Plastic Mold Polishing Techniques

Plastic Mold Polishing Techniques is very important for plastic molds manufacturing. It affect a lot on the plastic molds` quality. It helps to get a beautiful surface and makes ejection smooth. Mold polishing generally uses oil stone, sandpaper, polishing paste, etc. to grind the cavity surface of the mold, so that the working surface of the mold can be as bright as a mirror. It is called mold polishing.

Mold polishing has two purposes; one is to increase the finish of the mold, so that the surface of the product from the mold is smooth, beautiful, and beautiful, and the other is to make the mold easily demolded, so that the plastic is not stuck on the mold and cannot be taken off .

Polishing is a very important process in the mold making process. With the increasingly widespread use of plastic products, the appearance quality of plastic products is also getting higher and higher. Therefore, the surface polishing quality of plastic mold cavity should be improved accordingly, specially Mirrors and high-gloss high-gloss surfaces require higher mold surface roughness and therefore higher polishing requirements. Polishing not only increases the aesthetics of the workpiece, but also improves the corrosion resistance and wear resistance of the surface of the material. It also facilitates subsequent injection processing, such as making plastic products easy to demold and reducing production injection cycle time.

Mold polishing is a polishing method that obtains a smooth surface by cutting and plastic deformation of the material surface to remove the polished convex parts. Generally, whetstone strips, wool wheels, sandpaper, etc. are used, mainly by manual operation. Special parts such as the surface of the rotating body can be Use auxiliary tools such as turntables, and those with high surface quality can use the method of super-finishing polishing. Super-precision grinding and polishing is the use of special grinding tools, in the grinding and polishing liquid containing abrasives, tightly pressed on the surface of the workpiece to be processed for high-speed rotation. Using this technology, the surface roughness of Ra0.008um can be achieved, which is the highest among various polishing methods, and this method is often used in optical lens molds.

The mold polishing in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, the polishing of molds should be called mirror processing. It not only has high requirements for polishing itself but also has high standards for surface smoothness, smoothness and geometric accuracy. Surface polishing generally only requires a bright surface. The standard of mirror processing is divided into four levels: A0 = Ra0.008um, A1 = Ra0.016um, A3 = Ra0.032um, A4 = Ra0.063um, it is difficult to accurately control the geometric accuracy of parts due to electrolytic polishing, fluid polishing, etc Degree, and the surface quality of methods such as chemical polishing, ultrasonic polishing, magnetic grinding and polishing can not meet the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.

Polishing tools

1. Auxiliary grinding and polishing tools
1) Handheld electric straight rod rotary grinding and polishing tool
As shown in the figure, the chuck with the abrasive polishing tool rotates at high speed to achieve abrasive polishing. A special diamond grinding wheel with p2 ~ p12 can be arranged on the chuck to grind and polish the concave arc surface with different curvature. It can also be equipped with R4 ~ RI2 plastic grinding and polishing sleeves or felt polishing wheels, which can grind and polish cavity or hole with complex shapes.

2） Electric elbow rotary handheld electric elbow rotary abrasive polishing tool
As shown in the figure, it can be extended into the cavity to grind and polish the angled corners and corners.

2. Whetstone
Strip-shaped consolidating abrasives pressed and sintered with abrasives and bonding agents. Whetstone is usually lubricated when it is used, hence the name. Whetstone is generally used for manual grinding parts, and it can also be clamped on the machine tool for honing and super-finishing. There are two types of whetstone, man-made and natural. Man-made whetstone has two types of structure due to different abrasives, as shown in Figure 8.2.3.

2. Sandpaper
Sandpaper is made of aluminum oxide or silicon carbide and other abrasives bonded to paper. It is mainly used for rough polishing. The abrasives such as 400 #, 600 #, 800 # and 1000 # are commonly used according to particle size.

3. Grinding and polishing paste
Grinding and polishing paste is composed of abrasives and abrasive liquids, divided into hard abrasives and soft abrasives. The abrasives in hard abrasive polishing pastes include alumina, silicon carbide, boron carbide, and diamond. The abrasives and fine powders with a common particle size of 240 # and W40; soft abrasive abrasive polishing pastes contain oily active substances and are available when used Kerosene or gasoline dilution. Mainly used for fine polishing.

4. Polishing fluid
It is an abrasive material used for super-finishing. It is made of W0.5 ~ W5 chromium oxide and emulsion. It is mostly used for polishing product molds with high appearance requirements, such as optical lens molds.

+++++++++++++++++++++++++++++++

Mould Polishing Process

1. Mould Polishing Process Orders

Mold polishing generally uses rough oilstone to roughen the surface of the machined mold cavity, to remove the mark of the machine tool, then use a fine oilstone to remove the rough oilstone to achieve the mark, and then use fine sandpaper The polished surface of the fine oil stone is then polished, and finally the polishing cavity or the polishing paste is used for the final fine polishing of the cavity surface of the mold, and finally the effect as bright as a mirror is achieved. This is the general process of polishing the mold. Of course, if possible, you can use an ultrasonic polisher to polish the mold, which is more efficient. People are also more effortless.

To obtain a high-quality polishing effect, the most important thing is to have high-quality polishing tools and auxiliary products such as oilstone, sandpaper and diamond abrasive paste. The choice of polishing program depends on the surface conditions after the pre-processing, such as mechanical processing, EDM, grinding and so on. The general process of mechanical polishing is as follows:

+++++++++++++++++++++++++++++++++++++++++++
① The surface after rough polishing, milling, electric spark, grinding and other processes can be selected for polishing with a rotating surface polishing machine or an ultrasonic grinding machine with a speed of 35000 to 40000rpm. A common method is to use a wheel with a diameter of 3mm and WA # 400 to remove the white electric spark layer. Then there is manual whetstone grinding, with strip whetstone plus kerosene as a lubricant or coolant. The general order of use is # 180 ~ # 240 ~ # 320 ~ # 400 ~ # 600 ~ # 800 ~ # 1000. Whetstone polishing method, this operation is the most important and difficult operation, according to the different specifications of the processed products, the cross-grinding is carried out in a balanced angle of about 70 degrees. The ideal round-trip range is about 40 mm to 70 mm. Whetstone operations also vary depending on the material of the processed product. Many mold makers choose to start with # 400 in order to save time.

② Semi-precision polishing Semi-precision polishing mainly uses sandpaper and kerosene. After the whetstone operation is sandpaper operation, pay attention to the generation of round edges, rounded corners and orange peels of the mold kernel. Therefore, the whetstone process is as fine as possible. Emphasis on sandpaper polishing. Sandpaper is combined with a harder wooden rod to grind at a cross angle of about 70 degrees like a whetstone operation. The number of sandpaper grinding on one side is about 10 to 15 times. If the grinding time is too long, the grinding force of the sandpaper will be reduced, which will cause unevenness on the processed surface (this is also one of the reasons for the orange peel).
In the sandpaper operation, bamboo chips are generally used for grinding. It is most ideal to use a wooden rod with a low elasticity or an aluminum rod with a low hardness at about 45 degrees. No rubber or highly elastic materials can be used for the polishing surface, and sharp angles can be used for shapes that cannot be polished at a 45-degree angle. The order of sandpaper is: # 220 ~ # 320 ~ # 400 ~ # 600 ~ # 800 ~ # 1000 ~ # 1200 ~ # 1500. In fact, # 1500 sandpaper only uses mold steel suitable for hardening (above 52HRC), but not for pre-hardened steel, because it may cause burns on the surface of pre-hardened steel parts.

③Precision polishing mainly uses diamond polishing paste. If the polishing cloth wheel is mixed with diamond grinding powder or grinding paste, the usual grinding sequence is 9um (# 1800) ~ 6u m (# 3000) ~ 3um (# 8000) 9um diamond abrasive paste and polishing cloth wheel can be used to remove the hair-like scars left by # 1200 and # 1500 sandpaper. Then use sticky felt and diamond abrasive paste for polishing, the order is 1um (# 14000) ~ 1 / 2um (# 60000) ~ 1 / 4um (# 100000).
The polishing process with an accuracy requirement of more than 1um (including 1μm) can be performed in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is necessary. Dust, smoke, dandruff, and saliva foam can all be scrapped after a few hours of high-precision polished surfaces.

+++++++++++++

CAUTIONS and SKILLS of Mechanical Polishing:

I. Should pay attention to the following points when polishing with sandpaper:

① Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing round or spherical surfaces, using cork rods can better match the curvature of the round and spherical surfaces. The harder wood strips, like cherry wood, are more suitable for polishing flat surfaces. The end of the wooden strip is trimmed to keep it consistent with the shape of the surface of the steel piece, so that the sharp angle of the wooden strip (or bamboo strip) can contact the surface of the steel piece and cause deep scratches.
②When changing to different types of sandpaper, the polishing direction should be changed from 45 ° to 90 °, so that the streak shadow left by the previous model of sandpaper after polishing can be identified. Before changing to different types of sandpaper, the polishing surface must be carefully wiped with 100% pure cotton and a cleaning solution such as alcohol, because a small grit left on the surface will destroy the entire polishing work. When changing from polishing with sandpaper to polishing with diamond paste, this cleaning process is equally important. Before polishing continues, all particles and kerosene must be completely cleaned.
③ In order to avoid scratches and burns to the workpiece surface, special care must be taken when polishing with # 1200 and # 1500 sandpaper. Therefore, it is necessary to apply a light load and use a two-step polishing method to polish the surface. When polishing with each type of sandpaper, it should be polished twice in two different directions, each time between two directions 45 ° ~ 90 °

II diamond grinding and polishing should pay attention to the following points:

① This kind of 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 # 8000 abrasive paste, the common load is 100 ~ 200g / cm2, but it is difficult to maintain the accuracy of this load. To make this easier, you can make a thin and narrow handle on the wooden strip, such as adding a copper sheet; or cut a part of the bamboo strip to make it more soft. This can help control the polishing pressure to ensure that the mold surface pressure is not too high.
②When using diamond grinding and polishing, not only the working surface needs to be clean, but also the hands of the workers must be carefully cleaned.
③ The polishing time should not be too long each time. The shorter the time, the better the effect. If the polishing process is too long, it will cause “orange peel” and “pitting”.
④ In order to obtain high-quality polishing effect, polishing methods and tools that are easy to generate heat should be avoided. For example: polishing wheel polishing, the heat generated by the polishing wheel can easily cause “orange peel”.
⑤ When the polishing process is stopped, it is very important to ensure that the surface of the workpiece is clean and carefully remove all abrasives and lubricants, then a layer of mold anti-rust coating should be sprayed on the surface.

Because mechanical polishing is mainly done manually, polishing technology is still the main reason for the polishing quality. In addition, it is also related to the mold material, the surface condition before polishing, and the heat treatment process. High-quality steel is a prerequisite for obtaining good polishing quality. If the hardness of the steel surface is uneven or the characteristics are different, it will often cause polishing difficulties. Various inclusions and pores in the steel are not conducive to polishing.

(3) The impact of different hardness on the mold polishing process
The increased hardness increases the difficulty of grinding, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly. At the same time, the hardness increases and the possibility of excessive polishing decreases accordingly.

(4) The influence of the surface condition of the workpiece on the polishing process During the crushing process of steel cutting, the surface layer will be damaged by heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary mechanical processing or heat treatment. Therefore, before the end of EDM, precision quasi-EDM should be used for finishing, otherwise the surface will form a hardened thin layer. If the electric spark refining standard is not properly selected, the depth of the heat-affected layer can be up to 0.4mm. The hardness of the hardened thin layer is higher than the hardness of the substrate and must be removed. Therefore, it is better to add a rough grinding process to completely remove the damaged surface layer to form an average rough metal surface, which provides a good basis for polishing.
The most important point in ultra-mirror polishing is the size of the polishing. The distance between the grindstone and the mirror polishing is 10 & micro ;. (The ideal is to start grinding # 1500 (# 1000) oilstone. The number of sandpaper is: (# 320 ~) # 400 ~ # 600 ~ # 800 ~ # 1000 ~ # 1200 ~ # 1500 ~ # 01 ~ # 02 ~ # 03 ~ # 04 ~ # 05 ~ # 06 ~ # 015) The most basic principle, before proceeding to the next step, the mold kernel must be thoroughly cleaned to achieve good results. Be very careful not to repeat changes (repeated changes will affect quality issues)

2. Mould Polishing Process Measures

(1) Selection of tool materials
Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing a round or spherical surface, using a cork rod can better match the curvature of the round and spherical surfaces; while the harder wood strip is suitable for polishing a flat surface. The end of the wooden strip is trimmed to keep it consistent with the shape of the surface of the steel piece, which can avoid the deep angle of the wooden strip (or bamboo strip) contacting the surface of the steel piece and causing deep scratches.

2) Selection of polishing direction and cleaning of polishing surface
When changing to different types of sandpaper, the polishing direction should be changed from the previous polishing direction by 50 ° ~ 45 ° to polish, so that the streak shadow left by the previous type of sandpaper after polishing Can be distinguished. For the sub-mold mold, the final polishing pattern should be consistent with the demolding direction of the plastic part.
Before changing to different types of sandpaper, it is necessary to carefully wipe the polishing surface with a cleaning solution such as alcohol and cotton wool. The polishing paste of the previous process is not allowed to enter the next process, especially in the fine polishing stage. When changing from sandpaper polishing to diamond paste polishing, the cleaning process is even more important. Before polishing continues, all particles and kerosene must be completely cleaned.

(3) Possible defects and solutions during polishing When grinding and polishing, not only the work surface needs to be clean, but also the hands of the worker must be carefully cleaned; each polishing time should not be too long, the shorter the time, the more effective it is good. “Overpolished”
“Orange peel” and “pitting” will occur. In order to obtain a high-quality polishing effect, polishing methods and tools that are prone to heat should be avoided. For example, the heat generated during polishing and excessive polishing force will cause “orange peel”, or impurities in the material will be separated from the metal structure during the polishing process, forming “pitting corrosion”.

The solution is to increase the surface hardness of the polishing material, use soft polishing tools, high-quality alloy steel; apply appropriate force during polishing, and use the shortest time to complete the polishing. When the polishing process is stopped, it is important to ensure that the surface of the workpiece is clean and carefully remove all abrasives and lubricants. At the same time, a layer of mold anti-rust coating should be sprayed on the surface.

+++++++++++++++++++++++++++++++++++++++++++++++++

3. Factors affecting mold polishing quality

Since the general polishing is mainly done manually, the polishing technology is still the main reason for affecting the polishing quality. In addition, it is also related to the mold material, the surface condition before polishing, and the heat treatment process.
(1) The effect of different hardness on the polishing process
The increased hardness increases the difficulty of grinding, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly. At the same time, the hardness increases and the possibility of excessive polishing decreases accordingly.
(2) The influence of the surface condition of the workpiece on the polishing process During the mechanical cutting process of the steel, the surface of the workpiece will be poor due to heat, internal stress or other factors; a thin layer of hardening will form on the surface after EDM. Therefore, it is best to add a rough grinding process before polishing to completely remove the poor surface layer of mud on the surface of the workpiece to provide a good basis for polishing.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

2. Electrochemical polishing

Also known as “electrolytic polishing”, is a polishing method that uses the phenomenon of electrochemical anode dissolution of metals in the electrolyte to dissolve the surface of the workpiece to form a smooth surface. The principle is shown in Figure 8.2.4.
The electrochemical polishing process is divided into two steps:
The first step is macro leveling. The dissolved product diffuses into the electrolyte, and the geometric roughness of the material surface decreases, Ra> 1um.
In the second step, the shimmer is smooth, the anode is polarized, and the surface brightness is increased, Ra <1um. · Its characteristics are as follows.
① Electrolytic polishing is not limited by the material and performance of the workpiece. Carbide, powder metallurgy, hardened steel, stainless steel, heat-resistant steel, etc. can be polished.
② After electrolytic polishing, a dense and firm oxide film is often formed on the surface of the workpiece, which can improve the corrosion resistance of the workpiece surface without generating new metamorphic layer and surface residual stress.
③High polishing efficiency, such as polishing margin is 0.10 ～
At 0.15mm, the electrolytic polishing time is 10 ~ 15min, which is several times the efficiency of manual polishing.
④The process is simple, the operation is easy, the equipment is simple, and the investment is small.
⑤ Electrochemical polishing cannot eliminate the “coarse ripples” on the original surface. Therefore, there should be no ripples on the surface of the parts before electrochemical polishing.

Factors affecting the quality of electrochemical polishing
· Electrolyte, composition and ratio of electrolyte.
· Current density, the current density is too high, the anode precipitates oxygen and boils.
· Electrolyte temperature, electrolyte filtration, temperature uniformity. · Polishing time, specific metal materials have an optimal polishing time.
· Metal structure of metal materials.
· The original roughness of the polished surface, the best when the original roughness reaches Ra0.8-2.5.

++++++++++++++++++++++++++++++++++++++

3. Ultrasonic polishing

· Vibration waves with a frequency exceeding 16,000 times per second are called ultrasonic waves. The ultrasonic frequency for ultrasonic polishing is 16000 Hz to 25000 Hz. The characteristics of ultrasonic waves different from ordinary sound waves are: high frequency; short wavelength; large energy; significant phenomena such as reflection, refraction, resonance, and loss during propagation. Vibration waves with a frequency exceeding 16,000 times per second are called ultrasonic waves.
· Ultrasonic polishing is the use of ultrasonic energy to transmit high energy. The ultrasonic vibration device is used to drive the end face of the tool to make ultrasonic frequency vibration, forcing the abrasive suspension between the tool and the workpiece to hit the part to be added to the soil a way. Ultrasonic choke is suitable for the polishing of brittle and hard materials. Its basic principle is shown in Figure 8-7.

Put the workpiece in the abrasive suspension and put it in the ultrasonic field together, relying on the oscillation of the ultrasonic wave to make the abrasive grinding and polishing on the surface of the workpiece. Ultrasonic processing has a small macro force and will not cause deformation of the workpiece, but tooling is difficult to manufacture and install. The picture on the right is a schematic diagram of the principle of ultrasonic polishing.

During mold polishing processing, add working fluid with abrasives to the polishing area, and maintain the polishing tool with a certain static pressure (3N ~ 5N) to the workpiece, and push the polishing tool to reciprocate parallel to the surface of the workpiece, with a movement frequency of 10 times per minute ~ 30 times. The ultrasonic transducer generates ultrasonic frequency longitudinal vibration above 16000Hz, and amplifies the amplitude to about 10um ~ 20um with the aid of a horn, driving the polishing tool end face for ultrasonic vibration, forcing the abrasive particles suspended in the working fluid at a great speed and The acceleration constantly strikes and grinds the surface to be processed, crushing the material in the processing area into very fine particles and striking it off the material. Although there is very little material struck each time, there are still certain accelerations due to the number of hits per second reaching more than 16,000. At the same time, the high-frequency, alternating hydraulic positive and negative shock waves and “cavitation” generated by the ultrasonic vibration of the end face of the working fluid prompt the working fluid to drill into the micro-cracks of the processed material, which exacerbates the mechanical damage It means that when the tool end face leaves the workpiece surface with great acceleration, negative pressure and partial vacuum are formed in the machining gap, and many air pockets are formed in the working liquid. When the tool end face approaches the workpiece surface with great acceleration, the bubble bursts. Causes extremely strong hydraulic shock waves, causing cavitation on the vibration surface and the corresponding processing surface.

Cavitation has two functions: first, when the bubbles generated by the cavitation rupture, the surrounding medium is subjected to a large impact force at an instant, and this force produces a small mechanical erosion effect on the surface of the workpiece; Due to the impact of the abrasive on the surface and the micro-cracks caused by cavitation, the fine particles are stripped off the surface of the workpiece by the attracting effect of cavitation in the subsequent instant. This shows that the ultrasonic cavitation can strengthen the processing process. In addition, the positive and negative alternating hydraulic shocks also force the suspension working fluid to circulate in the machining gap, so that the dull abrasive particles are updated in time, and the chips can be removed in time. Ultrasonic vibration makes the tool self-cutting, which can prevent the pores of the abrasive tool from clogging and improve the grinding performance.

(1) Surface quality of ultrasonic polishing and its influencing factors
Ultrasonic polishing has good surface quality, and will not produce surface burns and surface modification layers. Its surface roughness can reach Ra less than 0.16um, which can basically meet the surface roughness requirements of plastic molds and other molds. The surface roughness of the ultrasonically polished surface depends on the size of the dents left after each abrasive impinges on the surface of the workpiece, the true diameter of the abrasive particles, the nature of the material being processed, the amplitude of the ultrasonic vibration The composition of the abrasive suspension working fluid is related.

Abrasive particle size is the main factor that determines the surface roughness value of ultrasonic polishing. With the decrease of the abrasive particle size, the roughness of the workpiece surface also decreases. Using the same size of abrasive and different super-reed amplitudes, the resulting surface roughness is also different. Table 5.15 shows the maximum surface roughness that can be achieved with various abrasive grain sizes under three different ultrasonic amplitudes: large, medium and small.
When the size of the abrasive particles is small, the hardness of the workpiece material is large, and the ultrasonic amplitude is small, the degree of the processed surface will be improved, but the productivity will be reduced, and the efficiency will be improved.

2) Selection of working fluid
The working fluid for ultrasonic polishing can be kerosene, gasoline, lubricating oil or water. The influence of the performance of the abrasive suspension working liquid on the surface roughness is more complicated. Practice shows that replacing water with kerosene or lubricating oil can improve the surface roughness. When the surface of the workpiece is required to reach the brightness of the mirror surface, the dry throwing method can be used, that is, abrasives are used, and no working fluid is added.

3) Polishing speed, polishing margin and polishing precision
①Polishing speed
The level of ultrasonic polishing speed is related to the workpiece material, hardness and abrasive material.
Generally, the surface roughness is small from Ra5um to Ra0.04um, and the polishing speed is 10min / cm2 ～ 15min / cm2.
②Polishing allowance
When ultrasonic polishing the EDM surface, the minimum polishing margin should be greater than the depth of the electromachined metamorphic layer or the etched pockets, so as to throw away the heat-affected layer. Therefore, the thickness of the polished removal is also different due to the different standards used for electrical machining. The polishing amount of rough machining by EDM is about .15mm. The polishing amount of precision machining in EDM is 0.02mm ~ 0.05mm. In order to ensure the polishing efficiency, the surface roughness Ra after electrical processing is generally required to be less than 2.5um, and the maximum should not be greater than Ra5um.

· Ultrasonic polishing has the following characteristics.
· ①Ultrasonic polishing is suitable for processing all kinds of hard and brittle materials, especially non-conductive non-metallic materials, such as glass, ceramics, quartz, diamond, etc.
· ②The ultrasonic polishing equipment is simple, easy to use and maintain, and can be made into a polishing tool with a complex shape from a softer material than the workpiece, and the complex surface and cavity can be carried out without complicated relative movement between the tool and the workpiece during processing. polishing.
· ③ The force and heat influence on the material are small, no deformation, burn and metamorphic layer will occur. Thin walls, thin slices, narrow slits and low-rigidity parts can be polished. The processing accuracy can reach 0.01 ~ 0.02mm and the surface roughness. The value can reach 0.63 ~ 0.08ulm.

++++++++++++++++++++++++++++++++++++++++++++++++++++

4.Squeeze grinding polishing

Also known as abrasive flow processing, it uses a sludge-like abrasive polishing agent composed of an abrasive and an organic polymer medium with a large viscosity to pass through the processed surface under the pressure, and the microscopic unevenness of the processed surface is removed by the abrasive scraping Material technology. The principle of extrusion grinding and polishing is shown in the figure.

Squeeze grinding polishing has the following characteristics.
· ①_Wide range of use. Because the abrasive polishing agent is a semi-fluid viscous substance, it can pass through the complicated surface to be processed. At the same time, a wide range of processing materials, metal materials, ceramics and hard plastics can be processed.
· ②Good mold polishing processing quality. This is due to the good scraping uniformity, which can remove the surface modification layer and microscopic inequality defects formed in the previous process without destroying the original shape accuracy of the workpiece. Generally, the dimensional accuracy of extrusion grinding and polishing can reach 0.01 ~ 0.0025mm, and the surface roughness R value can reach 0.04 ~ 0.025ulm.
· ③_High processing efficiency. Extruding, grinding and polishing of the 6 towers of the tight 6 takes 5 minutes and 19 minutes, it is manual.
On the other hand, it is possible to achieve simultaneous polishing of multiple pieces.

2. Technological characteristics of squeeze grinding polishing

(1) Scope of application Since viscous abrasive is a viscoelastic material in a semi-flow state, it can be used for polishing and deburring of various complex surfaces, such as various types of holes and profiles. Moreover, it can process almost all metal materials, as well as ceramics and hard plastics.

(2) Polishing effect The surface roughness after polishing is related to the original state. It can reach 1/10 of the original surface roughness, which can be improved by about 3 levels. The best surface roughness can reach the mirror surface of Ra0.025um. Abrasive flow processing can remove the surface residual stress at a depth of 0.025mm; it can remove the surface modification layer and other surface micro-defects formed by the previous processes (such as EDM, wire cutting).

(3) Processing efficiency The material removal amount is between 0.01mm and 0.1mm, and the processing time is usually 1min-5min, which can be completed in a maximum of ten minutes. Compared with manual polishing, the processing time can be reduced by more than 90%. For some small parts, multiple parts can be processed at the same time, and the efficiency can be greatly improved.

(4) Processing accuracy Squeeze grinding is a surface processing technology, so it cannot correct the shape error of the parts. The cutting uniformity can be kept within 10% of the cutting amount, therefore, the original shape accuracy of the part is not damaged. Due to the small amount of removal, high dimensional accuracy can be achieved, and the general dimensional accuracy can be controlled in the order of microns.

3. Viscous abrasive media
Viscous abrasive media is a mixture of abrasive and special matrix media. Its role is equivalent to that of cutting tools. It is the most critical factor for machining. Its performance directly affects the polishing effect.

(1) The performance of viscous abrasive media
① It has a certain fluidity and viscoelasticity to adapt to the processing of different surfaces, so that when passing through the hole of the part, the abrasive can produce sufficient cutting force on the hole wall.

② Abrasive particles are evenly dispersed in the medium, and the medium is required to have strong cohesion.

The medium cannot stick to the abrasive particles, so that a film is formed on the abrasive particles, which wraps the cutting edges of the abrasive particles, so that it cannot play a good polishing role.

③ Must have very little internal friction. Only in this way can the abrasive itself transmit pressure well, so that the abrasive can smoothly pass through the surface of the part for reciprocating cutting movement. Increase the friction of the abrasive against the parts and reduce the internal friction of the abrasive, thereby improving the efficiency of polishing.

④Good stability and long service life. When reciprocating the viscous abrasive in the upper and lower abrasive chambers and on the surface of the parts, it is necessary to overcome the resistance, and heat must be generated. Reduce polishing effect.

⑤Although the abrasive is soft after preparation, it must not be sticky. It should not stick to the parts, causing difficulty in cleaning, nor sticking to the operator’s hand, affecting the normal work of the operator.

⑥The cutting effect is strong, the processing speed is fast, and there must be a significant polishing effect. The medium has no corrosive effect on the parts, no harm to the human body, and will not cause irritation and cause skin allergies.

① Matrix medium It is a semi-solid, semi-fluid polymer. Its composition belongs to a viscoelastic rubber-like polymer compound, which mainly plays the role of binding abrasive particles.

When processing a surface with a large pore size or a relatively simple pore shape, a more viscous matrix medium is generally used. When processing small holes, long curved holes or fine holes, and narrow slits, a matrix medium with low viscosity or relatively easy flow should be used.

②Additives This is an ingredient added to the matrix medium in order to obtain ideal viscosity, consistency and stability. Its types include thickeners, viscosity reducers, lubricants, etc. ③ Abrasives Generally, alumina, boron carbide, and silicon carbide are used. When processing hard materials such as cemented carbide, diamond powder can be used. The abrasive particle size range is 80 # ~ 1200 #; the mass fraction range is 10% ~ 60%. The specific abrasive type, particle size and content should be determined according to different processing objects.

Coarse abrasive can get faster removal speed, fine abrasive can get smaller surface roughness. Therefore, fine abrasives are generally used for polishing, and coarse abrasives are used for deburring. Use finer abrasives for polishing small holes. In addition, the fine abrasive can also be used as an additive to adjust the consistency of the matrix medium. In actual use, it is often a mixture of several abrasive sizes to obtain better performance.

4. Squeeze grinding fixture
The fixture is an important tool that uses squeeze grinding to achieve the ideal polishing effect for the working parts of the mold. The fixture is an important part of extrusion grinding, and also a relatively flexible factor in processing. It needs to be designed according to the specific workpiece shape, size and processing requirements, but sometimes it needs to be determined through experiments.

The main function of the fixture is not only used to install, clamp parts, contain media and guide it through the parts, but also more importantly to provide one or several “disturbances” during the flow of media to control the flow of the media. . Because the flow of viscous abrasive media is the same as that of other fluids, it is easiest to pass through the paths with the shortest path, the largest cross-section, and the least resistance. In order to guide the medium to the required parts for cutting, you can use specially designed fixtures to block, bend, and interfere in certain parts to force the viscous abrasive to pass through the desired polished parts. The seal inside the fixture must be reliable, because small leaks will cause the wear of the fixture and the workpiece, and affect the polishing effect.

The concave model surface of the mold is divided into four types: through hole, stepped hole, cavity, convex model surface, etc. Table 5.17 shows the processing principle diagram of the 4 types of mold surface and fixture.

During the plastic mold polishing of the stepped hole, due to the different resistance when the medium flows through the large and small diameter holes, if the core jig as shown in FIG. 3 is not provided, it is difficult to produce the grinding pressure at the A and B parts. The flow medium near the corner is also reduced, and only part C can achieve the grinding effect. Therefore, it is necessary to place a core jig as shown in the figure to adjust the cross section of the medium to achieve the purpose of uniform grinding pressure.

PICTURE ——————–

As shown in the gear mold, it is possible to squeeze and grind the tooth surface at the bottom A, and a core jig should be equipped to form a channel. Figure 5.35 shows the clamp for squeezing and grinding the cavity that does not pass through the bottom. When the shape of the punch is squeezed and ground, the fixture is made into a structure that surrounds the punch.

5. Post-processing of extrusion grinding
After the squeezing and grinding, the workpiece and fixture are filled with media, which needs to be taken out. Due to the strong binding force of the medium itself, when it is difficult to peel the medium, a small amount of medium can be prepared to be combined with the medium on the work piece and simply led out. Use the compressed air to clean the place where the hand is out of touch. If the convex and concave mold surfaces blown with compressed air are immersed in an organic solution, the medium is easily dissolved and cleaned better. It will be better if combined with ultrasonic cleaning methods.

6. Squeeze grinding polishing process parameters and process rules
Extrusion grinding process parameters in addition to the thick viscosity of the viscous abrasive medium, abrasive type and particle size, the main ones are extrusion pressure, abrasive medium flow rate (or unit time media flow and processing time, etc. extrusion pressure, flow, processing The time is controlled by the extruder. The extrusion pressure is generally controlled in the range of 3MPa ~ 15MPa, the flow rate is generally controlled in the range of 7L / min ~ 25L / min, and the processing time is in the range of several minutes to tens of minutes.

+++++++++

5. EDM and ultrasonic compound polishing

In order to improve the surface roughness R. The polishing speed of the workpiece above 1.6um is combined with ultrasonic and special high-frequency narrow pulse high peak current pulse power for polishing. The impact of ultrasonic wave and the corrosion of electric pulse act on the surface of the workpiece at the same time, which quickly reduces the surface roughness. It is very effective for rough and hard surfaces processed by turning, milling, EDM and wire cutting.

6.Glass bead jet polishing

The glass bead spray method is a kind of spray processing method. It sprays small diameter glass beads together with compressed air, and is widely used in the polishing process of removing flash of parts, descaling, descaling and forming a dark glossy surface.
Figure 5.45 shows a schematic diagram of the glass bead spray method. The processing principle is the same as the sand blasting method, and it is especially suitable for polishing the softened surface like the wire cutting surface. After the wire is cut, the parts covered by the melt become uniform and smooth after being sprayed. The surface roughness value can be reduced by about 50%, and it is particularly suitable for the polishing of free-form surfaces.

PICTURE ———

(1) The effect of the glass bead particle size on the polishing quality When the surface roughness value of the wire cutting processing surface is large, the glass bead polishing with a larger particle size is used, and the effect is more obvious. If the surface roughness value of the original electro-machined surface is small, glass beads with a small particle size should be used; otherwise, due to the large particle size of the glass beads, the impact force and polishing effect are too strong, resulting in a rougher surface. Therefore, there is an optimum glass bead particle size for the roughness of the wire-cut surface.

PICTURE ———
(2) Influence of injection pressure on polishing quality
As shown in Figure 5.46, it is better to use a higher jet pressure for rough wire-cut surfaces. For finer wire-cut surfaces, jet pressure has little effect on the polishing effect.

(3) Effect of spray time on polishing quality Figure 5.47 shows the relationship between the spray processing time and the surface roughness of the wire cutting surface. For any wire-cut surface, within a few seconds of the initial spraying, the surface roughness value decreases sharply, and thereafter, with time, the surface roughness value hardly changes. For the finished surface with a small surface roughness value, the blast processing time is too long, but the surface tends to be rough. The improvement of the surface roughness of glass beads in the initial short time (5s ~ 20s).

(4) The influence of the glass bead spray method on the machining accuracy of parts. For the heat-treated mold steel, the glass bead spray method can remove the covering and the electric erosion part of the wire cutting processing surface. However, it is difficult to completely remove the metamorphic layer generated during electrical processing. The reason is that the hardness of the glass beads used is about 500HV, which is slightly lower than the hardness of the die steel of the processed material. Therefore, the shape of the processed parts will not be damaged, and at the same time, it is beneficial to the control of the size and shape accuracy.

Feel Free to Contact Us if Any Questions

 Previous Post

Difference between three-plate mold and two-plate mold

Next Post  

Cycle Time Calculation for Plastic Parts Injection Molding