First, the injection molding machine has two basic components:
Injection device and mold closing device for melting and feeding plastic into mold.
The function of the mold closing device is as follows: 1. The mold is closed under the condition of bearing the injection pressure; 2. Take the product out of the injection device and melt the plastic before injecting it into the mold, and then control the pressure and speed to inject the melt into the mold.
There are two types of injection devices currently used: screw pre-plastifier or two-stage device and reciprocating screw.
Screw pre-plastifier uses pre-plastifying screw (first stage) to inject molten plastic into injection rod (second stage). The screw preplasticizer has the advantages of constant melt quality, high pressure and speed, and accurate injection control (using mechanical thrust devices at both ends of the piston stroke). These advantages are required for transparent, thin-walled products and high production rates. Disadvantages include uneven residence time (resulting in material degradation), higher equipment costs and maintenance costs.
The most commonly used reciprocating screw injection device does not require a plunger to melt and inject plastic.
Second, extrusion blow molding
Extrusion blow molding is a method for manufacturing hollow thermoplastic parts. Widely manufactured blow molding objects include bottles, barrels, cans, boxes and all containers for packaging food, beverages, cosmetics, medicines and daily necessities. Large blow-molded containers are usually used for packaging chemical products, lubricants and bulk materials. Other blow molded products include balls, bellows and toys. For the automobile industry, the fuel tank, car shock absorber, seat back, center bracket, armrest and headrest covers are all blow molded. For machinery and furniture manufacturing, blow-molded parts include shells, door frames, frames, pottery jars or boxes with an open face.
Iii. polymers
The most common plastic material for blow molding 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, polycarbonate and other thermoplastics can also be used for blow molding.
Recently, engineering plastics have been widely accepted in the automobile industry. Material selection is based on mechanical strength, weather resistance, electrical properties, optical properties and other properties.
IV. Technology
At present, 3/4 of the blow molded products processed by plastic mold injection molding are manufactured by extrusion blow molding. The extrusion process is to force the material to pass through a hole or die to produce a product.
The extrusion blow molding process consists of four steps:
1. Plastic embryo (extrusion of hollow plastic pipe);
2. Close the flap mould on the mould blank, clamp the mould and cut off the mould blank;
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;
4. Trimming the flash to obtain the finished product.
Five, polymer mixing
Polymer blending is defined as a process of upgrading polymers or polymer systems by melt blending.
The blending process ranges from the addition of a single additive to the treatment of multiple additives, polymer alloy and reactive blending. It is estimated that one third of the polymer production in the United States has to go through blending. Mixed ingredients can be customized according to the performance requirements of the final application. Mixed products have mixed properties, such as high gloss and excellent impact strength, or precision moldability and good rigidity.
The blended polymer is usually pelletized for further processing. However, the industry is increasingly interested in combining blending with the next process, such as profile extrusion, to avoid reheating the polymer.
VI. Mixing
People use various types of melt mixing equipment, from roll mills and batch mixers to single-screw and twin-screw extruders. Continuous blending (extruder) is the most commonly used equipment because it can provide products of consistent quality and reduce operating costs.
There are two mixed types:
1. The distributed mixture can be evenly distributed without adopting high shear stress in remarriage batching. This kind of mixture is called extensional mixing or laminar mixing.
2. Dispersed mixing is also called strong mixing, in which high shear stress is applied to break up cohesive solids. For example, when the additive mass is broken, the actual particle size becomes smaller. Mixing operations often require two types of mixing in one process.
Seven, rotational molding
Rotational molding, also known as rotational molding, is a processing method for manufacturing hollow seamless products of various sizes and shapes. Traditionally, it is mainly applied to thermoplastic materials. In recent years, rotational molding of thermosetting materials such as cross-linkable polyethylene has also developed rapidly. Since rotational molding does not require higher injection pressure, higher shear rate or accurate compound meter. As a result, molding and machines are relatively cheap and have a long service life. Its main advantages are: high performance/price of the machine; Complex parts need not be assembled after forming. Multiple products and multiple colors can be formed at the same time. The cost of the mould is lower; Color and materials are easy to change; Less scrap loss. .
The basic processing process is very simple. The powdery or liquid polymer is placed in a mold, heated while rotating around two vertical axes, and then cooled. At the beginning of the heating stage, if powdered material is used, the surface of the mold is formed into a porous layer, and then gradually melts along with the circulation process to form a homogeneous layer with uniform thickness. If liquid material is used, it flows and is coated on the surface of the mold first, and when the gel point is reached, the flow is completely stopped. The mold is then transferred to the cooling area, cooled by forced ventilation or water spray, and then placed in the working area, where the mold is opened, the finished parts are taken away, and then the next cycle is carried out.
