How are moulding presses rated? A Guide to Clamping Force, Platen Size, and Key Specs

Moulding presses are primarily rated by their clamping force, which is measured in tonnage. This single metric indicates the maximum force the press can apply to keep a mould closed during the moulding process. However, a complete specification also includes platen size, daylight, stroke, and, for injection moulding, shot size.

While tonnage is the headline number, a press's true capability is defined by the interplay between force, physical dimensions, and material capacity. Selecting the right press requires matching all its specifications—not just one—to the specific requirements of your mould, your material, and your final part.

The Core Ratings Explained

To fully understand a press's capabilities, you must look beyond a single number and evaluate the key metrics that define its operational limits.

Clamping Force (Tonnage)

Clamping force is the amount of pressure, expressed in US tons or metric tonnes, that a press uses to hold the two halves of a mould shut.

This force is critical because it must counteract the immense pressure of the material being injected or compressed into the mould cavity. Insufficient clamping force will allow the mould to separate slightly, causing a defect known as flash, where material escapes at the parting line.

A common rule of thumb for injection moulding is to require 2 to 5 tons of clamping force for every square inch of the part's projected area. The exact value depends heavily on the material's viscosity and the complexity of the part.

Platen Size and Tie Bar Spacing

The platens are the large steel plates onto which the halves of the mould are mounted. The platen size dictates the maximum physical footprint of the mould that the press can theoretically hold.

However, the more practical limitation is the distance between the tie bars. These large steel rods provide structural support and guide the moving platen. Your mould must physically fit between them, making this a critical dimension for compatibility.

Daylight and Stroke

Daylight refers to the maximum distance between the two platens when the press is fully open. Stroke is the total distance the moving platen can travel.

These two dimensions determine if you can not only fit your mould but also operate it. The daylight must be greater than the mould's height when closed (shut height) plus the distance required to open the mould and successfully eject the finished part.

Shot Size (Injection Moulding)

Specific to injection moulding, shot size is the maximum volume of plastic resin the machine can inject into a mould in a single cycle. It is typically measured in ounces (oz) or grams (g).

This rating is crucial because the total volume of your part and its runner system must be less than the machine's maximum shot size. For optimal quality and process control, it is recommended to use between 20% and 80% of the machine's rated shot capacity.

Understanding the Trade-offs

Selecting a press is not about finding the machine with the highest numbers. It's about finding a balanced system where the specifications work in harmony for your specific application.

Tonnage vs. Platen Size

A press can have extremely high tonnage but small platens, making it suitable only for small, high-pressure parts. Conversely, a press with massive platens but low tonnage can accommodate large moulds but may be limited to low-pressure applications or materials. The two must be appropriate for your intended moulds.

The Risk of "Oversizing" Your Press

Using a press with far more tonnage than required is inefficient. A larger machine consumes more energy to operate and may have slower clamp movements, increasing cycle times. For injection moulding, using a machine with an oversized shot capacity can lead to material degradation, as the plastic resin sits in the heated barrel for too long.

The Danger of "Undersizing" Your Press

Using a press with insufficient tonnage is a direct cause of quality failure. The injection pressure will force the mould open, creating flash on the parts, leading to high scrap rates and wasted material. In extreme cases, it can put undue stress on the press and mould, risking damage.

Making the Right Choice for Your Application

The goal is to find the most efficient and cost-effective machine that meets all the technical requirements of your project. This requires a holistic view of the press ratings.

• If your primary focus is producing a specific part: Start with the part's projected area and material type to calculate the required tonnage, then verify that your mould fits within the press's platen, tie bar, and daylight specifications.
• If your primary focus is selecting a versatile press for a job shop: Prioritize a machine with a generous platen size and daylight to accommodate a wide range of customer moulds, ensuring its tonnage is sufficient for common engineering-grade materials.
• If your primary focus is maximizing efficiency for high-volume production: Precisely match the press to the part. Select the minimum tonnage that provides a safety margin and a shot size where your part volume falls within the ideal 20-80% capacity range.

By understanding how these ratings function as an interconnected system, you can select a moulding press that is not just powerful, but perfectly suited to your operational goals.

Summary Table:

Rating Metric	What It Measures	Why It's Important
Clamping Force (Tonnage)	Maximum force to hold mould closed.	Prevents defects like flash; critical for part quality.
Platen Size & Tie Bar Spacing	Physical space for mounting the mould.	Determines the maximum mould size that can be used.
Daylight & Stroke	Maximum opening distance and travel.	Ensures the mould can be installed and parts ejected.
Shot Size (Injection)	Maximum volume of material injected per cycle.	Must match part volume for optimal process control.

Select the Perfect Moulding Press with KINTEK

Choosing the right press is critical for your production quality and efficiency. The interplay of tonnage, platen size, and other specifications must be perfectly balanced for your specific moulds, materials, and parts.

KINTEK specializes in lab equipment and consumables, serving precise laboratory needs. Our experts can help you analyze your requirements to recommend a moulding press that delivers optimal performance and value, avoiding the risks of oversizing or undersizing.

Contact us today to discuss your application and find the ideal solution for your laboratory.

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