When it comes to manufacturing high-quality plastic parts, selecting the right plastic injection molding press is one of the most crucial decisions in the injection molding process. Whether you’re producing small precision components or large structural plastic parts, choosing the correct press size ensures product integrity, consistency, and cost-efficiency.
With over 70 years of experience in the plastics industry, UGP Plastics has built a reputation across Canada and North America as a trusted leader in plastic injection molding machine operations and custom molded part manufacturing. In this guide, we’ll break down what press sizing means, why it matters, and how to make the right selection for your project.
What Is Press Size in Plastic Injection Molding?
The size of a plastic injection molding press refers to its clamping force, usually measured in tons. This clamping force is what keeps the mold shut during the injection phase to prevent plastic leakage or flash.
The range of press sizes extends from 5 tons to more than 3,000 tons. The ability to maintain high pressure closure of the mold increases with larger tonnage machines which is essential for producing big or intricate parts. Using a press size that exceeds actual requirements leads to increased energy consumption and elevated operational expenses.
Why Press Sizing Matters in the Injection Molding Process
Selecting the appropriate press size is essential for:
- Part quality: Undersized presses may allow mold separation, causing defects.
- Efficiency: An oversized plastic injection molding machine can lead to increased cycle times and energy consumption.
- Cost-effectiveness: Right-sized equipment minimizes wear and optimizes material use.
The injection molding process requires exact calibration of press tonnage based on part design and material selection and mold configuration.
How to Calculate the Correct Press Tonnage
The rule of thumb for calculating the necessary press size is:
Projected Area of the Part (in²) × Tonnage Factor = Required Tonnage
The tonnage factor depends on the material being used. For example:
- Polypropylene may require 2–3 tons/in²
- Polycarbonate could need 4–6 tons/in²
So, if your part has a projected area of 40 in² and the material requires 4 tons/in², the ideal plastic injection molding press would be 160 tons.
Always add a 10% safety margin to the calculated tonnage to ensure optimal performance and mold longevity.
Filling Pressure vs. Clamping Force
While clamping force prevents the mold from opening, filling pressure is what injects the molten plastic into the cavity. These are often confused but serve different functions. Filling pressure is affected by:
- Melt viscosity
- Flow rate
- Gate and runner design
- Wall thickness
The monitoring of both values is necessary for quality control and to prevent part deformation or flash during the injection molding processing cycle.
At UGP Plastics, we’ve been refining our injection molding process and equipment sizing strategies for over seven decades. Our expert engineers analyze each project to determine the most efficient and cost-effective plastic injection molding press configuration—ensuring durability, consistency, and client satisfaction across every part we produce.
Looking for precision in every press stroke? Contact UGP Plastics today and request a quote tailored to your project needs. Let us bring 70+ years of trusted experience into your next production run.
Frequently Asked Questions (FAQ):
What size press for injection molding?
The size of the press is based on the part’s projected area and the material used. Smaller parts can use presses under 100 tons, while larger parts may require presses over 1,000 tons.
How to calculate press tonnage for injection molding?
Multiply the projected area of the part (in²) by the material’s tonnage factor (tons/in²). Add 10% to this value for safety and precision.
How do you calculate pressure for injection molding?
The calculation of injection pressure depends on flow simulation software or empirical testing which takes into account mold design and part geometry and resin viscosity. The processing stage requires monitoring and adjustment of injection pressure.
What is the filling pressure for injection molding?
Filling pressure can range from 8,000 to 30,000 psi depending on the material and part complexity. It must be high enough to fill the mold but not so high that it causes flash or damage.
