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Compression Molding

Compression molding is a versatile manufacturing process. It is employed in the production of a wide range of components and products in various sectors, particularly those demanding intricate shapes, high strength, and complex textures. This process can be described as squashing a soft and semi-liquid thermoset charge into a simple two-part cavity, wherein it is cured with heat and time to make a solid part that precisely reflects the cavity. Manual extraction from the tool is simple, as these parts are generally made line-of-draw (without undercuts), except that rubber parts can be stretched and extracted from moderate undercuts simply by hand pulling.

This article will discuss the process of compression molding and the types of manufacturing that use the technique.

What Is Compression Molding?

Compression molding is an elegant process with light and simple tooling requirements that can reproduce potentially large volumes of thermoset parts in various rigid and rubber materials. It is a multi-step process employed on specialist compression molding machinery, for mid- to high-volume manufacture of complex, net shape components that offer good accuracy and repeatability and a low production and tooling cost.

How Does Compression Molding Work?

In the simplest terms, a prepared (soft, uncured) charge is placed in an array of lower cavities cut into a preheated platen tool, potentially with up to 100 or more cavities. The upper plate of the multi-cavity tool is then pressed down to precisely locate the lower tool, pressed closed, and squeezed to expel excess material and force the charge to conform to the shape of the cavity.

Retained pressure and the action of the hot tool platens then force the charge material to complete its polymerization cross-linking process to form a non-plastic component that faithfully reflects the net shape of the cavity. This component may be of a rigid or elastomeric material, depending on the nature of the charge.

Once the cure is sufficiently advanced, the tool is opened and the shaped and rigid or rubber parts are peeled out of the tool, generally by hand.

Which Type of Manufacturing Uses the Compression Molding Process?

Compression molding serves to make parts across a wide range of industries and product types. It is suited to making rigid parts in thermoset polyurethanes, polyesters, and epoxies. It also serves in molding composite materials for high-strength, rigid parts. Finally, it is also used for molding complex thermoset rubber parts.

Industries and product sectors that exploit this manufacturing method include:

Electrical: Rigid plastic or composite enclosures for electrical components, providing protection and insulation. This is common in connector parts, insulators, and wire-guides.
Automotive: Parts ranging from exterior buffer panels to dashboards, from interior trim/comfort components to isolators/gaskets/diaphragms and engine mounts in rubber.
Aerospace: Composite structural components, interior comfort components, and panels for aircraft and spacecraft are compression molded. Rubber and high-temperature rubber engine seals, gaskets, and piping components are also commonly compression molded.
Medical: Enclosures for medical devices and equipment are compression molded, with reduced tooling costs compared to alternative technologies. Consumables and single-use rubber parts such as syringe seals are also made this way.
Consumer goods: Rigid and rubber parts for consumer products, such as: appliance components, handles, pipe connections, shock mounts, etc. are made this way.
Sporting goods: Elements of sports equipment, such as: the rigid shells of helmets, protective gear, buffer pads, and elastic components can be compression molded.
Construction materials: Some rigid construction materials like: panels, facades, and architectural components are made this way.
What Types of Materials Can Be Used for Compression Molding?
Compression molding applies to a wide range of materials, each selected based on the desired properties of the final product. Common materials include:

Thermosetting plastics, such as: phenolics, melamine, and epoxy resins, due to their ability to harden irreversibly when cured.
Composites, combining fibers like glass or carbon with resins, are also manufactured this way.
Rubber compounds, particularly in the production of gaskets and seals, are among the most widely produced parts.
Some thermoplastics, particularly polypropylene and polyethylene, can be compression molded, though they are more frequently associated with injection molding. This is an option to consider when volumes are too low to make injection mold tooling commercially viable.

What Products Can Be Made From Compression Molding?

Wherever plastic or rubber parts are required, compression molding is a possible option:

Small- to medium-sized rubber components such as: gaskets, diaphragms, cable protectors, shock absorbers, etc.
Foamed buffer parts and comfort panels.
Larger panels such as car and aircraft interior parts.
Higher-stress structural components such as: brackets, mounts, and reinforcers, made in glass fiber/polymer composites and less commonly carbon fiber-reinforced parts.
Electrical insulators and connector housings.
Lower-volume consumer goods housings.

Where To Look for Molding Services

In seeking compression molding services, you can explore several avenues to find reputable providers. Utilize online directories and platforms that specialize in connecting manufacturers with service providers. Services such as XTJ® list compression molding services and facilitate contact, RFP/RFQ processes, and supplier management. Attendance at industry-specific trade shows and exhibitions can provide direct access to potential compression molding service providers. This allows for direct communication and a firsthand look at their capabilities and approach.

Connecting with industry associations related to specific needs can be very helpful in cutting through the marketing clutter. These organizations often have their directories or can provide recommendations for reliable service providers, based on their own members’ recommendations/experiences. Seek recommendations from colleagues, industry contacts, or other businesses in your network. Word-of-mouth referrals can be valuable in finding trusted service providers to trusted associates. Explore local manufacturing hubs or industrial areas in which it is common to find companies offering compression molding services. Take greater care in evaluation, where no introduction, recommendation, or reviews are available.

What Is the Process of Compression Molding?

Compression molding is a relatively simple process that is essentially identical for large and small parts of most materials. The closure force of tools and the cure times vary according to the nature of the material charge, and equipment varies with tool platen size and charge volume.

The mold, typically made of steel or aluminum, is coated with a release agent to facilitate easy removal of the finished product. This is commonly a silicon oil spray.
The material, usually a pre-measured quantity of thermosetting resin or rubber compound, is placed into the mold cavity either by hand or by an automated dispensing system.
The mold is closed, and pressure is applied to force the material to flow and conform to the mold’s shape.
Heat and pressure are maintained for an appropriate curing time, allowing the material to harden sufficiently for extraction, having taken the shape of the mold.
The mold is opened, and the finished product is removed either by hand or machine.

How Efficient Is Compression Molding?

Efficiency is a complex measure for an industrial process, as it depends heavily on the expectations of the user. This is considerably lower than for comparable injection molding tools for essentially identical components — often cost differentials are a factor of ten in favor of compression molding. Most polymer components are priced on a materials-cost basis, with the manufacturing cost being a smaller proportion of the price, reflecting machine/labor and amortization costs. It is common for compression molded parts to have lower machine and amortization costs but higher labor burdens, so the part-price benefit must be analyzed on a case-by-case basis. Compression-molded parts are slower to produce than the primary alternative of injection molding. Compression molding is commercially viable at quite low volumes and can become cost-challenging at high volumes without automation investment. For many parts, wall sections are too thin to allow compression molding to be reliable, requiring alternatives such as transfer molding or injection molding.

Overall, for parts of moderate to high volume, where a thermoset material is required and the part is relatively thick-walled, compression molding offers a very efficient option to consider for manufacture.

Is Compression Molding More Cost Effective Than Blow Molding?

There is no simple answer to a cost-effectiveness comparison between compression molding and blow molding. It depends hugely on the specific requirements of the project. For high-volume production of simple, hollow parts, blow molding is cost-effective. Compression molding can be a cost-effective solution whenever volumes are lower, parts have complex shapes, tooling costs are a significant factor, and thermoset materials are required. It is worth noting that cases in which compression molding and blow molding are interchangeable options will be quite rare, as blow molding is generally used for making bottle-type components that cannot be compression molded.

To learn more, see our full guide on the Blow Molding Process.

What Are the Advantages of Compression Molding?

Compression molding offers a range of advantages over the clear alternative manufacturing methods of injection molding including:

Lower tooling costs than injection molding.
Suitability for larger parts with complex geometries and varying thicknesses, which may be challenging or costly with injection molding.
Compression molding accommodates a wider spectrum of materials, including: thermosetting plastics, composites, rubber compounds, and even some thermoplastics.
Reduced material waste, compared to injection molding, as sprues and runners for injection molding are generally larger in volume than the overflow/flash of compression molding.
Compression molding suits low-volume production, with low setup and teardown costs.
Compression molding excels in producing thick-walled parts with uniform density.

What Are the Disadvantages of Compression Molding?

Compression molding is a powerful tool, but there are things it cannot do such as:

It is poorly suited for high volume, and the cost of automation can rise to the level of injection molding setup and beyond.
It is a slow process in which in-tool cure is required, reducing productivity.
It cannot mold thin-walled parts.
Tolerances are good, but lower than the precision that can be achieved by injection molding.
Parts may require post-molding cure time to complete their thermosetting process.
Material distribution may be uneven and of poor repeatability, affecting part quality.
While tooling costs are low for small parts, they rise rapidly for larger parts.
Flow in the tool is limited, so poor charge preparation or placement can affect part quality.
Fine detail and surface finishes can be hard to reproduce, as the flow and pressure are both limited.
How Much Does the Compression Mold Cost?
The cost of compression molding varies significantly, based on part complexity, material selection, production volume, and tooling requirements. As a rule, compression molding is more cost-effective for lower production volumes, smaller parts, and simpler part geometries.

Tooling costs for compression molding are often lower than for comparable injection molded parts, making it an economical choice for applications that fit the optimal criteria.

For high-volume production and intricate designs, injection molding offers better cost efficiency.

How Is Compression Molding Tonnage Calculated?

Compression molding tonnage is calculated based on the projected area of the part being molded and the flow characteristics of the material to be molded.

The formula is:

Tonnage = (Projected Area × Material Pressure) x Safety Factor

The safety factor accounts for uncertainties in material flow and other variables. This aspect requires some experience and a good understanding of the tooling, materials, and aspects of processing.

What Is the Cycle Time of Compression Molding?

The cycle times for compression molding vary significantly based on factors such as: the material being used, the complexity of the part, mold design, and the molding equipment employed.

However, typical cycle times for compression molding usually range from around two minutes per cycle, for small parts, and can be up to 10 minutes per cycle for heavier sections that take longer to sufficiently cure.

Higher-cure temperature materials may require longer curing times, impacting the overall cycle time.

When Should Compression Molding Be Initiated?
Where the part conforms to the general requirements of compression molding, an evaluation and RFQ process can be initiated. The basic requirements are:

XTJ  rubber, rigid, or composite components are required.
Parts are of suitable minimum wall thickness to suit the processing parameters of the required material.
The profiles are simple enough to be reproduced in a limited flow, low-pressure process.
Undercuts are absent (for rigid materials) and of modest proportions for elastomers.
The parts are required in appropriate volumes — where small parts can be multi-cavity molded for higher processing throughput, but larger parts are single-cavity tooled.
How Can XTJ® Help You Find Compression Molding Suppliers?
XTJ® offers a comprehensive platform that connects businesses with manufacturing services, among which specialist compression molding suppliers are a distinct and well-represented category.

By accessing XTJ Maching, interested parties can access an extensive database of suppliers, filter by location, capabilities, certifications, and a wide range of other criteria. The platform’s search functionality allows users to locate reliable compression molding suppliers, view detailed company profiles, read client reviews, and assess supplier capabilities.

XTJ® provides a streamlined process for supplier discovery, enabling users to compare services, request quotes, and make informed decisions in comparing competing offers. This resource is valuable for efficiently finding trusted suppliers that align with specific project requirements and quality standards.

What Is the Difference Between Compression Molding and Injection Molding?
Both processes seek to form polymeric materials to conform with an openable cavity, converting the polymer from a viscous fluid to a rigid or elastomeric solid in the molding process.

However, the differences are profound. Injection molding tools are large, complex, and self-aligning machines that are close to receiving liquid polymer injection and then open as the polymer cools to allow ejection. Compression molding tools are two simple, machine-located platens with the cavity split between the two plates.

Injection molding introduces semi-liquid material through a small injection gate and fills the cavity under high pressure, retaining pressure as the charge cools, to compensate for shrinkage. Compression molding charges the lower cavity plate with a prepared volume of part-cured material and then closes the tool to squeeze the charge into the cavity, where it is generally heat-cured to partial hardness. Shrinkage rates are lower but they are not compensated for, allowing some reduction in dimensions.

Once formed, the injection mold tool opens and built-in ejector pins push the finished part and any attached feeder structure out of the cavity. When the compression molded part has cured, the tool is opened and the part is extracted by hand.

Injection molded parts are trimmed to remove feeder residues and they are then considered finished. Compression molded parts may have vent and flash trimming to be performed and then they are commonly put through a final cure process.


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