Urethane casting is a low-cost molding process for the fabrication of rigid or rubber-like plastic parts. Suitable for prototypes and certain end-use parts, urethane casting does not require costly hard tooling, instead using silicone molds to make a limited number of castings.
Customers have several options when it comes to urethane casting. In addition to the choice between rigid or flexible parts, they can easily add color to their castings — either during or after manufacturing — and can even use transparent materials to create items like clear electronics casings and presentation boxes.
This article discusses some of the key aspects of urethane casting: what it is, what kind of materials you can use, how you can add color or transparency to cast parts, and examples of when urethane casting is an appropriate technique to use.
What is urethane casting?
Urethane casting is a production-quality manufacturing process for short-run batches of parts. It is similar to injection molding, but the tooling used is soft and flexible rather than metal. Because of this, urethane casting is much more cost-effective than injection molding, especially for low-volume production, and offers shorter lead times.
Urethane casting produces parts that are generally less strong than injection molded parts but which have an excellent surface finish. These high-quality parts can be made to fairly tight tolerances and require minimal post-processing.
The urethane casting process goes like this:
- We obtain a positive master pattern or fabricate one using a technique like Stereolithography 3D printing or CNC machining.
- We set the master pattern in a container of liquid silicone, creating a negative mold.
- We cut the fully cured silicone mold in half and remove the master pattern.
- We fill the silicone mold with urethane and allow it to cure to make the cast part. We can reuse the silicone mold around 20 times before disposing of it.
Urethane casting may sometimes be referred to as vacuum casting or urethane molding. However, it is not always necessary to apply the vacuum procedure to make urethane parts.
Read our article on how to make a silicone mold for more information about the silicone mold making process.
Urethane casting materials
Urethane casting does not offer a huge number of material options when compared to, for example, CNC machining (compatible with a wide range of thermoplastic polymers). However, it does work with two distinct types of urethane casting resin, giving manufacturers an important element of flexibility in their product development.
It should be fairly obvious whether a part requires a rigid or elastomeric material depending on its end use. Rigidity is essential for most mechanical functions, while elasticity can be useful for shock absorption, gripping, and other uses.
Shore D urethanes are a group of rigid urethane plastics that are often tough and durable and which can be used for final parts like enclosures, cases, consumer electronics, toys, packaging, lenses, medical and scientific devices, and panels.
Shore A urethanes are a group of soft and urethane plastics that are flexible with good impact resistance. They share physical properties with rubber. These urethanes can be used for items like rubber grips and skins, overmolds, buttons and key pads, cable connectors, and even rubber boots.
Colored urethane cast parts
Although urethane casting does not offer a wide material selection, it does offer excellent coloration options, including cast-in colors.
This can make urethane casting a good choice for toys, consumer goods, and branded products that need to match a company’s corporate color scheme. It also allows R&D departments to prototype products in a range of colors.
Because urethanes are injected into the silicone mold in liquid form, it is relatively easy to add pigments to the liquid mixture.
Taking this approach to colored urethane parts produces several benefits. The applied color will not chip or peel, since it is mixed with the plastic right through the part, not just on its surface. Furthermore, it does not affect the surface texture of the part, which may be important for some applications.
On the other hand, it can be difficult to establish the correct amount of dye to add to the urethane prior to casting, so it may be necessary to discard test castings before the correct hue is found.
Color can also be applied to urethane parts after casting is complete, in the form of paints applied to the surface of the part.
Painting urethane parts gives manufacturer access to a wider range of colors and allows them to adjust gloss level. Paint can also be used to smooth over the surface texture of a part. Additionally, high-quality paints are less likely to fade after exposure to UV light than internal pigments.
The drawback of painting urethane parts is that paints are liable to chips and scratches. It also demands an entire extra step in the manufacturing process, adding to the overall project budget.
Transparent urethane cast parts
One of the benefits of urethane is its availability in clear formulations. Clear resins are useful for items like electronics enclosures, lenses, and lighting components.
In most cases, a vacuum chamber is used on clear parts to remove air bubbles, which would otherwise diminish the appearance of the transparent cast parts.
Clear materials can also be mixed with pigments for colored clear parts.
Urethane casting compared with alternatives
Urethane casting vs Stereolithography
Like urethane casting, another common manufacturing process for the creation of clear parts is Stereolithography (SLA), a laser-based additive manufacturing (3D printing) process. SLA is used in dentistry, jewelry making, and other disciplines that require detailed clear parts.
SLA can produce more complex geometries and finer features than urethane casting; after all, urethane casting sometimes uses an SLA printed part for the master pattern, and there will be some loss of detail after the molding and casting.
But urethane casting may be better if several copies of the part are required. Furthermore, it is not possible to add pigments to SLA resins, while it is possible for casting urethanes.
Urethane casting vs CNC machining
Urethane casting and CNC machining are vastly different production techniques, but they can occasionally be used to produce similar parts. Both can make rigid plastic parts like enclosures, for example, and both are suited to low-volume production runs.
CNC machining is much more precise than urethane casting and can create more detailed features. It is also compatible with a diverse array of materials, unlike urethane casting.
The urethane casting process is much cheaper in any volume, but especially when multiple units are required.
Urethane casting vs injection molding
The processes of urethane casting and injection molding share many similarities, with the key difference being the mold material: urethane casting has a soft silicone mold, while injection molding requires metal tooling.
Injection molding is much more suitable for medium to large order volumes and for parts that need to be stronger.
When dealing in small quantities, urethane casting is a good low-budget alternative to injection molding, since no expensive metal tooling is needed.
Urethane casting applications
Urethane casting is widely used for prototyping, but it is also a production-quality process suitable for certain end-use applications. Because although urethane cast parts are not especially strong, they possess other desirable qualities.
Similar to food-safe products, medical devices can benefit from urethane casting due to the non-toxic materials involved.
However, it is important to remember that not all castable urethanes are biocompatible.
Housings and enclosures
Coloration and transparency options make urethane casting suitable for the fabrication of housings and enclosures for items like consumer electronics and custom medical devices.
It is possible to achieve overmolding with urethane casting, which may, for example, be useful for setting electronic components within a rubberized shell.
Some polyurethanes are suitable for the production of sporting goods like small wheels, dumbbell grips, underwater components, and helmet padding.
Low-volume production parts
Urethane casting is regularly used to low-volume production in batches of up to 20 units. It can be especially useful for large parts like car bumpers, which would be very expensive to machine or mold with metal tooling.
Due to its speed and low cost, urethane casting is a fairly versatile solution to various prototyping tasks. It may be used to prototype parts that will eventually be mass produced using injection molding.