How to polish the injection mold?

In the process of industrial products to diversify and high-grade development, how to improve the quality of the mold that directly affects product quality is an important task.

In the mold manufacturing process, the smooth processing and mirror polishing stage processing after the shape processing is called parts surface grinding and polishing processing, it is an important process to improve the quality of the mold.

Mastering the reasonable polishing method can improve the quality and service life of the mold, and then improve product quality.

Common polishing methods and working principle

1.1 Mechanical polishing

Mechanical polishing is by cutting or plastic deformation of the material surface to remove the workpiece surface projection to get a smooth surface polishing method, generally using oil stone, wool wheel, sandpaper, etc., mainly by hand, the surface quality requirements of high can be used to the ultra-precision polishing method.

Ultra-precision polishing is the use of special abrasives, in the polishing fluid containing abrasives, pressed on the surface of the workpiece being processed, for high-speed rotary motion.

The use of this technology can achieve Ra0.008 μm surface roughness, which is the best surface roughness of various polishing methods. Optical lens molds often use this method. Mechanical polishing is the main method of injection mold polishing.

1.2 Chemical polishing

Chemical polishing is the material in the chemical medium so that the microscopic convex part of the surface is preferentially dissolved than the concave part, to obtain a smooth surface.

This method can polish workpieces with complex shapes and can polish many workpieces at the same time with high efficiency. The surface roughness obtained by chemical polishing is generally Ra10 μm.

1.3 Electrolytic polishing

The basic principle of electrolytic polishing is the same as chemical polishing, that is, by selectively dissolving the tiny projections on the surface of the material to make the surface smooth. Compared with chemical polishing, it can eliminate the effect of cathodic reaction, the effect is better.

1.4 Ultrasonic polishing

Ultrasonic polishing is a processing method of polishing brittle and hard materials through abrasive suspension by using ultrasonic vibration of the tool section.

The workpiece is put into the abrasive suspension and placed in the ultrasonic field together, and the abrasive is ground and polished on the surface of the workpiece by relying on the oscillating effect of ultrasonic waves.

Ultrasonic processing macro force is small, and will not cause deformation of the workpiece, but the tooling production and installation are more difficult.

1.5 Fluid polishing

Fluid polishing is to rely on the flow of liquid and the abrasive particles it carries to scour the surface of the workpiece to achieve the purpose of polishing.

Fluid power grinding is driven by hydraulic pressure, the medium is mainly used in the lower pressure flow through the good special compounds (polymer-like substances) and mixed with abrasives made, abrasives can be used silicon carbide powder.

1.6 Magnetic grinding and polishing

Magnetic grinding and polishing are to use magnetic abrasives to form abrasive brushes under the action of the magnetic field to grind and process the workpiece.

This method has high processing efficiency, good quality, and easy to control the processing conditions. With suitable abrasives, the machined surface roughness can reach Ra0.1 μm.

1.7 EDM ultrasonic compound polishing

To improve the surface roughness Ra1.6 μm above the polishing speed of the workpiece, the use of ultrasound and special high-frequency narrow pulse peak current pulse power for diamond compound polishing.

By ultrasonic vibration and electric pulse of corrosion at the same time on the surface of the workpiece, quickly reduce its surface roughness, which is obvious and very effective for polishing the rough surface of the injection mold after processing by turning, milling, EDM and wire-cutting processes.

Mold polishing tools and specifications commonly used categories

Commonly used tools for injection mold polishing are: sandpaper, oil stone, felt wheel, abrasive paste, alloy file, diamond grinding needle, bamboo flake, fiber oil stone, and round rotation polisher.

Sandpaper:150#,180#,320#,400#,600#,800#,1,000#,1,200#,1,500#.

Oil stone:120#,220#,400#,600#.

Felt wheel: cylindrical, round vertebrae, square spike.

Abrasive paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple)

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

Diamond sharpening needle: generally 3/32 shank or 1/8 shank, with round waveform, cylindrical, long straight column, long round vertebral shape.

Bamboo piece: various shapes to suit the operator and mold shape and made, the role is to press the sandpaper, grinding on the workpiece, to achieve the required surface roughness.

Fiber oil stone: 200# (black) 400# (blue) 600# (white) 800# (red)

Polishing process

3.1 Rough polishing

The surface finish after fine milling, EDM, grinding, and other processes can be polished by choosing a rotary surface polishing machine with a speed of 35 000 to 40 000 r/min. The manual oil stone polishing, strip oil stone with kerosene as lubricant or coolant. The order of use is 180#→240#→320#→400#→600#→800#→1 000#.

3.2 Semi-finishing polishing

Semi-fine polishing mainly uses sandpaper and kerosene. Sandpaper number in order: 400#→600#→800#→1 000#→1 200#→1 500#. #1 500 sandpaper is only suitable for hardened die steel (above 52 HRC), but not for pre-hardened steel because it may lead to surface damage of pre-hardened steel parts and cannot achieve the expected polishing effect.

3.3 Fine polishing

Fine polishing mainly uses diamond polishing paste. If a polishing cloth wheel is used for grinding with diamond grinding powder or paste, the usual grinding sequence is 9 μm (1 800#) → 6 μm (3 000#) → 3 μm (8 000#).

9 μm diamond grinding paste and polishing cloth wheel can be used to remove hairline abrasions left by 1 200# and 1 50 0# sandpaper. Next, polishing is done with sticky felt and diamond abrasive paste in the order of 1 μm (14 000#) → 1/2 μm (60 000#) → 1/4 μm (100 000#).

Working environment for polishing

The polishing process should be completed separately in two workplaces, i.e. the rough grinding processing place and the fine polishing processing place are separated, and care should be taken to clean the grit left on the workpiece surface from the previous process.

Generally from the rough polishing after finishing with oil stone to 1 200# sandpaper, the workpiece needs to be transferred to the dust-free room for polishing to ensure that there are no dust particles in the air sticking to the injection mold surface.

Polishing processes requiring precision of 1 μm or more (including 1 μm) can be performed in a clean polishing room. For more precise polishing, an absolutely clean space is necessary, as dust, smoke, dandruff, and saliva may ruin the high-precision polished surface.

After the polishing process is completed, the surface of the workpiece should be well protected from dust. When the polishing process is stopped, all abrasives and lubricants should be carefully removed to ensure that the surface of the workpiece is clean, followed by a spray layer of mold rust-proof coating on the surface of the workpiece.

Factors affecting the polishability of the surface

5.1 Workpiece surface condition

The surface layer of the material will be damaged by heat, internal stress, or other factors during mechanical processing, and improper cutting parameters will affect the polishing effect.

The surface after EDM is more difficult to grind than the surface after machining or heat treatment, so EDM finishing should be used before the end of EDM, otherwise the surface will form a hardened thin layer.

If the EDM finishing gauge is not selected properly, the depth of the heat-affected layer can be up to 0.4 mm. the hardened thin layer is harder than the base hardness and must be removed. Therefore, it is better to add a rough grinding process to provide a good basis for the polishing process.

5.2 Quality of steel

High-quality steel is a prerequisite for good polishing quality. Various inclusions and porosity in the steel will affect the polishing effect.

To achieve a good polishing effect, the workpiece must be specified at the beginning of mechanical processing polished surface roughness, when a workpiece determines the need for mirror polishing, must be selected polishing performance of good steel and are heat-treated otherwise can not achieve the desired effect.

5.3 Heat treatment process

If the heat treatment is not appropriate, the steel surface hardness is not uniform or there are differences in characteristics, it will cause difficulties for polishing.

5.4 Polishing technology

Since polishing is mainly done by hand, human skills are still the main reason affecting the quality of polishing.

Generally, believe that polishing technology affects the surface roughness good polishing technology but also with high-quality steel and the correct heat treatment process, to get a satisfactory polishing effect; conversely, the polishing technology is not good, even if the steel is good can not do the mirror effect.

Different types of polishing should pay attention to the matters

6.1 Mold sandpaper polishing and oil stone grinding

(1) For the injection mold surface with high hardness, only clean and soft oil stone polishing tools can be used.

(2) When changing the grit level in sanding, the workpiece and the operator’s hands must be cleaned to avoid bringing the coarse grit to the next finer level of sanding operation.

(3) In each sanding process, the sandpaper should be sanded from a different 45-degree direction until the sand pattern of the previous level is eliminated, and when the sand pattern of the previous level is cleared, the sanding time must be extended by 25% before switching to the next finer sanding level.

(4) Changing different directions when grinding can avoid the workpiece from producing waves and other high and low unevenness.

6.2 Matters to be noted for diamond grinding and polishing

Diamond grinding and polishing must be carried out under light pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with 8 000# grinding paste, the common load is 100-200 g/cm2, but it is difficult to maintain the accuracy of this load.

To facilitate this, a thin and narrow handle can be made on a wooden bar, or a part of the bamboo bar can be cut off to make it more flexible.

This will help control the polishing pressure to ensure that the pressure on the surface of the mold is not too high. When using diamond grinding and polishing, not only the work surface must be clean, but also the worker’s hands must be very clean.

6.3 Plastic mold polishing should pay attention to matters

The polishing of plastic injection molds is very different from the surface polishing required in other industries, strictly speaking, the polishing of plastic molds should be called mirror surface processing.

It not only has high requirements for polishing itself but also has high standards on surface flatness, smoothness, and geometric accuracy.

Mirror polishing standards are divided into 4 levels: A0 = Ra0.008 μm, A1 = Ra0.016 μm, A3 = Ra0.032 μm, A4 = Ra0.063 μm, due to electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of the parts.

While chemical polishing, ultrasonic polishing, magnetic polishing and other methods of surface quality can not meet the requirements, so precision The mirror surface processing of the injection mold or mechanical polishing is the main focus.

Polishing in the notes is as follows:

(1) When a new mold cavity begins processing, should first check the surface of the workpiece, and clean the surface with kerosene, so that the oil stone surface will not stick to the dirt caused by the loss of cutting function.

(2) When grinding coarse grain should be done in the order of difficult first and then easy, especially some difficult to study the dead ends, deeper bottom to study first, and finally the sides and large planes.

(3) Part of the workpiece may have more than one group together to research light, to first separately research a single workpiece rough or sparkle pattern, and then all the workpiece together to smooth research.

(4) Large plane or side plane of the workpiece, with oil stone research to remove the coarse grain and then use a flat steel piece to do a translucent inspection, check whether there is the uneven or inverted buckle of the bad situation, such as inverted buckle will lead to difficulties in the release of the parts or the parts strains.

(5) To prevent the molded workpiece research buckle or have some laminated surface need to protect the situation, can be used to diamond paste the saw blade or use sandpaper on the edge, an edge that you can get the desired protective effect.

(6) Grinding mold plane with a back and forth pull, dragging the handle of the oil stone as flat as possible, do not exceed 25 degrees, because the slope is too large, the force from the top downward impulse, easy to lead to the research of many rough lines on the workpiece.

(7) If the plane of the workpiece with copper or bamboo sheets t pressed against the sandpaper polishing, sandpaper should not be larger than the area of the tool, otherwise it will study to where it should not be studied.

(8) Try not to use the grinding machine to repair the parting surface, because the grinding wheel head to repair the parting surface is relatively rough as well as wave height unevenness, such as the necessary use, the grinding wheel head must be glued to repair to concentricity balance.

(9) The shape of the grinding tool should be close to the same shape as the surface of the injection mold, to ensure that the workpiece is not deformed by the grinding.

7 How to solve the common problems in polishing

7.1 Over polishing

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

7.2 Workpiece appears “orange peel” reason

The irregular rough surface is called “orange peel”, “orange peel” has many reasons, the most common reason is due to overheating or over-carburization of the mold surface caused by excessive polishing pressure and polishing time is too long to produce ” orange peel” is the main reason.

For example polishing wheel polishing, the heat generated by the polishing wheel will easily cause “orange peel”. Harder steel can withstand the polishing pressure will be larger, and relatively soft steel is prone to over-polishing, research has proved that the polishing over time will be different due to the different hardness of steel.

7.3 Eliminate the workpiece “orange peel” measures

When the surface quality is found to be poorly polished, many people will increase the polishing pressure and extend the polishing time, this practice will often make the surface quality becomes worse.

The following methods can be used to remedy this:

(1) Remove the defective surface, grinding grit than the previous use of sand number slightly coarse level, and then grinding, polishing strength than the previous lower.

(2) Stress relieving at a temperature lower than the tempering temperature of 25 °C, grinding with the finest grit before polishing until satisfactory results are achieved, and finally polishing with a lighter intensity.

7.4 Reasons for the formation of “pitting” on the surface of the workpiece

Due to some non-metallic impurities in the steel, usually hard and brittle oxide, in the polishing process from the steel surface is pulled out, the formation of micro-pits or pitting, “pitting” of the main factors are the following.

(1) The polishing pressure is too large, polishing time is too long.

(2) The purity of the steel is not enough, the content of hard impurities is high.

(3) The surface of the mold polish is rusted.

(4) Black skin material is not removed.

7.5 Measures to eliminate pitting of workpiece

(1) Carefully regrind the surface with a slightly coarser grit size than previously used and use a soft and sharp oil stone for the final step of the grinding before proceeding with the polishing procedure.

(2) Avoid using the softest polishing tools when the grit size is less than 1 mm.

(3) Use the shortest possible polishing time and the smallest possible polishing effort.

Conclusion

Polishing cavities in the process of mold manufacturing is a very important process, it is related to the quality and life of the plastic injection mold, but also determines the quality of products.

Mastering the working principle and process of polishing, and choosing reasonable polishing methods, can improve the quality and life of the mold, and then improve the quality of the products.