Blog Post

Posted: 3/15/2024

Cast Urethane vs. Injection Molding

Urethane Casting vs. Injection Molding

How do I know which is right for my project?


Cast Urethane and Injection molding have both benefits and drawbacks. When creating a plastic part, it can be complicated to differentiate which process is best for your application, volume, size and complexity. It is important to understand the advantages and weaknesses of each before deciding on a production process that is right for your project.


Cast urethane is the use of a silicone mold to produce a plastic part using the force of gravity to fill the part. The process begins by creating a master pattern, typically by 3D printing or in some instances, machining. This pattern is then placed into a constructed wooden frame which silicone is poured into, making the mold. This mold is then used to re-produce high quality prototype or production level parts with a two-part polyurethane mixture poured into the mold. Once the part has set, it is then removed from the mold, where it often gets finished and painted, correcting any voids or flash as a result of the low-pressure process. These silicone molds can be re-used up to twenty times and can easily be regenerated as needed.

Injection molding is the creation of a plastic part using a steel or metal mold or ‘tool’ and injecting a molten plastic into the tool. A tool is constructed based on the part dimensions, material selection, and part quantity among other part specifics and is inserted into an injection molding press for molding. In this press, the thermoplastic material, which is selected based on part requirements, is dried, melted, and injected into the mold via a reciprocating screw. The material cools in the mold and is ejected from the machine where it is ready to be handled. Tooling and finishing are then carried out if necessary.


Pros and Cons

Understanding where one process excels beyond the other can help drive the part designer to properly plan for the correct manufacturing process. Below are a few topics to consider when creating a part, and how these processes lend themselves to each.


Receiving prototype parts in a quick and inexpensive way is critical when creating, iterating and launching a product. Cast urethane is a great solution for this since the molds are made of silicone, keeping cost and manufacturing time to a minimum while making it easier to iterate the design. At this stage, getting quick part feedback can help uncover and offer the chance to correct part issues, reducing risk for costly tool modifications or retrofitting if left until the injection molding phase.


Upfront Tooling Costs

While injection molds can be amortized over parts in high volume to flip tool-to-part cost ratio to it’s advantage, the high pressure and temperature environment tends to result in higher tooling costs in comparison to cast urethane. This is due to the need for a complex steel or metal mold with internal systems built into the tool to produce quality parts for a long, repeatable and reliable lifetime. Since cast urethane tools are much more basic and designed for low volumes, the upfront costs tend to be a fraction of their injection molding counterparts. Because of these differences, it is important

to consider estimated annual part volumes and stability of part design when choosing between the two processes.


Part Volume

When it comes to part production, annual and lifetime volumes can quickly help designate what process is best for your application. While the quick turn and low cost tooling benefits of cast urethane outweigh injection molding in those categories, injection molding takes the win when it comes to production volume cost savings. Not only will tool price amortization yield long term savings across a higher part volume, but the cost per part is significantly reduced in injection molding. As a result of the high level of complexity in an injection molding tool and press, the cycle time to create a part can be as short as 30 seconds to minutes. In urethane, the process typically yields one to two parts per day. Injection molding automates the manufacturing process, and you are able to get hundreds to thousands of parts in the matter of days to weeks, all while having the long term benefit of a high quality tool that will last you for years to come.


Why Mack Prototype?

Our company is unique because we have the means to do both urethane casting and injection molding – among other processes – in the same facility. Typically, we recommend starting with the cast urethane to prototype parts, make or approve part design and confirm material selection. Due to the iterative nature of prototyping, our team works closely with customers until they are satisfied with their urethane prototype part.

Once the customer has given approval and frozen the design, we then move forward into injection molding to scale up production volumes. Injection molding has the advantage of manufacturing a medium to high volume of parts in a thermoplastic material consistently and accurately. Our 104 years of expertise in injection molding makes us an excellent extension of your manufacturing arm for plastic parts.


Want more information on which process is right for your project? Contact us!