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Compression Molding Plastic Parts – How Plastics are Compression Molded from Resin to Final Products

Updated: Apr 25

Since the very beginning of the plastics industry, plastic parts manufacturers have used the compression molding process to manufacture high quality parts from thermosets, thermoplastics and other polymers. This simple process of molding polymer parts through heat, pressure and time continues to be one of the most popular and cost-effective ways to manufacture plastic parts and products.

Compression molded plastic parts are ubiquitous and are used in nearly every industry – from molded plastic electrical components that require dimensional and thermal stability, to oil, gas, mining and transportation components that require corrosion and chemical resistance, to defense and aerospace parts that require wear resistance.

Agricultural and turf management equipment utilizes dozens of compression molded plastic products and parts, from compression molded UHMW Stalk Stompers used to protect combine harvester tires and tracks from corn stubble damage to compression molded skid shoes that allow growers to operate their headers closer to the ground to maximize yield. In addition, hundreds of household and recreational products feature compression molded plastics, from boat bumpers to the legendary C&A Pro high performance snowmobile skis which are manufactured from compression molded UHMW right here at Pride Solutions in Hutchinson, Minnesota.

How Compression Molded Plastic Parts are Manufactured

Compression molded plastic parts are manufactured by placing special synthetic thermoset, composite or thermoplastic resins such as UHMW into a specially designed metal mold cavity and then applying heat, typically around 300°F, and hydraulic pressure using a vertically oriented hydraulic press for a set period of time to form a plastic part that takes on the exact shape of the mold cavity. The heat and pressure are applied until the finished part has cured or set and then the part is removed, hand-clea

ned of any resin flash around the edges, and inspected prior to shipping to the customer.

UHMW resin used to compression mold plastic parts
Plastic parts are compression molded from UHMW resin.

Many plastic parts manufacturers have compared the compression molding process to making waffles, only the resin used is typically in a powdered form, and great care goes into engineering the mold, determining the correct temperature, pressure, and length of time to process a part, developing pigments with polymer suppliers to attain desired colors, mixing the resin and pigment, filling the mold with exactly the right amount of resin, and cleaning and preparing the mold between parts to ensure high quality final parts. The process requires trained and highly skilled compression molding experts with an eye for detail, like we have at Pride Solutions, where we have nearly 50 years of experience compression molding and fabricating plastic parts.

compression molding UHMW snowmobile skis
Molding Supervisor Nick Gewerth making C&A Pro snowmobile skis.

molded plastic snowmobile skis
Custom C&A Pro aftermarket snowmobile skis.

Overmolding or Insert Molding Parts with Thermoplastics and Rubber

Manufacturers often require metal parts and circuitry to be sealed with plastic or rubber to protect them from wear, friction, heat, moisture, chemicals and other elements. In these applications, metal parts can be overmolded or insert molded with thermoplastic or liquid silicone rubber, giving them added wear resistance and protection. In these processes, the preformed metal part is placed inside the mold and is overmolded with thermoplastic resin or rubber.

UHMW poly capped bolts are an excellent example of the overmolding process. Pride Solutions’ poly capped bolts are manufactured by placing stainless steel, zinc or carbon steel bolts into a specially designed mold and then overmolding the bolt head with UHMW plastic. Learn more about Pride Solutions poly capped bolts here.

In other applications, we have completely overmolded preformed metal parts with rubber or plastic for our customers. We have in-house cobot-assisted CNC metal turning capabilities, making us a one-stop shop for overmolded metal parts. Keeping our metal turning production process in-house allows us to have more control over quality, and save time and money – savings that we pass along to our customers.

overmolded UHMW plastic capped elevator bolt
UHMW Poly Capped Elevator Bolts can be custom ordered.

The advantages of compression molding polymers

Compression molding plastic and other polymer parts is a very popular manufacturing process because it has numerous advantages.

The benefits of compression molding plastic parts:

  • Suitable for advanced composites. Because there are no sprues, runners or gates like in injection molding, compression molding shops are able to work with advanced composites that contain fillers and fiber reinforcements, and manufacture plastic parts that are often more durable, wear resistant and corrosion resistant than metal.

  • Compression molded parts can be manufactured from a wide range of engineered plastics including UHMW, as well as other polymers such as rubber and silicone, and composites that are heavily loaded with fillers and fiber reinforcements.

  • Parts compression molded from UHMW-PE are extremely wear resistant, impact resistant and corrosion resistant and can withstand extreme cold and high temperatures. Read more about UHMW benefits here:

  • Compression molded parts are more durable, dimensionally stable, have a lower co-efficiency of friction, as well as enhanced dielectric properties.

  • Little Waste - Compression molded plastic parts are manufactured to near net shape so very little material is wasted. The compression molding process is ideal for working with expensive compounds. Any leftover resin can be easily recycled.

  • Compression molding allows for more intricacies in geometry. Compression molding shops are able to manufacture parts with holes or other features, often eliminating the need for secondary operations.

  • Many traditionally metal parts are easy to convert into compression molded plastic parts because it is possible to match the parts’ geometry. As an example, snowmobile skis once made out of steel are now primarily manufactured by compression molding UHMW plastic.

  • Compression molding can be used to manufacture large, bulky plastic parts that cannot be manufactured using other processes, and can manufacture parts from polymers that are not suitable for other processing techniques.

  • Compression molding is an excellent choice for plastic parts that have a variable wall thickness.

  • Compression molding is seamless and produces fewer knit lines on final products than other techniques.

  • Lower Cost – The compression molding process is very cost-effective compared to other plastic parts manufacturing techniques because the process of compression molding plastic parts from resin is fairly simple compared to other processes such as injection molding, and production of molding tools is less expensive.

  • Suitable for High Volume Production – Cycle times are longer than with injection molding, but it is less expensive to manufacture compression mold cavities and the parts are highly repeatable for mass production.

  • Compression molding is, by far, the best way to process large, intricate plastic parts.

  • Compression molded parts are extremely strong and durable and have excellent dimensional stability and machining accuracy.

What Polymers and Composites Can be Used for Compression Molding?

The compression molding process is suitable for a wide range of engineered plastics and composites.

At Pride Solutions, we specialize in compression molding Ultra-High Molecular Weight Polyethylene (UHMW-PE). We have nearly 50 years of experience working with UHMW.

What’s the Difference Between Compression Molding and Injection Molding?

Injection molding and compression molding are similar advanced molding processes that are used to create countless plastic products and parts.

Compression molding uses heat, pressure and time to force thermoset or thermoplastic polymers into the mold cavity to form a part in the shape of the cavity. Compression molding features a precision machined two-part metal tool that is placed in a vertical hydraulic press and heated. Resin, typically in powder form, is poured inside the mold cavity. The tool is then compressed at very high pressure for a set period of time, until it cools and the cured plastic part is removed. The compression molding operator then inspects the parts and removes resin flash around the edge of the part with a hand tool.

Injection molding is a high-volume and highly repeatable plastic parts manufacturing process that produces a wide variety of thermoplastic and thermoset parts.

Plastic injection molding involves heating and injecting melted resin into a mold cavity under high pressure until it is full. The mold, which is typically precision machined from steel or aluminum, is then cooled and the finished parts are released. An injection molding machine features a clamp unit and an injection unit. The clamp holds the mold closed while the resin is injected at high pressure by the injection unit. Examples of injected molded plastics include countless household items such as plastic bottles, TV remotes and toys; numerous plastic automotive components such as automotive intake valves, plastic bumpers, and automotive interior components; commercial products such as electronic housings, equipment housings and machine components; and dozens of plastic agricultural parts such as poly pickup bands for hay balers and drainage filter risers.

Pride Solutions offers in-house compression molding and thermoforming services, and has access to quality injection molding services and injection molded plastic parts through our supplier network.

How to Determine if a Plastic Part is Suitable for Compression Molding vs. Injection Molding

Compression molding and injection molding both have their place. Compression molding often is used to manufacture larger flat or curved plastic parts that require repeatability and durability, or complicated parts that require features like holes. It is best used for parts that need to be manufactured at high volume because the initial investment in the mold is expensive, but the cost of the parts is low when manufactured in high volume. Whereas, highly complex parts and parts that require mass production such as electronic casings, car parts, and plastic bottles are better suited for injection molding. Many of the household plastic items you use every day were manufactured using these processes.

Examples of Parts Suitable for Compression Molding

  • Metal parts that need to be replaced with lighter, but strong and durable plastic parts, such as automotive parts

  • Near-net shape plastic parts to custom specification

  • Custom plastic parts with features such as holes

  • Plastic rods and tubes

  • Seals and rings

  • UHMW plastic bushings

  • Skid Shoes for agricultural equipment

  • Electrical housings, handles and switches

  • Plastic gears

  • Electrical parts

  • And much more

If you are curious if your plastic or composite part can be produced using the compression molding process, please give us a call or fill out our contact request form. Our skilled engineering team can answer all of your questions and even assist you with designing your custom plastic parts using the latest SolidWorks CAD/CAM software.

Request a Quote for Compression Molded Plastic Parts

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