With sag problems (mostly) history, TPOs seem set for big future

Thermoforming grades of TPO entered the scene over the past several years to displace FRP (fiberglass reinforced thermoset plastics) and other materials in low volume, large part applications. “First generation TPOs received mixed acceptance due to processing issues,” said Hogan. “You can’t take an injection molding grade TPO and translate that to thermoforming as it is difficult to process.” Dow introduced its Engage TPO for thermoforming of automotive interior applications. These polyolefin elastomers are for applications requiring low gloss (less than 1.5% post-thermoformed gloss at 60 degrees). Engage also offers a broad processing window, he said, with enhanced bubble stability for thin-gauge sheet thermoforming, and provides controlled sag across a broad temperature range, reduced thinning and reduced tear. Thermoformable sheets of Engage also demonstrate good scratch/mar resistance, extended weather exposure without surface degradation, and resistance to common cleaning agents.

Problems with processing TPO include excessive sag and inconsistent forming lot-to-lot. “The amount and consistence of the sheet sag has been the number one barrier for TPOs in cut sheet thermoforming,” noted Hogan. “The rate of sag is also an issue,” he added. Dow’s Inspire EFP, a thermoformable grade of TPO, is said to provide reduced sheet sag to enable the forming of larger parts not possible with conventional TPOs. Once formed, TPO parts can take a lot of abuse, making them good for automotive applications.

Spartech Corp. (St. Louis, MO), one of the world’s largest sheet extruders, introduced its newest thermoplastic olefin family, Extreme, last spring. Extreme is comprised of three grades: Extreme LG, Extreme HG and Extreme D TPO sheet. These products also are said to address traditional TPO thermoforming challenges by avoiding problems caused by excessive sagging due to poor melt strength. “Until now, chronic thermoforming inconsistencies have limited the potential of TPOs,” said Michael Mahan, marketing product development manager for Spartech. “With more reliable performance and higher quality than traditional thermoplastic olefins, our Extreme TPO sheet products are opening the door to new applications. This is particularly the case in replacing fiberglass and metal in recreational vehicle, bus, truck and marine components where excellent aesthetics, strength, chemical and UV resistance, durability, and processing flexibility are required. In addition, due to improved processing, Extreme TPO sheet delivers higher throughput and reduced waste.”

The issue of moving FRP applications to TPO continues, Mahan noted. “But we’re actually starting to see the change because of the issue with VOCs (volatile organic compounds) from FRP,” he said.

When designing with TPOs, Brad Rickle, manager of materials development for sheet extruder Premier Material Concepts (Findlay, OH), said that optimization of sheet flatness is the key to good forming. Also, he advised not to “cut” TPO with PP or talc, as it “will degrade the properties needed for the material’s integrity. All TPOs are not created equally - choose the right grade for the application, keeping the proper specs and end-use demands in mind when making a selection.” Also, avoid over-engineering the product, “a practice which is rampant in this industry,” he said.