How to Calculate Injection Pressure in Injection Molding? A Practical Engineering Guide for Stable Production

A customer once asked a very direct question during a mold trial:

“We set the machine pressure, but why do we still get short shots in some cavities?”

This is one of the most common misunderstandings in injection molding. Many people assume that injection pressure is simply a machine setting. In reality, it is a calculated engineering parameter influenced by material flow behavior, mold design, gate size, temperature, and part geometry.

Injection pressure in injection molding is calculated based on the resistance required to fill the mold cavity, considering material viscosity, flow length, wall thickness, gate design, and temperature conditions. It is not a fixed value but a dynamic requirement that ensures molten plastic can fully and consistently fill every section of the mold without defects.

At Sanken, we work closely with injection molding and material converting systems in automotive and industrial applications, where pressure control directly impacts part quality, dimensional stability, and mass production consistency.

Why Injection Pressure Matters So Much

Injection pressure directly affects:

• Fill completeness (no short shots)
• Surface quality
• Dimensional accuracy
• Internal stress
• Cycle time stability
• Mold life

If pressure is too low → incomplete filling and defects.
If pressure is too high → flash, stress, and mold damage.

So the goal is not maximum pressure — but correct pressure.

Basic Formula Concept of Injection Pressure

A simplified engineering expression is:

Injection Pressure ≈ Flow Resistance × Flow Length / Wall Thickness

In practical terms:

• Longer flow distance → higher pressure required
• Thinner wall → higher pressure required
• Higher viscosity material → higher pressure required

However, real injection molding is more complex because multiple variables interact simultaneously.

Key Factors That Determine Injection Pressure

1. Material Viscosity

Different materials flow differently:

Higher viscosity = higher injection pressure requirement.

2. Flow Length-to-Thickness Ratio (L/T)

This is one of the most important design parameters.

• L = flow length
• T = wall thickness

If L/T ratio increases:

• Flow resistance increases
• Pressure requirement increases sharply

For example:

• Short thick part → low pressure
• Thin long part → high pressure

3. Gate Design

Gate size and type significantly affect pressure.

• Small gate → high resistance → higher pressure
• Large gate → smoother flow → lower pressure
• Submarine gate → moderate resistance
• Fan gate → improved distribution

Poor gate design is one of the main reasons for pressure instability.

4. Mold Temperature

Higher mold temperature:

• Improves flow
• Reduces viscosity
• Lowers required pressure

Lower mold temperature:

• Increases resistance
• Requires higher pressure

5. Melt Temperature

Hotter melt:

• Flows easier
• Reduces pressure requirement

However, overheating may cause:

• Material degradation
• Flash issues
• Dimensional instability

How Engineers Estimate Injection Pressure in Practice

In real production environments, engineers do not rely only on formulas.

They use a combination of:

1. Simulation (Mold Flow Analysis)

CAE software estimates:

• Flow front behavior
• Pressure distribution
• Filling balance
• Potential defects

This is the most accurate pre-production method.

2. Trial Molding Adjustment

During mold trials:

• Start with lower pressure
• Gradually increase until full fill is achieved
• Observe part quality changes

Key indicators:

• Short shot elimination
• Flash appearance
• Surface quality stability

3. Machine Monitoring Data

Modern injection machines provide:

• Peak injection pressure
• Holding pressure
• Screw position curves
• Fill time data

These values help optimize repeatability.

Simplified Engineering Example

Let’s say:

• Material: ABS
• Thin wall part
• Long flow path
• Small gate

Expected result:

• High flow resistance
• Medium-to-high injection pressure required

If defects occur:

• Increase melt temperature slightly
• Optimize gate size
• Adjust injection speed profile
• Improve mold temperature balance

This is why pressure cannot be treated as a single fixed number.

Common Mistakes in Injection Pressure Control

Mistake 1: Using excessive pressure to fix design issues

→ Leads to flash and stress

Mistake 2: Ignoring material viscosity differences

→ Causes inconsistent production

Mistake 3: Poor gate design compensation through pressure

→ Unstable filling behavior

Mistake 4: Not balancing temperature and pressure

→ Leads to unpredictable results

Why Injection Pressure Is Linked to Part Quality

Injection pressure affects micro-level structure inside the part:

• Molecular alignment
• Shrinkage behavior
• Warpage tendency
• Surface finish quality

That means pressure is not just a machine setting — it is a material behavior control parameter.

Why This Matters for Industrial Manufacturing

In high-volume production such as:

• Automotive components
• Electronic housings
• Precision molded parts
• Functional structural components

Small pressure variations can cause:

• Batch inconsistency
• Assembly issues
• Dimensional drift
• Increased rejection rate

At Sanken, we understand this connection because injection molding often integrates with:

• Precision die cutting
• Foam converting
• Adhesive assembly systems
• Multi-material integration

All of which require stable dimensional control.

How Professional Manufacturers Optimize Injection Pressure

Experienced manufacturers do not rely on pressure alone.

They optimize the entire system:

• Material selection
• Mold design
• Gate engineering
• Cooling system design
• Injection speed profile
• Holding pressure strategy

This holistic approach ensures stable production rather than reactive adjustments.

Conclusion

Injection pressure in injection molding is not a simple machine setting but a calculated result of material behavior, mold design, flow resistance, and processing conditions. Understanding how to estimate and optimize injection pressure is essential for achieving stable quality, reducing defects, and ensuring scalable production. At Sanken, we support manufacturing customers through integrated engineering and precision processing solutions that help improve consistency across complex industrial and automotive applications.