Introduction
You open the mold. The part looks incomplete. One section is missing. The plastic did not reach the far end of the cavity. This is a short shot.
Short shots are among the most common defects in injection molding. They waste material, energy, and machine time. If 20% of your parts are short shots, you lose 20% of your production capacity to defective products. That is lost revenue and higher costs.
Understanding why short shots happen is the first step to eliminating them. This guide covers the causes—material flow issues, gate design problems, air trapping, and machine-related factors. It then provides practical solutions you can apply today.
What Is a Short Shot?
A short shot occurs when molten plastic fails to fill the mold cavity completely. The final product is incomplete. Parts of the intended shape are missing or under-formed.
A plastic toy with a complex shape might have one appendage partially formed or completely absent. An electronic device housing may have a section missing, making it impossible to assemble internal components.
Impact on Product Quality
Short shots create products that do not meet dimensional specifications. In an electronic housing, a short shot causes misalignment. Internal components will not fit properly.
Structural integrity is also compromised. Areas with insufficient material are weak. They break more easily under normal use or stress testing.
Impact on Production Efficiency
Every short shot wastes raw materials, energy, and machine time. If your line produces 1,000 parts per hour but 200 are short shots, your usable output drops by 20%. You spend extra time identifying and discarding defective parts.
Frequent short shots require machine adjustments and troubleshooting. This disrupts production flow and reduces overall efficiency.
What Causes Insufficient Material Flow?
Material flow is critical. When flow is insufficient, short shots follow.
High Material Viscosity
Some plastics have naturally high viscosity. High-performance engineering plastics like polyphenylene sulfide (PPS) flow less easily than common plastics like polyethylene (PE) .
A study found that PPS processed at 300°C had a flow length only 50% to 60% of PE processed at 200°C under the same injection pressure and time.
Low Melt Temperature
As melt temperature increases, viscosity decreases. Flow improves.
For most thermoplastics, a 10°C to 15°C increase in melt temperature leads to a 10% to 20% increase in flow length. But be careful. Too high a temperature causes material degradation.
Low Injection Pressure
Insufficient injection pressure means the melt cannot overcome resistance in the mold. It never reaches all areas of the cavity.
In a complex mold with thin-walled sections, pressure below 80 MPa may cause short shots. Increasing to 100 MPa to 120 MPa often ensures complete filling.
How Does Gate Design Cause Short Shots?
The gate is the passage through which molten plastic enters the cavity. Its design is critical.
Gate Size Too Small
A gate that is too small restricts flow. In one example, a mold with a 0.8 mm gate diameter produced short shots in 70% of runs. Increasing the gate to 1.2 mm dropped the rate to 20% .
The table below shows the relationship:
| Gate Size (mm) | Short Shot Probability for Material A (%) | Short Shot Probability for Material B (%) |
|---|---|---|
| 0.8 | 70 | 65 |
| 1.0 | 50 | 45 |
| 1.2 | 20 | 15 |
Poor Gate Location
Gate location determines how far the plastic must travel. If the gate is at one end of a long, thin-walled cavity, the plastic may cool and solidify before reaching the far end.
A gate placed strategically—such as at the center—improves flow. For long, rectangular parts, placing the gate on a long side instead of a short end reduces travel distance.
Insufficient Gates
Complex shapes or large parts may need multiple gates. A case study of a large plastic panel showed that increasing from one gate to three gates completely eliminated short shots.
What Role Does Air Trapping Play?
Air trapped in the mold prevents plastic from filling completely.
How Air Traps Form
As molten plastic enters the mold, it displaces air. If there is no way for the air to escape, it gets trapped. In complex geometries—deep recesses or sharp corners—air pockets form.
Real-World Example
In production of plastic toys with intricate internal structures, air was trapped in internal cavities. Short shots occurred frequently. The missing sections made the toys non-functional because critical features were incomplete.
What Machine Issues Cause Short Shots?
Low Injection Speed
If injection speed is too low, the melt cools during its journey through the mold. For a medium-sized part, speed below 30 mm/s can cause a 40% chance of short shots. Increasing speed to 50 mm/s to 60 mm/s drops the probability to 10% to 15% .
Worn Screw
A worn screw cannot convey and compress material effectively. A screw with 20% wear reduces conveying capacity by 15% to 20% . Less material reaches the cavity. Short shots follow.
What Are Effective Solutions?
Material Adjustments
Increase melt temperature: For most thermoplastics, raising temperature by 10°C to 20°C improves flow. If polyethylene is running at 200°C, try 210°C to 220°C. Monitor for degradation. Polycarbonate degrades above 320°C to 330°C for common grades.
Switch to lower-viscosity material: If temperature adjustment does not work, consider a material with better flow. A company using high-viscosity polypropylene (PP) for a complex part switched to lower-viscosity PP. Short shot rate dropped from 30% to under 5% .
Gate Design Optimization
Increase gate size: A general rule: gate diameter should be 1% to 1.5% of the largest part dimension. For a 100 mm part, gate diameter of 1 mm to 1.5 mm works as a starting point. Adjust based on material and mold complexity.
Optimize gate location: Place gates where plastic can flow evenly throughout the cavity. For long rectangular parts, center placement on a long side works better than placement on a short end.
Add multiple gates: For complex shapes or large parts, use multiple gates. The large panel example showed three gates eliminated short shots.
Venting Improvements
Add exhaust grooves: Place grooves where air is likely trapped—end of flow paths, complex geometries. Typical groove depth: 0.02 mm to 0.04 mm . Width: 5 mm to 10 mm .
In a housing mold with internal ribs, adding exhaust grooves with 0.03 mm depth and 8 mm width reduced short shot rate from 35% to 15% .
Use breathable steel: Breathable steel has tiny pores—1% to 5% porosity by volume. It allows air to escape while preventing plastic seepage. One company using breathable steel for intricate components nearly eliminated short shots.
Machine Maintenance and Parameter Tuning
Regular maintenance: Replace worn parts like screws, barrels, and seals. Replacing a screw with 30% wear reduced short shot rate from 25% to 5% .
Adjust injection speed: Increase speed if too low. For ABS parts, increasing speed from 30 mm/s to 50 mm/s reduced short shot probability from 30% to 10% . Do not go too high—excessive speed causes flash or jetting.
Adjust injection pressure: Increase pressure by 10% to 20% if short shots occur. If initial pressure is 80 MPa, try 88 MPa to 96 MPa. Stay within machine and mold limits.
The table below summarizes solutions:
| Cause | Solution |
|---|---|
| High viscosity | Increase melt temperature; switch to lower-viscosity material |
| Small gate | Increase gate size |
| Poor gate location | Optimize placement; add multiple gates |
| Air trapping | Add exhaust grooves; use breathable steel |
| Low injection speed | Increase speed |
| Worn screw | Replace screw; regular maintenance |
| Low injection pressure | Increase pressure within limits |
What Does a Real-World Example Look Like?
A manufacturer produced plastic caps with a persistent short shot problem. The defect rate was 40% . Investigation revealed the gate diameter was 0.8 mm—too small for the material.
The gate was increased to 1.2 mm. The short shot rate dropped to 10% . Additional venting improvements—adding exhaust grooves at the end of flow paths—brought the rate down to 2% .
Production output increased by nearly 40% without adding machine time. Material waste dropped significantly. The solution paid for itself within weeks.
Conclusion
Short shots are preventable. They happen when material flow is insufficient, gate design is poor, air is trapped, or machine parameters are off.
Solutions exist for each cause. Adjust material temperature or switch to lower-viscosity materials. Optimize gate size, location, and number. Improve venting with exhaust grooves or breathable steel. Maintain machines and tune injection speed and pressure.
A systematic approach—identifying the specific cause and applying the corresponding solution—eliminates short shots. The result is higher quality, less waste, and more efficient production.
FAQ
What is the most common cause of short shots in injection molding?
Insufficient material flow is the most common cause. This can result from low melt temperature, high material viscosity, or low injection pressure. Material flow issues account for the majority of short shot cases in typical production environments.
How do I know if my gate size is causing short shots?
Check if short shots occur in areas farthest from the gate. Measure gate diameter against part dimensions. A gate diameter less than 1% of the largest part dimension may be too small for many materials. Increase gate size incrementally and monitor short shot rate.
Can air trapping cause short shots even with proper gate design?
Yes. Air trapping occurs independently of gate design. If air cannot escape from the cavity, it blocks plastic flow. This is common in molds with deep recesses, sharp corners, or complex internal structures. Proper venting is essential regardless of gate design.
What injection speed should I use to prevent short shots?
Optimal speed depends on material and part geometry. For many medium-sized parts, speeds between 50 mm/s and 60 mm/s work well. Start at lower speeds and increase until short shots stop. Watch for flash or jetting at higher speeds.
How often should I check machine components for wear?
Inspect screws, barrels, and seals during scheduled maintenance. For high-volume production, check every 3 to 6 months. A worn screw reduces conveying capacity and can cause short shots before other symptoms appear.
Contact Yigu Technology for Custom Manufacturing
At Yigu Technology , we solve short shot problems every day. Our engineers conduct flow analysis before building molds to identify potential issues early. We optimize gate design, venting, and cooling to prevent short shots.
We also maintain our machines rigorously. Worn components are replaced before they affect production. Our operators are trained to tune parameters—temperature, pressure, speed—for each job.
If you are experiencing short shots or other injection molding defects, we can help. We deliver high-quality, consistent parts for automotive, medical, electronics, and industrial applications.
Contact Yigu Technology today to discuss your injection molding project.
