Plastic Molds

A short shot refers to the phenomenon where plastic molds are incompletely filled due to insufficient plastic flow, resulting in incomplete plastic products. The main reason is improper injection pressure and speed (including excessive pressure loss caused by resistance). This is primarily influenced by the following aspects:

I. Injection Molding Machine

Insufficient Injection Capacity – This can be caused by either an overly high capacity of the injection molding machine or insufficient plasticizing capacity or injection volume. To increase the insufficient plasticizing capacity, the heating time can be extended, the screw speed increased, and the back pressure raised. If the injection volume is not high enough, an injection molding machine with a larger injection capacity can be used to solve the problem.

II. Improper Plastic Mold Design

Local Short Shots Caused by Unbalanced Flow in Each Cavity of Multi-Cavity Molds – When the injection molding capacity of the machine is sufficient, this defect is mainly caused by uneven flow in each gate, i.e., uneven distribution of the mold cavities. Excessive Melt Flow Range and High Flow Resistance – Obstacles to melt flow include the nozzle, gate, runner, 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 low flow resistance. Poor Ventilation – When filling the mold cavity, air can be trapped, creating a reaction force. As the melt is injected into the cavity, the cavity is sealed by the melt from the beginning, trapping air in unfilled local areas. Similarly, due to the fast filling speed, sometimes the air does not have enough time to escape through the parting surface out of the mold but is compressed, resulting in partially unfilled areas in the cavity and shortening the production time of the molded part.

III. Improper Molding Process

Improper Injection Molding Process – Low barrel temperature, slow injection speed, short injection time, and insufficient back pressure lead to plastic shortage. Excess Plastic Supply – If too much plastic enters the barrel, injection pressure loss occurs due to particle compression, reducing the pressure required to inject the melt from the nozzle, which is necessary for injection molding, resulting in insufficient injection pressure. The solution is to adjust the feed rate, i.e., the melting volume, to exactly match the molding requirements. Unstable Production Cycle – Frequent machine stoppages and production that does not align with the normal cycle cause some plastic to stay in the barrel for too long, reducing density and viscosity, leading to short shots. Improper Plastic Mold Temperature Resulting in Low Injection Speed and Plastic Filling.

IV. Plastic Selection

Poor Plastic Flowability – If the plastic material does not have good flowability, it will solidify before reaching the farthest end of the cavity or flowing into the overflow tank, often resulting in insufficient flow. To eliminate these defects, the melt/mold temperature can be increased, and the injection pressure/speed can be raised to ensure the melt reaches the end of the cavity before solidifying. In this case, good plastic flowability is particularly important, so selecting a plastic with good flowability is also a solution. If the flow coverage is too long and the plastic cannot properly fill the injection-molded parts, it is recommended to change the gate position to reduce plastic flow.

V. Improper Product Design

During product design, the principle of uniform wall thickness must be followed. If uniform wall thickness cannot be maintained, the plastic mold design should be changed promptly. Increase the number of runners or ribs in areas with difficult feeding to avoid defects of insufficient filling due to overly thin walls. In summary, the causes of short shots are multifaceted and mutually influential. To reduce and correct this defect, it is necessary to comprehensively consider the relationships between these aspects, continuously practice, and accumulate experience to quickly determine the cause of insufficient filling, thereby reducing resource waste and increasing product output.