Blow-moldable TPVs give silicone rubbers the boot

Problem is, conventional low-durometer (<45 Shore D) materials suitable for blow molding have notoriously been plagued by one significant limitation: Heat resistance below 284°F (140°C). This has kept them out of many high-temperature automotive boots, bellows, and ducts, though these part shapes are ideal candidates for blow molding.

A recently developed thermoplastic vulcanizate (TPV), Zeotherm 120-90B, from Zeon Chemicals LP, Louisville, Ky., survives temperatures from 40 to 347°F (40 to 175°C) while retaining softness and flexibility. This opens the door for designers to economically mold parts using a number of blow-molding processes including:

Press blow molding is typically used for small parts, such as automotive boots (constantvelocity joint, rack and pinion, driveline). The process gives extremely good control of wall thickness from top to bottom of the part. It can also mold extra features into the top of parts.

Suction blow molding uses a vacuum to draw the TPV parison (tube-shaped molten resin) through the mold. The process creates geometrically complicated parts — i.e., charged air ducts for passenger cars — with minimal flashing. Most moderngeneration, suction-blow-moldingmachines are configured for two or more layers and can regulate the ratio of rigid-to-soft material in the part. With care, this socalled sequential molding technique lets designers retain good flexibility in the bellow area of a part while the rest remains rigid.

3D sequential coextrusion is an older version of suction blow molding. It is the same as suction blow molding except the parison goes into the mold manually or by robot rather than by vacuum. Coextrusion blow-molded Zeotherm 120-90B ducts, for example, outperform ethylene-propylene-diene-monomer (EPDM)/ polypropylene (PP) and copolyester at high temperatures. This technique is commonly used to make clean-air intake ducts.

Additionally, the TPV is a candidate for automotive suspension and steering parts, including inboard constant velocity joint (CVJ) and rack-and-pinion boots. As temperature demands in these types of applications rise to over 284°F the new TPV will let Tier One and OEM firms eliminate the need for heat shield as avoid the expensive use of silicone or ethylene-acrylic rubber boots.

Air-induction systems in turbocharged diesel passenger cars and heavy trucks are candidates for the TPV as well. Here, the ability to blow-mold complex 3D shapes will let designers use a single TPV part to replace entire duct assemblies made from combinations of rubber/metal/clamps or rubber/plastic/clamps.

Side-by-side functional tests of automotive boots made from Zeotherm 120-90B show the boots can survive higher speed rotation and higher temperatures than versions made from copolyester. Tests also show that the TPV better resists aggressive (low-friction) greases compared to HTR-series copolyester.

In side-by-side testing with EPDM/PP TPVs, Zeotherm 120-90B reportedly showed it could stand up to 302°F, (150°C) heat better than EPDM/PP TPVs at 248°F (120°C), and had superior grease resistance.

Unlike EPDM/PP TPVs, the 120-90B TPV extracts little oil from lubricating greases used in rack-and-pinion and CVJ boots, thus letting the grease retain good lubrication properties. In contrast, EPDM/PP TPV and copolyester degraded grease lubricity.

The 120-90B TPV has been optimized for the dominant makes and designs of blow-molding machines that produce automotive boots and air ducts. These include press blow, injection blow and mono and coextrusion blow (3D, sequential, and suction) molding. To ensure part flexibility and ideal NVH (noise, vibration, hardness) properties, Zeotherm 120-90B has been designed with a hardness of 90 Shore A — much lower than copolyesters.

A few processing and design tips for blow molding Zeotherm TPVs

TYPICAL PROCESSING PARAMETERS:
Zeotherm 120-90B and 130-90B have a plastic phase of nylon (polyamide) 6. They are typically blow molded using melt temperatures of 482 to 518°F (250 to 270°C), depending on the part and the particular machine. Because nylon absorbs moisture, it is critical to properly dry the TPVs before blow molding. If the material is not properly dried, the parison may exhibit poor melt strength and have regions of porosity.

TYPICAL PART CONSTRUCTION:

Automotive boots, made from Zeotherm TPVs are typically constructed as monolayer (100% Zeotherm) parts.
For automotive air-duct applications, Zeotherm TPVs are typically used with a high-viscosity, blow-molding grade of nylon. The TPV furnishes flexibility and sealing, while the nylon provides rigidity. Via a sequential blow-molding process, it's possible to regulate the placement of soft and rigid sections in the part. The combination also gives good economics, as nylon costs less than the Zeotherm TPVs.

PART DESIGN RECOMMENDATIONS:

For automotive boots, a rounded convolute shape yields the best results in terms of part durability. The convolute shape is more typical of a rubber-boot design.
For automotive ducts, part design can typically follow what has been commonly used for TPO (thermoplastic polyolefin)/PP ducts. Wall thicknesses are typically 1.5 to 2.5 mm in automotive ducts.

WHY ZEOTHERM TPVs VERSUS OTHER MATERIALS:

For automotive boots, the materials resist heat to 175°C, features dramatically improved hot-grease resistance (Zeotherm is not degraded by hot grease nor are the greases degraded by contact with Zeotherm), and the TPV is softer and more flexible than copolyesters. On vehicles where the automotive boots are close to a heat source (i.e., exhaust manifold, exhaust pipe routing, engine), TPV temperature performance is advantageous.
For automotive air ducts, the TPV is more flexible than so-called flexibilized nylons. It also costs less than silicone rubber + aluminum — which has been the approach of choice at higher temperatures. The material also better resists oil vapors than TPO/PPs and copolyesters.

Zeotherm TPVs in combination with nylon have been a good fit in North America for 2007 model-year turbo-diesel-powered vehicles. New EPA regulations for 2007 are requiring redesign of diesel engines. And, with the redesign, the temperature demands on clean-air intake (air cleaner to turbocharger) and cold-side charged-air duct (innercooler to throttle body) are increasing above where conventional materials can function.

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MAKE CONTACT
Zeon Chemical LP, (866) 493-8765, zeotherm.com