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Cast vs Injection Molding

Product designers who are considering material choices for a product or components must keep manufacturing processes in mind as they make their selection. Two common manufacturing processes are injection molding and cast molding. Injection molding is the process by which a material is introduced into a mold under high pressure while in a liquid state. Cast molding, on the other hand, employs the force of gravity to introduce the material into the mold. Each of these processes plays an important role in manufacturing, but there are differences that must be considered when making design decisions. Below are a few of the most important differences between cast urethane vs injection molding.

Cast vs. Injection Molding

Material Compatibility

Each of these manufacturing processes is optimized for a set of materials. In the case of injection molding, which requires materials with very low viscosities in their liquid state, thermoplastic elastomers are most often used, such as thermoplastic polyurethane (TPU) and other plastics. This allows the material to shape. Cast molding most often employs thermoset materials, which are irreversibly shaped as the material cures, as is the case with thermoset polyurethanes. 

Due to their chemistry, thermoset and thermoplastic elastomers display big differences in material properties. Thermoset polyurethanes, for example, have a much wider range of hardnesses, improved abrasion resistance and enhanced resistance to chemicals than its thermoplastic cousin, TPU. The physical properties required for the operation of the product or component being designed will help determine the right material for the application, and in turn the appropriate manufacturing process. 

Tooling and Manufacturing Costs

Due to the high pressures and temperature differentials involved in injection molding, tooling costs can sometime be higher. Tools for injection molding are most often closed metal molds that include complex systems to regulate temperature and material flow. These molds usually require expensive machining and finishing steps. Due to their cost, it may take a long time to amortize the expense of injection molding tools, often requiring very high production volumes.

Cast molding tends to use much simpler mold tooling, and this simplicity usually comes with much lower tooling costs. Cast molding can employ open molds, close molds, compression molds, and many other types of mold designs. This provides for a lot of flexibility when designing the right molding process for a product. As a result, cast molding is often a great solution for products with low to high volumes.

Rapid Prototyping

Prototyping products and components in the intended material in a fast and inexpensive way can speed up your product development process; the quicker you can validate the material in the application the quicker you can bring the product to market. The injection molding process requires hard tooling for prototyping, which comes with higher costs and longer lead times. Moreover, if the material does not satisfy the needs of application, you might find yourself with a pricey mold you can not use, or may be costly to retrofit. The low complexity of cast molding offers the possibility of making  prototypes using soft molds made from a variety of materials. Prototype molds for casting can be easily and quickly made, allowing for rapid and iterative prototyping. 

Reaction Injection Molded part
 A Word About RIM

As a result, this process allows product designers to experiment with various materials, geometries and features. Moreover, it allows you to validate the molding concept prior to investing in costly production-quality tools. Reaction Injection Molding RIM employs a similar process to injection molding, by introducing material under high pressure into a closed mold. RIM, however, is used in combination with thermoset polyurethanes, allowing for the chemical reaction to occur in the mold cavity. This technology allows product designers to take advantage of a much more robust material set, while also enjoying some of the advantages of injection molding. RIM is most often used for manufacturing larger parts, intricate geometries or unique surface finishes that may be difficult to obtain from a casting process.

Considering Reaction Injection Molding  RIM to manufacture your product?                                                                                           Check out our recent blog post, 5 Design Tips for a successful RIM design. 

Conclusion 

When evaluating different manufacturing methods, it is important to consider the materials requirements for a product, tooling and production costs, and how the manufacturing process fits into your product development cycle. Injection molding and cast molding both have their own advantages and disadvantages and understanding these manufacturing methods can help a product designer in making material and design decisions. Identifying the right manufacturing process early in the design process can be the difference between an on-time launch and costly delays. 

______________________________________________________ Other Related Topics ______________________________________________________ 

Thermoset Polyurethane Foam 

Thermoset Polyurethane Conductivity

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