Elastomers in Product Development: Elastomer Fabrication Processes

Elastomers are the hard-working hero of many a product design. In this series of posts, we’re exploring the flexible properties and use cases for a range of elastomers and examining their use in new product development.

Porticos has extensive knowledge and experience with the integration of elastomers into physical devices. We’re here to help; please get in touch!

Read Previous Post: Introduction To Engineering Elastomers

One of the most important contributors to cost is the manufacturing process used to make an elastomeric part. Consider three contenders:

  • An Injection Mold is an expensive tool, but can make complex shapes very quickly and inexpensively.
  • An Extrusion Die is very inexpensive and makes parts very quickly, however, the complexity of the part is severely limited.
  • A Reaction Mold permits complex geometry in high-performance materials, but it makes them rather slowly, which contributes to raising the cost of the parts.

There are dozens of proven (sometimes proprietary) processes. The most common ones have strengths and weaknesses that will influence your material decisions.

Net Shape (Additive) Processes

Casting

Casting is the original process for creating “rubber” parts, and still very common. A liquid thermoset is simply poured into a cavity and allowed to cure.

In reality, “casting” covers a whole category of processes. At one end of the spectrum, hobbyists pour silicone or urethane into hand-made or 3D-printed dies. At the other extreme, highly automated systems dispense the pre-cured material into multi-cavity dies that then apply a vacuum to de-gas the mix before curing.

Image credit: Ariel Cornejo, CC BY-SA 4.0, via Wikimedia Commons

Injection Molding

Injection molding will be familiar to most Product Development engineers. The same process that makes rigid plastic parts is suitable for most thermoplastic elastomers.

Injection molding is fast, repeatable, and easily automated, making it a great choice for high-volume production. However, the sophisticated tooling required involves longer lead times and higher initial costs. It is only suitable for thermoplastics.

Reaction Injection Molding

A conceptually similar process known as Reaction Injection Molding (RIM) accommodates thermosets instead of thermoplastics. This involves forcing uncured thermoset liquid into a cavity, where it cures. 

RIM tooling and molding machines can be less expensive than those used in traditional injection molding because the pressures and temperatures are lower. But curing time is relatively long which usually increases the ultimate cost of parts.

Image credit: Ariel Cornejo, CC BY-SA 4.0, via Wikimedia Commons

Compression Molding

In compression molding, a slug of pre- or partially-cured thermosetting material (“charge”) is placed between two mold halves that are pressed together. The charge is sized to fill the cavity with minimal overflow. The heat does not melt the charge but does accelerate the curing.

Compression molding has some real advantages when geometric complexity is relatively low. “Rubber” keypads are a common example.

Image credit: Ariel Cornejo, CC BY-SA 4.0, via Wikimedia Commons

Transfer Molding

Transfer molding offers a hybrid approach between injection and compression molding. Thermoset material is placed into a “pot” and pressed via a “plunger” into the mold cavity. This process offers slightly faster cycle times and better dimensional stability than compression molding. Most importantly, it accommodates high geometric complexity equivalent to injection molding.

Extrusion

Extrusion involves forcing an elastomer through a die to create continuous profiles. It is applicable to both thermoplastic and thermoset processes. It is extremely inexpensive and results in very little waste, but is limited to constant cross-sectional geometry. Rubber hoses are a common example.

Insert Molding and Overmolding

Most net-shape processes can be modified to add elastomer to existing components. For example, the vinyl insulation on electrical wires is created by being overmolded on an extrusion die. When the substrate is rigid, we call it Insert Molding. When a single, sophisticated mold makes both the rigid substrate and the elastomer, we call it Overmolding. Overmolding is a very common use of thermoplastic elastomers; your toothbrush probably has an overmolded handle.

Subtractive Processes

Die Cutting

This method involves a “cookie cutter” blade that cuts shapes from a sheet or roll of elastomer. This is a common way to make flat gaskets and some finished products like silicone trivets.

Laser / Water Jet Cutting

Large or complex shapes may be cut on a CNC machine, using either a laser or an abrasive water jet. Laser and water-jet cutting can be used to cut out flat elastomeric parts without the need for any additional tooling. 

Porticos has used our own CO2 laser to prototype gaskets and flexible spacers created with an array of different elastomers.

Traditional Machining

Very high durometer (hardness) elastomers can be machined. However, the difficulty associated with fixturing flexible components makes this process expensive and tricky. It is rarely used.

Comparing Solutions

As stated at the outset, the multitude of processes each comes with its own pros and cons. The table below helps highlight the characteristics of each solution and offers a quick comparison of likely contenders.

Next Up

There are a lot of elastomers out there. What justifies so many contenders? How many niches can there be? In the next post, we’ll look at the unusual or exemplary material properties that guide you towards a selection.

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About Porticos

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Porticos, Inc. is a Product Engineering and New Product Development firm located in Research Triangle Park, NC.

Established in 2003, Porticos produces innovative and effective solutions for their clients and the markets they serve. Porticos provides broad expertise in development, planning, and production. 

Contact us for more information or support bringing your idea to market.