Injection Molding vs. Compression Molding—What’s the Difference?
All about these traditional plastic part-making processes.
Both injection molding and compression molding are considered traditional methods of making plastic parts. They both rely on heat and pressure but use different approaches to shaping the materials. So what’s so different about them, and when would you use either one? Let’s find out.
What is Injection Molding?
The injection molding process involves injecting molten thermoplastic into a mold cavity. The machine melts the plastic resin pellets inside its heated barrel, then a screw inside turns, forces, and compresses the pellets forward into smaller spaces at high pressure. The screw itself isn’t heated, but the mechanical force it generates, along with the heat from the barrel, is enough to keep the plastic in melted form. The following image gives you an idea of what a simple setup looks like:
The process hasn’t changed much since the first screw injection molding machine was developed by an American inventor named James Watson Hendry in 1946, and most modern equipment is still based on his design. Hendry’s design was based on the design of the first injection machine made in 1872 by another inventor from the U.S., John Wesley Hyatt. But this didn’t have an advanced screw mechanism.
The mold typically has two parts (sometimes more) that are held in place by hydraulic rams so that no plastic can escape through the cracks. There are also cooling channels that help the plastic harden before it’s time to take it out to avoid it getting damaged. You’d be hard-pressed to find another method as effective for mass production; it can tackle huge runs – we’re talking millions of parts per machine per year.
This method has another advantage, too: it’s highly automated and very fast, which ultimately makes its parts relatively cheap. Some of the potential drawbacks, though, include the fact that the mold needs to be able to withstand the high pressure, so tooling could work out a little more expensive. Also, for large, thin parts like vehicle panels, you’re probably better off using a different manufacturing technique.
What is Compression Molding?
Considered one of the most popular ways of creating parts from pliable materials, compression molding was first developed in 1905 as a way of making bakelite components. This type of molding usually uses a two-part mold, with the bottom part securely attached to the base of the tool and the top half free to move up and down. They’re generally heated to help cure the piece (sometimes called the charge) and get it ready for compression. The charge is weighed and placed inside the bottom half of the mold before the top half moves down and squeezes it into shape. The heated mold then stays closed for a few minutes to allow the charge inside to cure. After that, the part is removed and moved to post-processing, where any flash (excess material that builds up at the parting line) is removed. This is really common when it comes to compression molded parts.
This process is used extensively to create reinforced panels. For instance, fiber-filled plastic sheets are laid out in the bottom half of the mold, and when the top half compresses the material to take its shape, it creates a composite part. That’s how large plastic car bumpers are made. You can read our guide on compression molding to dig a little deeper.
Compression molding has lots going for it. For example, it can use SMC (sheet molding compound) and BMC (bulk molding compound) to make composite panels. Instead of having a continuous string of fiber running through them, they have chopped strands that are distributed throughout the bulk material. Other processes, like injection molding, can’t make composite parts. Another advantage is that compression molding can work out a bit cheaper because it doesn’t rely on extreme internal pressure. The downsides? It’s not the fastest procedure out there, and it can’t really make highly complex parts. That’s because the materials tend to be rather viscous and won’t flow very well into tiny little cavities to make intricate designs. The following image further illustrates how this works:
Frequently Asked Questions on Injection and Compression Molding
What materials are compatible with these processes?
Injection molding is fine with rigid thermoplastics, thermoplastic elastomers, and urethanes. Compression molding is usually used in conjunction with flexible elastomers, like rubber and silicone, but it can also produce rigid composite components, as mentioned earlier with SMC and BMC.
Are there any mutual alternatives?
The closest is probably transfer molding. It has certain characteristics of both and works by pressure-forcing the material into an enclosed mold. Sometimes, the material is actually solid but melts because of the pressure. Transfer molding is fine to be used with materials like rubber, silicone, and even normal thermoplastics.
Injection molding vs. blow molding—what’s the difference?
Blow molding is another way of fulfilling high-volume orders by transforming thermoplastics into other products. It’s more commonly used when trying to create hollow containers with thin walls. We actually have a whole article on the differences between the two. Why not check it out?
Compression molding vs. urethane casting—what’s the difference?
Although they sound nothing alike, urethane casting is somewhat similar to compression molding in that it creates parts in small batches and can make detailed components, too. When it comes to plastics, it usually works well with polyurethane or nylons. Molten plastic is poured into a mold at normal air pressure and removed when it’s hardened.
How Xometry Can Help
If you want to know anything else about molding or other manufacturing techniques, you can reach out to one of our representatives who would be happy to help. Xometry also offers a huge range of related services including CNC machining, 3D printing, and laser cutting. Why not request a free, no-obligation quote on our website and get your project started right away?
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