Large Tonnage Injection Molding: Low- and High-Pressure Processes

Large plastic products can be produced in two ways, large-tonnage injection molding and large-tonnage structural foam molding. Both can produce products that can replace metal, wood, concrete, or fiberglass. Each process, however, has its benefits and specific applications, so it is important to understand the processes and their benefits.

Injection molding can be used to produce large plastic products with thinner walls. These products tend to be lighter than those produced by structural foam molding, usually weighing less than 20 pounds.

Structural foam molding also allows for the production of very large plastic products with thicker walls. These products are generally heavier than injection molded products, weighing 50 pounds or more. However, structural foam molded products are still more lightweight than solid plastic parts.

Low-Pressure Structural Foam Molding

Low-pressure structural foam molding requires nitrogen gas and/or a chemical blowing agent to be mixed with the melted resin. This mixture is then injected into a mold under significantly lower pressure than traditional injection molding techniques. Structural foam molding produces very large parts with thicker walls, making them very durable.


The most common resins used in structural foam molding are polyethylene and polypropylene. Both resins possess advantageous properties that make them ideal for this process.

Polyethylene is a durable and lightweight resin that offers excellent load-bearing, shock absorption, vibration-dampening properties, and resistance to chemicals, grease, and moisture. Polypropylene is a rigid and lightweight resin offering excellent compression and impact resistance, as well as chemical, temperature, and moisture resistance.

Structural foam molding and these two resins are great for creating large industrial totes and grilles for lawn and garden tractors due to their durability and resistance to heat, chemicals, and moisture.


Highly Moldable

Low-pressure structural foam molding can create parts with variable wall thicknesses. However, cycle time will be determined by the thickest part.  Uniform wall thickness is always best.  Also, due to the low pressure used in the process, there is a reduced risk of warping and deformation during the manufacturing process.

High Strength-to-Weight Ratio

These products are lighter weight than solid plastic products, however, they maintain their strength and durability.

High Thermal Stability

Structural foam’s ability to withstand thermal expansion makes it ideal for products that will be exposed to a wide range of temperatures. Whether it is heat or cold, these products will not warp or lose their properties if exposed to extreme temperatures.


Due to the lower pressure used in this molding process, the tooling used does not need to be as durable as high-pressure injection molding tools, saving you money.

High-Pressure Injection Molding

Injection molding requires plastic resin pellets to be melted down in the barrel of the injection molded machine. The melted resin is then forced into the mold through a high-pressure nozzle, forcing the material into all parts of the mold cavity. As the plastic cools and hardens, it retains the shape of the mold.


There are three common resins used in high-pressure injection molding: HDPE, ABS, and polypropylene. Again, each has its own advantages and is chosen based on the application of the product being produced.

HDPE, or high-density polyethylene, is a flexible and waterproof resin. It has good low-temperature resistance, chemical resistance, and corrosion resistance, and is easily processed at an affordable price.

ABS is a very durable resin with high durability. It also has high tensile strength and is very resistant to physical impacts and chemicals.

Polypropylene is a lightweight, cost-effective, rigid resin. Its chemical and heat resistance along with its high tensile strength and impact tolerance make it a great choice for a wide range of products.

High-pressure injection molding is used to create many outdoor products, including DuraTrac outdoor floors and lawn or garden carts.


Repeatability with High Detail

This manufacturing process can produce identical parts over and over. Additionally, the high-pressure forces used force the plastic against the mold walls, creating highly complex designs with intricate details.


Cycle times for injection molding are very short—sometimes only lasting a handful of seconds. It is also possible to use a multi-cavity to produce several different parts in one run, saving even more production time.

Produces Less Waste

Luckily, while the injection molding process doesn’t produce much waste, any unused or waste plastic produced can be recycled for future products.

Can be Automated

Many parts of the injection molding process are being automated, including loading and unloading, inspection and quality control, assembly, and more. Using automation and robotics during the manufacturing process helps to reduce labor costs and production time, increasing your cost savings.

Work with a Plastic Injection Molder With Large Tonnage Injection Molding and Structural Foam Molding

Working with a manufacturer who has large tonnage capabilities in both low-pressure structural foam molding and high-pressure injection molding grants you a wider range of manufacturing options so you get the best product possible.

Port Erie’s large-tonnage injection molding creates large parts with walls thinner than 0.25 inches. Our machines are capable of handling up to 1,650 tons and can create parts up to 20 pounds.

Our large tonnage structural foam molding produces parts up to 50 pounds with walls thicker than 0.25 inches. These parts are heavy but would weigh significantly more if they were solid plastic.

At Port Erie, we will assess your part and help you determine which process is best for your product’s functionality and your budget. If you need, our engineering team can get involved to help optimize your design, so it is as compatible with its manufacturing process as possible.

Port Erie Can Handle All of Your Structural Foam Needs

At Port Erie Plastics, we are dedicated to providing our clients with the highest quality plastic products possible. From engineering to injection molding or structural foam molding and value-adding secondary operations plus distribution and warehousing, we have you covered.

Contact our team today with any questions, or request a quote if you are ready to get started.

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