— Let’s shape up
Wednesday, January 21, 2015
The thermoforming process typically consists of heating thermoplastic sheet, film or profile to its softening point and then forcing the hot, flexible material against the contours of a mold in three ways:
- pneumatic means — differentials in air pressure are created by pulling a vacuum between the plastic and the mold, or the pressure of compressed air is used to force the material against the mold
- mechanical means — plug, matched mold, etc.
- a combination of pneumatic and mechanical means
Recent advances in thermoforming have come in both software and machinery. For example, injection molders are not the only plastic processors that can benefit from process simulation software. Accuform SRO has developed T-SIM, a thermoforming simulation software package. A consortium of 11 European thermoforming companies supported the development of the software.
T-SIM simulates positive or negative forming with or without plug assist. The software predicts the final wall thickness distribution based on the specified processing parameters (the pressure level, the speed of tools, the sheet temperature distribution, etc.). Time-dependent sheet sagging is included.
T-SIM is also able to predistort images for printing them on the flat sheet, so that once thermoformed, the images appear true. An image projection manager enables projection of multiple images using various projection methods (planar, cylindrical and spherical projection).
Let’s delve further starting with custom thermoformers like the Profile Plastics Corporation, who are specialists in heavy-gauge, custom-molded, highly engineered plastic parts manufactured via vacuum, pressure and twin-sheet thermoforming processes. The company has prospered for 50-plus years by steadily investing in and innovatively applying the latest technology.
Geiss thermoforming machine halogen heating element.
Profile’s use and development of an unconventional vacuum-forming machine from Geiss Thermoforming USA is an example. The unit was one of the first halogen-heated, in-line closed-chamber-style machines in the U.S.
Profile specializes in large, technical parts for medical, analytical and electronic equipment, as well as appliances and materials-handling components. The acquisition of the Geiss machine with its halogen heating has helped Profile to maintain its momentum by extending its use of heat-sensitive materials.
Of the successive operations (clamping, heating, forming, cooling and trimming) carried out to produce a thermoformed part, heating appears to be the critical step. The sheet has to be evenly heated at the proper temperature.
Heating is conventionally performed by means of medium-wave (quartz) or long-wave (ceramic) radiators, but the Philips infrared halogen lamp is providing a great improvement in the heating process. For a given installed power, infrared halogen lamps are more efficient as they create a higher irradiance at a given distance from the plastic sheet, compared to both quartz and ceramic.
Although it takes minutes for quartz and ceramic radiators to reach their operating output, infrared halogen lamps (short wave emitter) only require a few seconds to get the same level of energy. Infrared halogen lamps also generally give a better temperature gradient over the plastic thickness. This is due to short wave radiation, a unique feature of infrared halogen lamps that has been shown to be more penetrating than long wave or even medium wave radiation.
Infrared halogen lamps can be instantly adjusted to the optimized heat level by simple dimming. Adaptation to various kinds of shapes and colors is not a problem. Infrared halogen lamps are 100 percent dimmable, which allows fine tuning of the process.
Profile’s goal is to thermoform parts equal to injection molding in look, quality and precision, while outdoing them in design ingenuity and economy. Such an objective was not feasible 20 years ago, as it was not possible to trim parts with precision equal to injection molding, and it was difficult to measure part dimensions to guarantee quality on a repeatable basis.
Multiaxis CNC routers provided a way to address trim-speed and precision, and Profile pioneered their use in thermoforming. All of Profile’s parts are now CNC-trimmed. Profile also addressed the need for precision measurement by investing in coordinate-measuring machines, computer-aided devices for measuring critical part parameters on a repeatable basis. The move allowed Profile to pursue more demanding applications.
Next, Berry Plastics Corporation’s original purchase of Landis Plastics Inc. has turned out over time to bring Berry a larger injection molding/packaging base. It also gave Berry Landis’ polypropylene (PP) thermoforming technology. Putting the companies together brought together Berry’s state-of-the-art technology for deep-draw, post-trim thermoforming and Landis’ PP trim-in-place thermoforming technology that is also state-of-the-art.
Within the plastics industry, it was believed that PP could not be drawn deeper than 4 inches, but Berry developed a new PP thermoform drink cup line that draws just over 8 inches deep, yielding drink cup sizes of 22-44 ounces.
A polypropylene drink cup that unlike other materials won’t crack, leak or break is attractive to consumers, while retailers are also interested in a value-added product that provides premium image potential yet is cost competitive with traditional plastic, paper and foam cups. Barry’s new technology provides the fountain industry with a drink cup that can be crushed and straightened out and will still hold liquid.
The new drink can also spell profits for retailers who have discovered that reusable polypropylene drink cups — viewed as premium products by consumers — are a cost-effective alternative to traditional paper, plastic or foam cup materials.
Other companies are thermoforming PP, too. PP is expected to grow 9.1 percent in thermoforming applications. Less than 100 million pounds of PP was used in thermoforming 10 years ago, and about 750 million pounds is expected to be used this year. By combining its deep-draw PP thermoformed product with PP trim-in-place thermoforming technology, Berry is advancing in all sorts of single-service products including ready-to-eat packages, salad packages and snack containers.
Finally, custom thermoformer, Kintz Plastics — a recognized leader in heavy-gauge thermoforming — offers three thermoforming processes:
- vacuum forming (with design flexibility at reduced tooling costs)
- pressforming (a cost-effective alternative to injection molding)
- twin-sheet forming (which compares favorably to blow molding, in both function and cost).
The company, which produces plastic parts for the medical, computer and transportation industries, has installed the largest thermoforming machine in North America. Partly customized in-house, the four-station rotary machine dubbed “jumbo” makes parts of up to 9-by-13-by-5 feet from a single sheet of plastic.
Kintz forms a full range of large parts with the machine, including products such as exterior vehicle panels, spas, pool stairs, office tabletops, kiosk and vending machine panels. Some captive thermoformers make parts of equal or larger size, such as truck bed and refrigerator liners.
The machine can be operated in vacuum, pressure or twin sheet modes. Kintz foresees considerable potential in large twin-sheet hollow parts, such as underground storm water drainage vessels.
Another market of significant interest to Kintz is unpainted decorative parts. This market segment is experiencing high growth utilizing Bayer MaterialScience’s and Sabic Innovative Plastics’ weatherable, high-gloss sheet products. These applications also benefit from a push to replace thermoset fiberglass composites in various recreational and agricultural vehicles, riding mowers, spas and showers, and also window and siding profiles.
About the Author
The Plastics Institute of America (PIA) at the University of Massachusetts Lowell is a not-for-profit educational and research organization dedicated to providing service to the plastics industries since 1961. The PIA is led by Prof.-Dr. Aldo Crugnola, executive director; Prof.-Dr. Nick Schott, secretary and director of educational programs; and Dr. Donald Rosato, publications chairman (pictured).
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