December 5, 2022
Common sense of plastic machinery and technology
Thermoplastic injection molding This method is to melt the plastic material and then inject it into the membrane cavity. Once the molten plastic enters the mold, it will be cooled into a certain shape according to the mold cavity sample. The shape obtained is often the final product, and no other processing is required before installation or use as the final product. Many details, such as bulge, rib and thread, can be formed in one step of injection molding.
The injection molding machine has two basic components: an injection device and a clamping device for melting and feeding plastics into the mold. The function of the mold mixing device is to: 1. close the mold under the injection pressure; 2. take out the products from the injection device to melt the plastic before it is injected into the mold, and then control the pressure and speed to inject the melt into the mold. At present, there are two designs of injection molding: screw type pre plasticizer or two-stage device, and reciprocating screw. The screw type preplasticizer uses a preplasticizing screw (first stage) to inject molten plastic into the injection rod (second stage).
The screw preplasticizer has the advantages of constant molten mass, high pressure and speed, and accurate injection volume control (using mechanical thrust devices at both ends of the piston stroke). These advantages are required for transparent, thin-walled products and high production rate. Its disadvantages include uneven residence time (leading to material degradation), high equipment costs and maintenance costs.
The most commonly used reciprocating screw injection device does not need a plunger to melt and inject the plastic.
2、 Extrusion blow molding
Extrusion blow molding is a method of manufacturing hollow thermoplastic parts. The blow molding objects widely made by people include bottles, barrels, cans, boxes and all containers for packaging food, beverages, cosmetics, medicines and daily necessities. Large blow moulded containers are usually used for packaging chemical products, lubricants and bulk materials. Other blow molded products include balls, bellows and toys. For the automotive industry, the fuel tank, car shock absorber, seat back, center bracket, armrest and headrest cover are all blown. For machinery and furniture manufacturing, blow moulded parts include shells, door frames, frames, pottery pots or boxes with an open surface.
The most common raw material for blow molding and extrusion is high-density polyethylene, which is usually used in most milk. Other polyolefins are also often processed by blow molding. Depending on the application, styrene polymers, polyvinyl chloride, polyester, polyurethane, polyacrylate and other thermoplastic plastics can also be used for blow molding.
Recently, engineering plastics have been widely accepted in the automotive industry. Material selection is based on mechanical strength, weather resistance, electrical performance, optical performance and other properties.
Three quarters of the blown beaver products are manufactured by extrusion blow molding. Extrusion process is to force materials to pass through a hole or die to produce
The extrusion blow molding process consists of five steps: 1. plastic molding embryo (extrusion of hollow plastic pipe): 2. close the split mold on the molding embryo, clamp the mold and cut the mold embryo; 3. blow the mold to the cold wall of the mold cavity, adjust the opening and maintain a certain pressure during cooling, open the mold and write down the blown parts; 5. trim the flash to get the finished product-
Polymer blending is defined as a process of upgrading polymers or polymer systems through melt blending The mixing process ranges from the addition of a single additive to the treatment of multiple additives, polymer alloys and reactive mud culture. It is estimated that one third of polymer production in the United States goes through blending. The mixing ingredients can be customized according to the performance requirements of the final application. Blended products have mixed properties, such as high gloss and excellent impact strength, or precision molding and good stiffness.
The polymer prepared from the mud is usually pelleted for further processing. However, the industry is increasingly interested in mixing with the next
Step processes are combined, such as profile extrusion, to avoid reheating the polymer.
Various types of melt mixing equipment are used, ranging from rolling mills and batch mixers to single screw and twin screw extruders Continuous mud dispensing (extruder) is the most needed equipment, because it can provide products with consistent quality and reduce operating costs. There are two hybrid types:
The distributed mud compound can be uniformly distributed in the remarriage ingredients without using high shear stress. This kind of mixture is called extensibility mixing or laminar mixing.
Dispersive mixing is also called strong mixing, in which high shear stress is applied to break the cohesive solids. For example, when the additive pellets are broken, the actual grain size becomes smaller.
Mixing operations often require two types of mixing in one process.
3、 Rotational molding
Rotational molding, also known as rotary molding, is a processing method for manufacturing hollow seamless products of various sizes and shapes. Traditionally, it is mainly used in thermoplastic materials. In recent years, the roll molding of thermosetting materials such as crosslinkable polyethylene has also developed rapidly. Because rotational molding does not require high injection pressure, high shear rate or accurate compound metering. Therefore, the molding and machine are relatively cheap and have a long service life. Its main advantages are: high performance/price of the machine; The forming of complex components does not require post assembly; Multiple products and colors can be molded at the same time. The cost of the mold is low: the color and material are easy to change; the leftover material loss is small.
The basic processing process is simple. The powder or liquid polymer is placed in the mold, heated and rotated around two vertical axes at the same time, and then cooled. At the beginning of the heating phase, if the powder material is used, a porous layer is formed on the surface of the mold Then it gradually melts with the circulation process to form a homogeneous layer of uniform thickness. If liquid material is used, the light flow and coating will be on the surface of the mold. When it reaches the gel point, it will stop completely: flow. The mold is then transferred to the cooling work area, cooled by forced ventilation or water spraying, and then placed in the 「. work area, where the mold is opened, the finished products are taken away, and the next cycle is carried out
4、 Cast plastic
Nylon - 6 has high strength, abrasion resistance, abrasion resistance and resilience. Pouring reaction is mostly conducted under atmospheric pressure or very low pressure, so nylon produced by casting has obvious advantages over nylon produced by extrusion or molding. For example, compared with extruded nylon, cast nylon has higher degree of knot and molecular weight, better dimensional stability, easy machinability, higher modulus and hot deformation temperature.
Because the size and shape of cast nylon are not limited, it has greater flexibility. The cast nylon with simple shape can be produced with low-cost tools, and the production cycle is also short; Complex parts need to be produced in complex tools. Try not to use expensive plastic casting machines in the club. The lactam monomer undergoes anionic polymerization in the mold and is finally converted into a polymer. Therefore, the reaction of anhydrous acid catalytic process is poor and the conversion efficiency is low, so people often choose catalytic reaction to get better results. Since the 1950s, cocatalysts have been developed. In the late 1960s, more effective production machinery has been developed to accelerate the commercialization of cast nylon.
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