Application of New Injection Moulding Technology in Household Appliances

The moulding processing methods for plastic products for home appliances mainly include injection moulding, extrusion moulding, thermoforming, hollow moulding, lamination moulding, compression moulding, transfer moulding, foam moulding, casting moulding, inlay moulding, encapsulation, etc., among which mainly is injection moulding. In recent years, some new plastic processing technologies and new equipment have been widely used in the plastic moulding of household electrical appliances, such as precision injection moulding, rapid prototyping technology, melt injection moulding technology, gas-assisted/water-assisted injection moulding technology, electromagnetic dynamic injection moulding technology and coating membrane injection moulding technology.

1. Precision injection moulding
Precision injection moulding can ensure high precision and repeatability in terms of size and weight of plastic injection moulded parts. Injection moulding machines using this technology can achieve high pressure, high speed injection. Since the control method is usually open-loop or closed-loop control, high-precision control of the injection moulding process parameters is possible. In general, precision injection moulding requires high precision for the mould. At present, small and medium-sized precision injection moulding machines have been produced in China.

2. Gas-assisted / water-assisted injection moulding
Gas-assisted/water-assisted injection moulding is widely used to form a variety of injection moulded parts, and its typical product is TV casing. When injection moulding, gas or superheated water can be injected into the cavity almost simultaneously with the plastic melt. At this time, the plastic melt is coated with gas or superheated water, and the formed plastic product is a sandwich structure. After the plastic part is shaped, the gas or water can be discharged to release the mould. Such plastic products have the advantages of material saving, small shrinkage, good appearance and good rigidity. A key part of the moulding equipment is the gas-assisted or water-assisted device and its control software. In recent years, although there are many researches in China and the application speed is fast, domestic equipment is still unstable.

3. The fuse core injection moulding
Fuse injection moulding techniques are commonly used to shape shaped cavity plastic parts that have high requirement on cavity roughness and precision, and that can't' be processed by hollow moulding or rotational moulding processes. At present, the application of this technology in foreign countries is relatively mature, but it is still in an individual application state in China. The processing principle of this technology is that the core shape constituting the cavity is first formed, and then the core type is used as an insert for injection moulding. Under the heating of injection moulded part, the core type is melted and flowed out, thereby forming a cavity. The most important point in using this technology is the need to master the melting point of core materials and plastic parts. Generally, the core material may be a general-purpose plastic, a thermoplastic elastomer or a low-melting metal such as lead, tin or the like depending on the case.

4. Rapid prototyping technology
Rapid prototyping technology is rapidly developed along with the development of home appliances in various diversification directions and continuous upgrading. It is mainly used in plastic casings for home appliances. The advantage of this technology is that small batch production of plastic parts can be achieved without the need for a mould. At present, more mature rapid prototyping methods include laser scanning moulding and liquid photo curing moulding, among which laser scanning moulding is widely used. Laser scanning device is composed of laser light source, scanning device, dusting device and a computer. The processing process is that the laser head controlled by the computer scans according to a certain trajectory. At the position where the laser passes, the plastic micro-powder is melted and bonded together by heat, and each time the scanning is finished, the micro powder device is sprinkled with a thin powder, so that after repeated scanning, the plastic products having a certain shape and size is formed. At present, some enterprises in China have been able to produce laser scanning moulding machines and plastic micro-powders, but the performance of the equipment is unstable, and the number of micro-powders is also small.

5. Electromagnetic dynamic injection moulding
Electromagnetic dynamic injection moulding technology was invented by Mr. Jim from South China University of Technology. This technique causes the screw to reciprocate in the axial direction by the action of electromagnetic force. Since the plastic is micro plasticized in the pre-plasticizing stage, the plastic structure of the pressure-holding stage is made denser and the internal stress of the product is reduced. This technology can be used to form more demanding products such as optical discs. When used in the moulding of common products, the quality of the product can be improved.

6. Film injection moulding technology
The application of film injection moulding technology and equipment in foreign countries has been relatively mature. When using this technology, it is necessary to sandwich a special printed decorative plastic film into the mould before injection moulding. After being deformed by heat, the printed film can be attached to the surface of plastic parts, which is not only beautiful and firm, but also eliminates the post-decoration step.

In general, the demand for plastic moulds for plastic products used in household appliances is very large. For example, a plastic mould for a refrigerator or a fully automatic washing machine usually exceeds 100 pairs, and an air conditioner requires more than 20 pairs, one color TV requires 50-70 plastic moulds. At the same time, the technical requirements for plastic moulds are relatively high, and often the injection moulding cycle time is required to be as short as possible, thereby greatly promoting the development of mould design and modern mould manufacturing technology. In addition, some of the more difficult moulds, such as hot runner injection moulds, laminated injection moulds, etc., and are gradually increasing in China.

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