Visteon Corp. has lately achieved positive results using negative thermoforming to produce thermoplastic olefin (TPO) skins for automotive instrument panels. Rather than stretching TPO sheet over a positive tool, which remains the more common approach, negative thermoforming instead draws a heated sheet down into a cavity with help from a vacuum system below the sheet and air jets above it.
Rather than stretching a TPO sheet over a positive tool, negative thermoforming draws a heated sheet down into a cavity for improved cosmetic results.
According to Visteon engineering manager Aaron Wisniewski, negative thermoforming has cosmetic advantages that could further bolster the use of TPO as an alternative to the vinyl that has historically covered instrument panels. "The key is craftsmanship," he says. "Our goal is to produce olefin-based instrument panels with perfect craftsmanship." Negative thermoforming supports that goal in three ways: For starters, the process directly imparts a variety of patterns to the finished parts' visible surfaces as they come in contact with the tool. Wisniewski says that process can produce a wide variety of grains, including checked, leather, or basketball. "We can also get multiple grains on the same part," he says.
And because the method creates the grain during the forming process, it avoids grain loss around corner sections. Wisniewski notes that this problem plagues positive thermoforming—which achieves its grain through pre-patterned sheets rather than the tools. "With negative thermoforming, we get nice sharp corners without grain washout," he says.
Finally, negative thermoforming further supports the integration of seamless passenger-side airbag doors. Wisniewski explains that the custom-formulated TPO capable of a controlled sag into the tooling also happens to have better heat-age properties than previous materials. "That results in airbag doors that open without any fragmentation in a wider variety of conditions," he says.
Right now, skins made in the negative process use conventional foam-in-place urethanes before going over a rigid polyolefin substrate. And that foam seems like a temporary step back from all-olefin instrument panels made from Visteon's in-mold lamination process—which combines a positive thermoformed foamed TPO skin with an injection molded olefin substrate. But Wisniewski hints that Visteon may soon be able to cut out urethanes by using negative thermoforming method to produce foamed TPO skins. "We're getting very close to that," he says.
Visteon started using the new process last year on the Hyundai Tiburon Coupe. And instrument panels with negative thermoformed skins will soon appear on North American vehicles too, Wisniewski says.