Prototype with Overmolding, Liquid Silicone Rubber, and Multi-Axis Machining
John Hayes posted on February 12, 2016 |

Engineers often find themselves in a rush to get a prototype of their part. They can find a local 3D print service, hope a machine shop will fit their single part onto a CNC machine, or hunt down a custom mold shop. Alternatively, they can upload their part file to one of many online services.

We recently caught up with Rob Bodor, VP and General Manager of Proto Labs, to learn about some new materials and processes available for rapid prototyping within their typical 1-3 delivery window.

Overmolding Available as a Rapid Prototyping Service

Overmolding is the injection molding process that molds one material (typically a TPE) onto a more rigid substrate. For example, a toothbrush handle is made with a rigid material that also has a soft material overlaid to provide grip. 

This knob is an example of a part made through overmolding.
This knob is an example of a part made through overmolding.

To achieve this result, the prototype shop first shoots the substrate from a rigid plastic. It then molds another softer material on top of that using either the same mold or another mold.

Rob said, “Customers have long been asking for overmolding. Until now, Proto Labs has been meeting their requests occasionally through a custom one-off process.”   Overmolding is a difficult process to introduce at scale because of the complexity of both the algorithms and the user interface that together determine how many molds are required. The company has now invested in a system that tracks overmolded prototypes from end to end with little human interaction, according to Bodor. The service is expected to launch later in 2016.

From a customer’s perspective, they will upload their CAD part files and the assembly file onto the Proto Labs web site to receive a quote.  If they choose to proceed, they confirm materials and a delivery date.

In the knob shown above, there is a harder substrate with a softer plastic overmold that is chemically bonded to the substrate. In this example, you can see that the substrate forms the structure and the shiny top of the knob, whereas the overmold provides the tactile control for a softer touch.

Liquid Silicone Rubber for Lenses

Liquid silicone rubber (LSR) is making headway in lens applications. LSR lenses can be flexible and deformable but return to their original shape. There are other advantages to LSR lenses:

  • Non-corroding
  • Withstand a wide range of temperatures without becoming brittle
  • Can be designed to be self-sealing. 
Automotive lens in liquid silicone rubber.
Automotive lens in liquid silicone rubber.

There are many other applications for LSR, all of which are now available as a rapid prototyping service.

CNC Turning

Turning parts on a lathe is practically as old as machining, and now Proto Labs has added it to its rapid prototyping services. Previously, the company had made parts with cylindrical features on a milling machine. Given that turning is more efficient for cylindrical parts, and results in a superior surface finish, you may wonder that it took Proto Labs so long.

Part created on a combination lathe/4-axis milling machine by Proto Labs.
Part created on a combination lathe/4-axis milling machine by Proto Labs.

The reason is that Proto Labs relies on automation to efficiently scale their quoting process and prototype production, so they had to develop sophisticated software for CNC turning. For example, when a customer uploads a CAD file, Proto Labs software will analyze whether the product should be made by turning or milling before providing a quote to the customer.

Let’s say you have a part like the one shown above. It is cylindrical around a center axis, but it also needs additional features. If there is a clear benefit to milling over turning, the software recommends that option. If, however, it is not so clear-cut, Proto Labs offers the customer a choice. 

Part turned and machined at Proto Labs.
Part turned and machined at Proto Labs.

Bodor said developing the software for turning is not trivial: “We were doing machining for seven years before we introduced turning.” With milling, the part is stationary and the cutting head moves. With turning, the roles are reversed and the process is quite different.

Proto Labs has standardized its procedure with HAAS machines with three milling axes plus turning. This allows the machines to turn a cylindrical part and then add a feature by machining it.

Proto Labs has grown by automating in unlikely places

The company has an impressive $1.4B market value because it has been able to create repeatable prototyping services at scale. It has shared some secrets to its success with other manufacturers, such as:

  1. Automating certain manual low-value processes, such as quoting.
  2. Standardizing the machining processes when you can’t standardize the parts.
  3. Using the Internet to find ideal customers.
  4. Charging a premium for doing something that nobody else wants to do, such as quick turn prototypes.

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