What Causes Short Shots in Injection Molding? Root Causes and Prevention

In the precision world of plastic injection molding, achieving a fully formed, flawless component every single cycle is the ultimate goal. However, manufacturing challenges can arise. One of the most common and frustrating defects encountered on the production floor is the short shot.

A short shot occurs when molten plastic resin fails to completely fill the mold cavity before it cools and solidifies, resulting in an incomplete, malformed plastic part.

At AluPrototype, we specialize in precision manufacturing, rapid prototyping, and custom injection molding. Understanding the root causes of short shots is essential for optimizing machine parameters, perfecting mold design, and ensuring consistent product quality. Here is a detailed breakdown of what causes short shots and how to prevent them.

1. Machine and Process Parameter Causes

Many short shots stem from incorrect machine settings or insufficient injection power during the production cycle:

• Insufficient Injection Pressure: If the hydraulic or electric pressure driving the screw forward is too low, the molten plastic lacks the force required to push through narrow runners, gates, and complex cavity corners.

• Low Injection Speed: Injecting the material too slowly allows the leading edge of the plastic melt to cool prematurely as it travels through the cold mold, increasing viscosity and halting flow.

• Inadequate Shot Size (Feed Settings): If the machine is set to feed too little material into the barrel, there simply won't be enough plastic volume available to completely fill the mold cavity.

• Low Melt or Mold Temperature: If barrel heater bands are set too low, the resin won't achieve optimal fluidity. Similarly, a cold mold can cause the plastic to freeze before the cavity is fully packed.

• Excessive Material Viscosity: Using a resin with a melt flow index (MFI) that is too low for a thin-walled or intricate part will restrict flow and cause incomplete filling.

2. Mold Design and Tooling Causes

Sometimes the physical architecture of the mold itself restricts material flow:

• Poor Gate Placement or Small Gate Size: Gates that are too restrictive or positioned incorrectly can bottleneck the flow of plastic, causing the material to lose pressure or freeze off too early.

• Inadequate Venting: If trapped air inside the mold cavity has nowhere to escape, it creates high back-pressure (air traps). This compressed air cushion can block the incoming plastic from completely filling the void.

• Imbalanced Runner Systems: In multi-cavity molds, if the runner layout is asymmetrical, plastic will reach some cavities faster and easier than others, starving the furthest cavities of material.

• Excessive Flow Lengths: Thin-walled parts with long flow paths from the gate to the extremities of the part are notoriously prone to short shots if wall thickness isn't optimized.

3. Material-Related Causes

The condition and type of raw polymer resin play a critical role in flow behavior:

• Material Degradation: Overheating certain polymers in the barrel can cause chain scission or cross-linking, altering viscosity and hindering flow.

• Moisture Contamination: Hygroscopic engineering plastics (like Nylon, PC, or ABS) that aren't properly dried before molding can generate steam bubbles and pressure inconsistencies inside the barrel, disrupting steady injection.

How AluPrototype Prevents Short Shots

Preventing short shots requires a proactive, engineering-first approach from the initial design phase through final production:

• Advanced DFM (Design for Manufacturing) Reviews: At AluPrototype, we evaluate wall thicknesses, gate locations, and flow paths before cutting any metal to ensure smooth, uninterrupted plastic flow.

• Rigorous Process Optimization: Our technicians meticulously calibrate injection speeds, pressures, and melt temperatures to match the specific rheological properties of your chosen polymer.

• Proper Mold Venting & Maintenance: We engineer optimal venting channels into every mold to eliminate air traps and ensure complete cavity packing.

Partner with AluPrototype for Flawless Injection Molding

Whether you are launching a new product line, transitioning from rapid prototypes to high-volume manufacturing, or troubleshooting persistent molding defects, expert guidance makes all the difference.

At AluPrototype, we deliver high-precision CNC machining, rapid tooling, and custom injection molding tailored to your exact specifications.Contact AluPrototype todayto discuss your project and get a fast, competitive quote!