Urethane Casting vs. Injection Molding: Which One is Better?
Learn more about the differences and comparisons of each process.
Casting and injection molding are two common manufacturing methods that are used to fabricate many of the products we encounter every day. Urethane, a thermoset plastic, is commonly used to make parts such as gaskets in automotive settings, casters and rollers for medical carts, and various hardware such as pulleys, bushings, shock absorbers, and scraper blades.
To determine whether urethane casting vs. injection molding is better for your particular application, consider whether the part is for prototyping or mass production and identify how many parts you intend to create. This article will describe the similarities and differences between casting vs. injection molding urethane to help you determine which process is best for your project.
What Is Urethane Casting?
Urethane casting is the process of creating parts by pouring liquid urethane into a mold. The mold is typically made from a soft material such as silicone. Molds filled with liquid urethane are then placed in an oven to cure. However, not all urethanes require heating to cure. Some can cure at room temperature over time. Once cured, the mold is opened and the casted part can be removed. Urethane casting is often used to create finished parts for small production runs or for the rapid prototyping of a product that will eventually be injection molded.
The soft molds for urethane casting are created from a “master pattern.” A master pattern is a 3D-printed or CNC-machined replica of the part to be cast. The master pattern is then dipped in liquid silicone which then cures and sets. Once set, the master pattern is cut in half and the 3D-printed or CNC-machined portion is removed. This process leaves behind two mold halves whose internal cavities, taken together, are identical to the part to be cast.
How Is the Lead Time of Urethane Casting for Production?
Lead times of production-level urethane casting usually run from 3-4 weeks. While the master pattern and mold for urethane casting can be completed in one to two days, satisfying production volumes can take longer. This is because urethane must cure before it can be removed from the mold, and curing time can’t be abbreviated. The production output for a single urethane casting mold is generally about 1-10 parts per day. The output largely depends on factors such as the type of urethane and part geometry.
How Is the Volume Produced by Urethane Casting?
The volume of products produced by urethane casting is small compared to other manufacturing methods. This is primarily because urethane casting is a labor-intensive, manual process. It contrasts sharply with injection molding or CNC machining processes which are heavily automated and require little manual intervention. Urethanes must be prepared properly before casting. Some urethanes may be purchased already in their liquid state. Others are purchased solid and must be melted in an oven.
Once liquified, urethanes must be mixed at the appropriate ratios with different chemicals to facilitate proper curing and to obtain desirable characteristics such as a particular color. Additionally, these ratios can be adjusted to achieve properties such as abrasion resistance or increased compressive strength. While some fast-setting urethanes can cure within 30 minutes, others may take several hours to cure. All these aspects make the process suitable for prototyping and small-scale production runs, but not large manufacturing operations.
How Is the Tools and Parts Cost for Urethane Casting?
The tools and parts required for urethane casting are significantly cheaper than those associated with other manufacturing methods. Silicone for the molds is inexpensive and it’s much easier to create these flexible molds than to make die sets for injection molding. Additionally, urethane resins and curing agents are affordable. The main barrier tends to be the master pattern since SLA printers or CNC machines are very costly. Many manufacturers outsource their master-pattern production to third parties. This, in many cases, is still significantly more cost-effective than other manufacturing methods like injection molding.
How Is the Tolerance of Urethane Casting?
The tolerance of urethane casting is usually +0.015” but can depend on several factors. The type of urethane, part size, and part complexity can all influence the tolerance of a urethane cast part. Tolerances can vary on a case-by-case basis.
Is Urethane Casting Expensive?
No, urethane casting is not expensive. It is cost-effective for both prototyping and manufacturing (depending on the production volumes required).
What Is Injection Molding?
Injection molding is the process of forming highly precise products by forcing molten plastic material into the cavities of a mold at very high pressure. This is unlike the casting process where gravity helps the urethane resin fill out the mold cavity. In injection molding, the pressurized filling of cavities and rapid cooling of parts makes it a significantly faster and repeatable process than casting. As such, it’s an ideal process for large-scale manufacturing operations.
Molds for plastic injection molding are fabricated through methods like CNC machining and electric discharge machining (EDM). These manufacturing processes can form cavities to tolerances as tight as +0.005” on average and, in some cases, may get as low as +0.001” depending on the application. Because of the high pressures and temperatures involved, molds for injection molding are typically made from hardened steel or aluminum.
How is the Lead Time for Injection Molding for Production?
Lead times for the actual injection molding tooling are long. The precision necessary in the metal molds requires time to perfect. Once the molds are completed, however, injection molding production needs less lead time than casting. Large quantities of parts can be completed at a much faster pace. The high pressure and rapid cooling make for a much faster cycle time. While some die sets for injection molding have only one or two cavities, others may have well over 100. This means injection molding can produce hundreds, if not thousands, of parts per day to fulfill production orders.
How Is the Volume Produced by Injection Molding?
The volume of parts possible through injection molding is significantly higher than that produced by casting. Injection molds can have multiple cavities for identical parts and overall cycle times are short.
How Is the Tools and Parts Cost for Injection Molding?
The tools and parts required for injection molding are much costlier than those needed for urethane casting. Die sets for injection molding are typically fabricated from metals such as hardened steel or aluminum and are made to precise tolerances (+0.005” or better). Several machining processes and post-processes are required to create a viable mold. Die sets are typically composed of many different parts such as ejection pins, springs, and limit switches. The cost of manufacturing and assembling die sets for injection molding quickly runs into the thousands, or in some cases, hundreds of thousands of dollars.
How Is the Tolerance of Injection Molding?
Injection molding tolerances are often +0.005” but can be as small as +0.001” depending on customer specifications.
Is Injection Molding Only for Plastics?
No, injection molding is not only for plastics. Metal injection molding is also possible. In this process, granulated metal powder and a binder substance are mixed together and then melted. This molten mixture is then injected into a mold to form parts.
How to Choose the Right Manufacturing Process
When deciding whether urethane casting vs. injection molding is the right manufacturing process, consider the items listed below:
- Production Volume: Urethane casting is better for prototyping or small production runs, while injection molding is better for medium-to-large production runs. Casting can produce 1-10 parts per day while injection molding can produce hundreds, if not thousands, depending on the number of cavities in your mold.
- Tooling Cost: The tooling for urethane casting is much cheaper than for injection molding. Urethane resin, curing agents, and silicone molds for casting are far less expensive than the hardened steel or aluminum die sets necessary for injection molding.
- Part Cost: Urethane cast parts are more expensive than injection molded parts because of the manual labor and low volumes produced by casting.
- Material: Only a limited set of curable urethanes can be used for casting while an extensive variety of thermoplastics and thermosets can be used with injection molding.
- Lifespan: Silicone molds for urethane casting are not as durable as the hardened steel or aluminum injection-ready molds that can last decades. Consider how long you expect to need your particular mold.
What Are the Differences of Urethane Casting and Injection Molding?
Table 1 below shows the differences, advantages, and disadvantages of urethane casting vs. injection molding:
|Urethane Casting||Injection Molding|
Commonly Used For
Does Urethane Break Down Over Time?
Yes, urethane breaks down over time. However, its rate of degradation depends on factors such as extreme temperatures, exposure to UV radiation, and moisture.
Can you Injection Mold Urethane?
Yes, urethane can be injection molded in the form of polyurethane. Polyurethane is a polymerized version of urethane that behaves similarly to common thermoplastics such as polypropylene and polyethylene.
Is Urethane Banned?
No, urethane is not banned. However, there are numerous regulations related to its use and production, especially within the European Union.
Is PVC Good for Injection Molding?
Yes, PVC is a common material for injection molding to make automotive parts, food packaging, and bottle caps, among other things.
This article presented urethane casting and injection molding, explained what they are, and discussed when to use each process best. To learn more about urethane casting and injection molding, contact a Xometry representative.
Xometry provides a wide range of manufacturing capabilities, including casting and injection molding, and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.
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