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.
1. Reasons and troubleshooting methods for main runner mould sticking during injection moulding.
(1) The cooling time is too short and the main runner has not yet solidified.
(2) The slope of the main runner is insufficient, and the draft angle should be increased.
(3) The mismatched size of the main runner bushing and the nozzle causes leakage.
(4) The main runner is rough and has no cooling wells and pulling hook.
(5) If the nozzle temperature is too low, the temperature should be raised.
The hot runner injection moulding method was introduced in the 1950s. After a long period of promotion, its application penetration rate has increased year by year. In the mid-1980s, hot runner injection moulds in the United States accounted for 15% to 17% of the total number of injection moulds, 12% to 15% in Europe and about 10% in Japan. However, in the 1990s, hot runner injection moulds accounted for more than 40% of plastic injection moulds produced in the United States, and accounted for more than 90% of injection moulds for large products.
It can be seen that the 3 coloring methods of injection moulded parts have their own advantages. The injection moulding companies needs to select the appropriate coloring method according to its own products to make the processed products have good quality, which can not only meet the requirements of customers but also reduce the cost, to maximize economic benefits.
In recent years, the manufacturing cost of injection moulds has continuously increased, so how to reduce the cost of injection moulds has become the hot topic that we discuss. Therefore, this article tries to analyze from the various aspects of mould manufacturing, in order to reduce the cost of injection mould manufacturing. Quality, price, cycle and service are 4 major factors that mould producers and users are very concerned about. In recent years, mainly due to the continuous increase in labor costs and rising raw material energy prices, resulting in injection mould manufacturing costs are also raising, and opposite of this, the profit rate of injection moulding company is constantly falling. Some injection moulding manufacturer reflect that the quality and level of some injection moulds currently produced are worse than they were five years ago. This is really puzzling: In the past five years, there has been a lot of technological progress. How does the quality level drop? They explained that because some users (or injection mould purchasers) unilaterally and simply demanded price reductions, they were indifferent to the injection mould quality level, so they had to be forced to use low-priced materials, low-grade mould standard parts, and unreasonable simplification of the injection mould structure. Use the methods of reducing the manufacturing process and processing costs to deal with. Therefore, it is not that injection moulding companies can’t produce high-quality and high-level injection moulds. Although this phenomenon exists in the injection moulding industry, this kind of abnormally low-cost manufacturing can’t be promoted, and should be guided correctly and suppressed. Therefore, promoting low-cost manufacturing under the premise of ensuring and improving the quality and level of injection mould.
The stage of injection mould pressure preservation is also a stage of continuous pressure application. The density of plastic also increases with the increasing pressure of the injection moulding machine. In the process of injection mould pressure preservation, the cavity is slowly filled with plastic, resulting in the injection mould back pressure gradually increased. In the process of pressure preservation, the screw of the injection moulding machine moves forward slowly, and the rate of plastic is relatively slow. In this case, the flow is called pressure preservation flow. In the injection mould pressure preservation stage, the plastic is cooled, resulting in accelerated solidification, increased melt viscosity, and large resistance in the cavity. At the later stage of pressure preservation, the material density continues to increase, and the plastic parts are also moulded by injection moulding. When the gate solidifies, the stage of pressure preservation is finished, and the pressure in the cavity reaches the maximum value.
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