Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity.
The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand.
The advantage of compression molding is its ability to mold large, fairly intricate parts. Also, it is one of the lowest cost molding methods compared with other methods such as transfer molding and injection molding; moreover it wastes relatively little material, giving it an advantage when working with expensive compounds. However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding.
Compression-molding is also suitable for ultra-large basic shape production in sizes beyond the capacity of extrusion techniques. Materials that are typically manufactured through compression molding include: Polyester fiberglass resin systems (SMC/BMC), Torlon PAI, Vespel PI, Meldin PI, Ryton PPS, and many grades of PEEK.
Compression molding was first developed to manufacture composite parts for metal replacement applications, compression molding is typically used to make larger flat or moderately curved parts. This method of molding is greatly used in manufacturing automotive parts such as hoods, fenders, scoops, spoilers, as well as smaller more intricate parts.
The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Bulk molding compound (BMC) or sheet molding compound (SMC), are conformed to the mold form by the applied pressure and heated until the curing reaction occurs. SMC feed material usually is cut to conform to the surface area of the mold. The mold is then cooled and the part removed. Materials may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder.
Materials are heated above their melting points, formed and cooled. The more evenly the feed material is distributed over the mold surface, the less flow orientation occurs during the compression stage.
Thermoplastic matrices are commonplace in mass production industries eg. automotive applications where the leading technologies are Long Fibre reinforced Thermoplastics (LFT) and Glass fiber Mat reinforced Thermoplastics (GMT).
In compression molding there are six important considerations that an engineer should bear in mind:
Determining the proper amount of material.
Determining the minimum amount of energy required to heat the material.
Determining the minimum time required to heat the material.
Determining the appropriate heating technique.
Predicting the required force, to ensure that shot attains the proper shape.
Designing the mold for rapid cooling after the material has been compressed into the mold.
Compression molding is a manufacturing process used to create plastic and composite parts. Through a mixture of heat and high pressure, it squeezes materials—like thermosetting polymers or thermoplastic compounds—into set shapes. It’s the heat and pressure that allows solid materials to soften and reform into new structures that are wholly even and cured. During the process, curing triggers a chemical reaction, which helps give the final product strength and durability.
The Importance of Compression Molding
This is one process that’s helpful when it comes to turning pre-impregnated intermediate products into semi-structural and structural composite components. It’s also key for developing fibrous materials that have been impregnated with thermoset and thermoplastic matrices. Not only that, but compression molding is a great process for companies looking to cut costs, minimize waste, and make numerous products. Compression molding is also better suited for manufacturing certain part geometries, such as thick walls.
How the Compression Molding Process Works
There are a few steps in the compression molding process. Here’s how it works, according to our engineers:
Step 1: The first step in compression molding is creating the molds, which are responsible for shaping the end product. These are usually made of steel or aluminum and, once they’re put together, have a cavity, an upper mold, and a lower mold.
Step 2: After the mold is created, the machine is set up for the process. Users will clean the mold, input the settings, and turn on the heat. Keeping the temperature controlled is essential for preventing defects and warping.
Step 3: For thermosets, the next step involves placing a charge made of fiber-reinforced resin, silicone, or rubber into the cavity. The mold is then closed while heat, pressure, and speed settings are locked in to begin the molding process.
It’s slightly different for thermoplastics. For these materials, temperature-controlled cooling molds are used instead.
Step 4: Once the shapes are complete, they cool and any extra edges or excess material are removed. Altogether, the process can take 1 to 5 minutes, but it largely depends on how thick the parts are.
Applications of Compression Molding
Thanks to its customizable parts and settings and its flexibility with materials, compression molding spans many different industries. It has a lot of different uses, with just a small chunk of those listed below.
Kitchenware: The staples in your cooking space may exist thanks to compression molding. This process can make products like bowls, cups, plates, and utensils and create versions that are resistant to heat and breaks.The ever-popular melamine plates used for eating outdoors are often made this way.
Automotive parts: Both small and large components for vehicles like cars, trucks, and tractors can be made through compression molding. As examples, you can get door panels, dashboards, and parts for engines.
Electrical components: With the previous materials listed, it’s no surprise that compression molding can be used to make electrical components. Manufacturers can get precise shapes, reliable functionality, and consistency across the board.
Video games and computer devices: The devices we spend hours on wouldn’t function the same without the help of compression molding. This process can produce keypads for computers, video game controllers, and parts that provide electrical insulation.
Medical and dental components: You could probably name quite a few tools and devices your doctor and dentist rely on, but without some finer details and tiny components, they wouldn’t be usable! Compression molding can create plastic and silicone parts like syringe stoppers and pieces for respirator masks.
Advantages and Disadvantages
There are pros and cons to think about when you’re weighing up compression molding and other similar processes. Here are some of the advantages and disadvantages that Xometry makes their customers aware of when considering the process:
Advantages include:
Ultra-strong parts that last through immense wear and tear and heavy-duty use.
Molds are highly customizable and can be created for intricate designs or more basic styles.
Compression molding can handle various materials, including highly viscious materials, and therefore make a wide range of different products.
Molds are efficient when it comes to using material, which saves money in the long run.
The parts made from compression molds have excellent finishes.
Batch production is possible and users can create settings and cycles for maximum efficiency.
Compression molding machines can use recycled and eco-friendly materials—a perk for sustainably-minded companies.
Disadvantages include:
Complex parts are no problem, but intricate components with thin walls is a no-go.
Pressure range limits make it difficult to create detailed shapes
While it only takes a few minutes, compression molding is slower than other processes
Compression molding utilizes a variety of materials. Some of the most common materials used in compression molding are discussed below:
1. Epoxy
Epoxy resins are popular in compression molding due to their excellent mechanical properties, high heat resistance, and dimensional stability. They flow easily when melted, ensuring complete cavity filling during compression. The cured epoxy parts exhibit high strength and durability, making them ideal for applications requiring robust molded components, such as aerospace parts, electrical insulation, and structural composites.
2. Silicone
Silicone materials are well-suited for compression molding because of their exceptional temperature resistance, flexibility, and excellent electrical insulation properties. They can maintain their physical properties across a wide temperature range. Silicone material can easily flow and conform to intricate mold cavities, allowing for the production of precision seals, gaskets, medical devices, and automotive components.
3. Melamine
Melamine resins offer outstanding heat resistance, hardness, and chemical resistance, making them suitable for compression molding. Melamine molds easily and produces finished parts with excellent surface finishes and dimensional stability. Melamine is commonly used in compression molding for manufacturing kitchenware, decorative laminates, electrical components, and heat-resistant utensils.
4. Urethane
Urethane, or polyurethane, materials are favored in compression molding for their exceptional toughness, abrasion resistance, and impact strength. They can be formulated to exhibit a wide range of physical properties, making them versatile for various applications. Urethane flows well during compression molding, allowing for intricate mold designs and the production of items such as automotive parts, rollers, wheels, and industrial seals.
5. High-Density Polyethylene (HDPE)
HDPE is a thermoplastic material known for its high strength, chemical resistance, and rigidity. It can be easily melted and flows readily during compression molding, filling complex mold cavities with precision. High-density polyethylene parts exhibit excellent impact resistance and dimensional stability. Compression molding is commonly used for manufacturing products such as automotive components and industrial parts.
6. Polyphenylene Sulfide (PPS)
PPS is a high-performance thermoplastic with exceptional chemical resistance, flame retardancy, and dimensional stability. It has good flow characteristics when melted, ensuring complete cavity filling during compression molding. PPS parts exhibit high strength and stiffness, making them suitable for demanding applications in electrical components, automotive parts, and industrial equipment.
7. Polytetrafluoroethylene (PTFE)
PTFE is a non-stick fluoropolymer with exceptional chemical resistance and high-temperature stability. PTFE has a low coefficient of friction and excellent electrical insulation properties. It can flow easily during compression molding, allowing for the production of complex shapes and precise parts like gaskets, seals, bearings, and electrical insulation components.
Compression molding machines typically consist of the following:
A large tonnage press, ranging from 150 tons to 2500 tons.
A heated mold consisting of two halves, an upper and a lower, which are precision machined to create the desired shape of the final product. The mold cavities are designed to accommodate the material and allow for proper flow and distribution during the molding process.
Separate heating chamber or an oven for the materials to be heated to a specific temperature to achieve their molten state.
Long, slender cylindrical electric resistors known as heating lines as the primary heating elements.
Shop air for cleaning purposes, such as removing debris or excess material from the mold cavity or part surfaces. Compressed air can be directed through nozzles or air blasts to blow away any loose particles or contaminants.
Automation systems such as: conveyor systems, hoppers, or robotic arms, temperature control and monitoring devices, and mechanical ejection systems.
Molding cycle times in compression molding are typically influenced by the thickness of the part being molded. However, they generally fall within the range of 60-300 seconds, making compression molding a fairly fast molding process for thermoset materials.
Compression molding finds application in processing pre-impregnated intermediate products to create semi-structural and structural composite components. Recently, compression molding has been adopted for platelet-like materials, which are fiber strands or bundles pre-impregnated with a thermoset or thermoplastic matrix. These materials bear similarities to waste thermoset prepregs or recycled thermoplastic materials.
Compression molding also allows for the production of large complex composite components with minimal material wastage. It is an attractive option for numerous industries seeking robust and cost-effective manufacturing solutions.
Compression molding is a manufacturing process that produces various plastic or composite parts. It involves placing a pre-measured and preheated material, such as a thermosetting polymer or thermoplastic compound, into an open mold cavity. The mold is then closed, and high pressure is applied to compress the material and shape it to the desired form. Heat is often used in conjunction with pressure to facilitate the curing or melting of the material. The combination of heat, pressure, and time causes the material to soften and flow, filling the mold cavities and taking on the desired shape. The pressure applied helps ensure that the material is evenly distributed and consolidated within the mold. Once the material has cured and solidified under pressure, the mold is opened, and the finished product is removed. The curing process typically involves the material undergoing a chemical reaction, such as cross-linking, which gives it its final strength and durability.
compression molding is one of molding method, thanks for your introduction