Introduction to plastic molding principle

Injection Molding

Plastic injection molding technology uses an extruder to inject melted plastic material into a metal mold, pour the melted plastic into this enclosed space and let it cool and solidify to obtain a plastic product with the same shape as the enclosed space. This part manufacturing method Called, [Injection Molding]. It is the most important and commonly used technology among various plastic molding methods. This manufacturing method requires a plastic injection machine.

Principle of injection molding

The principle of injection molding is to pour solid raw materials (pellet) as large as mung beans into the barrel of the extruder. The raw materials are stirred by a rotating screw device and heated and melted in the heating barrel of the extruder. Under the alternating operation of spiral rotation and oil pressure, the liquid plastic is injected from the barrel at high pressure to fill the space in the metal mold, and then immediately send cold water to the metal mold to lower the temperature. After the molded product has cooled and hardened, the mold can be opened and taken out. These processes can be fully automated, with high-speed continuous production, and the speed is very fast. On average, a cup can be completed in about 10 seconds, and a ball pen tube can be completed in 1 second, even for large items such as car dashboards in 3-4 minutes. Among all the molding methods, the fastest and suitable for mass production. Moreover, the size of the molded product is accurate and the quality is stable.
From simple shape cups to complex shapes of car dashboards, from 0.01g clock pinions. to over 20Kg bathtub-like molded products, it can be manufactured. The loss of raw materials is small, the finishing process is convenient, and there is color when forming, and the appearance is beautiful. In addition, the cost can be greatly reduced in mass production. The plastic used can be both thermoplastic and thermosetting. But the mold is different. Thermoplastics make up the majority. Commonly used are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and ABS.
Plastic Injection

Thermoforming (vacuum forming)

The vacuum forming method is the most common method of thermoforming. The thermoplastic sheet is first fixed on the mold frame for molding, heated to soften it, and the air remaining between the mold and the sheet is drawn out with a vacuum pump. The vacuum at this time The suction force makes the thin plate stretch and deform and stick to the mold. After cooling and hardening, the air is reversely filled between the mold and the thin plate, and the formed thin plate will be pushed away from the mold surface by air pressure and demolded. For the molded product, the excess edge part must be cut off (the sheet can be cut by a punching machine, and the thicker can be cut with a band saw or circular saw) and other secondary processing (such as drilling, punching, etc.) can be completed. Aluminum molds are used for mass production, and plaster molds, plastic molds or electroformed molds are used for small-scale production.

Vacuum forming method

The vacuum forming method can manufacture finished products of various sizes, and can use automated equipment to reduce product costs, and the equipment costs are also lower than other forming methods. However, vacuum forming can only produce thin shells, and cannot produce complex-shaped structures. Because the sheet is stretched during the forming process, it is difficult to control the average thickness. Therefore, it is not suitable for manufacturing high-precision molded products. Therefore, the previous molding experience should be used as much as possible when designing.

Draw out molding 

This is a molding method that can produce a continuous strip of structural material with exactly the same cross-sectional shape. The molding process uses an extrusion mechanism equipped with a screw. The thrust of the spiral rotation is used to press and heat the melted plastic plastic liquid to pass through the first metal extrusion mold with a cross-sectional shape installed in front of the extrusion cylinder , The material has been roughly solidified after passing through the mold, but the temperature is still high and easy to deform. Therefore, the shape is corrected by a forming die and extruded into the correct cross-sectional shape. Finally, the hardening operation is completed in the cooling water tank. The equipment used is similar to the aluminum extrusion equipment, but with the same cross-sectional shape, the cost of the plastic extrusion mold is much more expensive, and the design of the mold relies heavily on experience.
This type of molding method is commonly used in continuous molding of sheets, tubes, and other structures with complex cross-sectional shapes. For small and simple construction materials, metal molds are naturally cheap; but the structure is complicated and strange in shape.

Compression molding

This manufacturing technology is mainly aimed at the molding method of thermosetting plastics (such as: melamine resin, urea resin, epoxy resin, phenol resin and polyester). Commonly used for parts that require heat resistance, fire resistance, and electrical insulation, such as tableware and kitchen utensils for children, and other electrical parts such as switches and sockets. Unlike the previous two molding methods, this molding method usually uses semi-automatic The equipment, the efficiency is accelerated, and the quality is stable. The mold is divided into upper and lower parts, the upper part is the female mold, and the lower part is the male mold. The female mold of the metal casting mold is heated first, and the pre-calculated volume is poured into the female mold cavity Thermosetting plastic materials; for example, polyester and epoxy resin are liquid raw materials. Put glass fiber into a metal mold and then pour the resin. Sometimes the glass fiber and resin are mixed to form a paper clay-like raw material. Knead a ball and weigh it and place it directly in the mold for molding.

Compression molding machine

The compression molding machine operates vertically, using hydraulic pressure to push the mother mold upwards, and then pressurize it after being combined with the daughter mold. After heating and pressurizing, the molding material will become a fluid state, fill the metal mold, and at the same time produce a chemical reaction to harden. Compression molding is prone to burrs. Therefore, the final finishing treatment is often required. It can manufacture helmets, chairs, and auto parts. A medium-sized object with a simple shape, but the molding speed is not fast enough.

Blow molding

Plastic blow molding, put the extrusion mechanism on the top, pass the softened thermoplastic plastic tube extruded downwards between the open metal molds, wait until the appropriate position, close the mold, so that the soft plastic tube is clamped in the metal mold In the space, send air into the soft plastic tube; the pressure of the air causes the soft plastic tube to expand like a balloon, and its membrane expands until it hits the inner wall of the metal mold. After it hardens, take it out (hard plastic balloon ) Hollow products with the same shape as the inner wall of the metal mold can be made. This is blow molding. Blow molding is mainly used to make bottle-shaped hollow thin-wall molded products.
Kettles, trash cans, gasoline cans, and even chemical barrels with a capacity of up to one ton, and fishery product transport boxes. The thickness of the blown product is uneven, and the surface is easy to leave traces of plastic flow, so it is rough and not refined, and the shape cannot be made into a small and complex structure. However, it can also be designed with practical experience to produce an integrated oil drum with a handle. , And a complicated double-walled box with hinges. The most used resin is PE, and others include PVC, polycarbonate, PP, and nylon. Blow molding products have very thin walls, which can save a lot of materials and reduce costs. Therefore, they are widely used in packaging products such as seasonings, detergents, and shampoos.
Blow-molded products are usually used for cheap and bulky products, so such products are not easy to transport and have considerable locality.

Foam molding

The thickness of the molding is 1.5 mm at the minimum and 450 mm at the maximum; the foaming magnification ranges from several times to 70 times, with a wide range of options. Using these characteristics, it is possible to manufacture structural materials and molded products that require heat insulation, cushioning, light weight, and floating. EVA foam is mainly used as foam board, and the foam board has hard skin on both sides. It is usually cut into slices of fixed thickness with a wheel knife machine, and the shape is punched out by a punch. It can be used for foam flooring and puzzles. , Heat insulation, cushioning materials, etc., you can also first heat it to a softened state, force it to deform with a mold, and then it can become a specific shape after cooling. It is widely used in shoe soles, baseball catcher armor, and helmets. Inner cushion.

Styrofoam (EPS)

This is the expansion of polystyrene (PS) with a blowing agent. This molding method is to first heat the raw material particles to expand the air in the preliminary expanded particles. When the particles swell to a certain density, fill Into the aluminum mold, heating with water vapor, the heat of water vapor (100-200 degrees C) also dissolves the surface of the foam particles and sticks together. The method of forming raw materials. Dissolution lines appear on the surface, which is a major disadvantage of the product. Because water vapor is used as a heat source. It is also commonly referred to as "steam forming".

Rotary molding

Its principle is similar to that of condensed layer forming but the mold used is not fixed. Generally, the powder raw material of PE is appropriately enclosed in the metal mold of thin steel plate, and at the same time, two rotating shafts are used to make a star-like rotation. Heating outside. The heat causes the powder material in the mold to gradually dissolve because of the rotation of the mold. The molten material rolls through the entire mold and adheres to the inner wall. A certain thickness is formed. Then it is cooled and hardened to form a molded product. In addition to powdered raw materials, PVC sol and polyester mixed with glass wool can also be used as raw materials. The advantage of the rotational molding method is that it can manufacture large hollow molded products with low-cost equipment. However, the productivity is low, and it is suitable for medium to small quantity production. The shape cannot be as fine and complicated as the injection molding method, and the dimensional precision is not high. There are few such molding methods. Commonly produced products include water tanks, water tanks, etc. Recently, I heard that there is a successful technical experiment of rolling with PE and foaming agent. It is possible to make a large-sized barrel with an insulating layer effect, but the thickness is uneven.

Injection molding

The material of the mold is not limited, plaster mold, wood mold, plastic mold, metal mold, silicone mold are all available. Silicone grease is most commonly used for manufacturing. Because the silicone mold has flexibility, it can manufacture molded products with a reverse draft angle design. The mold cost is low, suitable for small-scale production. It can be used to make jewelry (earrings, brooches, etc.), home accessories imitation handicrafts, because the finished product is thick and the material can be transparent, it is often used as a jade imitation art. If the glue is poured, if the coloring agent is deliberately not mixed evenly, it will be poured. The molding will produce a pattern of liquid flow, such as wood-like or jade finished products.

Lek casting molding (gel molding)

Utilizing the characteristics of PVC solution (sol/plastisol) that it solidifies on the surface of the contact object after being heated, the mold coagulation layer is suitable for making soft hollow molded products, such as PVC plastic dolls, animals and the like. In addition, the processed product is immersed in the sol and then lifted, so that the sol adheres to it, and it is heated and solidified, cooled and hardened, and formed into a protective or soft layer, such as the handle of a small file or a wire hanger.

Reference from wiki:

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