Plastics is a new type of material that is artificially synthesized by people using chemical methods. The use of various plastics processed products has become an indispensable production and living material for various sectors of the national economy and people's lives. Injection molding is the most commonly used method in plastic molding processing. Injection molding can realize production automation, high-speed, and has extremely high economic benefits. In the face of huge market demand, various businesses have invested large amounts of money in the injection molding industry, purchased injection molding machines and raw materials, then, how can we turn raw materials into products? This involves the injection molding processing technology. Only by mastering this technology can we process high-quality products and create higher production efficiency.
Injection molding is a cyclic process, and each cycle includes the following: raw material pretreatment quantitative feeding - melt plasticization pressure injection injection mold cooling cooling mold removal and product handling. The injection molding processing technology is also the parameter required for control and the problem to be noticed in each process of the entire cycle. The following is a brief description of the process technology in each process.
The first is the pretreatment of raw materials, including the drying and color mixing of raw materials. Some plastics, such as polycarbonate (PC), polyamide (PA), and polyethylene terephthalate (PET), because they contain hydrophilic substances on their macromolecules, are easily hygroscopic, causing them to have different levels of moisture. When the moisture content is higher than the specified level, the surface of the product is accompanied with defects such as silver thread, markings, and air bubbles. When the moisture content is high, the polymer material is degraded. In general mass production of plastics should be used boiling or air drying, each plastic has the best drying temperature and drying time, too much time, the effect is not good, some plastic drying temperature and time in Table 1 (to letter Easy SDL straight hopper as an example): Table 1
No.
raw material
Dry humidity (degrees)
Time (hours)
1
PE, PS, PP
80
2-3
2
NYLON SERIV
75
4-8
3
ABS, AS
80-852-44
PC1204-75
PVC70
1-5
However, if a hydrophilic plastic such as PET is dried, it is impossible to use only an ordinary drying hopper. A vacuum oven or a dehumidifier is also required, and a dehumidifier can be used for continuous production. When the company processed PET preforms, it used a Sidson SD-90 dehumidifier to continuously dry at 140 degrees for 5 hours, removing 98% of the moisture content. The effect was very significant. As for the color mixing, generally used mixer, if you need to accurately control the masterbatch content, you can use masterbatch mixer, such as Sweden's CF series can be controlled according to the operation of the machine to control the masterbatch measurement.
The second is the quantitative addition, which is related to the accuracy of the machine.
The third is melt plastication. The main points to be mentioned in this process are the following three aspects: 1. The correct selection of barrels and screws; according to the processing performance of plastics, choose the right barrel and screw. Our company has developed a number of special screws and barrels for customers to choose from for the characteristics of some plastics, such as: exhaust screw, separate screw, special screw and barrel for thermosetting plastic, PVC screw, PC screw and double metal material Tubes, screws and so on. 2. The setting of heating temperature; The choice of barrel temperature is related to the characteristics of various plastics. Each plastic has its own viscous flow temperature (T1) and melting point (T2), in addition to a decomposition temperature (T3). The optimum temperature of the barrel should be between T1-T3 or T2-T3. If the range is smaller, the barrel temperature should be lower, otherwise it can be increased. The temperature of several commonly used plastics is shown in Table II.
Table 2 Plastic Name Melt Temperature Inlet Temperature Barrel Temperature Die Temperature ABC200-26050-6024060PS180-26040-6022020LDPE180-26060-8021030HDPE200-26060-8024020PA230-28070-8525080PC280-32080-9030090POM190-22060-7020590PP200-26060-8024050PVC150-22040 -6016560PET220-26060-7024060
Increasing the temperature of the barrel will increase the roughness and impact strength of the product, but it can reduce the shrinkage, warpage, orientation and internal stress of the product, so the temperature of the barrel can be increased as much as it allows. As for the nozzle The temperature is usually slightly lower than the barrel temperature. 3, set the appropriate back pressure: set a little back pressure during the pre-plastic, can increase the shear screw melt, discharge the melt inside the air, so that color mixing, plasticization more uniform; but if the back pressure is too high It will cause excessive screw and irregular offside conditions, which will make the amount of injected rubber extremely unstable, and will cause decomposition when processing materials such as PVC. The following is a list of recommended back pressures for some thermoplastics, see Table 3: Table 3
plastic
Back pressure (bar)
Note
PS ABS
100-200
Insufficient back pressure causes burning of plastic parts (caused by the presence of air or other gases in the melt)
PA6 PA66 PBT
10-19
Accurately set back pressure to achieve correct melt uniformity LDPE HDPE PP
100-300
Correct back pressure setting reduces changes in finished product weight
PMMA SAN
120-400
When the back pressure is too low, the melt and the finished product may have bubbles.
PVC CP CPVC
100-300
Back pressure values need to be carefully set, these plastics are sensitive to heat.
The fourth is pressure injection, divided into two stages of injection and pressure maintenance. The processing of thin-walled products requires high-speed injections, or even the addition of auxiliary devices such as accumulators to speed up the injection. However, if the speed is too fast, it may cause the melt to scorch when it flows through nozzles, gates, etc. If processing plastics that are very sensitive to heat, low-speed injections should be used. Many machines now have a multi-stage injection function that switches different injection speeds at different screw positions. Applying this function can not only improve the quality of the finished product, but also avoid the appearance defects, burns and flashes of the injection parts, reduce the concentration of internal stress, shorten the injection cycle by 15-40%, and increase the productivity when the finished product is injected. At 95-98% of the time, the pressure holding operation begins. The value of the holding pressure is generally 25-65% of the injection pressure. When thin-walled injection parts are produced, the value of the holding pressure may be as high as 95-100% of the injection pressure. Nowadays, there are gas-assisted injection technology, which specializes in processing the following products: 1. Large-planar, high-precision products, such as: laptop front cover (need to install liquid crystal display); 2. Handle class, completely solve the plastic microcosm, shrink wall Such defects, and save 20% of raw materials at the same time, increase the strength of the product; 3, the type of water cage, can greatly reduce the complexity of the mold. This technique of two companies in Bartonfield (Germany)
