ABC’s of Blow Molding
By Michael Legault
" Blow molding of complex parts used to be an art, now it's more of a science," says Dr. Changize Sadr, ABC Group vice president at the company's Tech Centre in North York, Ont.Advancement in software...
” Blow molding of complex parts used to be an art, now it’s more of a science,” says Dr. Changize Sadr, ABC Group vice president at the company’s Tech Centre in North York, Ont.
Advancement in software used to simulate process conditions (see box-p.14) has been a major factor in giving part designers the ability to take much of the guesswork out of blow molding, notes Sadr. Programs developed in recent years are allowing the designer to visualize how the parison will form during extrusion, as well as how it will stretch and fill the mold during blowing. Computer-assisted engineering is being used to predict part thickness profiles and structural properties before any prototyping trials have been run. This permits fine-tuning of the part’s design before investment in tooling, after which it becomes more costly to make changes.
The net result of these and other breakthroughs, says Sadr, has been the transformation of blow molding from a technology primarily used to make bottles, to a technology capable of competing with the other advanced plastic forming technologies for a variety of industrial and automotive applications.Today, blow molded exterior and interior automotive parts are able to meet the most stringent safety, performance and aesthetic requirements, while providing cost-effective, value-added features.
Sadr says blow molded automotive parts provide several significant advantages:
Tooling cost savings
Part elimination through integration
Feature incorporation of wiring, duct work or structural components into hollow cavities
Tooling, materials and machinery breakthroughs extend applications .
One specific example of the value-added benefits often afforded by blow molding is an under-the-hood, pressurized radiator surge tank (see picture opposite page) soon to go into production. The tank is designed with baffles which were previously welded together at the wall bends. After prolonged use, high pressures encountered during operation would often cause a rupture of these welds. Sadr says ABC Group Tech Centre collaborated with ABC Group’s tooling and machine building division, Supreme Tooling Group, to produce an innovative tool that permits blow molding of the complex-shaped tank as one single piece. Wall baffles are integral and fully joined without welds or stress risers, thus eliminating the risk of rupture under pressure.
ABC Group began blow molding knee bolsters (the lower section of the instrument panel) for first time on the 1996 Chevrolet Astro and GMC Safari vans. These are the first blow molded parts used on an interior structural application. Changes in the design of vehicle interiors in general, and instrument panels in particular, have been mandated by Federal Motor Vehicle regulations requiring a greater level of passive safety by the 1999 model year. The knee bolster was originally an injection molded part with steel reinforcement. The conversion to blow molding was accomplished by a combination of innovations in design, machinery, tooling and materials, says Sadr.
Three-dimensional (3D), sequential blow molding is the latest technological development to extend automotive blow molding applications. Characteristically, ABC is not only using the technology, but leading in its development by building and using its own patented line of 3D sequential blow molding machines and processes.
Three dimensional blow molding refers to a process in which the blown parison is manipulated in the tool cavity; while sequential blow molding refers to a process which integrates rigid and soft regions on a single tube-shaped part. Sdar says originally both technologies, first developed in Japan, were commercially unfeasible because of long cycle times and inconsistent part thickness. Currently ABC Group has a number of 3D sequential blow molding machines world-wide which are used to make a variety of under-the-hood components. One typical piece made by the technology seamlessly integrates bellows and resonator sections of a manifold duct, eliminating, in the process, three clamps and reducing the number of parts from four to one. The bellows, or soft section is made of a highly cross-linked, vulcanized thermoplastic material, and the resonator, or rigid section is made of filled polypropylene.
FUTURE LOOKS BRIGHT
One walk through ABC’s Technical Centre showroom is enough to convince a person the future is not only in plastics, but blow molded plastics: joint boots, fluid bottles, truck running boards, seat backs, moldings, interior trim components, door pillars, gas tanks….and much more. Sadr notes that about 60 percent of automotive gas tank systems are now made of plastic, and he expects the percentage to grow as new regulations controlling gas permeation rates due to take effect next century are even more stringent than current regulations. Tanks made of multi-layer combinations of polyethylene and ethylene-vinyl alcohol (EVOH) will provide one of the most effective means of meeting these new regulations, says Sadr. CPL