Twin sheet forming enables lightweight surgical helmet design

Developing new production techniques results in award-winning medical device

Original award-winning twin-sheet formed surgical helmet part shown with cutaway.

Twin sheet forming offers several advantages for manufacturers looking for lightweight yet strong hollow plastic parts. Case in point: a medical device OEM partnered with Profile Plastics to create a wearable device for surgeons.

This extreme project pushed the boundaries beyond what most customers need in twin-sheet parts. Even with multiple design changes, the result was an early-to-market commercial success while keeping capital investment low for the OEM. It also won multiple awards for Profile.

Project Background

An existing customer was developing a method to cool orthopedic surgeons while wearing personal protection equipment (PPE). The concept was a helmet with a small built-in fan and air plenum. The OEM was familiar with Profile’s advanced thermoforming capabilities.

The product design team wanted to try twin-sheet forming on this project. The only alternative was an expensive multipart injection molded assembly. The twin-sheet process potentially offered a cost-effective way to produce rigid hollow parts that were extremely light.

The product would have little competition in the market at the time. The device would keep doctors comfortable while supporting a face shield and cover to protect them from contamination.

How twin-sheet forming works

The process involves heating two plastic sheets and using vacuum to pre-form them between two opposite molds. Pressing the molds together fuses the plastic sheets around the outside, forming a single plastic piece without adhesives or fasteners.

The twin sheet forming technique offers some unique capabilities for making hollow parts. It compares favorably to both rotational molding and blow molding processes.

Twin sheet forming, developed from pressure forming, was a relatively new thermoforming method when this project started. Profile gained valuable experience developing new techniques. See several award-winning projects on our awards page.

Challenge: Complex geometry required new levels of accuracy

The central concern for the development team was creating a rigid helmet strong enough to support the fan and attached PPE while staying extremely light. The goal was to use the thinnest walls possible. No other twin-sheet parts in production at the time came close to this level of lightness.

Design tweaks are normal during product development. The helmet, however, required a lot of adjustment because the complex shape had sealed and trimmed edges in multiple planes. The design specified ABS, which offers strength for the material thickness.

Horizontal sealed edges around a twin-sheet formed part, called pinches, are relatively easy. But pinches on vertical surfaces are very tricky. They require highly accurate molds that include some draft along with a lot of testing.

Solution: Develop new processes and more accurate tooling

Profile has a great deal of experience taking on challenging projects. For the helmet, engineers worked closely with both the customer and the mold maker to adjust the design and the tooling. They ended up creating a new forming process to achieve the customer’s vision.

Early samples were produced on a thermoforming machine that was hard to manage. The issue was controlling the sheet. During development, Profile installed a new thermoforming machine that improved sheet control, optimizing the system for twin-sheet forming.

Improving alignment

Accuracy is often problematic for twin-sheet jobs, requiring looser tolerances. Profile’s engineers and the toolmaker jointly developed better tooling. Over the project’s lifetime, the team tried various alignment methods till they found an effective yet affordable system.

Controlling pinches was the key innovation in this project. Improved accuracy was critical for making pinches in multiple planes at the same time. The new process also took advantage of the natural sheet sag, when the heated thermoplastic sheet starts to stretch under its own weight.

Adding lightness

The thin walls made getting the vertical pinches right even more demanding. The original design specified 0.062” starting thickness. In later versions, the gauge was reduced to just 0.050” but the part’s performance and approval rate remained the same.

The extreme thinness was only possible because of the improved alignment Profile developed. Without it, the two sheets might not pinch fully or get pressed so thin the material could tear. Other design changes further lightened the design.

Detailed trimming

Once the helmet was formed, it needed to be trimmed. A trimming fixture holds each piece in place for the CNC machine to work on. A complex part that needs a lot of trimming may require more than one fixture to hold the part at different angles.

The complex geometries of the helmet presented an additional challenge. Profile created a custom trimming method just for this project.

Results

The advancements that Profile created during the surgical helmet’s development resulted in a successful product launch for the satisfied OEM. Twin-sheet forming allowed them to open a new market while keeping their overall capital expenditure low.

Profile was also able to keep up with product design changes, including switching from white to custom silver ABS. The helmet went through 5 generations over the product’s 20-year run. More than 10,000 parts were twin-sheet formed per year during that time.

This extreme project pushed twin-sheet forming to new levels of performance and was recognized with three awards in the SPE Thermoforming Division’s part competition. That included the People’s Choice award, voted on by conference attendees. See it on our awards page.