Plastic Injection Molding Process

What are the Advantages of Injection Molding in Mold Production?

• High production efficiency: The injection molding machine has fast production speed, can inject multiple products at one time, and has a high degree of automation.
  There is less manual operation and the production efficiency is improved.
• Stable product quality: The injection molding machine automatically controls factors such as temperature, pressure, time, etc., and the products produced. The product quality is stable and meets the requirements.
• Low production cost: The injection molding process uses thermoplastic particles as raw materials, which are low-priced and can be recycled.
  Utilization reduces raw material costs.
• High production accuracy: By adjusting the parameters of the injection molding machine, the dimensional accuracy and weight accuracy of the product can be controlled.
  Produce high-precision products that meet requirements.
• Environmentally friendly production: The injection molding process uses plastic particles as raw materials, avoiding the hazards caused by traditional processing methods.
  Dust, wastewater, exhaust gas, and other environmental problems meet environmental protection requirements.
• Wide scope of application: The injection molding process is suitable for parts of various shapes and sizes, and can produce various plastic products, such as toys, household items, electronic product casings, etc.

Molding Cycle During Injection Molding Process

The molding cycle directly affects the labor rate and equipment utilization. Therefore, the production process should be under the premise of quality assurance, and try to shorten the molding cycle in the relevant time. In the whole molding cycle, injection time and cooling time are the most important, they have a decisive impact on the quality of products. Injection time in the mold filling time is directly inversely proportional to the rate of mold filling, the production of mold filling time is generally about 3 to 5 seconds.

Injection time in the pressure time is the pressure time of the plastic cavity, the whole injection time accounts for a larger proportion, generally about 20 to 120 seconds (extra thick parts can be as high as 5 to 10 minutes). In the gate at the melt sealing before the amount of holding time, on the product dimensional accuracy has an impact, if the later, it has no effect. The holding time also has an optimum value, which is known to depend on the material temperature, the mold temperature, and the size of the main runner and gate. If the dimensions of the main runner and gate as well as the process conditions are normal, the pressure value that results in the smallest fluctuation in the shrinkage of the product is usually taken as the optimum value. The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature. The end of the cooling time should ensure that the products do not cause changes in the principle of demolding, cooling time is generally about 30 to 120 seconds between the cooling time is too long is not necessary, not only to reduce productivity, for complex parts will also cause demolding difficulties, forcible demolding will even produce demolding stress. Other time in the molding cycle is related to whether the production process is continuous and automated and the degree of dualization.

Depending on the material to be processed. Injection molding of thermoplastics includes the processes of charging, plasticizing, injecting, holding pressure, cooling, and demolding. Thermosetting plastics and rubber molding also include the same process, but the barrel temperature is lower than that of thermoplastic plastics, the injection pressure is higher, the mold is heated, and the material injected in the mold needs to go through the process of curing or vulcanization, and then take advantage of the heat to remove the film.

Comparison of Thermoset and Thermoplastic Injection Molding Conditions

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Pressure Control In Injection Molding

Plasticizing Pressure: (back pressure) When using a screw injection machine, the pressure on the melt at the top of the screw when the screw rotates and retreats are called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted through the relief valve in the hydraulic system. In injection, the size of the plasticizing pressure remains unchanged with the screw speed. Increasing the plasticizing pressure will increase the temperature of the melt, but will reduce the plasticizing speed. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color materials can be mixed evenly, and the gas in the melt can be discharged. In general operations, the plasticizing pressure should be determined as low as possible while ensuring excellent product quality. The specific value varies with the type of plastic used, but it usually rarely exceeds 20kg/cm2.

Injection Pressure: In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil pressure). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, give the molten material a filling rate, and compact the molten material.

Injection pressure is divided into injection and holding pressure, usually 1 to 4 stages + 1 to 3 stages of holding pressure. Generally, the holding pressure is smaller than the injection pressure and should be adjusted according to the plastic material used to achieve the best result. physical properties, appearance, and size requirements.

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