 000 #). 9 μm diamond paste and polishing cloth wheel can be used to remove hair marks from 1 200# and 1 50 0# sandpaper. Polishing is then carried out with a felt and a diamond paste in order of 1 μm (14 000 #) → 1/2 μm (60 000 #) → 1/4 μm (100 000 #).

Polished working environment

Polishing process should be carried out separately at two working locations, that is rough grinding processing location and fine polishing processing location are separated, and be careful to clean sand particles remaining on the surface of workpiece in previous process.
Generally, after rough polishing with oil stone to 1200# sandpaper, workpiece needs to be polished to a clean room to ensure that no dust particles adhere to mold surface. Accuracy requirements above 1 μm (including 1 μm) can be performed in a clean polishing chamber. For more precise polishing, it must be in an absolutely clean space, as dust, smoke, dandruff and water droplets can scrap high-precision polished surfaces.
After polishing process is completed, surface of workpiece should be protected from dust. When polishing process is stopped, all abrasives and lubricants should be carefully removed to ensure that surface of workpiece is clean, and then a layer of mold anti-rust coating should be sprayed on the surface of workpiece.

Factors affecting surface finish of mold

(1) Surface condition of workpiece

During mold manufacturing process, surface layer may be damaged by heat, internal stress or other factors, and improper cutting parameters may affect polishing effect. Surface after EDM is more difficult to grind than surface after machining or heat treatment. Therefore, EDM should be finished before end of EDM, otherwise surface will form a hardened thin layer. If EDM is not properly selected, depth of heat affected layer can be up to 0.4 mm. Hardness of hardened layer is higher than hardness of substrate and must be removed. Therefore, it is best to add a rough grinding process to provide a good basis for polishing.

(2) Quality of steel

High-quality steel is a prerequisite for good polishing quality, various inclusions and pores in steel will affect polishing effect. In order to achieve a good polishing effect, workpiece must be marked with surface roughness of polishing when starting machining. When a workpiece is determined to require mirror polishing, it is necessary to select a steel with good polishing properties and heat treatment, otherwise desired effect is not achieved.

(3) Heat treatment process

If heat treatment is not proper, surface hardness of steel is uneven or characteristics are different, which may cause difficulty in polishing.

(4) Polishing technology

Since polishing is mainly done manually, human skills are still main reason for quality of polishing.
It is generally believed that polishing technique affects surface roughness. In fact, good polishing technology is matched with high-quality steel and correct heat treatment process to obtain a satisfactory polishing effect. Conversely, polishing technique is not good, even if steel is good, mirror effect is not achieved. .

Different types of polishing should pay attention to matters

(1) Matters needing attention in mold sanding and oilstone grinding

1) For harder mold surfaces, only clean and soft oilstone sanding tools should be used.
2) When changing sand level in grinding, workpiece and operator's hands must be cleaned to avoid carrying coarse sand to the next fine grinding operation.
3) When performing each grinding process, sandpaper should be sanded from different 45° direction until upper sand grain is removed. When upper sand grain is removed, sanding time must be extended by 25% before next fine sand number can be converted.
4) Change different directions during grinding to avoid waves and other unevenness of workpiece.

(2) Matters needing attention in diamond polishing

Diamond grinding and polishing must be carried out under lighter pressures, especially when polishing pre-hardened steel parts and polishing with fine abrasive pastes. When polishing with 8 000# abrasive paste, common load is 100-200 g/cm2, but it is difficult to maintain accuracy of this load. To make this easier, you can make a thin and narrow handle on strip or cut a part of strip to make it softer. This helps control polishing pressure to ensure that mold surface pressure is not too high. When using diamond grinding and polishing, not only working surface is required to be clean, but workers' hands must also be very clean.

(3) Matters needing attention in plastic molding polishing

Polishing of plastic molding is very different from surface polishing required in other industries. Strictly speaking, polishing of plastic molding should be called mirror processing. Not only does it have high requirements for polishing itself, but it also has a high standard for surface flatness, smoothness and geometric accuracy.
Standard for mirror polishing is divided into 4 levels:
A0=Ra0.008 μm
A1=Ra0.016 μm
A3=Ra0.032 μm
A4=Ra0.063 μm
Because electropolishing, fluid polishing and other methods are difficult to precisely control geometric accuracy of part, and surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet requirements, so mirror processing of precision plastic molding is mainly based on mechanical polishing.

(4) Precautions in polishing are as follows:

1) When a new cavity begins to be machined, surface of workpiece should be inspected first, and surface should be cleaned with kerosene so that surface of oilstone does not stick to dirt and lose function of cutting.
2) When roughing grain, it should be carried out in the order of difficulty and easyness. Especially some dead corners that are difficult to study, deep bottom must be studied first, and finally side and big plane.
3) Some workpieces may have multiple pieces assembled together for grinding. It is necessary to separately study rough or fire pattern of a single workpiece, and then all workpieces are ground and smoothed.
4) For workpieces with large planes or side planes, use oil stone to grind rough lines and then use straight steel sheets for light transmission inspection to check whether there are any uneven or inverted buckles. If there is a inverted buckle, it will cause difficulty in demoulding or strain on part.
5) In order to prevent mold workpiece from being out of buckle or having some bonding surfaces to be protected, it can be pasted with a saw blade or sanded on the side, so that ideal protection effect can be obtained.
6) Grinding mold plane with pulling back and forth. Drag handle of the oil stone as far as possible, not to exceed 25°. Because slope is too large, force is pressed from top to bottom, which may lead to development of many rough lines on the workpiece.
7) If plane of workpiece is polished with copper or bamboo pressed sandpaper, sandpaper should not be larger than tool area, otherwise it will be ground to a place where it should not be ground.
8) Try not to use sander to repair mold surface. Because parting surface of grinding wheel head is rough and waves are uneven, if necessary, grinding wheel head must be repaired to concentricity balance.
9) Shape of tool to be ground should be close to surface shape of mold to ensure that workpiece is not deformed by grinding.

How to solve common problems in polishing

(1) Over-polishing

The biggest problem encountered during daily polishing is “over-polishing”, which means that the longer polishing time, the worse quality of mold surface. There are two phenomena of over-polishing: “orange peel” and “pitting”. Excessive polishing occurs mostly in mechanical polishing.

(2) Reasons for appearance of "orange peel" on workpiece

Irregularly rough surfaces are called "orange peels". There are many reasons for producing "orange peels". The most common cause is due to overheating or excessive carburization of mold surface. Excessive polishing pressure and long polishing time are main reasons for production of "orange peel". For example: polishing with a polishing wheel, heat generated by polishing wheel can easily cause "orange peel". Harder steels can withstand higher polishing pressures, and relatively softer steels are prone to over-polishing. Studies have shown that excessive polishing time will vary depending on hardness of steel.

(3) Measures to eliminate "orange peel" of workpiece

When surface quality is found to be bad, many people increase polishing pressure and extend polishing time. This practice tends to make surface quality worse.
Following methods can be used to remedy:
1) Remove defective surface. Particle size of grinding is slightly higher than previous sand number, and then grinding is performed. Polishing intensity is lower than previous one.
2) Perform stress relief at a temperature lower than tempering temperature of 25℃, and grind it with finest sand number before polishing until a satisfactory effect is achieved, and finally polish with a lighter force.

(4) Reasons for formation of "pitting" on the surface of workpiece

Due to some non-metallic impurities in steel, usually hard and brittle oxides, which are pulled out from surface of steel during polishing process to form micro-pits or pitting, main factors causing "pitting" are as follows:
1) Polishing pressure is too large and polishing time is too long.
2) Purity of steel is not enough and content of hard impurities is high.
3) Surface of mold is rusty.
4) Black leather is not removed.

(5) Measures to eliminate pitting corrosion of workpiece

1) Carefully re-grind surface. Grit size is slightly larger than previously used particle size. Final step of grinding is performed with soft and sharp oil stone before polishing process.
2) The softest polishing tool should be avoided when grit size is less than 1 mm.
3) Use the shortest polishing time and minimum polishing force possible.
Polishing of cavity during mold manufacturing process is a very important process, which is related to quality and life of mold, and also determines quality of product. Mastering working principle and process of polishing, and selecting a reasonable polishing method can improve quality and life of plastic molding, and thus improve quality of product.

Gud Mould Industry Co., Ltd