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Plastic molds (tools for processing plastics to shape them)

Plastic molds (tools for processing plastics to shape them)

Issue Time:2021-04-01
Basic introductionedit
A combined plastic mold for compression molding, extrusion molding, injection molding, blow molding and low-foaming molding. It mainly includes a cavity with a variable cavity composed of a concave mold combined substrate, a concave mold component and a concave mold combined card board The concave mold is a convex mold with a variable core composed of a convex mold combined base plate, a convex mold component, a convex mold combined card board, a cavity cut-off component and a side cut combined board. Coordinated changes of mold convex, concave mold and auxiliary molding system. Series of plastic parts of different shapes and sizes can be processed. In the plastic processing industry, it is matched with plastic molding machines to give plastic products a complete configuration and precise size tools. Due to the wide variety of plastics and processing methods, and the complex and simple structures of plastic molding machines and plastic products, the types and structures of plastic molds are also diverse.
With the rapid development of the plastics industry and the continuous improvement of general and engineering plastics in terms of strength, the application range of plastic products is also expanding, and the amount of plastic products is also rising.
Plastic mold is a tool for producing plastic products. It is composed of several groups of parts, and there is a molding cavity in this combination. During injection molding, the mold is clamped on the injection molding machine, the molten plastic is injected into the molding cavity, cooled and shaped in the cavity, and then the upper and lower molds are separated, and the product is ejected from the cavity and out of the mold through the ejection system, and finally the mold is closed For the next injection, the entire injection molding process is carried out cyclically.
Generally, a plastic mold is composed of a movable mold and a fixed mold. The movable mold is installed on the movable template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine. During injection molding, the movable mold and the fixed mold are closed to form a pouring system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated to take out the plastic products.
Although the structure of the mold may vary widely due to the variety and performance of plastics, the shape and structure of plastic products, and the type of injection machine, the basic structure is the same. The mold is mainly composed of a pouring system, a temperature control system, forming parts and structural parts. Among them, the pouring system and molded parts are the parts that are in direct contact with the plastic and change with the plastic and the product. They are the most complex and the most variable parts in the plastic mold, requiring the highest processing finish and precision.
The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, cold material cavity, runner and gate, etc. Molded parts refer to the various parts that make up the shape of the product, including movable molds, fixed molds and cavities, cores, molding rods, and vents.
my country's plastic molds are driven by high technology and driven by the application requirements of pillar industries, forming a huge industrial chain, from the upstream raw and auxiliary materials industry and processing, testing equipment to downstream machinery, automobiles, motorcycles, home appliances, electronic communications , Construction and building materials and other major application industries, the development of plastic molds is full of vitality.
Main categoriesedit
According to the different molding methods, the types of plastic processing molds corresponding to different process requirements can be divided, mainly including injection molding molds, extrusion molding molds, blister molding molds, high-foamed polystyrene molding molds, etc.
Plastic mould

Plastic mould
1. Plastic injection (plastic) mold
It is mainly a molding mold that is most commonly used in the production of thermoplastic products. The corresponding processing equipment for plastic injection molds is a plastic injection molding machine. The plastic is first heated and melted in the heating cylinder at the bottom of the injection machine, and then melted in the screw of the injection machine. Or driven by the plunger, it enters the mold cavity through the nozzle of the injection machine and the pouring system of the mold, the plastic is cooled and hardened to form, and the product is demolded. Its structure is usually composed of molding parts, pouring system, guide parts, push-out mechanism, temperature control system, exhaust system, support parts and other parts. Manufacturing materials usually use plastic mold steel modules, and the commonly used materials are mainly carbon structural steel, carbon tool steel, alloy tool steel, high-speed steel, etc. The injection molding processing method is usually only suitable for the production of thermoplastic products. The plastic products produced by the injection molding process are very wide. From daily necessities to various complex machinery, electrical appliances, and transportation parts, they are all molded by injection molds. It is the most widely used processing method in the production of plastic products.
2. Plastic compression mould
It includes two structural mold types: compression molding and injection molding. They are a type of molds mainly used to form thermosetting plastics, and their corresponding equipment is a pressure forming machine. According to the characteristics of the plastic, the compression molding method heats the mold to the molding temperature (generally 103°—108°), then puts the measured compression powder into the mold cavity and the feeding chamber, and closes the mold. The plastic is under high heat and high pressure. It is a softened viscous flow, solidified and shaped into the desired product shape after a certain period of time. The difference between injection molding and compression molding is that there is a separate feeding chamber. The mold is closed before molding. The plastic is preheated in the feeding chamber and is in a viscous flow state. It is adjusted and extruded into the mold cavity under pressure to harden and form. Compression molds are also used to form some special thermoplastics, such as hard-to-melt thermoplastics (such as polyvinyl fluoride) blanks (cold forming), resin lenses with high optical performance, slightly foamed nitrocellulose car steering wheels, etc. . The compression mold is mainly composed of a cavity, a feeding cavity, a guiding mechanism, an ejecting part, a heating system, etc. Injection molds are widely used for packaging electrical components. The material used in the manufacture of compression moulds is basically the same as that of injection moulds.
3. Plastic extrusion mould
A type of mold used to mold and produce continuous-shaped plastic products, also called extrusion molding heads, is widely used in the processing of pipes, rods, monofilaments, plates, films, wire and cable coatings, profiled materials, etc. The corresponding production equipment is a plastic extruder. The principle is that the solid plastic is melted and plasticized under the conditions of heating and the screw of the extruder to rotate and pressurize, and the cross-section is the same as the shape of the die through a specific shape of the die. Continuous plastic products. Its manufacturing materials mainly include carbon structural steel, alloy tools, etc. Some extrusion dies will also be inlaid with wear-resistant materials such as diamond on the parts that need to be wear-resistant. The extrusion process is usually only suitable for the production of thermoplastic products, and its structure is obviously different from injection molds and compression molds.
4. Plastic blow mould
It is a kind of mold used to form plastic container hollow products (such as beverage bottles, daily chemical products and other packaging containers). The forms of blow molding are mainly extrusion blow molding hollow molding and injection blow molding hollow molding according to the process principle. , Injection stretch blow molding hollow molding (commonly known as "injection stretch blow"), multilayer blow molding hollow molding, sheet blow molding hollow molding, etc. The corresponding equipment for blow molding of hollow products is usually called a plastic blow molding machine. Blow molding is only suitable for the production of thermoplastic products. The structure of the blow mold is relatively simple, and the materials used are mostly made of carbon.
5. Plastic blister mold
It is a kind of mold that uses plastic plates and sheets as raw materials to form some simpler plastic products. Its principle is to use the vacuum method or compressed air molding method to make the plastic plates and sheets fixed on the concave mold or the convex mold. Under the condition of heating and softening, it is deformed and pasted on the cavity of the mold to obtain the desired molded product, which is mainly used in the production of some daily necessities, food, and toy packaging products. The blister mold is made of cast aluminum or non-metallic materials because of the low pressure during molding, and the structure is relatively simple.
6. High-expansion polystyrene molding die
It is a mold for using expandable polystyrene (beads composed of polystyrene and foaming agent) raw materials to form various desired shapes of foam packaging materials. The principle is that the expandable polystyrene can be steam-formed in the mold, including two types of simple manual operation molds and hydraulic straight-through foam plastic molds, which are mainly used to produce industrial packaging products. The materials used to make this kind of molds are cast aluminum, stainless steel, bronze and so on.

Design elementsedit
Mold design and manufacturing are closely related to plastic processing. The success or failure of plastic processing depends largely on the effect of mold design and mold manufacturing quality, and the design of plastic molds is based on the correct design of plastic products.

The structural elements to be considered in the design of plastic molds are:
① Parting surface, that is, the contact surface where the concave mold and the convex mold cooperate with each other when the mold is closed. The selection of its location and form is affected by factors such as product shape and appearance, wall thickness, molding method, post-processing technology, mold type and structure, demolding method, and molding machine structure.
②Structural parts, namely sliders, inclined tops, straight top blocks, etc. of complex molds. The design of structural parts is very critical, and it is related to the life of the mold, processing cycle, cost, product quality, etc. Therefore, the design of the core structure of a complex mold requires a higher comprehensive ability of the designer, and the pursuit of simpler, more durable, and more economical is as much as possible. Design.
③Mold precision, namely avoiding jams, precise positioning, guide posts, positioning pins, etc. The positioning system is related to the appearance quality of the product, mold quality and life. According to the different mold structure, different positioning methods are selected. The positioning accuracy control mainly depends on processing. The internal mold positioning is mainly considered by the designer to design a more reasonable and easy-to-adjustable positioning the way.
②Gating system, that is, the feed channel from the nozzle of the injection molding machine to the cavity, including the main runner, the runner, the gate and the cold cavities. In particular, the selection of the gate position should be conducive to the molten plastic filling the cavity in a good flow state. The solid runner and gate cold material attached to the product are easy to eject from the mold and be removed when the mold is opened. Except for road model).
③ Plastic shrinkage rate and various factors that affect the dimensional accuracy of products, such as mold manufacturing and assembly errors, mold wear, etc. In addition, when designing compression molds and injection molds, the matching of the process and structural parameters of the molding machine should also be considered. Computer-aided design technology has been widely used in plastic mold design. [1] 
Mold accessories editor
Punch needle, punch, guide post, guide sleeve, precise positioning, ejector pin, thimble, cylinder, slider device, inclined top device, steel ball sleeve, date stamp, plastic mold spring, sealing ring, cooling joint, water plunger, Positioning ring, sprue bushing, pull pin, pull rod, limit nail, garbage nail, support column, shutter, mold opening sequence controller (mold clamping buckle assembly), etc.
Structural parts editing
1. Composition
The structure of blow molds, casting molds and thermoforming molds is relatively simple.
Compression molds, injection molds, and transfer molds are more complicated in structure, and there are many parts that make up such molds.
The basic parts are:
①Molding parts, including concave molds, convex molds, and various molding cores, are parts of the inner and outer surfaces or upper and lower end faces, side holes, undercuts and threads of the molded product.
②Support fixed parts, including mold base plate, fixed plate, support plate, cushion block, etc., to fix the mold or support pressure.
③Guiding parts, including guide posts and guide sleeves, are used to determine the relative position of the movement of the mold or ejection mechanism.
④ Core-pulling parts, including diagonal pins, sliders, etc., are used to pull out the movable core when the mold is opened to demold the product.
⑤Push out parts, including push rod, push tube, push block, push piece plate, push piece ring, push rod fixing plate, push plate, etc., to demold the product. Standard mold bases are commonly used for injection molds. This mold base is a complete set of basic parts that have been standardized and serialized in structure, form, and size. The mold cavity can be processed by itself according to the shape of the product. The use of standard mold bases is beneficial to shorten the molding cycle.
2. The role of common mold base parts
Fixed mold base plate (panel): Fix the front mold on the injection molding machine.
Runner plate (nozzle plate): Remove the waste handle when opening the mold to make it fall off automatically (three-plate mold).
Fixed mold plate (A plate): the front mold part of the molded product.
Movable mold fixing plate (B plate): the back mold part of the molded product.
Cushion: Mould foot, its function is to make the top plate have enough space for movement.
Push plate: When opening the mold, push out the product from the mold by pushing out parts such as ejector rods, top blocks, and inclined tops.
Movable mold base plate (bottom plate): Fix the back mold on the injection molding machine.
Guide post and guide sleeve: play the role of guiding and positioning, assisting the opening of the front and rear molds, and the basic positioning of the mold.
Support column (support head): Improve the strength of the B plate, effectively avoiding the deformation of the B plate caused by long-term production.
Top plate guide column (Zhong Tuo Division): guide and position the push plate to ensure smooth ejection.
Material requirementsedit
The working conditions of plastic molds are different from those of cold stamping dies. Generally, they must work at 150°C-200°C. In addition to being subjected to a certain pressure, they also have to withstand temperature. According to the different use conditions and processing methods of plastic molding molds, the basic performance requirements of steel for plastic molds are roughly summarized as follows:
Plastic mould
Plastic mould
1. Sufficient surface hardness and wear resistance
The hardness of the plastic mold is usually below 50-60HRC, and the heat-treated mold should have sufficient surface hardness to ensure that the mold has sufficient rigidity. When the mold is working, due to the filling and flow of the plastic, it has to withstand greater compressive stress and friction. The mold is required to maintain the accuracy of the shape and the stability of the dimensional accuracy to ensure that the mold has a sufficient service life. The wear resistance of the mold depends on the chemical composition and heat treatment hardness of the steel, so increasing the hardness of the mold is beneficial to improve its wear resistance.
2. Excellent machinability
In addition to EMD processing, most plastic molding molds require certain cutting processing and fitter repairs. In order to extend the service life of cutting tools, improve cutting performance, and reduce surface roughness, the hardness of the steel used for plastic molds must be appropriate.
3. Good polishing performance
For high-quality plastic products, the surface roughness of the cavity is required to be small. For example, the surface roughness value of the injection mold cavity is required to be less than Ra0.1~0.25, and the optical surface requires Ra<0.01nm, and the cavity must be polished to reduce the surface roughness value. For this reason, the selected steel requires less material impurities, fine and uniform structure, no fiber directionality, and no pitting or orange peel defects during polishing.
4. Good thermal stability
The shape of the parts of the plastic injection mold is often complicated and difficult to process after quenching. Therefore, it should be selected as much as possible with good thermal stability. When the mold is formed by heat treatment, the linear expansion coefficient is small, the heat treatment deformation is small, and the dimensional change caused by the temperature difference The rate is small, the metallographic structure and the mold size are stable, and the processing can be reduced or no longer needed to ensure the mold size accuracy and surface roughness requirements.
45 and 50 grades of carbon steel have certain strength and wear resistance, and are mostly used as mold base materials after quenching and tempering. High-carbon tool steel and low-alloy tool steel have higher strength and wear resistance after heat treatment, and are mostly used for forming parts. However, high-carbon tool steel is only suitable for manufacturing small-sized and simple shaped parts due to its large heat treatment deformation.
With the development of the plastics industry, the complexity and precision of plastic products have become more and more demanding, and higher requirements have been placed on mold materials. For the manufacture of complex, precise and corrosion-resistant plastic molds, pre-hardened steel (such as PMS), corrosion-resistant steel (such as PCR) and low-carbon maraging steel (such as 18Ni-250) can be used, all of which have good Cutting, heat treatment and polishing performance and high strength.
In addition, when selecting materials, you must also consider preventing scratches and bonding. If there is relative movement between the two surfaces, try to avoid choosing materials with the same structure. Under special conditions, one side can be plated or nitrided to make the two sides have different The surface structure.
Material selectionedit
1. Working conditions of plastic molds
Due to the development of the plastics and plastic molding industry, the quality requirements for plastic molds are becoming higher and higher, so the failure of plastic molds and their influencing factors have become an important research topic. The main working parts of plastic molds are molded parts, such as punches, concave molds, etc., which form the cavity of the plastic mold to form various surfaces of plastic parts and directly contact the plastic, and withstand pressure, temperature, friction and corrosion, etc. effect.
2. Analysis of failure causes of plastic mold materials
Generally, mold manufacturing includes mold design, material selection, heat treatment, machining, debugging and installation. According to the investigation, among the factors of mold failure, the material and heat treatment used in the mold are the main factors affecting the service life. From the overall qualityHair-like wear marks left by No. 500 sandpaper. Then use sticky felt and diamond paste for polishing, the order is 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000). The polishing process with an accuracy requirement of 1μm or more (including 1μm) can be carried out 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 can all scrap the high-precision polished surface obtained after hours of work.
2. Rough toss
After milling, EDM, grinding, etc., the surface can be polished with a rotating surface polisher or ultrasonic grinder with a speed of 35 000-40 000 rpm. The commonly used method is to remove the white EDM layer with wheels with diameter Φ3mm and WA # 400. Then there is manual whetstone grinding, and the strips of whetstone are added with kerosene as a lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many mold manufacturers choose to start with #400 in order to save time.
3. Semi-precision throwing
Semi-precision polishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper only uses die steel suitable for hardening (above 52HRC), not pre-hardened steel, because it may cause burns on the surface of pre-hardened steel.
⒋Fine throwing
For fine polishing with abrasive paste, 8000 and 10000 will reach the mirror surface.
Polishing methodedit
Mechanical polishing
Mechanical polishing is a polishing method that relies on cutting and plastic deformation of the material surface to remove the polished convexities to obtain a smooth surface. Generally, oil stone sticks, wool wheels, sandpaper, etc. are used, and manual operations are the main ones. Special parts such as the surface of the rotating body can be used. Use the turntable and other auxiliary tools, and the ultra-precision polishing method can be used if the surface quality is high. Ultra-precision polishing is the use of special abrasive tools, which are pressed tightly on the processed surface of the workpiece in a polishing fluid containing abrasives for high-speed rotation. Using this technology, the surface roughness of Ra0.008μm can be achieved, which is the highest among various polishing methods. Optical lens molds often use this method.
Chemical polishing
Chemical polishing is to make the surface microscopic convex part of the material in the chemical medium dissolve preferentially than the concave part, so as to obtain a smooth surface. The main advantage of this method is that it does not require complex equipment, can polish workpieces with complex shapes, and can polish many workpieces at the same time, with high efficiency. The core problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is generally several 10 μm.
Electrolytic polishing
The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, by selectively dissolving tiny protrusions on the surface of the material to make the surface smooth. Compared with chemical polishing, the effect of cathode reaction can be eliminated, and the effect is better. The electrochemical polishing process is divided into two steps: (1) Macroscopic leveling The dissolved products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra>1μm. ⑵ Low-light leveling: Anode polarization, surface brightness is improved, Ra<1μm.
Ultrasonic polishing
Put the workpiece in the abrasive suspension and put it together in the ultrasonic field, relying on the oscillation effect of the ultrasonic, so that the abrasive is ground and polished on the surface of the workpiece. Ultrasonic machining has a small macroscopic force and will not cause deformation of the workpiece, but it is difficult to manufacture and install tooling. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, so that the dissolved products on the surface of the workpiece are separated, and the corrosion or electrolyte near the surface is uniform; the cavitation effect of ultrasonic in the liquid can also inhibit the corrosion process and facilitate surface brightening.
Fluid polishing
Fluid polishing relies on high-speed flowing liquid and abrasive particles carried by it to wash the surface of the workpiece to achieve the purpose of polishing. Commonly used methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding and so on. Hydrodynamic grinding is driven by hydraulic pressure to make the liquid medium carrying abrasive particles flow back and forth across the surface of the workpiece at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives. The abrasives can be made of silicon carbide powder.
Magnetic grinding and polishing
Magnetic abrasive polishing is to use magnetic abrasives to form abrasive brushes under the action of a magnetic field to grind the workpiece. This method has high processing efficiency, good quality, easy control of processing conditions and good working conditions. Using suitable abrasives, the surface roughness can reach Ra0.1μm. 2 Mechanical polishing based on this method The polishing mentioned in the processing of plastic molds is very different from the surface polishing required in other industries. Strictly speaking, the polishing of the mold should be called mirror processing. It not only has high requirements for polishing itself, but also has high standards for surface flatness, smoothness and geometric accuracy. Surface polishing generally only requires a bright surface. The standard of mirror surface processing is divided into four levels: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm. It is difficult to precisely control the geometric accuracy of parts due to methods such as electrolytic polishing and fluid polishing. However, the surface quality of chemical polishing, ultrasonic polishing, magnetic abrasive polishing and other methods are not up to the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.
Basic procedure
To obtain a high-quality polishing effect, the most important thing is to have high-quality polishing tools and auxiliary products such as whetstone, sandpaper and diamond lapping paste. The choice of polishing procedure depends on the surface condition after pre-processing, such as machining, EDM, grinding and so on.
Mold development editor
China's annual plastic mold output value reached 53.4 billion yuan? Do you believe it? Because of the rapid development of China's economy, the requirements for the plastic mold industry are becoming more and more stringent, which provides a huge impetus for the development of the plastic mold industry. According to accurate statistics, China's annual plastic mold output value reached 53.4 billion yuan, which is an existing fact.
With the rapid development of automobile manufacturing and IT manufacturing, the domestic mold industry has achieved rapid development. It is understood that the proportion of plastic molds in the mold industry in my country can reach 30%. It is expected that in the future mold market, plastic molds will account for the total mold The proportion of the quantity will still be gradually increased, and the development speed will be faster than other molds.
It is reported that the mold industry has grown rapidly at a rate of 20% per year since 2000, which has promoted the improvement of mold grades. Sophisticated mold manufacturing equipment has provided a guarantee for the improvement of mold technology.
The analysis believes that because of the majority of imported molds, precision, large, complex, and long-life molds, from the perspective of reducing imports and increasing the localization rate, the market share of such high-end molds will gradually increase. The rapid development of the construction industry has made various profile extrusion molds and PVC plastic pipe joint molds become new economic growth points in the mold market. The rapid development of highways has also put forward higher requirements for automobile tires. Therefore, radial rubber tire molds , Especially the development of active molds will also be higher than the overall average; replacing wood with plastic and metal with plastic will make the demand for plastic molds in the automobile and motorcycle industries huge; the household appliance industry will be in the “Twelfth Five-Year Plan” period. There is a big development, especially the demand for plastic molds for refrigerators, air conditioners, microwave ovens, etc.
At the same time, the pace of structural adjustment of the plastic mold industry is accelerating, and the number and capabilities of market-oriented professional plastic mold manufacturers are also growing rapidly. According to the analysis of the production, sales, market conditions, industry structure, products and import and export of the plastic mold manufacturing industry, and with reference to the development trend of plastic mold related industries, predict the future development direction of my country's plastic mold manufacturing industry. How much development potential the mold manufacturing industry has, these need to be verified.