Urethane Casting vs. Injection Molding: Which One is Better?

Many of the products we use in everyday life are made through injection molding, and their prototypes were most likely made using urethane casting. While both of these methods involve shaping plastics into a final product using molds, they work in very different ways. For starters, injection molding forcefully inserts the melted material into the mold’s cavities, while urethane casting typically lets gravity do all the hard work. When it comes to manual labor, though, casting is a pretty hands-on process, while injection molding is heavily automated. Their differences don’t end there. To let you decide for yourself which method is better (or better suited to your particular needs), we’re laying out all the facts on these two processes below, and there’s even a side-by-side comparison table. Read on to see how they measure up against each other. 

What is Urethane Casting?

Urethane casting is the process of creating parts by mixing and pouring a liquid form of urethane—a thermoset plastic—into a mold that’s made from silicone or other soft material. The filled molds are then placed in an oven for the material to cure, although some urethanes don’t need heating and can cure at room temperature over time. Curing takes a few hours, but some fast-drying urethanes can do it within half an hour. The mold can be opened up when the material has set, revealing the cast part, which is then removed. 

Urethanes need to be properly prepared before they can be cast. Most come as two-part liquid resins that must be mixed in precise ratios before being poured into molds. Once in liquid form, the material needs to be mixed with different chemicals at just the right ratios so that it will cure properly and get the desired properties, for instance, a certain color. The ratios can be adjusted for different perks like more compressive strength or abrasion resistance.

Urethane casting molds are created from a “master pattern”—a replica of the final part that’s usually made with CNC machining or 3D printing. The master pattern then gets liquid silicone poured over it. When it cures, the silicone is carefully cut in half so that the master can be taken out. This leaves behind two mold halves with internal cavities which, when put together, are identical to the final part. 

A single mold can make around 1–10 parts per day, but this depends on the type of urethane used and the shape and size of the part. While the making of the master pattern and mold is fairly quick, taking only one to two days, the same cannot be said of the production process—this can take up to four weeks. 

Advantages

• Tooling lead times are faster as they’re made with 3D printing or CNC machining and liquid silicone
• Cost-effective for prototyping and low-volume manufacturing
• Affordable (tooling costs, silicone for molds, urethane resins, curing agents)
• Flexible silicone molds easy to create

Disadvantages

• Urethane can break down over time, especially in extreme temperatures, UV radiation, and moisture exposure
• Low production volume compared to other methods
• Labor-intensive, manual process that drives up cost per part
• Not suitable for large manufacturing ops
• Master pattern needs expensive SLA printers or CNC machines (better to outsource)
• Silicone molds not as durable as metal molds
• Long production lead times
• Post-processing and trimming needed after part is removed from mold

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 a manufacturing method that forms highly precise products by forcing—or injecting, if you will—molten plastic material into the cavities of a mold at very high pressure. The parts go through rapid cooling which, combined with the forceful material placement, makes this process very fast and repeatable, and ideal for large-scale manufacturing projects. Although it’s mainly used for plastics, there are variations like Metal Injection Molding (MIM) that use metal powders with a binder.

The molds used need to be durable enough to withstand the high pressure and temperatures in the plastic injection molding process, which is why they’re commonly made of hardened steel or aluminum via CNC machining or EDM (electric discharge machining). These methods can create cavities with tolerances typically around ±0.002” to ±0.005” and in some cases, as tight as +0.001”. 

Die sets usually have many different parts like ejection pins, springs, cooling channels, runners, and gates, so manufacturing and assembling these costs a lot of money—anywhere from a few thousand to hundreds of thousands of dollars.

The longest part of injection molding is the making of metal molds because these need a lot of precision and time to perfect. They also need several machining processes and post-processes. Once they’re made, though, the parts can be made very quickly with short cycle times. Some molds have only one or two cavities, so they can make only one or two products per cycle. Others, however, can have anywhere from 50–100 (or more!) and can churn out hundreds, if not thousands, of identical parts a day.

Advantages

• Large and rapid production capacity
• Highly automated process makes price per part low
• Parts are ready for use straight after molding—no curing time needed
• Molds are durable and long-lasting

Disadvantages

• Making the mold takes a long time and needs a lot of precision
• Expensive upfront costs for tooling and die set

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:

1. 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.
2. 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.
3. Part Cost: Urethane cast parts are more expensive than injection molded parts because of the manual labor and low volumes produced by casting.
4. 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.
5. 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 Between Urethane Casting and Injection Molding?

Both urethane casting and injection molding are widely used and relied-upon manufacturing processes, but their pros and cons make them suitable for very different needs. To help you choose the right one for your projects, we’ve put all the necessary information on these two methods in the below side-by-side comparison table.

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