In order to ensure the smooth process of injection moulding and make the plastic moulded products quality meet the requirements, a series of preparatory work must be done before plastic injection moulding, including the inspection, coloring and drying of raw materials, the cleaning of material barrel, the preheating of inserts and the selection of demoulding agents.
Injection moulding is a method including injection and moulding. At a certain temperature, the completely molten plastic material is stirred by screw, and the molten is injected into mould cavity by high pressure, after cooling and solidifying, the injection moulded parts are obtained. There are six stages in the injection moulding process: mould clamping, injection moulding, pressure holding, cooling, mould opening, and plastic parts removal. These six stages directly determine the quality of injection moulded product, and these six stages are a complete continuous process.
ABS is commonly known as acrylonitrile butadiene styrene and is a copolymer of three monomers of acrylonitrile, butadiene and styrene. Due to the different ratios of the three monomers, it has different properties and melting temperatures. If the ABS with flow performance is mixed with other plastics or additives, the use and properties of ABS will be expanded, such as impact grade, heat resistance, flame retardant grade, transparent grade, enhanced grade, electroplating grade, etc.
Injection mould polishing is also called mould grinding, is generally a process in which the surface of mould cavity is polished by using oil stone, sandpaper, polishing paste, wool wheel, etc. so that the working surface of injection mould can be bright as a mirror. Injection mould polishing has 2 purposes, one is to increase the smoothness of plastic injection mould, so that the surface of the plastic moulded product is smooth and beautiful. And the other is making the injection moulded products easy to demould and reducing the injection moulding cycle time.
The deformation, bending and twisting of injection moulded parts are mainly caused by the fact that the shrinkage rate of the flow direction during plastic moulding is larger than that of the vertical direction, so that the shrinkage ratio of the parts is different and causing warpage and deformation. Warpage is also caused by that there is greater internal stress remaining inside the injection moulded part during injection moulding. All of them are manifestation of deformation caused by high stress orientation. Therefore, fundamentally speaking, mould design determines the warping tendency of injection moulded part. It is very difficult to suppress this tendency by changing the moulding conditions. Finally, the problem must be solved from the mould design and improvement. Warpage of injection moulded parts is mainly caused by following 3 aspects.
There are many factors affecting the dimension precision of injection moulded parts. First of all, the manufacturing precision of mould has the biggest influence on it. The second is the fluctuation of plastic shrinkage. Third, in the injection moulding process, the mould wear and other reasons cause the constant change in mould size, will also affect the size of injection moulded parts. Fourth, the change of technological conditions, the change of flash thickness and the demould slope will also affect the precision of injection moulded parts.
Not all injection moulded parts have to be post-treatment. The necessary post-treatment is usually carried out only for plastic parts with metal inserts. And the internal stress of injection moulded parts with large temperature range, high dimensional precision, large wall thickness is not easy to eliminate by itself, so it is necessary to be post-treated.
In the four processes of injection moulding products production, the plastic injection moulding is the most important, and it is an indispensable process for the production of all injection moulding products or profiles. The other three processes depend on the requirements of injection moulding products, that is, not every plastic product has to go through these processes completely. Mechanical processing is used to complete the moulding process can not be completed or not completed accurately. Modification is mainly to beautify the surface or appearance of injection moulding products. Assembly is the process of connecting or matching finished plastic parts into a finished product. The latter three processes are called secondary or post-processing. Secondary processing, by contrast, is often secondary.
In the plastic injection moulding, there are many factors affecting the quality of plastic parts. However, after the plastic raw material, injection moulding machine and mould structure are determined, the selection and control of injection moulding process conditions is one of the key factors to ensure the smooth moulding and the quality of plastic parts. The most important technological conditions of injection moulding are temperature, pressure and moulding cycle.
The gating system is also known as the runner system, which is a set of feed channels that direct the plastic melt from the nozzle of injection moulding machine to the cavity, typically consisting of main runner, branch runner, gate and cold slug well. It is directly related to the injection moulding quality and production efficiency of plastic products.
Since the shrinkage rate of plastic injection moulding is greatly affected by the moulding conditions, the shrinkage rate is not a fixed value, but a range. In the specific processing process, if various processing conditions can increase the moulding shrinkage rate, we can take 4 % as the specific shrinkage rate. If the specific moulding conditions are favorable for the reduction of moulding shrinkage ratio, 2.5% can be selected as the specific shrinkage ratio. Of course, the latter method is more difficult.
In injection moulding, the cooling time of injection moulded part accounts for approximately 80% of the time of entire injection moulding cycle. Poor cooling often result in warpage of injection moulded parts or surface defects, which will affect the dimensional stability of injection moulded parts. Properly arranging injection, pressure holding and cooling time can improve product quality and productivity.
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