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Urethane casting vs. injection molding

Many of the objects we encounter in everyday life are made using traditional manufacturing processes. From hardware and automotive parts to electronics, plastic packaging, and more, much of what surrounds us has been thoughtfully designed and manufactured to prioritize functionality.

Two of today’s most common manufacturing methods are urethane casting and injection molding. Both involve adding molten plastics into molds to create parts, but one or the other may be preferable for use with different applications and production run volumes. In this piece, we’ll discuss how they work, the differences between the two, and the types of parts best suited for each process.

Urethane casting: A versatile approach to plastics

The process of urethane casting, sometimes called vacuum casting, begins with creating a master pattern, which is essentially a 1:1 replica of the final part that can be 3D printed or CNC machined. The pattern is then placed in a sealed box, covered in liquid silicone, and allowed to cure. Once set, the block of silicone is cut in half, freeing the master pattern. At this point, the mold is ready for use.

To create parts, urethane casting resin — typically a polyurethane material that mimics the physical properties of the production-grade plastics used in injection molding — is poured into the mold and cured in ovens.

The versatility of the silicone mold-making process allows manufacturers to create parts featuring different colors, textures, shapes, and mechanical properties with highly detailed surface finishes. In many cases, urethane casting produces parts that are as strong — if not stronger than — parts made with injection molding, with greater abrasion and chemical resistance as well.

This makes urethane casting an excellent manufacturing method for producing rubber and plastic parts with joints, logos, and overlapping features, or parts that require high flexibility. This includes applications like electronics coverings, medical equipment, wheels, robotics, and conveyor systems.

Silicone molds can be created quickly and affordably; sometimes for as little as a few hundred dollars, versus the expensive metal molds used in injection molding, which can take weeks and thousands of dollars to create. However, silicone molds are less durable than those used for injection molded parts, meaning they’re more ideal for prototyping and small- or medium-volume production runs.

Injection molding: Mass production made cost-effective

Injection molding differs from urethane casting in a few notable ways. While both use molds to create parts, those used in the injection molding process are typically made of hardened steel or aluminum. Creating these molds is time-consuming and expensive, but because the molds are highly durable, they’re able to withstand repetitive use over multiple production runs. This durability means the molds often pay for themselves by reducing the cost per part over longer periods of service.

Unlike urethane casting, which uses gravity to fill the molds, injection molding uses a pressurized nozzle to fill the cavity with molten plastic. The molds are designed so that the parts cool and can be ejected quickly, allowing manufacturers to rapidly produce high-volume production runs with consistent part quality.

Another difference between urethane casting and injection molding is that injection molding can be used to manufacture both plastic and metal parts. Plastic injection molding predominantly uses production-grade plastics like polycarbonate, PVC, and ABS to mass produce identical items like water bottles, automotive parts, and containers.

Metal injection molding, on the other hand, is cost-effective for manufacturing with expensive metals like tungsten carbide and titanium alloys —  the process generally produces far less waste material than traditional metal manufacturing processes. Due to the expensive costs associated with using metal, the economic advantage of metal injection molding comes in the production of small, complex geometries often found in firearm, medical, dental, automotive, and aerospace parts.

Choosing the right manufacturing process

When it comes to urethane casting vs. injection molding, here are the key takeaways to keep in mind:

  • Cast urethane molds can be created quickly and affordably, but they are less durable. Urethane casting is a better choice for prototyping or creating small production runs of plastic and rubber parts. The versatility of the process allows for a wide range of part specifications, durometers, and geometries not always found in injection molding.
  • Injection molding comes with expensive upfront costs. Creating the mold takes time and resources to ensure that it produces parts of consistent quality and is able to withstand the heat and pressure involved in the process. However, the increased durability of the mold means that it can be used to create higher volumes of parts at speed, which drastically reduces the cost per part.

When you partner with Fast Radius, you can rest easy knowing our teams of experts will work with you during every phase of the production process. We’ll ensure that your designs are optimized and that your choice of materials and manufacturing method align with the desired application of the final part. Ready to get started? Contact us today.

Visit our resource center for more information about urethane casting, injection molding, 3D printing, and the other manufacturing services we provide.

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