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Injection Molding Introduction Of The Injection Process
Oct 30, 2017

Injection molding, refers to a certain temperature, through the screw stirring the complete melting of plastic materials, high-pressure injection into the mold cavity, after cooling solidification, the method of getting the molding products. This method is suitable for mass production of complex shape parts and is one of the most important processing methods. The main mold, injection, pressure, cooling, open mold, product removal 6 stages.

Injection process

Temperature control

⒈ Barrel Temperature: The injection molding process needs to control the temperature of the barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plastic and flow of plastics, and then a temperature mainly affects the flow and cooling of plastics. Each plastic has a different flow temperature, the same kind of plastic, due to different source or brand, its flow temperature and decomposition temperature is different, this is due to the weight and molecular weight distribution, plastic in different types of injection molding process is different, so choose the barrel temperature is not the same.

⒉ Nozzle Temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, this is to prevent the melt in the straight-through nozzle may occur "casting phenomenon." Nozzle temperature can not be too low, otherwise it will cause premature condensation of the molten material will be blocked nozzle, or due to early coagulation injection mold cavity and affect the performance of products.

⒊ mould temperature: The mold temperature has a great influence on the intrinsic performance and apparent quality of the product. Mold temperature depends on the crystallinity of plastic, product size and structure, performance requirements, as well as other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

Pressure control

During injection molding, pressure includes plasticizing pressure and injection pressure, and directly affects the plastic and product quality.

⒈ plasticizing Pressure: (back pressure) using screw-type injection machine, screw the top of the melt material in the screw rotation back when the pressure is called plasticizing pressure, also known as back pressure. The size of the pressure can be adjusted by the overflow valve in the hydraulic system. In the injection, the size of the plastic pressure is with the screw design, product quality requirements and different types of plastics to be changed, if these conditions and the speed of the screw are unchanged, then the increase in plasticizing pressure will enhance the shearing effect, that is, will increase the melt temperature, but will reduce the plasticizing efficiency, increase the current and leakage flow, increase the driving power.

In addition, the increase of plasticizing pressure can often make the melt temperature uniformity, the mixing of color materials and the discharge of gas in the melt. In general operation, the decision of plasticizing pressure should be in the premise of guaranteeing the quality of the product is good, the lower the better, the specific value of the plastic with the variety of different, but usually rarely more than 20 kg/cm.

⒉ injection pressure: In the current production, almost all injection presses are injected with plunger or screw on top of plastic

The pressure applied (by the oil circuit pressure conversion) is the same. The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, to give the rate of the melt filling and compaction of the molten material.

Molding cycle

The time required to complete an injection molding process is called the molding cycle, also known as the mold-plastic cycle. It actually includes the following parts: Molding cycle: Molding cycle directly affects labor productivity and equipment utilization ratio. Therefore, in the production process, should ensure the quality of the premise, as far as possible to shorten the molding cycle of the relevant time. During the whole molding cycle, the injection time and cooling time are the most important, and they have a decisive effect on the quality of the products. The filling time in injection time is directly proportional to the mold filling rate, and the mold filling time in production is generally about 3-5 seconds. The holding time in injection time is the pressure time of plastic in the cavity, the proportion of the injection time is larger, generally about 20-120 seconds (the extra thick workpiece can be up to 5-10 minutes). At the gate of the melt before freezing, the number of holding time, the accuracy of the product size, if in the future, there is no impact. The retention time also has the most value, and it is known to depend on the material temperature, mold temperature and the size of the sprue and gate. If the size and process conditions of the sprue and gate are normal, the pressure value of the minimum fluctuation range of the product shrinkage rate is usually obtained. The cooling time mainly depends on the thickness of the product, the thermal and crystallization properties of the plastics, and the mold temperature. The end of the cooling time, should be to ensure that the product demoulding without causing changes in principle, cooling time is generally about 30-120 seconds, cooling time is not necessary, not only to reduce production efficiency, the complex workpiece will also cause demoulding difficulties, forced demoulding will even produce release stress. Other time in the molding cycle is related to the continuity and automation of the production process and the degree of continuity and automation.