Design great hollow plastic parts with twin sheet forming

 Why twin-sheet form rather than blow mold or rotomold?

Twin-sheet formed hollow parts can be light yet rigid, as shown in this cutaway

Twin sheet forming expands the benefits of pressure forming into new dimensions. It’s the only thermoforming process that can deliver hollow plastic parts. For OEMs, it offers lightweight and highly detailed parts that provide extra rigidity.

If you need hollow components with the high performance and low capital cost of thermoformed parts, twin-sheet forming may be your ideal solution. Let’s consider the advantages of the process and how it compares to blow molding or rotomolding.

What is twin-sheet forming?

The core of the process is simple. Two thermoplastic sheets are heated at the same time then formed between two opposite pressure forming tools.

Vacuum is applied to preform each sheet. Clamping the molds together fuses the plastic sheets around the outside. The sealed edge is called a pinch. The applied pressure essentially welds the two sheets into a single plastic piece without adhesives or fasteners.

Compressed air (20-100 psi) is then injected into the space between the two sides. This fully conforms the plastic to the contours of the molds, giving both sides a detailed surface. After forming, the part is CNC trimmed like any other thermoformed piece.

The resulting hollow part resembles a blow molded or rotomolded part yet is lighter and stronger with better surface aesthetics. Twin-sheet forming offers unique capabilities unavailable with those other processes. It’s also cost effective for low to moderate volumes up to 10,000 pieces/year, depending on the size of the part.

How the process developed

Vacuum forming, the original thermoforming process, is effective for single-sided parts. It can use a positive or negative mold.

Pressure forming was developed and commercialized in the 1980s. Adding a sealed pressure box or plate behind the plastic sheet allowed air pressure to better conform the sheet to the mold. This improved detail and quality on the side of the sheet contacting the mold. The technique uses negative molds.

The pressure box was central to the success of pressure forming. It quickly became evident that the box could be exchanged with a second mold. Twin sheet forming was a natural extension of pressure forming and gained commercial acceptance in the 1990s.

Evolving thermoforming techniques from vacuum to pressure to twin-sheet forming required better equipment, tooling, and controls. Achieving a high level of repeatable accuracy demands technical expertise and experience.

Unique technical advantages

The advantage of twin-sheet forming is creating hollow parts with highly detailed aesthetic surfaces that rival injection molding.  These parts deliver all the features inherent to pressure forming, with unique abilities that add value compared to other 3D processes.

• Multi-colored parts, using different colors on each side
• Ability to combine different material thicknesses or even types
• Can use multi-layer materials
• Flexibility in seam line placement
• Can be filled or foamed post-process
• Handles difficult draw ratios

Twin sheet forming offers a cost-effective solution for OEM components. For the customer, twin-sheet forming is another thermoforming technique, not a more capital-intensive process. The strength of the technology is in the machine and the company’s abilities.

How does it compare to blow molding or rotomolding?

A comparison table of twin-sheet forming and rotomolding, click the image to download a PDF version.

Consider how the specific benefits of twin-sheet forming make it a key alternative to other processes used for hollow 3D parts.

Rotationally molded (or rotomolded) parts tend to be heavier, with thick corners but thin walls. Twin-sheet parts offer:

• More uniform wall thickness
• Better surface appearance
• Use less resin
• Faster cycle times
• Higher production volumes

The blow molding process also faces limitations. Twin-sheet forming delivers:

• Better surface quality
• Ability to use dissimilar top and bottom colors and/or materials
• Wider parts
• Lower tooling costs
• Cost-effective at lower volumes

Typical applications

A variety of applications in medical, industrial, automotive, or food service markets all benefit from twin-sheet forming. Typical products include:

• Airducts
• Insulated caps or tops
• Door panels or trays
• Engineered shock absorbers
• Enclosed containers, like gas cans
• Recreational products, like kayaks

Examples that push boundaries

Profile’s engineering and manufacturing teams gain new insight from every twin-sheet project, allowing us to further optimize our processes. Our experience has resulted in numerous award-winning OEM projects. On the page, you’ll see these examples like:

• Medical diagnostic equipment air ducts. Profile designed an “inside pinch” for smooth hose connections plus a formed-in attachment tab to reduce assembly time. This product is still in production.
• Body protection cushions. Panels of elastomeric thermoplastic TPU absorb large amounts of energy at a controlled rate. Profile developed sophisticated tools and processes to produce non-planar pinch geometry and close-tolerance vent hole cuts.
• Tankless water heater pressure regulator. Profile engineered a process and advanced multi-cavity tooling to produce pressurized internal air cells with improved yield and service life.
• Surgical helmet. Profile developed unique vertical pinches to produce a complex shape that was extremely light yet rigid. Read our full case study to learn more.

Work with a dedicated thermoformer

Make no mistake. Producing high quality and cost-effective twin-sheet parts is hard. Manufacturers focused on other plastic processes like injection molding usually offer only limited thermoforming capabilities. They’re unlikely to have the expertise and sophistication required for superior twin-sheet forming.

You want to work with a dedicated thermoforming supplier, especially for twin-sheet forming projects with advanced requirements. Benefit from Profile’s deep knowledge and experience.