This article will take an in-depth look at compression molding.
The article will give a better understanding of the following topics:
Compression molding is a manufacturing process where compressive force is used to shape a material to fit the contours of a mold composed of two halves, an upper and a lower part. When the mold halves are pressed together, they create a cavity that shapes the material into the desired form. The design of the mold ensures that the product can be easily ejected once the material has set and cured.
Originally developed for synthetic materials, compression molding is the most cost-effective technique for processing thermosetting plastics. For thermoplastics, however, injection molding is generally preferred.
Compression molds come in several types, including flash, positive, landed positive, and semi-positive molds, with the flash mold being the most commonly used. Compression molding processes are categorized into bulk and sheet molding, with bulk molding involving a blend of fillers, catalysts, stabilizers, pigments, and fiber reinforcers. The key advantages of compression molding for thermoset plastics include enhanced strength, reduced weight, and high resistance to corrosion.
Compression molding is a durable and cost-effective technique used in manufacturing.
Unlike disposable molding methods, the molds used in compression molding are designed for repeated use across several production runs.
Below, you'll find a detailed overview of the main stages in the compression molding process.
Initially, the mold must be set up. Common preparation steps involve:
Compression molding can be applied to a variety of materials, leading to a diverse range of shapes, sizes, compositions, and conditions.
Preparation transforms the material from its initial state into one better suited for the compression process. This preparation may involve any of the following steps:
Charge preparation is typically the most labor-intensive phase of the compression molding process, largely due to its lower level of automation.
This step involves positioning the charge in the lower section of the mold to achieve optimal compression results. The charge is then distributed within the mold according to its shape, desired thickness, and other considerations.
The two mold components move relative to each other, approaching as closely as needed. As they close, they compress the charge progressively. This compression process aims to achieve one or more of the following:
Three key parameters are critical during the compression process:
This stage of the molding process aids in solidifying the compressed charge into the finished product. It might involve cooling the material or applying hardening agents and catalysts to achieve the desired setting and hardening.
Various types of curing methods include:
Curing agents such as Dimethyl stannane and Tetraethoxysilane are utilized for materials including resins, polyurethane, and silicone. These agents facilitate condensation-type curing processes.
Additional curing agents, like organopolysiloxane, are employed to cure various silicones.
Other commonly used agents are Benzoyl peroxide, Peroctoate, and t-butyl perbenzoate.
Cooling serves several important purposes, including:
Ejection involves removing the product from the mold once curing is complete. This process can be either manual or automated. Manual ejection is often used in smaller-scale or hobbyist molding applications, such as the production of medical accessories. Automated ejection usually employs a plunger that extends from beneath the mold or utilizes a separate suction mechanism.
Ejection typically involves the use of a release agent or coating applied to the mold to prevent the product from sticking and to facilitate its removal. This process, sometimes referred to as mold curing, is distinct from the curing discussed earlier. The ejection phase plays a crucial role in determining the geometry of the compression molded products. While products with threads, holes, and grooves can be molded, these additional features can complicate ejection and make automation more challenging.
Examples of release agents include:
Typically, the charge is loaded slightly in excess of the required product volume. This excess material is expelled from the mold cavity at the partition lines as the charge is compressed. After ejection, the excess material, known as flash, remains attached to the product and must be removed during the subsequent de-flashing stage.
De-flashing can be performed either manually or automatically. Manual de-flashing, which involves cutting off the excess material with a blade, is often used for very large molded products that are difficult to handle with automated equipment, or for cost reasons. Automated de-flashing methods include techniques such as water jets and ice blasting, often carried out under cryogenic conditions. Another automated approach is vibration tumbling. The orientation of the flash—vertical or horizontal—depends on the mold's parting line geometry, which is determined by how the two mold parts align.
Below are some of the different types of compression molds:
In flash compression molding, the charge is intentionally overfilled so that excess material, or flash, is produced at the end of the compression process. This causes a small gap between the mold parts, allowing the flash to escape. While this method can lead to considerable waste, it is less likely to result in blistering.
This method requires precise measurement of the charge and does not create a gap between the mold parts at the parting line. It has the following characteristics:
This method is more costly than the other two flash control methods but offers a blend of their advantages. It falls between the two approaches in terms of flash management.
Charge measurement with this method does not need to be as precise as with positive-type molds. Some excess material may be allowed to escape during compression.
As with other areas of industrial technology, compression molding is continually advancing, especially in terms of automation and environmental sustainability.
Various technologies are applied to different aspects of compression molding, including:
Hydraulic systems are commonly employed for pressing in most applications, though lighter presses may utilize pneumatic systems.
The pressing motion is typically vertical, which simplifies the design of the pressing mechanism and its support structure, especially given the considerable weight of the mold. While the pressing mechanism can technically be designed in any orientation, compression is generally applied through a telescoping action as the mold parts come together.
Compression molding employs durable, non-flexible molds, typically made from steel for commercial use. These molds are machined from solid blocks rather than cast, as they are usually not produced in large enough quantities to warrant casting.
Machining can be done manually (e.g., milling, drilling) or through automated processes (e.g., CNC). Steel is the primary material used for molds, and the machining process is generally subtractive rather than additive.
Compression molding is a versatile and scalable technique used across various applications, including:
Bench-top compressors are commonly used for experimentation, such as refining molding techniques and developing new materials. They are also utilized in prototyping, where they compete with 3D printing technologies.
This scale of molding is suitable for recreational use as well. Additionally, due to the relatively simple principles involved, it is possible to assemble the necessary components for demonstrations and educational purposes.
This could be part of auxiliary operations that are outside the core business functions of a company. For example, a firm specializing in servicing irrigation equipment might choose to mold some of the components in-house rather than outsourcing.
This approach is commonly adopted by companies whose primary business involves selling compression-molded products or components. However, the scale of production may be limited by economic considerations.
This scale encompasses both the mass production of molded products and the manufacturing of large-sized items. At this level, automation becomes essential to justify the production requirements.
This process involves combining fabrics with a liquid or molten charge, like epoxies, to create durable products. This technology is increasingly being adopted in the automotive industry.
This technology quickly reduces the pressure in the mold cavity during pressing, resulting in improved surface finishes on the products.
This technology is an advanced form of compression molding. In transfer molding, the charge is held in a transfer port. Initially, the two mold parts are brought together, and then the charge is transferred into the mold cavity. The key distinction from compression molding is the use of the transfer port in transfer molding, whereas in compression molding, the charge is placed in the mold cavity before the mold parts are closed.
Transfer molding is typically employed for producing more complex products that are not feasible with compression molding.
This process is similar to transfer molding, but with a key difference: the charge is injected into a partially closed mold. This partial closure means that the cavity volume during injection is larger than the final volume achieved after compression.
Once the injection is completed, the mold parts fully close to compress the injected charge. This method is generally faster than compression molding but requires more costly equipment.
In this process, a charge is compressed onto a pre-made component, known as the insert. The charge then encapsulates or attaches to the insert, which is prepared separately before being inserted into the mold for encapsulation.
For example, an insert might be a knife blade, and the insert molding process could result in attaching a plastic handle to it. Common inserts also include threads (sometimes referred to as nutsets) and electrical contacts.
Insert molding offers a convenient method for assembling components, particularly those that do not require disassembly during their service life. The advantages of insert molding include:
However, it also comes with the following disadvantages:
Overmolding involves the molding of material onto a pre molded component. It is usually done on materials with different mechanical properties to combine their advantages.
Examples would be a power tool handle and a toothbrush. To produce the handle, an elastomer can be over-molded onto a PTFE substrate (the PFTE substrate molded in some previous stage). The PTFE would provide rigidity for the handle, and the elastomer would improve the handle's grip, ergonomics, and aesthetics.
Although both methods are technically possible, over-molding is more frequently achieved through injection molding rather than compression molding. This is because the substrate for over-molding is typically produced using compression molding.
A variant of over-molding is two-shot molding, which is exclusively performed using injection molding.
Compression molding is recognized as one of the traditional molding techniques, with several others having emerged since. Below are some of the alternative molding methods:
Extrusion molding is well-suited for products with a consistent cross-section or for long items where creating a full-sized mold would be impractical. Even for smaller dimensions, extrusion molding is often faster than other methods, such as injection molding, as it allows for the production of products with a uniform cross-section, which can then be cut to size as needed.
Blow molding is used to create hollow objects, typically with an opening smaller than the final product's cavity. Bottles are a common example of blow-molded items. The process begins with placing a preform into the mold. The mold is then closed and heated, and the preform is blown to expand and fill the mold, taking on its shape in the process.
In injection molding, material is forced into a closed mold under pressure. The mold must be fully closed before the material is injected to ensure it fills the cavity properly. The material’s viscosity must be low enough to flow into the mold effectively under the applied pressure, so the charge is continuously heated. Preforms for blow molding are often produced using injection molding. In practice, any thermoplastic product suitable for compression molding can also be made with injection molding. Charge preparation is simpler with injection molding, as it typically involves using plastic pellets. While injection-molded components face similar geometric constraints as compression-molded ones, injection molding is generally more cost-effective for producing most thermoplastic products.
Thermoforming is employed to produce very thin products, such as fast food packaging. In this process, the material is first draped over the mold and heated. Then, a vacuum is used to draw the material tightly onto the mold's surface.
This method is used to create large hollow objects where blow molding might encounter technical or practical limitations.
Although 3D printing operates on a fundamentally different principle than molding, it competes with molding techniques in the realm of prototyping. Molding methods are typically better suited for mass production, while 3D printing excels in producing a diverse range of designs.
Casting involves pouring liquid material into molds under the influence of gravity. This method is not typically used for large-scale plastic production. Unlike other methods mentioned, casting can utilize pliable and temporary molds.
Most compression-molded products are made from thermosets, though rubber, thermoplastics, and polymer composites are also commonly used. The prevalence and scale of compression molding across different industries are largely driven by demand.
Compression molding is particularly effective for products that are typically flat or have solid, flat surfaces, such as:
Various materials are used in compression molding, including:
Thermosets are plastics that can be melted only once. Once hardened through an irreversible chemical reaction involving polymerization and cross-linking, they cannot be re-melted or recycled. When exposed to high heat, thermosets tend to smolder and char rather than melting.
The inability to recycle thermosets is a major disadvantage, making them particularly difficult to dispose of in an environmentally friendly manner. Despite this, thermosets offer specific properties that make them advantageous for certain applications:
Compared to metals:
Compared to thermoplastics:
Thermosets offer excellent dimensional stability and heat resistance. In molding processes, they are often combined with other materials, particularly carbon fibers, to create composites. Some common thermosets used in molding include:
This material is named for its chemical structure, which features a phenyl group in its monomer. Commonly known as Bakelite, it is valued for its excellent heat resistance and dimensional stability.
This category encompasses various substances due to the presence of an epoxide group in their chemical structure. They exhibit mechanical properties similar to those of phenolic molding compounds.
Polyester can be used as either a thermoset or a thermoplastic.
This is another type of thermosetting plastic.
Thermoplastics can be melted repeatedly. Polyester is one of the few materials to fit in both lists: thermosets and thermoplastics, depending on how it is hardened. Thermoplastics can be disposed of more sustainably. They are relatively low cost. However, their mechanical performance is bettered by thermosets.
Below are some of the thermoplastics used in molding:
Polypropylene foam is manufactured using compression molding with a chemical blowing agent (foaming agent).
This is another example of a thermoplastic.
Polyethylene can be combined with rubber to create a composite that can be molded as an elastomer.
Polyester acts as a thermoplastic if it is not combined with a hardening agent.
This thermoplastic is known for its very high viscosity and excellent non-stick properties.
Polyaryletherketones (PAEK) are used in compression molding to replace metals in specific applications. Polyetheretherketones (PEEK) and polyetherketoneketones (PEKK) are also part of this material family.
Fibers are incorporated into resins to create composite materials. These composites leverage the advantages of their constituent materials, offering improved properties compared to the individual components. For instance, sheet molding compounds are examples of glass-reinforced composites.
Carbon fibers perform a similar function to glass fibers but typically result in a more rigid composite and come at a higher cost.
This material usually consists of two layers of polymer resin, such as polyester, surrounding a layer of glass fibers. A polyethylene film covers the compound to facilitate handling; this film is removed before molding. The finished sheets are typically around 5mm thick. For products requiring greater thickness, multiple layers of SMC can be stacked. There is also a variant known as Thick-walled SMC, which can reach thicknesses of up to 50mm.
BMC is a dough-like mixture of polymer resins, chopped fibers (as opposed to the long fibers used in SMC), and a hardening agent. Loading BMC into the mold involves ensuring the correct amount of charge is used, making it more pliable than SMC.
Chemically, an elastomer is a polymer characterized by its viscoelasticity. Their applications are driven by their insulating properties and resistance to various substances.
Here are some examples of elastomers used in molding:
This is an acrylonitrile-butadiene rubber known for its oil resistance. It can be used in various molding processes, including compression molding, injection molding, transfer molding, and over-molding.
This rubber is resistant to water and performs well against organic acids. However, it has limited resistance to strong acids, ozone, and oils.
This elastomer is highly resistant to ozone and weather conditions, making it suitable for applications like sealing hot water. It also performs well with greases, alcohols, and detergents but is less effective with petroleum fuels.
Viton is one of the most durable and expensive elastomers, known for its high-temperature resistance and performance in exposure to fuels and water. It is commonly used in O-rings, fuel injectors, and boat propeller fittings.
This rubber is suitable for mechanically demanding applications, enduring significant stretching and temperature fluctuations. It also performs well at very low temperatures and is used in aerospace applications and electrostatic discharge protection.
Compression molding offers several benefits, including:
However, there are some drawbacks associated with compression molding, including:
Overall, the advantages of compression molding generally outweigh its drawbacks.
Below are some of the standards applied in compression molding:
Compression molding finds itself amidst a rapidly advancing manufacturing industry, to which new techniques are continually introduced. As a result, it has endured some moderate longevity compared with some methods competing in the same space. Its ability to meet the evolving needs of the industry has been aided by its adaptation into emerging trends, for instance, robotics. In the broader context of the manufacturing industry, compression molding is not an end in itself but a means to some other end.
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