Shenzhen Alu Rapid Prototype Precision Co., Ltd.

In injection molding, a short shot (also called a short fill) occurs when the molten plastic fails to completely fill the mold cavity, resulting in incomplete parts with voids, thin areas, or missing sections (e.g., an unfilled corner or a truncated feature on a component). This defect can compromise part quality, strength, and functionality, often leading to scrap or rework. Short shots are common in plastic injection molding and stem from issues in material, machine settings, or mold design.

Primary Causes of Short Shots

Based on industry expertise, here are the most common causes, grouped by category for clarity. These are drawn from manufacturing guidelines and troubleshooting resources.

1. Insufficient Injection Pressure or Speed

Low injection pressure: The force applied by the machine is too weak to push the molten plastic into all areas of the mold, especially thin or remote sections.

Low injection speed: If the plastic is injected too slowly, it may begin to solidify (freeze) before fully filling the cavity, particularly in complex geometries.

Remedy: Gradually increase pressure and speed while monitoring for overpacking or flash (excess material).

2. Temperature-Related Issues

Low melt temperature: The plastic is too cool when injected, increasing its viscosity and reducing flowability, which prevents it from reaching the mold's extremities.

Low mold temperature: A cold mold accelerates cooling, causing the material to "freeze off" prematurely.

Remedy: Raise barrel, nozzle, or mold temperatures incrementally, ensuring the material doesn't degrade from overheating.

3. Material Properties and Variations

High material viscosity: Thick or overly viscous resin (due to incorrect grade, moisture, or degradation) resists flowing through gates and cavities.

Material inconsistencies: Variations in batch quality, contamination, or insufficient drying can alter flow characteristics.

Remedy: Verify material specs, dry pellets properly, and test with a different batch if needed.

4. Mold Design and Venting Problems

Inadequate venting: Trapped air in the mold creates backpressure, blocking plastic flow and causing incomplete fills.

Poor gate design or location: Undersized, blocked, or poorly placed gates restrict material entry; thin wall sections exacerbate this.

Mold geometry issues: Complex shapes, sharp corners, or uneven wall thicknesses hinder uniform filling.

Remedy: Improve vents (e.g., add or enlarge them), optimize gate size/placement via Design for Manufacturability (DFM) reviews, or adjust wall thicknesses.

5. Machine or Process Malfunctions

Mechanical failures: Worn components, leaks, or blockages in the machine or mold (e.g., foreign debris in gates).

Incorrect process parameters: Mismatched settings for shot size, cushion, or hold time.

Remedy: Perform routine maintenance, clean molds, and validate settings against a process sheet.

Addressing short shots systematically—starting with process tweaks before mold changes—can resolve most issues efficiently.