Injection Molding Short Shot: Why It Happens and How to Fix It

 

Injection Molding Short Shot: Why It Happens and How to Fix It

A short shot occurs when molten plastic fails to completely fill the mold cavity.

This defect is common in injection molding and can lead to:

• Incomplete parts
• Weak structures
• High rejection rates

 Short shots are often a sign of flow limitation or process imbalance

What Causes Short Shot? (Engineering Breakdown)

At its core, short shot is caused by:

Insufficient material flow before solidification

This typically comes from three key areas:

1. Material-Related Causes

• Low melt flowability
• Material too cold
• High viscosity plastics (e.g., PC, nylon)

 Result: plastic cannot reach the end of the cavity

2. Mold Design Causes

Poor Gate Design

• Gate too small restricts flow
  ➡️ See gate design guide:
  /gate-design/

Unbalanced Runner System

• Uneven flow to cavities
  ➡️ Learn about runner design:
  /runner-design/

Poor Venting

• Air trapped inside cavity blocks filling

3. Process-Related Causes

• Low injection pressure
• Low injection speed
• Low melt temperature
• Insufficient holding pressure

 These directly reduce flow capability

How to Diagnose Short Shot Quickly (Practical Guide)

Use this table to identify the root cause efficiently:

Symptom	Likely Cause	Recommended Solution
Missing at end of part	Low pressure	Increase injection pressure
Incomplete thin walls	Low melt temperature	Increase melt temperature
Random short shots	Poor venting	Improve mold venting
Only one cavity affected	Runner imbalance	Balance runner system
Hesitation marks before short shot	Gate restriction	Enlarge gate size

👉 This diagnostic approach helps reduce trial-and-error during production.

Proven Solutions to Fix Short Shot

1. Increase Injection Pressure and Speed

• Improves flow reach
• Helps fill thin sections

2. Optimize Melt and Mold Temperature

• Higher temperature improves flowability
• Prevents early solidification

3. Improve Gate and Runner Design

• Increase gate size
• Balance runner system

 Poor flow design is one of the most common root causes

4. Improve Venting System

• Allow trapped air to escape
• Prevent flow blockage

5. Adjust Part Design

• Reduce flow length
• Increase wall thickness where needed

 Design optimization often provides long-term solutions

Related Design Factors That Affect Short Shot

Short shot rarely occurs alone—it is often related to other design factors:

• Gate design affects flow entry and pressure
  ➡️ /gate-design/
• Runner system controls material distribution
  ➡️ /runner-design/
• Cooling system influences solidification timing
  ➡️ /cooling-system/
• Warpage issues may occur together due to stress imbalance
  ➡️ /warpage-solutions/

 Optimizing all these factors together improves overall molding stability.

Real Case: Eliminating Short Shot in Thin-Wall Parts

A client producing thin-wall electronic components faced frequent short shots.

Problem:

• Small gate size
• Low melt temperature
• Long flow path

Solution:

• Increased gate size
• Raised melt temperature
• Optimized injection speed

Result:

• Short shot eliminated
• Improved part consistency
• Reduced scrap rate significantly

DFM Tips from Our Engineering Team

To prevent short shot issues early, we focus on:

• Flow length vs thickness ratio
• Gate size and position
• Runner balance
• Material flow properties

 Early DFM analysis can prevent most filling issues before tooling begins

Frequently Asked Questions

What is the main cause of short shot?

Insufficient material flow due to low pressure, low temperature, or poor mold design.

How do I fix short shot quickly?

Start by increasing injection pressure and temperature, then check gate and venting design.

Does gate size affect short shot?

Yes. A small gate restricts flow and is a common cause of short shots.

Can material selection cause short shot?

Yes. High-viscosity materials are more prone to short shot defects.

Get Expert Help to Solve Short Shot Issues

Facing short shot problems in your production?

 Send us your:

• 3D CAD file
• Material information
• Defect photos

We provide:

• Free defect analysis
• Mold and design optimization
• Process improvement recommendations

Contact us today to eliminate short shot defects and improve production efficiency.