The plastic material is one of the most commonly used materials for daily necessities. As an important means of production, injection molding is one of the most important factors affecting plastic products. The technological parameters of the molding process directly determine the flow state of the melt in the mold cavity, which has the most direct and far-reaching influence on the quality of the products. Therefore, it is an effective way to improve the quality of the plastic products to find the optimal technological conditions for the molding of the products and control the molding process. Here we will introduce the influence of injection molding process on product performance.
- Injection pressure
Injection pressure refers to the pressure exerted by the top of the screw or the plunger on the plastic melt during the injection process. Its role is to allow the melt to mix and plasticize, the screw (or plunger) must provide resistance to overcome the flow of solid particles and melt in the cylinder and nozzle. It makes the plastic melt fill the mold cavity at a certain speed, and compacts the mold cavity after it is filled with the melt, making the plastic part dense and filling the shrinkage caused by the cooling of the melt, so as to keep the plastic part inaccurate shape. The injection pressure is mainly determined by the type of plastic, the type of injection molding machine, the temperature of the mold, the mold structure, and the wall thickness of the plastic parts, among which the size and structure of the casting system have a great influence on the injection pressure.
- Injection time
Injection time is one of the parameters controlling the injection rate. The shorter the injection time, the higher the injection rate. Size of the injection rate has a great influence on the performance of the plastic parts. Increasing the injection rate can increase the filling pressure, which is beneficial to the filling process, and reduce the heat loss in the filling process. At the same time, it can reduce the shrinkage of products, reduce the core orientation of plastic parts and increase the strength of fusion joints. The improvement of injection rate is beneficial to the comprehensive performance of products, but too high injection rate will increase pressure loss, reduce the thickness of curing layer, improve the surface orientation of plastic parts, and even make the melt elastic turbulence, make plastic parts easy to form flash, surface crack, etc.. The experiment proved that too high or too low injection rate would lead to a decrease of impact strength. The low injection rate makes the weld strength decrease, the general orientation effect increase and the internal stress increase.
- Retaining Pressure
When the melt fills the cavity, the injection pressure compacts the melt in the mold. In actual production, the holding pressure should be equal to or less than the pressure used for injection. When the holding pressure is equal to the injection pressure, the shrinkage rate of plastic parts is often reduced and the stability and mechanical properties of plastic parts can be guaranteed. In general, the effect of holding pressure on product quality is similar to that of injection pressure. Most of the characteristics of plastic parts and parts of the structure, and to overcome the melt from the barrel to the mold cavity flow resistance, the molten material into the cavity and compaction, often accompanied by the increase in residual stress during demoulding, resulting in plastic parts demoulding difficulties, deformation, surface scratches, flanges and so on. Therefore, the selection of pressure when the need for careful selection.
Holding time and cooling time also have a direct impact on the quality of plastic parts. Shortening the holding time will reduce the pressure of the mold cavity, which may cause backflow, shrinkage cavity, depression and other defects, and affect the stability of the size of the plastic parts. Longer holding time can improve the dimensional stability of plastic parts and avoid the occurrence of the above defects. At the same time, it will increase the cavity pressure and change the internal stress caused by uneven temperature. But it will increase the demoulding difficulty, easy to produce surface scratch or plastic top bending. The length of plasticizing time can affect the plasticizing quality and directly affect product performance. Time is too short to make the plasticization uniform, consistent temperature, easy to produce hard, silver wire and so on, and too long will make the molten material due to the role of the screw decomposition, burns, and so on, also bring bad effects on product quality.
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