Why There is Short Shot during Injection

The reasons for this outcome are numerous and can be systematically categorized into the following major groups:

1. Material and Process Parameters (Most Common)

This is the most frequently adjusted aspect during production.

• Insufficient injection pressure/velocity: This is the most direct cause. Inadequate pressure or velocity prevents the molten plastic from overcoming flow resistance (especially in thin-walled, long runners), failing to fill distant cavities before solidification.
• Low melt temperature/mold temperature: Temperature critically affects plastic flowability. Low melt temperature increases viscosity and reduces flowability; low mold temperature causes rapid cooling upon contact with mold walls, thickening the solidified layer, narrowing the effective runner, and ultimately causing blockage.
• Insufficient holding pressure/time: After the injection phase, the holding phase compensates for volume loss due to cooling and shrinkage. Inadequate pressure or insufficient duration leads to short shots in cavity ends, ribs, and other areas where shrinkage isn't fully compensated.
• Insufficient metering (shot volume): If the melt volume stored in the injection machine's barrel is inadequate to fill the entire cavity, short shots naturally occur.

2. Mold Design and Structure

Molds form the foundation of molding, and design issues often cannot be fully resolved through process adjustments alone.

Poor runner system design:

•  Insufficient runner/gate dimensions: Creates significant flow resistance, causing pressure to dissipate entirely within the runner.
•  Improper gate location: Results in excessively long melt flow paths or uneven filling.
• Poor cold runner design: Low-temperature material (cold runner) at the nozzle tip enters the cavity, rapidly blocking the flow front.
• Inadequate venting: Insufficient or blocked vents at the cavity end or weld line area trap air, forming high-pressure air pockets that prevent complete filling. This often causes scorching (air-trapped scorch) in underfilled areas.
• Unbalanced cavity layout: In multi-cavity molds, if the runner system is not balanced, cavities cannot fill simultaneously, making distant cavities prone to short shots.

Product design issues:

• Excessively thin walls: Demands extremely high plastic flowability.
• Dramatic wall thickness variations: Thick walls cool slowly while thin walls cool rapidly, causing premature solidification of melt as it flows through thin sections.

3. Equipment and Operation Issues

•  Injection molding machine performance problems: Nozzle clogging, wear of check valves (sprue rings) causing leakage during injection, unstable hydraulic system pressure, etc.
• Mold Maintenance Issues: Vent channels clogged by oil residue or debris; worn ejector pins or slides causing flash (sprue), leading to localized pressure leakage.
• Material Issues: Using incorrect plastic grades (incompatible flow properties), raw material contaminated with impurities, excessive recycled material content, or insufficient drying (moisture content) all adversely affect flow and filling.

Standard Troubleshooting Approach (“From Simple to Complex”)

When short shots occur, field engineers typically follow this sequence for inspection and adjustment:

First, check the most obvious factors: Is the shot volume setting sufficient? Is the material properly dried? Are the mold vent holes clean?

Significantly adjust process parameters: Gradually increase melt temperature, mold temperature, injection pressure, and injection speed. This is the fastest and most commonly used method. If short shots improve after these increases, the issue likely lies here.

Check holding pressure: Observe if the underfilled areas are near the gate or in thick-walled sections. Try increasing holding pressure and holding time.

Inspect equipment and mold: Verify nozzle patency and check mold for wear/damage. Perform a dry shot to evaluate melt flow characteristics.

Revisit mold design: If all process adjustments yield limited results and the issue persists, mold modifications may be necessary—such as enlarging gates/runners, adding vent channels, or adjusting cooling channels.

In summary, short shots/short shots result from an imbalance between plastic “flowability” and “cooling rate.” Any factor reducing flowability or causing premature cooling may trigger short shots. The solution lies in systematically analyzing five key areas—material, machine, mold, process, and product design—to identify the most critical limiting factor.