Blow molding is a well-established process that lets designers glean more design freedom and drop part cost by easing assembly via parts consolidation.
What is in this article?:
- Blow-moldable TPVs give silicone rubbers the boot
- A few processing and design tips for blow molding Zeotherm TPVs
- Finding the right engineering materials and adhesives
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.