Injection Molding Clamping Force – Formula & Examples 2026

Introduction to Clamping Force

Clamping force calculation is the foundation of successful injection molding. This critical parameter determines whether your mold will stay closed during the high-pressure injection phase, directly impacting part quality, mold life, and production efficiency. In this comprehensive guide, we'll break down the exact formulas, provide step-by-step examples, and help you select the right Tederic machine for your application.

Whether you're a process engineer sizing a new mold or a production manager troubleshooting flash defects, understanding clamping force physics will save you thousands in scrap and downtime. We'll cover everything from the basic formula to advanced considerations like wall thickness effects and safety margins.

The Physics Behind Clamping Force

During injection molding, the molten plastic exerts tremendous pressure against the mold cavity walls. This pressure creates a separation force that tries to push the mold halves apart. The clamping force must be greater than this separation force to keep the mold closed and prevent flash defects.

The physics is straightforward: cavity pressure acts perpendicular to the projected area of the part. Every square inch of projected area generates a force equal to the cavity pressure multiplied by that area. The total clamping force required is the sum of all these individual forces across the entire part surface.

The Core Formula: F = P × A

The fundamental clamping force formula is elegantly simple:

F = P × A

Where:

• F = Clamping force (tons or kN)
• P = Cavity pressure (tons/in² or MPa)
• A = Projected area (in² or mm²)

This formula represents the minimum force needed to prevent mold opening. In practice, we add safety factors and material-specific multipliers to account for real-world variables like flow restrictions and pressure variations.

The Complete Engineering Formula

The more comprehensive formula used in industry is:

Tonnage = Projected Area (in²) × Clamp Factor (tons/in²) × Safety Factor

The clamp factor accounts for material viscosity, flow length, and processing conditions. Safety factors typically range from 1.1 to 1.5 to handle process variations.

Step-by-Step Clamping Force Calculation

Let's work through a practical example. We'll calculate the clamping force for a rectangular container measuring 6" × 4" × 2" tall with 0.125" wall thickness, molded in polypropylene.

Step 1: Calculate Projected Area

The projected area is the silhouette of the part when viewed from the parting line direction. For a rectangular box, this is simply length × width:

A = 6" × 4" = 24 in²

Step 2: Determine Clamp Factor

From material tables, polypropylene has a clamp factor of 2.5-3.5 tons/in². For this moderate-flow part, we'll use 3.0 tons/in².

Step 3: Apply Safety Factor

We add a 20% safety margin for process variations: SF = 1.2

Step 4: Calculate Required Tonnage

Tonnage = 24 in² × 3.0 tons/in² × 1.2 = 86.4 tons

You would need an injection molding machine with at least 90 tons of clamping force (rounding up for safety).

Material Clamp Factors Table

Clamp factors vary significantly by material viscosity and processing temperature. Use this reference table for initial calculations:

Material	Clamp Factor (tons/in²)	Typical Processing Temp (°F)	Notes
Polyethylene (LDPE)	2.0 - 2.5	350-450	Low viscosity, easy flow
Polyethylene (HDPE)	3.0 - 4.0	400-500	Higher molecular weight
Polypropylene (PP)	2.5 - 3.5	400-500	Semi-crystalline, good flow
Polystyrene (PS)	3.5 - 4.5	400-500	Good dimensional stability
ABS	3.0 - 4.0	400-500	Impact resistant
Polycarbonate (PC)	4.0 - 5.0	550-650	High viscosity, high pressure
Polyamide (Nylon 6)	3.5 - 4.5	500-550	Hygroscopic, moisture sensitive
PBT	3.5 - 4.5	450-550	Fast cycling capability
PVC (Rigid)	4.0 - 5.0	350-400	Thermal sensitive
Polyurethane (TPU)	3.0 - 4.0	400-450	Flexible, good flow

How to Calculate Projected Area

The projected area calculation requires careful consideration of part geometry and mold design. Here are the key methods:

For Simple Shapes

• Rectangular parts: Length × Width
• Circular parts: π × r²

For Complex Parts

Use CAD software to calculate the true projected area. The method:

1. Import the 3D model into CAD software
2. Project the part onto the XY plane (parting line direction)
3. Measure the area of the resulting 2D silhouette
4. Add runner and sprue contributions if significant

Impact of Wall Thickness & Flow Length Ratio

Wall thickness and flow length significantly affect cavity pressure and thus clamping requirements.

Wall Thickness Effect

Thinner walls require higher injection speeds and pressures to fill before freezing. The relationship is:

Pressure ∝ 1/Wall Thickness

Parts with 0.060" walls may require 2-3x the clamp factor of parts with 0.200" walls.

Flow Length Ratio (L/t)

The flow length ratio (flow length ÷ wall thickness) affects pressure drop. Long, thin flow paths create significant resistance:

L/t > 150:1 typically requires significantly more powerful machines.

Safety Factors & Margin Calculations

Safety factors account for process variations, material inconsistencies, and machine capabilities.

• General purpose parts: 1.1 - 1.2
• Precision parts: 1.2 - 1.3
• Multi-cavity molds: 1.3 - 1.4
• Thin-walled parts: 1.4 - 1.6

Consequences of Wrong Tonnage

Incorrect clamping force calculation leads to expensive problems and production delays.

Insufficient Clamping Force (Under-Clamping)

Flash formation: Molten plastic escapes through the parting line, creating excess material that must be trimmed. Consequences:

• Increased post-processing costs (deflashing labor)
• Reduced part precision and dimensional accuracy
• Mold damage from plastic intrusion into guiding components
• Production downtime for cleanup and mold repair

Excessive Clamping Force (Over-Clamping)

Vent crushing: Too much force compresses mold vents, trapping air and creating diesel burns. Consequences:

• Surface defects (burns, streaks)
• Weak weld lines from trapped air
• Accelerated wear on mold plates and machine tie bars

Tederic Machine Selection Guide

Once you've calculated the required clamping force, selecting the right Tederic machine series ensures optimal performance.

Series	Clamping Force Range	Best Applications
DE Series (All-Electric)	30 - 300 tons	Precision, medical, electronics
NEO Series (Toggle)	90 - 1000 tons	General purpose, packaging, technical
DH Series (Two-Platen)	500 - 4000 tons	Large parts, automotive

Summary & Key Takeaways

Mastering clamping force calculation is essential for injection molding success. The fundamental formula F = P × A provides the foundation, but real-world application requires material-specific factors, safety margins, and careful consideration of part geometry.

Remember: It's better to select a machine 10-20% more powerful than your calculation result to ensure long-term process stability.

Contact TEDESolutions for expert guidance on clamping force calculations and selecting the ideal Tederic machine for your needs.

See also our articles on Cycle Time Calculation and Production Cycle Optimization.