Short shot, also called under-fill, insufficient filling, plastic parts are not full, refers to the end of the material flow part of the incomplete phenomenon or a part of a mold multi-cavity product filling is not full , especially in thin-walled areas or at the end of the flow path. It is manifested as the solidification of the melt without filling the cavity, resulting in a shortage of material in the product.
The main reason for short shot is that the flow resistance is too high, resulting in the melt can not continue to flow. Factors affecting flow include:wall thickness of plastic parts, mold temperature, injection pressure, melt temperature and material composition.
The reasons for the shortage of plastic parts are as follows:
1、improper selection of injection molding machine equipment
When selecting plastic injection machine, the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic parts and the gate, and the total weight of the injection cannot exceed 85% of the plasticizing capacity of the injection molding machine.
2、2、2、Insufficient supply of plastic injection molding materials.
Currently used to control the charging method is a fixed-volume charging method, the amount of roll material and raw materials, whether the fruit of the warp is uniform, charging the bottom of the mouth with or without “bridging” phenomenon. If the temperature at the mouth of the feeding is too high, it will also cause the falling material is not smooth. In this regard, should be dredged and cooled charging port.
3、Poor fluidity ofinjection molding plastic materials.
When the fluidity of injection molding plastic materials is poor, the structural parameters of the mold are the main reason that affects short shot. Therefore, the flow stagnation defects of the mold casting system should be improved, such as reasonable setting of the injection gate position, enlargement of the injection port, runner and injection port size, or use larger nozzles. At the same time, an appropriate amount of additives can be added to the raw material formula to improve the flow performance of plastics.
If the amount of lubricant in plastic injection molding raw material is too much, and the injection screw non-return ring and barrel wear gap is large, the melt in the barrel will cause insufficient material supply, resulting in under-injection. In this regard, should reduce the amount of lubricant and adjust the barrel and injection screw and check ring gap, repair equipment.
5、Blockage of cold material impurities in material channel
When impurities in the melt block the nozzle or cold material blocks the gate and runner, the nozzle should be folded down for clean up or expand the mold cold material hole and runner cross-section.
6、Unreasonable design of shot plastic injection molding.
In the case of multi-cavity molds, the appearance defects of plastic parts are often caused by the unreasonable design of the gate and runner balance. When designing the casting system, should paid attention to the balance of injection gate, and weight of plastic parts in each cavity, so that each cavity can be filled at the same time. The gate position should be selected in the thick wall, or a design scheme with a balanced distribution of the runner can be used.
7、Poor exhaust of injection mold plastic.
When a large amount of gas remaining in plastic injection moldings due to poor exhaust is squeezed by the flow material,resulting in a high pressure greater than the injection pressure, it will prevent the melt from filling the cavity resulting in short shot. In this case, should check whether there is a cold material cavity or whether its position is correct. For molds with deep cavities, exhaust grooves or vent holes should be added to the short shot area. On the mold closing surface, a vent groove with a depth of 0.02~0.04mm and a width of 5~10mm can be opened, and the vent hole should be set at the final filling position of the cavity.
8、Mold for plastic injection temperature is too low
After the melt enters the low-temperature mold cavity, it will be cooled too quickly and can’t fill all the corners of the cavity. Therefore, before starting the mold must be preheated to the temperature required by the process, just open the injection molding machine, should be appropriate to control the amount of cooling water through the mold. If the mold temperature does not go up, should check the mold cooling system design is reasonable.
9、Melt temperature is too low
When plastic injection molding raw material temperature is lower than the process requirements of the temperature, should check the barrel feeder is intact and try to improve the barrel temperature. Just started, the barrel temperature is always lower than the barrel heater instrumentation to indicate the temperature, it should be noted that the barrel will be heated to the instrumentation temperature after a period of time is needed to start.
10、Nozzle temperatureof Plastic injection molding toolingis too low
In plastic injection molding manufacturing process, the nozzle is in contact with plastic mold, because the mold temperature is generally lower than the nozzle temperature, and the temperature difference is large, the two frequent contact will make the nozzle temperature drop, resulting in the melt in the nozzle at the freezing.
If the nozzle temperature is very low and can not rise up, should check whether the nozzle heater is damaged, and try to improve the nozzle temperature, otherwise, the pressure loss of the flow material is too large will also cause under-injection.
11、Insufficientpressure of injection plastic moldingor holding pressure
The injection pressure is proportional to the filling length of the cavity. If the injection pressure is too small, the filling length of the cavity will be short, and the cavity will not be filled. In this case, plastic injection pressure can be increased by slowing down the injection speed and appropriately extending the injection time. When the injection pressure cannot be further increased, the viscosity of the melt can be reduced by increasing raw material temperature and improving the flow performance of the melt.
12、plastic injection molding manufacturingspeed is too slow
Injection speed is directly related to the mold filling speed. If the injection speed is too slow, the melt is slow to fill the mold, and low-speed flow of the melt is easy to cool down, so that its flow properties further decline in shote injection. Should be appropriate to improve injection machine speed.
13、Unreasonable design of injection molded plastic components structure
When the thickness and length of injection molded plastic components are not proportional, shape is very complex, and molding area is very large, the melt is easily blocked at the entrance of the thin-walled part of the plastic part, making it difficult to fill the cavity.
In injection molded plastic components, thickness of the plastic part is mostly 1~3mm, and thickness of large plastic parts is 3~6mm. Recommended minimum thickness is generally: polyethylene 0.5mm, cellulose acetate and butyrate plastic 0.7mm, ethyl cellulose plastic 0.9mm, polymethy, methacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm, and polyvinyl chloride 2.3mm. Generally, the thickness of the plastic part exceeding 8mm or less than 0.5mm.
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