LSR Injection Molding Machine: How to Choose the Right Tonnage

Selecting the right LSR injection molding machine tonnage is essential for stable production and consistent product quality. Many manufacturers focus only on part size, but tonnage selection actually depends on multiple factors, including mold design, cavity number, and material behavior.

Liquid silicone rubber (LSR) is a low-viscosity material that flows easily but is highly sensitive to pressure imbalance. Insufficient clamping force leads to flash and defects, while excessive tonnage increases cost and energy consumption.

What Is Injection Molding Machine Tonnage

Definition of Clamping Force

Injection molding machine tonnage refers to the clamping force used to keep the mold closed during injection. This force prevents the mold from opening under internal pressure. The general formula used in engineering is:Clamping Force = Projected Area × Material Factor × Safety Margin

If the tonnage is too low, molten material escapes and creates flash. If too high, it can damage the mold and reduce efficiency. 

Why Tonnage Matters in LSR Injection

LSR requires more precise clamping control than thermoplastics due to its high flowability. Even small pressure variations can affect part quality, especially in thin-wall or multi-cavity designs.

Key Factors That Determine LSR Machine Tonnage

Projected Area

Projected area is the most important factor in tonnage calculation. It represents the surface area of the part from the mold opening direction. A larger projected area increases the force required to keep the mold closed.

Number of Cavities

Multi-cavity molds significantly increase total projected area and pressure load. As cavity count increases, tonnage requirements rise accordingly, making balance and machine stability more critical.

Material Behavior (LSR Characteristics)

LSR has lower viscosity than most plastics, which allows faster filling but also increases the risk of flashing. Materials with higher flowability require more precise clamping control rather than simply higher tonnage

Injection Pressure and Process Conditions

Higher injection pressure or faster filling speeds increase the force acting on the mold, which raises tonnage requirements

Case Study: iPhone 17 Pro silicone phone case

Project Background

iPhone 17 Pro silicone phone case project required high flexibility, smooth surface finish, and consistent wall thickness. The product had a large projected area and thin-wall structure, making it highly sensitive to clamping force.

Production Challenges

During initial production, the following issues occurred:

· Flash along the parting line

· Uneven filling

· Process instability

These problems were caused by insufficient clamping force combined with multi-cavity pressure imbalance.

Tonnage Selection Optimization

The engineering team evaluated:

· Total projected area of the phone case

· Multi-cavity configuration

· LSR material flow characteristics

Because thin-wall LSR parts require higher injection pressure, additional clamping force and precision control were necessary.

Final Results

After optimizing tonnage and system parameters:

· Flash was eliminated

· Filling became uniform

· Production stability improved significantly

This case demonstrates that correct tonnage selection must consider the entire system, not just part size.

Common Mistakes in Tonnage Selection

Only Considering Product Size

Ignoring cavity number, mold structure, and material leads to incorrect machine selection.

Choosing Excessive Tonnage

Oversized machines increase cost and energy consumption without improving product quality. Excess force may also damage mold components.

Ignoring LSR Behavior

Using thermoplastic standards for LSR projects often results in instability and defects due to different flow characteristics.

How to Choose the Right Tonnage

Accurate tonnage selection requires evaluating projected area, cavity layout, material properties, and injection pressure. A safety factor is typically added to ensure process stability.

Tonnage must match the entire production system, including mold design, machine capability, and process parameters. A balanced system improves yield, reduces defects, and lowers operating costs.

FAQs

Q:What happens if tonnage is too low?

A:The mold may open during injection, causing flash, short shots, and dimensional defects.

Q:Is higher tonnage always better?

A:No. Excessive tonnage increases cost and may damage the mold without improving quality.

Q:How is tonnage calculated?

A:Tonnage is calculated based on projected area, material factor, and injection pressure, usually with a safety margin included.

Q:Why is LSR tonnage selection different?

A:LSR flows more easily than thermoplastics, making it more sensitive to pressure imbalance and requiring precise clamping control.

Conclusion

Choosing the right LSR injection molding machine tonnage is critical for achieving high-quality and stable production. The correct approach is not simply selecting a larger machine, but optimizing the balance between projected area, material behavior, mold design, and process conditions.