In order to ensure the physical properties and appearance quality of PVC products, the traditional UPVC injection process requires the kneaded material to be pelletized and then produced using an injection molding machine. With the development of UPVC industry, the application of various new processing aids and the continuous improvement of special injection molding machines, the powder injection molding process has also gradually developed. When the amount of filler is large, the granulation efficiency is low, which also makes the enterprise develop towards powder injection molding. From the actual production, the author analyzes and compares the characteristics of the two processes of pellet injection and powder injection.

1. Pellet injection

The injection molding machine adopts single-screw melting glue, which has limited plasticizing ability, which is easy to cause cold spot peeling of the product, and the material is not equipped with an exhaust device, and the product is prone to generate bubbles (water droplets) and radial flow patterns. The twin-screw extruder used in granulation has a strong plasticizing ability and an exhaust device, which can just make up for the shortcomings of the injection molding machine. After the pellets are plasticized by the twin-screw extruder, the materials can reach molecular level dispersion, and the vacuum exhaust device can absorb excess water. The pellets stored in the air have stable chemical properties, which is of great help to the secondary plasticization of the injection molding machine.The specific performance is as follows: product density, physical properties are stable, it is not easy to produce cold spots and peeling, can effectively control the phenomenon of bubble flow, surface finish it is good. The pellets are easier to transport in the factory, the leakage loss is small, and the pollution to the workshop environment is less. However, the manpower and material resources of the granulation process are large, and the efficiency is low. If this process can be reduced, it will be conducive to energy saving and consumption reduction of the enterprise.

2. Powder injection
Powder injection molding reduces the intermediate process of granulation, which can reduce the input of resources such as manpower and equipment, reduce the energy consumption of hydropower, and thus reduce the manufacturing cost; the streamlining of the process also reduces the management cost of the workshop. Although the powder is kneaded by a high-speed mixer, all kinds of additives can only achieve agglomerate dispersion. The single screw injection molding machine cannot completely disperse the additives, and the melt cannot achieve the desired effect, thereby affecting the physical properties of the product. : The product is brittle, with rough surface and cold spots. Limited by the production site and environment, and in order to facilitate management, generally mixed powder may need to be stored for several days before being used for injection molding. Because the powder is easy to absorb moisture, it is affected by the geographical location of the workshop or seasonal changes. When the air humidity is large, it will cause the powder to have too much water content, and bubbles appear on the surface of the product, which affects the appearance quality. The above problems have little effect on small-size products, but there are obvious visual defects on large-size products, so small-size products may consider using powder injection molding. However, most of the small-size products are water supply fittings, which need to focus on controlling physical properties. Injection molding with pellets can more effectively control their physical properties, which should be paid attention to.

3. Factors affecting powder injection
3.1 Equipment
The powder injection molding machine is very different from the pellet injection molding machine, such as the design of the screw and barrel, the nozzle, the design and sealing of the hopper and the root of the screw, etc. To understanding. As far as the screw is concerned, it can be further subdivided according to the amount of filling, such as the screw equipped with the PVC-U powder injection molding machine of Datong Machinery Co., Ltd.
Powder injection molding has strict requirements on the injection molding machine and the control of the entire production process. If the powder injection molding machine is used to inject powder, there will be a narrow processing range and unstable production. If the quality of PVC resin, additives is unstable, and there are errors in the ingredients and kneading operations, a batch of products will fail. In addition, the leakage of powder in the general pellet injection molding machine is more serious. For powders with a larger filling amount, the problem of uneven blanking at a higher frequency may also occur, resulting in molds that cannot be filled or the density is too small and the shrinkage is too large. The larger the injection volume, the more obvious this phenomenon is. Old-style pellet injection molding machines that have been used for many years have poor plasticizing ability and unstable pressure, and cannot generally be used for powder injection molding.

3.2 Mold
Mold runner design has a great influence on powder injection. The main channel and the auxiliary channel are generally short and thick, with a circular cross section. The diameter of the cold feed well should be larger than the main channel, and the depth should be close to the diameter. In the past, the mold designed to save the gate material is not suitable for powder injection molding. The gate should be opened as large as possible, as long as the product does not cause the material to be depressed. For powders with a large filling amount, more attention should be paid to the design of the mold runner.

3.3 Recipe
Some granules are consumed during granulation, and the amount of lubricant in the formula can be appropriately reduced during powder injection molding; in addition, the powder injection molding is one-time plasticization, and the amount of heat stabilizer can also be reduced as appropriate; in terms of fillers, light Quality calcium carbonate has stronger moisture absorption capacity than heavy calcium carbonate. The larger the amount of calcium carbonate, the more obvious the appearance quality problems such as bubbles or radial flow marks on the surface of the product due to the moisture absorption of calcium carbonate.

3.4 Process
In order to make the additives disperse better, the mixing temperature should be properly increased or the mixing time should be extended. The hot mixing temperature should be 120~135. The loss and pollution caused by powder transportation in the workshop vary according to the technical level of the operator, and the main measure is to prevent leakage. In view of the characteristics of powder injection cold material and more gas, the screw speed should be lower. The injection process is mainly divided into 4 levels.
(1) Level 1 low speed, less injection volume (less than 10% of the total injection volume), so that the cold material of the nozzle enters the cold material well or adheres to the surface of the runner to avoid entering the cavity.
(2) The second level is fast and the injection volume is large (about 80% of the total injection volume), which quickly fills most of the cavity of the cavity without causing melt scorching, so as to reduce the melt in the long process The resistance caused by cold during the movement is reduced.
(3) The third level is slow, and the injection volume is small (about 5% of the total injection volume).The gas in the cavity is accumulated near the product fusion line due to the rapid filling of the previous level, which affects the bonding strength of the fusion line and The appearance of the product, so the main function of this stage is to exhaust and melt the melt together.
(4) The fourth-stage slow speed, high pressure, less injection volume (less than 5% of the total injection volume), play the role of filling. In addition, the injection time should be longer than the actual filling time to improve the physical properties of the product.
4 Conclusion
The advantages of pellet injection molding process make it widely used in UPVC pipeline industry. Compared with pellet injection molding, powder injection molding shows many deficiencies, such as the production process is difficult to control, the yield is low, the economic effect is not obvious, and the mechanical and physical properties of the product are unstable. However, powder injection molding can reduce resource investment and energy consumption, which is conducive to energy saving, emission reduction and sustainable development, and has certain research value. The overall recommendation is to use pellet injection, which is conducive to environmental protection and working environment improvement, and the yield is higher.