What is a short shot injection molding?
As the name implies, a “short shot” is when the mold stops short of completely filling the mold cavity, leaving voids or thin areas, and creating a deficient product. These holes are commonly found in very thin areas of the mold or at the edges of the final product, the last places a mold would normally fill.
Short shot injection molding, also known as underfill, refers to the phenomenon of partial incompleteness at the end of the material flow, or partial underfill in multiple cavities of an injection mold.
In the thin-walled area, the end of the flow path is prone to the phenomenon of injection molding short shot, which is usually manifested by the melt condensing without filling the cavity; after the melt enters the cavity, it does not fill well, resulting in a lack of material in the product.
The main reason for the short shot is excessive flow resistance, which will prevent the melt from continuing to flow.
The main factors affecting the length of melt flow are the wall thickness of the part, injection mold temperature, injection molding pressure, melt temperature, material composition, etc. If you cannot handle these situations well, it will cause a short shot injection molding.
Below are the 7 reasons cause the short shot injection molding
Improper equipment selection
In the selection of equipment, the injection volume of the injection molding machine needs to be greater than the total weight of the plastic parts and water spout, and the total weight of the injection cannot be greater than 85% of the plasticized volume of the injection molding machine.
Insufficient material supply
At present, the commonly used way to control the feeding is the fixed volume feeding method. The volume of its roll material and whether the grain size of the raw material is uniform, the bottom of the filling port has no bridging phenomenon.
If the temperature at the filling mouth is relatively high, it also leads to poor falling material. In order to cope with such a situation, the filling mouth should be unclogged and cooled.
If the raw material formula, the amount of lubricant is too much, and when the injection screw stops ring and the barrel wear gap is large, the molten material in the barrel backflow, serious will cause a lack of material supply, resulting in a short shot.
In order to cope with this phenomenon, the amount of lubricant should be reduced, adjust the barrel and injection screw and stop ring clearance, timely maintenance of equipment.
Injection mold in the production process of the pouring system design does not meet the requirements
In a mold with multiple cavities, the appearance of plastic parts is often defective because the balance and design of the gate and sprue do not meet the requirements. When designing the pouring system, pay attention to the gate balance.
The weight of the parts in each cavity should be proportional to the size of the gate so that each cavity can be filled at the same time. The location of the gate should be chosen at the thick wall, and the design scheme of balanced arrangement of the manifold can also be used.
If the gate or runner is small, thin, or long, the pressure of the fluff will be lost too much during its flow, so that the flow is hindered and it will be easy to produce poor filling. In order to cope with such a situation, the cross-section of the runner and the area of the gate should be enlarged, and the method of multi-point feeding can also be used according to the actual situation.
Bad exhaust of the injection mold
When a large amount of gas remains in the mold due to poor exhaust, it will be squeezed by the flowing material and produce a high pressure greater than the injection pressure, thus preventing the molten material from filling the cavity and causing a short shot.
In order to cope with this situation, we should check carefully whether there is a cold cavity set or whether its position is correct. For molds with deep cavities, we should add exhaust grooves and exhaust holes in the short shot area.
It is possible to open an exhausted groove of 0.02~0.04mm deep and 5~10mm wide on the closing surface of the mold, and the exhaust hole should be set at the mold filling of the cavity.
When using raw materials with too much moisture or volatile substances, it will also produce a lot of gas, which will lead to poor exhaust of the injection mold, we should dry the raw materials or clean up the volatile substances in the raw materials in time.
In addition, the mold production process can increase the mold temperature, slow down the injection speed, reduce the pouring system flow booster, reduce the mold force, increase the mold gap, etc., a good solution to the poor exhaust situation.
Nozzle temperature is too low
In the injection molding process, the nozzle is in contact with the mold. As the mold temperature is usually lower than the nozzle temperature, and the temperature difference is large. After frequent contact between the two, the temperature of the nozzle will drop, which will lead to melting freezing at the nozzle.
If there is no cold cavity in the mold structure, the cold material will solidify when it enters the cavity, so that the hot melt that helps to plug in the back cannot fill the cavity. Therefore, when opening the mold, it is necessary to separate the nozzle from the mold, so that the influence of the mold temperature on the nozzle temperature is reduced and the nozzle temperature meets the requirements.
If the nozzle temperature is very low and cannot rise, we need to check whether the nozzle heater is damaged, and then find a way to raise the nozzle temperature. Otherwise, the pressure loss of the flow material is too large, which will also cause a short shot.
Injection molding speed is too slow
Injection molding speed will have some influence on the mold filling speed, if the injection speed is too slow, the melt will fill the mold slowly. And the low-speed flow of the melt is easy to cool down, so that its flow properties further decline, resulting in a short shot.
In order to cope with such a situation, we need to increase the injection speed appropriately. However, we need to pay attention to the fact that if the injection speed is too fast, it will cause other failures.
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