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by Ralph Meyer
February/March 2008
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| High impact polystyrene (HIPS) sheet just prior to the forming stage. Notice the large sag in the sheet. |
Many IAPD distributors have turned to plastics fabrication in order to serve a broader market of customers that desire finished or semi-finished products. Distributors in our industry have panel saws for cut-to-size, and often edge finishers and line benders that allow us to fabricate a variety of products. We even have CNC routers and lasers for cutting intricate precision plastic parts. However, only about a fourth of us in IAPD participate in the vacuum forming process. Vacuum forming is a niche process, and we should thoroughly understand it so that we can offer it to our customers. This article will explain the vacuum forming process and why it is strategically important in the vast array of fabrication methods.
In the course of servicing customers with a full line of plastic sheet products, there will undoubtedly be opportunities to make or source finished parts. A plastic machine cover, a tote tray, returnable packaging, a medical device control panel, or a machine guard — the list goes on forever. Industries served include automotive, medical, sporting goods, appliance, point of purchase and many more. Vacuum forming is a great solution when the customer needs too many units to fabricate by hand and too few units to tool up for injection molding. It also helps when the product is too large and bulky to ship a long distance.
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| Kydex® thermoplastic sheet formed over a water-cooled, temperature-controlled aluminum mold to make a medical cover. |
Vacuum forming is a process where plastic sheet is heated to its transition temperature and placed in contact with some form of a mold. It uses vacuum for pressure to form the plastic to the new shape. This process is actually part of a larger group of processes called thermoforming, which generally describes any process that makes a plastic part where heat is used to soften flat sheet and form it into a new shape. In order to maintain some scope on these comments, this article relates specifically to heavy gauge vacuum forming of sheet as opposed to the roll-fed continuous processes commonly used in the packaging industry. Heavy gauge vacuum forming utilizes plastic sheet that is at least .030 inches thick up to .750 inches thick.
Heavy gauge vacuum forming is a good option in many situations because the cost of tooling is often the least costly of all possible processing solutions. As an alternative to injection molding, it can be 10 times less expensive. The larger the part, the more this advantage becomes a factor. A common plastic shoebox makes a good example: The customer needs 1,000 pieces each year. An aluminum temperature controlled vacuum forming tool could cost $1,500. A hardened steel injection molding tool would be closer to $20,000. Certainly this gives the customer pause to consider the vacuum forming process. The vacuum forming tool could be made in eight weeks or less, while it may require 12 or more weeks to produce a hardened steel tool for injection molding.
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| High impact polystyrene (HIPS) part being formed on a tool mounted on the top platten. The reason for running parts upside down is to take advantage of the sheet sag as it approaches forming temperature to prestretch the sheet. |
If tool cost and lead-time were the only factors to consider, it would be too easy. The advantage of lower tooling cost is somewhat mitigated by the extra labor required to finish a vacuum-formed part. Injection molding is a high-pressure process so parts are produced with very fine detail and tolerances. Vacuum forming is limited to 14.7 pounds per square inch, the weight of one atmosphere. This means less mold detail and greater size variation. A vacuum-formed part starts as a flat sheet and needs to be trimmed after it is formed. The injection-molded part is usually finished when it is ejected from the mold. Depending on the part geometry, a finished vacuum-formed part may be 30 to 90 percent of the starting sheet weight. Injection molding typically yields over 90 percent of the plastic in each part produced. In both instances, the trimmings, sprues and runners are recycled. The cycle time for vacuum forming is usually longer than injection molding. This is because it takes longer to heat the plastic to forming temperature and once formed, the part is cooled by forced air rather than a temperature controlled matched tool.
You may be wondering is vacuum forming competitive or not? Often it is, but that depends on the cost benefit relationship between the automation of injection molding and the more manual process of vacuum forming. In the last 10 years the technology of vacuum forming machines has developed rapidly to help reduce costs and improve the quality of the final product. Heating ovens have evolved from poorly controlled single zone top and bottom heaters to finite precise profiled heaters. This helps control the wall thickness of the final product. Machine mechanics used to be pneumatic and hydraulic and now are mostly electric, again improving their speed and precision of movement. Rapid setup systems for molds and clamp frames have reduced setup times from hours to under 30 minutes. A vacuum-formed part is also much more stress free than an injection-molded part. Often it just makes economic sense to start out a project devoting resources to part design and pilot runs rather than expensive tooling.
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| Kydex® thermoplastic sheet being heated in a quartz zoned oven. |
A vacuum forming department can be very capital intensive. In order to be productive, a shop needs modern forming machines, 3 and 5 axis CNC routers, mold temperature controllers, large vacuum pumps and storage tanks, very large electrical or gas services, a capable quality control department, and possibly an internal mold making department. This work is best left to experienced professionals because even with all the fancy equipment, learned technique is often the key to success.
Pressure forming is a similar process and a term that is used frequently and unknowingly in place of vacuum forming. Pressure forming is vacuum forming with the addition of air pressure to create more force on the sheet against the mold. The vacuum force forms the sheet from the mold surface and air pressure pushes on the other sheet side. This process improves the detail of the part and creates an appearance part that closely resembles an injection-molded part. Many office and medical equipment enclosures are produced using the pressure forming process. The additional pressure adds from 30 to 150 psi to the process and increases the forming pressure 3 to 10 times. Molds are more expensive than vacuum forming due to the additional detail and pressures involved, but still far less than injection molding. Vacuum forming machines also need to be more capable to withstand pressures. For example, a pressure forming mold for a part 3 square feet in size, may require 60,000 pounds of pressure instead of 6,350 pounds from just vacuum.
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| Urethane tool mounted on the top vacuum forming platten. |
Selling vacuum-formed parts is a great way to deliver an improved solution to your customer. Often you can improve part quality, replace a metal part, reduce the number of parts through combination, provide a lighter part or eliminate production costs through simplification. Consider adding vacuum forming to your list of value-added services.
Be sure to contact your equipment supplier for the proper procedures of your vacuum forming machine.
Ralph Meyer is president of Meyer Plastics, Inc. For more information, contact Meyer Plastics, Inc., 5167 East 65th Street, Indianapolis, IN 46220 USA; (800) 968-4131, (317) 259-4131, fax (317) 252-4687, rmeyer@meyerplastics.com, www.meyerplastics.com.
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