Short shot, also known as inadequate filling or short shot, refers to the phenomenon where the polymer does not completely fill all corners of the mold cavity. The causes of short shots and related solutions are as follows: 

1. Unreasonable selection of injection molding equipment When choosing an injection machine, the maximum injection volume of the machine should be greater than the weight of the product (including the molded part, runner, and flash). To achieve better results, the total injection volume should be kept below 85% of the maximum injection volume.

 2. Poor flow performance of the polymer In this case, additives should be added to the raw materials in appropriate amounts to improve the flow performance of the resin. Additionally, check the proportion of recycled materials in the raw materials and reduce the amount appropriately. Alternatively, consider improving the mold's injection system by adjusting the position of the runners, increasing the sizes of the gates, runners, and nozzles, in order to improve the flow defects in the mold's injection system. 

3. Actual unreasonable injection system A reasonable injection system considers the balance of gates and runners, with the cavity volume in each mold cavity proportional to the size of the gate, allowing the polymer to fill all cavities simultaneously. The gate position should be chosen at thick wall areas, and a balanced distribution design of the diverging runner layout can be adopted. In cases where the gate or runner is small, thin, or long, the pressure loss of the melt during flow is too great, causing flow resistance and easily leading to short shots. For this situation, the cross-section of the runner and the gate area should be enlarged, and multi-point feeding methods can be employed if necessary.

4. The material temperature and mold temperature are too low

Generally, there is a close positive proportional relationship between material temperature and filling length. Lower material temperature will decrease the flowability of the melt, resulting in a shorter filling length. When it is confirmed that the material temperature is low, it is necessary to check whether the barrel heater is intact and try to increase the barrel temperature.

If low-temperature injection is unavoidable to prevent melt decomposition, the temperature of the front section of the barrel can be appropriately increased, or the injection speed can be accelerated to reduce the cooling time of the polymer. However, it should be noted that too fast an injection speed may cause melt fracture, resulting in wrinkle defects.

A lower mold temperature can cause the molten polymer to cool prematurely, preventing it from filling the entire mold cavity. To address this situation, the mold should be heated to a temperature that meets the process requirements, and during the initial stage of injection molding, the flow rate of the cooling medium should be reduced. If the mold temperature remains low, the design of the cooling system should be changed. 5. Low injection nozzle temperature

During the injection process, the nozzle comes into direct contact with the mold. Since the mold temperature is generally lower than the nozzle temperature, and there is a significant temperature difference, frequent contact between the two causes the nozzle temperature to drop, leading to condensation of the melt at the nozzle (also known as dry tip).

To prevent the cold material from solidifying immediately after entering the mold cavity and blocking the subsequent hot melt material from filling the cavity, it is advisable to consider creating a cold material pocket on the runner. Additionally, during mold opening, ensure that the nozzle is separated from the mold to minimize the impact of mold temperature on nozzle temperature. 6. Insufficient injection pressure and holding pressure

The relationship between injection pressure and filling length is also close to being proportional. A low injection pressure will result in a short filling length, leading to under-injection.

In this case, the injection pressure can be increased by slowing down the forward speed of the injection rod and appropriately extending the injection time. If the injection pressure cannot be further increased, the melt viscosity can be reduced by appropriately increasing the material temperature (at this time, the material temperature should not cause the melt to decompose), thereby improving the melt flowability to make up for the deficiency.

In addition, if the holding time is too short, under-injection may occur. Therefore, selecting an appropriate holding time (generally controlled between 30~120s, and can be appropriately increased for thick-walled products) can prevent under-injection. At the same time, it should also be noted that an excessively long holding time can also cause difficulties in the automatic detachment of the product. 7. Irrational structural design of the product

When the overall dimensions of the product are disproportionate (such as the thickness being disproportionate to the length), or when the shape of the product is complex and the molding area is large, the melt is prone to flow resistance at the thin walls of the plastic part, resulting in insufficient filling. Therefore, when designing the physical structure of the plastic part, attention should be paid to the relationship between the thickness of the plastic part and the maximum flow length during melt filling.

In injection molding, the thickness of plastic parts is generally 1~3mm, and for large plastic parts, it is 3~6mm. Generally, plastic parts with a thickness exceeding 8mm or less than 0.5mm are not conducive to injection molding.

In addition, when molding complex structural plastic parts, necessary measures need to be taken in terms of process, such as appropriately adjusting the runner layout, reasonably determining the gate position, increasing the injection speed, raising the mold temperature and material temperature, and selecting resins with better flowability. 8. Poor ventilation

Poor venting can cause a large amount of gas to remain trapped within the mold cavity, which can then be squeezed by the flowing material, generating significant pressure. When this pressure exceeds the injection pressure, it can hinder the filling of the mold with melt, leading to under-injection.

In response to this phenomenon, it is necessary to check whether the cold material cavity is set up or whether its position is reasonable. For deep cavity molds, exhaust grooves or exhaust holes should be set up at the locations where under-injection occurs. The exhaust grooves can be opened on the parting surface of the mold, with a depth of generally 0.02~0.04mm and a width of 5~10mm, while the exhaust holes should be set up at the final filling position of the cavity. For polymers that are prone to volatilization or have high moisture content, volatile components should be removed or drying treatment should be carried out before injection molding.

The phenomenon of poor exhaust can also be improved by changing the process parameters of the mold system. For example, measures such as increasing mold temperature, reducing injection speed, decreasing clamping force, and increasing mold clearance can be taken.