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What is Compression Molding?

Learn more about how compression molding plastic works

Xomety X
By Team Xometry
 12 min read
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Compression molding is one of the most widely used plastic molding manufacturing techniques. Like injection molding, compression molding is an economical, time-tested solution for plastic fabrication. It is a molding process where preheated plastic material, usually referred to as charge, is placed on a heated open mold cavity and compressed under pressure to form the final shape of the product.

Compression molding is used to produce plastic products with large designs and can create complex parts. It outshines other techniques by the capacity to produce parts with composite material. Application of this old yet efficient technique ranges from vehicle bumpers and aircraft bodies to smaller consumer products such as shoe soles and buttons. In this article, we will cover the various benefits, materials used, and potential drawbacks of compression molding, and how it compares with other plastic molding manufacturing techniques. 

What is Compression Molding?

Compression molding is a technique where heat and pressure are used to form the final shape of the product. This method is usually suitable for silicone rubber, thermosetting and thermo-softening plastics, as well as advanced composite materials. Compression molding employs a heated cavity and vertical press mechanism which makes the necessary tooling easy to construct and maintain. It can be used in both low and high-volume production. Apart from this, the simplicity of compression molding machines and low tooling costs makes it attractive for functional prototyping and small-volume manufacturing.

Compression molding wastes less material than injection molding, as no runners, sprue, or gates are required to form the part. Consequently, this technique produces an aesthetically as well as a functionally superior surface finish. It is the go-to technique for large flat and curved components. This molding technique outperforms other manufacturing techniques when composite materials are used to give extra stiffness, corrosive resistance, durability, and other special material properties. Compression molding often uses a mixture of thermoset plastic and other materials called sheet molding compound (SMC) or bulk molding compound (BMC) to produce large and complex composite components with high-grade mechanical properties.

As with any manufacturing technique, compression molding comes with a few drawbacks. It has a longer time cycle and highly complex and intricate parts might be difficult to produce using this method. Sometimes, extra material (known as flash) will end up attached to the part during the molding process. Generally the excess material is cut or trimmed during post processing.

The Compression Molding Process

The compression molding process begins by preparing and weighing the raw material to be formed. Raw materials include resins in the form of pellets, powder, or preforms. This charge material, usually plastic or composite material, is often preheated and placed into a heated mold cavity, which is usually made from machined aluminum or steel. Afterward, the matching half mold or top plug is closed using a hydraulic press. Typically, this motion is vertically compressing and during this time, the pressure forces the plasticized charge to fill the mold uniformly, creating a homogenous structure of the part. The mold is kept closed and the high temperature cures the charge material into the final shape and chemistry. Once the curing reaction is finished, the upper mold is lifted, and the final product is removed.

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  • Xometry Image

Compression molding clamps a pre-form charge of rubber, thermoset, or silicone into a cavity to form the shape of the part.

The standard raw materials used for the compression molding manufacturing process are plastics resin, rubbers, and some types of fiber reinforcements. Glass and carbon fiber reinforced thermoset plastics are the industry standard for composite component fabrications using compression molding. Additionally, elastomers like natural rubber and silicone parts are used to make parts using compression molding processes.

It is worth noting that cold compression molding is also used. For this curing occurs at room temperature. Additional post processing where heat is applied might be used.

Compression Molding vs Injection Molding

The working principle of both techniques are similar but with few key differences. In injection molding, a molten molding material is injected into a closed mold at high pressure. In compression molding, preheated molding material is inserted in an open mold cavity and the top half matching mold is closed at a lower pressure than that of injection molding.

In injection molding, plastic feed material in the form of pellets is supplied into the machine where a screw auger and a heater melt the pellets. From there machined steel or aluminum mold parts are clamped tight at high tonnage (where dozens or hundreds of tons of weight’s worth of pressure is applied). They remain clamped until the pumping of the molten material into the closed mold is complete and the part material solidifies. Typically the mold opens within seconds, and the newly formed part is ejected.

The injection molding process becomes more economical as the volume of production increases, which makes it ideal for high-volume production runs. This is due to the high initial cost of tooling, and design optimization for injection molding. This production process allows the use of a variety of raw materials such as thermoplastics like polycarbonate, nylon, polyether ether ketone (PEEK), and thermosetting plastics like thermoplastic polyurethane (TPU) and elastomer (TPE). Depending upon the application, the precise material selection is carried out according to design requirements.

Even though injection molding operates at a high frequency, with a cycle that usually takes a few seconds, it cannot be used to produce larger parts and advanced composite components. The fact that less material is wasted due to the absence of gates, sprues, and runners, is one of the several advantages of the compression molding technique.

For smaller and highly complex parts, the injection molding technique is best suited. For successful injection molding, the design of the part and mold must be optimized with consideration to the necessary drafting, runners, gate, sprues, parting line, and material selection.

Compression Molding vs Injection Compression Molding

Injection compression molding is a hybrid of both compression molding and injection molding. It is a technique that takes the best of both molding methods by eliminating the disadvantages of long flow paths. In this process the molten plastic is injected into a slightly open mold cavity. This makes the fill pressure lower than injection molding process. Once the molten material is injected, the other matching half mold is closed, distributing the mold material uniformly to the entire cavity and maintaining the pressure. Consequently, the process lowers shear stress and shear velocity of the molten material and makes this method suitable to produce high precision parts with low residual stresses. Products like complex gears, where less variation in density and high strength is required, are produced using the injection compression molding method.

Compression Molding Composites and Other Materials

Compression molding technique is the fastest, most economical, and most mature production process for parts made of thermoset and thermo-softening plastics, rubbers, and silicone materials. By implementing silicone compression molding, prototypes and other silicone products can be fabricated relatively quickly. Silicone and natural rubber possess special properties that enable them to be molded in any shape. Rubber is a common material choice across various industries to address structural and functional requirements such as vibration absorption, high tensile strength and resilience, and elasticity. A variety of thermoplastics (thermo-softening), used across different industries for their special material properties, also take advantage of the compression molding method.

The heat used during compression molding cures thermoset plastics irreversibly. Due to their exceptional properties of durability and strength, thermoset plastics are the foundation of composite materials. Thermoplastics on the other hand get soft or melt when heated and are suitable for recycling. When compression molding is utilized, the slow, uniform flow of molten charge renders higher strength, structural integrity, homogeneity, and smoother surface finish than the injection molding process.

Here are some commonly used materials for compression molded parts:

  • Silicone rubber: used whenever padding, waterproof, slip-resistance, parts isolation, and temperature resistance are design criteria.
  • Ultra-High Molecular Weight Polyethylene (UHMW-PE): known for its exceptional properties of high abrasion, impact, and temperature resistance.
  • PEEK: a semicrystalline thermoplastic with remarkable mechanical and chemical properties.
  • Nylon: popular engineering thermoplastic for its high abrasion and wear resistance, as well as easy machining.
  • High Density Polyethylene (HDPE): widely used thermoplastic for its affordability, versatility, suitability for food and beverage, and ready recyclability.
  • Polypropylene (PP): a thermoplastic with excellent chemical and temperature resistance, good electrical insulation, and impermeable.
  • SMC and BMC: complex mixtures of resins such phenolics, vinyl ester, epoxy, polyester, fillers, catalyst, and thickeners, which are used for compression molding of composite components. SMC is suitable for stiff, large, and long components, whereas BMC is suitable for sturdy complex shapes.
  • Polyvinyl Chloride (PVC): used for precision, high impact strength, and leak-free requirements.
  • Other plastics include polyethylene (PE), polystyrene (PS), acrylics, polyesters, polyimides, and fluoropolymers.

Compression molding is the only reliable mass production method for most of the composite parts used today. In this technique, fiber is laid carefully according to specific orientation on the charge, so that its strength is higher in certain directions. This can use a mixture of polymer matrix, fillers, and additives. The heat and pressure of the process cures and consolidates those materials to increase the material’s integrity. This results in a composite component with high stiffness, durability, and low fiber degradation and knit lines. Carbon fiber reinforced polymer is used whenever strength-to-weight ratio and stiffness is critical. For fiberglass on the other hand, glass is used as a fiber source and produces similar outstanding properties such as strength and intricate shape moldability.

Typical composite materials used include:

  • Matrix: polyester, vinyl ester, or epoxy
  • Fiber: carbon fiber, glass fiber, or PTFE
  • Fillers: calcium carbonate, kaolin, or alumina trihydrate. These reduce cost and improve properties like fire retardance, and temperature resistance.

Compression Molding Product Examples

Compression molded parts and components are ubiquitous in our daily life. This method of plastic fabrication is widely utilized in aerospace and automotive industry, consumer electronics, kitchenware, medical industry, and in any other application where the exceptional benefits of the compression molding technique is required.

Aerospace

Composites play a major role in the aerospace industry. It is an industry where strength-to-weight ratio and surface finish is of utmost importance. Various composite components such as carbon fiber reinforced plastics (CFRP) and fiberglass reinforced plastics (GFRP) are used in aircraft bodies such as wings, aircraft engine casings, and blades.

Automotive

Similarly, in the automotive industry, weight is a design concern as well and hugely impacts fuel efficiency. Composites like CFRP and GFRP are used to manufacture several parts of vehicles  including engine components such as mounts and brackets, body parts such as bumpers and spoilers, and interior parts such as dashboards, door panels, and seat backs. It’s also used to make several electrical and mechanical components. Rubber parts play a crucial role as sealants, in vibration control, and production of O-rings.

Medical

In the medical industry, silicone rubber is often sought after for its unique biocompatibility properties. Compression molding is used to produce various parts for medical applications using silicone rubber. To mention a few: orthopedic surgical implants, diaphragms, lip seals of cylinders, vibration inhibitors, and silicone masks for Continuous Positive Airway Passage (CPAP) equipment, as well as parts and components of other medical equipment.

Electronics

Compression-molded products are also used in most electronic goods. A good example is a rubber housing for a feevr contactless thermal imaging device. The design of the case contains undercuts and uneven wall thickness in which compression molding stands out in fabricating from other molding techniques. The overall smooth surface finish is one of the many benefits of this technique. Compression molding is usually utilized in the production of several type of casings for smartphones and tablets, and in the fabrication of many of the electronic devices’ internal parts.

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  • The camera housing for the feevr device is compression molded

A compression molded rubber casing made by Xometry for Feevr imaging device.

For other industries, from agriculture to oil and gas, compression molded parts play a major role in producing key elements of machinery and systems. In the agricultural industry compression molded UHMW-PE made parts like chain guides, distributors, and couple wear plates. In the oil and gas field, compression molded parts play a significant role as seals, bushing and bearings, and structural and electrical components, to mention a few.

Compression Molding with Xometry

Compression molding is an established and widely used plastic and composite fabrication method utilized by manufacturers around the world. Its strongest abilities include production of parts and components with high tensile strength and stiffness, as well as other extreme properties. It is also a suitable production technique from low to high volume production as well as rapid prototyping. Compression molding can produce stunning surface finishes as well as handling complex geometries, and large bulky components. It’s also the only suitable technique for fabrication of composite parts. Moreover, compression molding techniques prevail whenever part material used is impossible to produce using other molding techniques and customized composites are fabricated.

Engineers at Xometry are delighted to help you decide whether to use compression molding or other molding techniques according to design requirements, material, and quantities for your project. Get a quote from Xometry today to get started.

 

Disclaimer

The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.

Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.