Injection Molding vs. Compression Molding: Differences and Comparison
Learn about the differences between these two processes.
Injection molding is a plastics manufacturing technology that is capable of very high production volumes. The process works by injecting liquid plastic into a mold at very high pressure. Compression molding, on the other hand, is better suited to medium-volume production and is typically used with rubbers and silicones.
Injection molding is better suited to complex parts, whereas compression molding is better suited to simple geometries and large panels. Injection molding is highly automated and does not need constant human supervision, while compression molding often requires human intervention to load a charge and remove completed products. Tooling is also cheaper for compression molding. This article will compare injection molding vs compression molding in terms of the manufacturing process, materials used, and production rates.
Injection molding works by injecting molten thermoplastic into a mold cavity. The first injection molding machine was developed in 1872 by John Wesley Hyatt. In 1946, James Watson Hendry built the first screw injection molding machine, which is what modern machines are based on.
Injection molding melts plastic resin pellets inside a barrel. The barrel contains a screw with a shaft whose diameter increases along its length. As the screw rotates, plastic is forced into an incrementally smaller volume. This compression is the primary mechanism responsible for melting the plastic. The barrel is also heated to supply additional heat. Once enough plastic to fill the mold has melted, the screw retracts, then is pushed forward to force the plastic into the mold at very high pressures.
Molds are made from at least two parts and are held together with hydraulic rams so that the high-pressure plastic injected into the mold cannot escape from the parting line. The mold has cooling channels that cool the part enough so that it can be ejected from the mold without damaging it. Injection molding can achieve high production volumes unmatched by any other plastic processing technique. Compression molding, on the other hand, is better suited to medium-volume production. For more information, see our guide on what is injection molding.
A simple injection molding setup is shown in Figure 1:
Injection molding setup.
Image Credit: Shutterstock.com/Moreno Soppelsa
Listed below are some advantages of injection molding compared with compression molding:
- Injection molding is a high-volume production method that can produce millions of parts per machine annually.
- Due to the high levels of automation and rapid manufacturing speed, injection molded parts are cheaper than compression molded parts.
Listed below are some disadvantages of injection molding compared with compression molding:
- Injection molding requires very high plastic injection pressures. The mold must therefore be built to withstand these high pressures. This adds cost to the tooling.
- Injection molding is not well suited to manufacture large, thin parts like vehicle panels.
Compression molding is a process most often used to manufacture parts from pliable materials. It was first developed in 1905 to produce bakelite parts. Compression molding typically employs a two-part mold, with one part fixed to the base of the compression molding tool while the top half is free to move up and down. The molds are usually heated to help cure the product while also heating the charge before compression. A precisely weighed-out charge is heated until it is pliable. It is then placed into the bottom half of the mold. The top half then moves down and forces the charge to conform to the shape of the mold. The heated mold stays closed while the material is allowed to cure. This typically takes a few minutes. Once cured, the part is removed from the mold. Post-processing is required to cut off any flash. Flash (or excess material at the parting line of the mold) is a common occurrence for compression-molded parts.
Compression molding is used for creating reinforced panels. The process involves laying out fiber-filled plastic sheets in the bottom half of the mold. Thereafter, the top half of the mold compresses and heats the material to force the plastic throughout the mold, creating a composite part. This is how large plastic car bumpers are made. For more information, see our guide on what is compression molding.
A simple compression molding setup is shown in Figure 2 below:
A rubber compression machine.
Image Credit: Shutterstock.com/Soon_Sound
Listed below are some advantages of compression molding compared to injection molding:
- Compression molding can make use of SMC (Sheet Molding Compound) and BMC (Bulk Molding Compound) to create composite panels. These panels do not have continuous fiber-impregnated sheets, but rather chopped fiber strands that are distributed throughout the bulk material. The injection molding process cannot make composite material parts.
- Compression molds do not need to withstand high internal pressures as do injection molds. Instead, the only load they experience is a compression force from above. This means that compression molds tend to be less expensive.
Listed below are some disadvantages of compression molding compared to injection molding:
- Compression molding is a very manual and slow process. Even if a robot arm is used to add the charge and remove the product, the cycle time between parts is still much slower than injection molding due to the need for the parts to be set before being removed.
- Compression molding cannot create parts with high levels of complexity. The materials used are typically very viscous and as a result, they don’t flow well into small, complicated features.
|Attribute||Impression Molding||Compression Molding|
High-volume production process
Thermosets, thermoplastics (including fiber and metal-filled), thermoplastic elastomers, metal-filled polymers, fiber-filled thermoplastics
Thermosets, thermoplastics, silicone, unvulcanized rubber, BMC, SMC
High tooling cost
Can produce large, thin-walled panels
Injection molding is better suited to high volume production whereas compression molding is better suited to pliable and flexible materials. Compression molding also has cheaper tooling costs.
When it comes to individual part cost, injection molding can produce inexpensive, complex parts. However, this process is only cost-competitive if production run volumes are large enough to spread the high cost of injection molding tooling over a sufficient number of pieces. At lower volumes, compression molding is cheaper than injection molding due to the lower initial mold costs.
Injection molding is significantly faster than compression molding. The cycle time for injection molded parts can be a few seconds, whereas compression molding cycle times can easily reach a few minutes. This is because compression molded parts often need to cure before they are removed from the mold.
Injection molding is a high-volume production method that is unmatched by any other manufacturing technology. Injection molding processes can be fully automated, while compression molding often requires a person to place the material into the mold, remove it, and post-process it.
Injection molded parts can be manufactured from. The process can make use of rigid thermoplastics as well as thermoplastic elastomers and thermoplastic urethanes. Compression molding more often makes use of flexible elastomers like rubber and silicone, but can also produce rigid composite components by making use of SMC and BMC.
Despite having distinct technology differences, a mutual alternative to injection molding and compression molding is:
- Transfer Molding: Transfer molding has characteristics of both compression molding and injection molding. The process works by pressure forcing the casting material into an enclosed mold. In some cases, the raw material is solid and melts due to the compression pressure. Rubber, silicone, and normal thermoplastics can be used in this process.
Listed below are some similarities between injection molding and compression molding:
- Injection molding and compression molding can both produce plastic parts.
- Compression molding and injection molding both make use of heat and pressure to produce plastic parts, albeit with different mechanisms.
Below is an alternative to injection molding:
- Injection Molding vs. Blow Molding: Blow molding is another high-volume production technology that transforms thermoplastics into products. Blow molding, however, is specifically suited to the manufacture of thin-walled, hollow containers. For more information, see our guide on Blow Molding vs. Injection Molding.
Below is an alternative to compression molding:
- Compression Molding vs. Urethane Casting: Urethane casting is a low-volume production method for producing parts. Casting in the plastics industry typically refers to the casting of polyurethane or nylons. The process involves pouring a liquid thermosetting polymer into a mold under atmospheric pressure to produce parts. The thermosetting polymer will then cure in the mold before it can be removed. Casting is capable of producing parts with fine features.
This article presented the comparison of injection molding to compression molding, explained what they are, and discussed the different attributes of each. To learn more about injection molding and compression molding, contact a Xometry representative.
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