Short Shots Injection Molding: Troubleshooting Causes & Solutions

Understanding Short Shots in Injection Molding

In the realm of plastic injection molding, a "short shot" is a critical quality defect where the molten plastic fails to completely fill the mold cavity. This results in an incomplete part that often lacks defined edges, prongs, or substantial sections of its geometry. Unlike minor surface blemishes, short shots typically render the product functionally useless, leading to significant material waste and production downtime. Identifying and resolving these issues immediately is essential for maintaining profitability and production efficiency.

Short shots occur when the flow of the polymer melt freezes off before the cavity is packed, or when the injection pressure is insufficient to push the material into the furthest reaches of the mold. While the visual symptom is simple—missing plastic—the root causes are often a complex interplay between material properties, machine parameters, and mold design. A systematic approach to troubleshooting is required to isolate the specific variable responsible for the failure.

Primary Causes of Short Shots

To effectively fix short shots, one must first categorize the potential origin of the problem. Generally, these causes fall into three distinct categories: material-related issues, mold design restrictions, and processing parameter errors.

Material Viscosity and Flow Properties

The flowability of the plastic resin is a fundamental factor. If the material is too viscous (thick), it requires higher pressure to push through the runners and gates. High-viscosity materials, or materials that have not been dried properly, often resist flow, cooling and solidifying before they reach the end of the cavity. Additionally, the use of regrind material can inconsistently alter the melt flow index (MFI), leading to unpredictable filling patterns.

Mold Design and Venting Constraints

The physical design of the mold plays a pivotal role in how plastic travels. "Gas traps" are a frequent culprit; as plastic fills the mold, the air inside must escape. If the venting is inadequate or blocked, the compressed air creates a pocket of high resistance that prevents the plastic from occupying that space. Furthermore, restrictive gate sizes or long, thin runner systems can cause excessive pressure drop, robbing the melt of the energy needed to fill the part completely.

Machine Process Settings

Even with the perfect mold and material, improper machine settings can guarantee failure. Common process-related causes include:

• Insufficient injection pressure or speed, preventing the melt from reaching the flow front quickly enough.
• Low barrel or nozzle temperatures, causing the plastic to cool prematurely.
• Inadequate shot size (cushion), meaning there is simply not enough volume of plastic injected to fill the cavity.

Step-by-Step Troubleshooting Guide

When a short shot occurs, operators should follow a logical sequence of adjustments to correct the defect without introducing new problems like flash or burning. It is recommended to change one variable at a time and observe the results for a few cycles.

1. Verify the Cushion and Shot Size

Check the "cushion" position on the injection molding machine. The cushion is the material remaining in the barrel after the injection phase. If the cushion is zero, the screw has bottomed out, meaning the machine physically cannot inject more material. In this case, increase the shot size (feed position) slightly to ensure there is enough material available to fill the mold and maintain holding pressure.

2. Increase Injection Speed and Pressure

Plastic cools rapidly once it touches the mold walls. If the injection speed is too slow, the flow front freezes before filling is complete. gradually increase the injection speed to push the material to the end of flow before it solidifies. If the machine is pressure-limited (hitting the maximum set injection pressure), increase the injection pressure limit to allow the screw to maintain the desired speed.

3. Inspect and Clean Vents

If the short shot consistently appears at the very end of the flow path or in a blind spot, check for "burn marks" or trapped gas. This indicates a venting issue. Clean the vents on the parting line to remove residue. If the problem persists, you may need to add new vents or vacuum venting to allow air to escape faster than the plastic enters.

4. Adjust Temperatures

Temperature affects viscosity. Raising the barrel temperature reduces the viscosity of the melt, allowing it to flow more easily. However, be cautious not to exceed the material's degradation limit. Similarly, raising the mold temperature helps keep the plastic molten for longer, facilitating flow into thin-walled sections.

Quick Reference Solution Table

The following table outlines common observations associated with short shots and their corresponding corrective actions.

Design Solutions for Long-Term Prevention

While process adjustments can solve immediate production issues, recurring short shots often point to a fundamental issue with the part or mold design. Addressing these during the DFM (Design for Manufacturing) stage or through mold modifications is the most robust solution.

• Wall Thickness Uniformity: Ensure that the part has consistent wall thickness. Molten plastic prefers the path of least resistance (thicker areas). If thin sections are located at the end of the fill, the material may hesitate and freeze. Increasing wall thickness in problematic areas can facilitate flow.
• Gate Placement: Locate gates to ensure the flow path is balanced and as short as possible. The material should flow from thick sections to thin sections, not the other way around.
• Runner System Sizing: Runners that are too narrow create high shear and pressure loss. increasing the diameter of runners and gates can significantly reduce the pressure required to fill the cavity.

Conclusion

Short shots in injection molding are a manageable defect if approached methodically. By distinguishing between machine limitations, venting issues, and material properties, manufacturers can quickly implement the correct fix. Regular preventive maintenance on molds—specifically keeping vents clear—combined with optimized process windows, will ensure consistent, high-quality production runs.