What Causes Short Shot in Plastic Injection Molding?

Short Shot is in the top 3 of all statistics in injection moulding. Compared with the top ranking of the material flowers, mixed colour and other bad appearance, will only affect the use of the appearance, no impact on the use of the function, but the Short Shot will not only affect the appearance, but also affect the function, in some key parts of the Short Shot, will directly lead to the use of the failure.

Short shot is a phenomenon where the plastic mould cavity is not completely filled due to insufficient plastic flow, resulting in an incomplete plastic product. The main cause is improper injection pressure and speed (including excessive pressure loss caused by resistance), which is influenced by the following.

Injection molding machine

Insufficient injection capacity

This is caused by excessive injection machine capacity, which may also be caused by insufficient plasticising capacity or insufficient injection volume.

In this case, the insufficient plasticizing capacity can be increased by extending the heating time, increasing the screw speed and raising the back pressure. If the injection volume is not high enough, the problem can be solved by using an injection machine with a higher injection volume.

Improper design of plastic moulds

Localised short shot caused by unbalanced flow in each cavity of a multi-cavity mould

When the injection moulding machine has sufficient injection moulding capacity, this defect is mainly caused by uneven flow in each gate, i.e. uneven distribution of plastic mould cavities.

The flow range of the melt is too long and the resistance to flow is too high.

Areas that impede the flow of the melt include nozzles, gates, runners, and thin walls of the product. The flow resistance of the nozzle can be reduced by increasing the nozzle diameter/temperature and using a nozzle with a small flow resistance.

Poor venting

When filling a cavity, air can become trapped to create reaction forces. When the melt is injected into the cavity, the cavity is closed by the melt from the start and air is trapped in the unfilled local area.

Similarly, because the filling rate is so fast, sometimes the air does not have enough time to exit the plastic mould through the parting face and is instead compressed, resulting in a partially unfilled area in the cavity, thus reducing the production time of the moulded part.

Improper moulding process

Improper injection moulding process

Low barrel temperature, slow injection speed, short injection time, insufficient back pressure leading to shortage of plastic.

Excess supply of plastic

If too much plastic enters the barrel, there is a loss of injection pressure due to compression of the pellets, which reduces the pressure required to inject the melt from the nozzle, which is necessary for injection moulding, resulting in insufficient injection pressure. The solution is to adjust the feed volume, i.e. the melt volume, and make it just right for the moulding.

Unstable production cycles

Frequent machine stoppages and production that does not coincide with the normal cycle allow some plastics to remain in the barrel for too long, resulting in a reduction in density and viscosity, which leads to plastic short shots.

Inappropriate plastic mould temperatures leading to lower injection rates and plastic filling

Selection of plastics

Poor plastic flow

If the plastic material does not have great flow, it will solidify before reaching the farthest end of the cavity or flowing into the overflow tank, which usually results in an undershot.

To eliminate these defects, the melt mould temperature can be increased and the injection pressure rate increased so that the melt reaches the end of the cavity before it solidifies.

In this case, a large plastic flow is particularly important, so choosing a plastic with good flow is also a solution. If the flow coverage is too long and the plastic does not fill the injection moulded part correctly, it is recommended to change the gate position to reduce the plastic flow.

Inappropriate product design

During product design, the principle of uniform wall thickness must be followed. If it is not possible to maintain a uniform wall thickness, change the plastic mould design promptly.

Increase the number of runners or ribs in areas where feeding is difficult to avoid underfilling defects caused by too thin a wall thickness.

Poor exhaust of the mould

When a large amount of gas remains in the mould due to poor venting and is squeezed by the flow of material, a high pressure greater than the injection pressure is generated, which prevents the melt from filling the cavity and causes a short shot.

To counteract this, we should check carefully whether there are cold cavities set up or whether they are correctly positioned. For moulds with deep cavities, we should provide additional exhaust grooves and holes in the short shot area.

Exhaust grooves of 0.02 to 0.04 mm deep and 5 to 10 mm wide can be opened on the closing surface, and exhaust holes should be set at the mould filling of the cavity.

When using raw materials with too much moisture or volatile substances, a large amount of gas can be generated, which can lead to poor exhaust of the mould.

In addition, the production process of the mould can increase the mould temperature, slow down the injection speed, reduce the flow of the pouring system to help, reduce the closing force, increase the mould gap, etc., a good solution to poor exhaust.

Nozzle temperature too low

In the injection process, the nozzle is in contact with the mould. As the mould temperature is usually lower than the nozzle temperature, and the temperature difference is large.

Frequent contact between the two causes the nozzle temperature to drop, which leads to the melt freezing at the nozzle.

If there are no cold cavities in the mould structure, the cold material will solidify when it enters the cavity, preventing the hot melt, which is helping to plug behind it, from filling the cavity.

Therefore, when opening the mould, the nozzle needs to be separated from the mould so that the influence of the mould temperature on the nozzle temperature is reduced and the nozzle temperature meets the requirements.

If the nozzle temperature is very low and does not rise, we need to check if 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 and can also cause short shots.

Injection speed is too slow

The injection speed will have some influence on the mould filling speed. If the injection speed is too slow, the melt will fill the mould slowly.

The low flowing melt can easily cool down, which further reduces its flow properties and creates short shots.

To cope with this, we need to increase the injection speed appropriately. However, we need to be careful: if the injection speed is too fast, it can cause other failures.

Summary

The causes of short shot are multiple and interrelated and interact with each other. Reducing and correcting this defect requires a thorough consideration of the relationship between these aspects, constant practice and experience in order to quickly identify the causes of underfilling, thus reducing wasted resources and increasing product yield.