Twin-sheet forming: how to get in on the action.

It can make hollow or double-walled structural parts that compete with injection and blow molding, structural foam, and rotomolding. But there's a lot you should know about machinery, tooling, and heating.

The technique is 30 to 40 years old, but twin-sheet forming is grabbing the interest of thermoformers and their customers just like it was brand-new. The process utilizes two sheets of material, which are formed in separate molds while being simultaneously fused or "knitted" together under pressure to make a part with hollow sections. Accomplishing this during one machine cycle eliminates the need for secondary bonding, glueing, or ultrasonic welding of separately formed parts - and reportedly produces higher strength than post-bonded structures.

Pallets have emerged as a potentially huge market where twin-sheet forming competes in price and strength with presswood or structural foam (see Technology News story, p. 17). Twin-sheet has also appeared recently in applications ranging from medical diagnostic equipment to commercial aircraft interiors. And twin-sheet parts are being tested in tabletops, trays, and refrigerator doors. "We see the auto industry as probably a natural for twin-sheet applications," says R.J. Williams Jr., CFO of thermoformer Cadillac Products in Sterling Heights, Mich. Other industrial markets include the textile, printing, and electronics industries.

Sales of twin-sheet forming machines at Brown Machine have doubled in the last few years to become 50% of its overall business, says Brown Machine product manager Richard Roe. Bruce Smith, president of Central Automated Machinery (CAM) Inc., says twin-sheet is 40% of CAM's business so far this year vs. 20% two years ago. Mold makers report a similar increase in activity. Portage Casting & Mold has built 500 twin-sheet molds in the past five years, double the rate of the prior five-year period, reports v.p. Ken Griep. At Miller Mold Co., sales and plant manager Tim Weldon says, "Making twin-sheet tooling is a small part of our business, but it's beginning to take off."

Twin-sheet today is still the province of a relatively small number of processors, but perhaps not for long. Lyle Shuert, president of Shuert Industries, Sterling Heights, Mich., one of the first companies to take the plunge into twin-sheet, says the process was initially limited by the low profile of industrial thermoforming. "Heavy-gauge thermoforming has historically been a small market," Shuert says. "I think the twin-sheet process is becoming bigger now because of the success of previous applications. The process is getting more visibility and the number of practioners is rising."

Steve Spooner, an owner of Arbor Craft Corp., a custom former in Plymouth, Mich., says he's been doing four or five twin-sheet projects a month for the last three or four years. "We are getting a good deal of response. We're seeing requests for twin-sheet applications even when it's not required. Customers just want to know whether it can fit into their process."

WHY NOW?

Why is a decades-old process the focus of attention right now? One reason seems to be growing appreciation of its structural potential because of its ability to incorporate internal ribs and bosses, as well as metal reinforcing inserts. "Twin-sheet gives the thermoformer the ability to make a structural component, where previously the thermoformer only considered making a covering like a panel, or guard," says Chris Grimm, general manager of Grimm Brothers Plastics Corp., a thermoformer in Wapello, Iowa. When combined with pressure forming, twin-sheet can produce parts with the crisp definition and sharp corners of injection molding or structural foam. It also can make hollow parts that would otherwise be possible only with blow molding or rotomolding. Twin-sheet's advantage over other processes is its tooling costs. "In general, the greater the area of the mold, the more the savings vs. injection- or blow-mold tooling, "says Art Buckel, thermoforming consultant at McConnell Co. Tooling lead times are much shorter for twin-sheet, as well.

What's more, twin-sheet adds versatility to thermoforming by allowing a part to incorporate sheets of different colors or gauges. Some processors believe that two sheets of entirely different resins could be used. There is some evidence that few companies have tried it. Kintz Plastics Inc., Howes Cave, N.Y., regularly twin-sheet forms with different gauge sheet and has used dissimilar materials, reports Michael Righi, engineering manager. Chuck Mulcahy of GE Plastics Structured Products group in Pittsfield, Mass., says his firm has also used dissimilar materials.

Until about 10 years ago, the equipment, tooling, and process know-how were somewhat of a mystery, say both processors and suppliers. Although the process is better understood today, there are still risks for beginners. "A lot of the knowledge of twin-sheet forming isn't written down," says Thomas Pintar, v.p. of sales and marketing at TriEnda Corp., a Portage, Wis., twin-sheet processor. Formers, moldmakers, and machine builders all say their knowledge was derived from much trial and error. "If you know how to do single-sheet forming it doesn't mean you can do twin-sheet," one processor cautions. For one thing, most processors say twin-sheet forming involves finer tuning and better control of the entire process. One processor confesses that he, like others who've tried twin-sheet forming, initially experienced a reject rate of at least 50%.

WHAT TYPE OF MACHINE?

Almost all processors interviewed agree that a first-time twin-sheet former should buy a new machine rather than retrofit an existing one. "It's not cost-effective to retrofit a machine," states Roger Fox of MAAC Machinery. Domestic twin-sheet machine builders include Brown Machine, Central Automated Machinery (CAM), Comet Industries, Custom Manufacturers, MAAC, and Z.M.D. In Europe, Adolf Illig, Paul Kiefel GmbH, Geiss Maschinenfabrik, and Cannon Shelley make twin-sheet systems but have not yet sold any here.

The large majority of twin-sheet forming uses heavy-gauge cut sheet 0.060-in. or thicker. Twin-sheet is processed chiefly on rotary machines or single-station shuttles, but it can be done on a side-by-side machine. There is presently only one twin-sheet processor known to be using a modified continuous roll-fed thermoformer. This unique and proprietary twin-sheet approach was developed by Spencer Industries Inc., Dale, Ind., which declines to discuss any details. However, Brown Machine offers continuous roll-fed twin-sheet forming according to Jim Allen, company's in-line product manager.

Richard Roe at Brown says, "There's no question that the equipment today is faster and more accurate." He says the sheet index system on Brown rotary machines index faster, hold a more constant speed, and are more accurate than previous models. CAM's Smith says his machine controls now have improved graphic interfaces, process monitoring, and recipe storage to facilitate faster and better machine setups.

Brown, Comet, CAM, and MAAC offer electrically driven platens that can be controlled by PLC to position the molds precisely and repeatably, which is crucial to achieving a good bond between the sheets. Platens can also be driven hydraulically or pneumatically, but with less precise control. Pneumatic drive is relatively slow when moving very large, heavy platens, says Bill Kent, Brown v.p. Hydraulic platen drive is said to be best suited to handling large, heavy molds.

Sources at Brown, Comet, CAM, MAAC, and Custom Manufacturers say the added clamping force needed to weld the two sheets together can be provided with air bags and bayonet clamps such as have been used on single-sheet pressure formers. The pressurizing system is usually attached to the lower platen in order to press the bottom mold up into the top tool during forming.

Portage Casting's Ken Griep says better control of platen movement can reduce the pressure requirements to make a part. He notes that some processors want to minimize pressure in order avoid to squeezing material out the sides of the bond area. "Sometimes you don't want full fusing, such as when you are making a cosmetic part. You want the sheet to lightly fuse, "says John Griep, president at Portage.

HEATING CAN BE TRICKY

The choice of rotary or single-station machine has important consequences for heating the twin sheets. A rotary machine heats each sheet in a separate clamp frame in a separate oven. This technique allows use of thicker sheet since each sheet can be exposed to simultaneous top and bottom heating.

A single-station machine is limited by the requirement that the two sheets are held in the same clamp frame in the oven. Because each sheet receives heat from only one side, most processors say single-station machines can form twin sheets up to a max. of 0.125 in. thick. In single-station machines there's also the risk that the two sheets could fuse together in the oven.

To hold the sheets apart, consultant Art Buckel of McConnell Co. recommends mounting a needle or thin nozzle device on the clamp flame to blow hot air between the sheets. Air temperature is typically 600-800 F, depending on the material. Processors say the sheet should typically be heated to 240-300 F before forming. Roe says Brown machines can be equipped with a special air nozzle developed by GE Plastics and manufactured by Convectronics. Grimm Brothers designed its own nozzle to keep the sheets apart.

Jeff Jenkins, an engineer at Furon in Seattle, suggests an alternative to hot-air nozzles for single-station operation. He says a PTFE-coated hot plate might be used for contact heating between the sheets. GE's Mulcahy says the company has tested this using a double damp frame design with a PTFE plate manually inserted and removed from between the sheets. It doesn't appear that anyone has yet tried this approach, but some processors speculate that if it works it could permit use of larger sheet in single-station twin-sheet forming.

Processors say it would be very difficult to form a twin-sheet product using dissimilar materials or gauges on a single-station machine. They expect that one sheet would almost inevitably be overheated or underheated. Comet Industries president Robert Kostur suggests using screen packs to block heat from the thin sheet.

The single-station process doesn't allow inserts to be placed between the sheets. But because of the short travel time between the heating and forming station, single-station machines favor twin-sheet forming of materials that tend to cool fast such as polycarbonate, modified TPOs or other high-temperature engineering materials.

A side-by-side (or double-ender) machine, which has two separate oven stations to the left and right of a single forming station, heats each sheet separately but simultaneously. Thus, it is better able to form sheet of different gauges or materials. The down side is that a double-ender costs more than a rotary of similar output capacity.

Virtually all sources interviewed agreed that close temperature control is even more important to twin-sheet than it is to standard thermoforming. Don Aldrich, owner/manager of Custom Manufacturers, says a more precise alternative to timed heating cycles is to use infrared sensors inside the oven to monitor sheet temperature directly. Infrared temperature data can be sent to the machine's PLC to advance the sheet to the next station when the proper temperature is reached. Infrared sheet-temperature monitoring is recommended by other machine builders (including Brown, CAM, and Comet Industries), as well as by some processors (Grimm Brothers and Kintz Plastics) and consultant Art Buckel. Bruce Smith, president of CAM, also suggests installing a photo-electric sensor in the tool to detect when the sheet sags enough to indicate it's ready for forming. Smith said it can be an alternative device for measurement when both sheets are similar.

TOOLING TIPS

The most significant factor in mold design is obtaining correct alignment of the two tools. Processors, who commonly design their own tools, say they have incorporated pins or other devices inside and outside the mold to ensure accurate alignment during each cycle. Griep of Portage Casting says the standard pin-and-bushing approach is the most effective. He adds that a twin-sheet mold should have a minimum of two pin-and-bushing sets.

All twin-sheet molds are built of cast or machined aluminum. According to Comet's Kostur, "The selection of aluminum alloy or mixing of zinc and lead will help in solidification of the molten metal." He adds the mixing ultimately aids material flow in the mold.

Rich Jackson, general manager of Solar Plastics in Tampa, Fla., says twin-sheet molds should have no more than 20-in. draw, otherwise the sheet may be difficult to pull down into the deepest corners before it cools. But Portage's Griep says the 20-in. draw limit applies more to ABS or styrenics than to HDPE, which retains heat longer and can be drawn as deep as 38 in.

Mold pinch-off points and sheet weld or knit lines should be designed so that when the mold halves come together not only do the two sheets bond adequately, but excess material is pushed inside the part, forming a bulb or bead along the knit line without marring the external part appearance.

According to a report from GE Plastics' (Twin-Sheet Forming of GE's High Performance Sheet Products, ref. no. TM-90-4B), a pinch-off area approximately 3/8 in. wide will produce excellent welds on most parts. The sheet pinch-off is achieved with a raised surface on one mold half, which forms a seal with the other mold half. GE recommends a pinch-off height at least 1.25 times the material thickness.

GE adds that a vent should be machined into the mold through the pinch-off area in a section of the part that will later be trimmed. This vent is to prevent the formed part from collapsing due to negative-pressure buildup in the part as it cools. Vacuum holes should be incorporated into the vent to ensure that the vent remains open during forming, the report says.

Consultant Buckel and Portage's Griep both recommend that molds have an electric heating element incorporated into the pinch-off areas to facilitate good fusion Of the two sheets when forming high-temperature engineering resins.

Griep adds that every twin-sheet mold should be designed for insertion of blow pins into the cavity of non-critical areas of a part. The blow pins should be as small as possible if in the critical areas of the part, Griep says. The blow pins exhaust the hot air locked into the hollow areas or chambers of the part. Exchanging the air inside the part speeds cooling and prevents warpage, Griep notes. He says the optimal number of blow pins depends on the size and depth of the part, but at least two are required.

Buckel recommends using blow pins to pressurize the space between the sheets to improve part definition and speed cooling by ensuring close sheet contact with the mold. Pressurized air at 35-100 psi (which can be ambient or chilled) should be delivered to the molds as they come together to fuse the sheets. The parts must be pressurized quickly, he says. For fast cooling, Buckel adds, "You want to get the air out as fast as you are putting it in." He says vacuum ports can be quite small, having orifices only 0.013 in. wide at the part contact area and then widen to 0.125 in. outside the contact area to permit movement of a sufficient volume of air. John Griep says blow pins with internal diameters of 3/16 in to 3/8 in are commonly used with twin sheet forming. Comet's Kostur says C[O.sub.2] can be used to quickly cool the part.

Twin-sheet processors recommend strict monitoring of mold temperature and water flow through the mold. Buckel and Griep says every 3 to 5 feet of tubing should return to the manifold for water-temperature control. Kostur of Comet Industries recommends zoned control for optimal results. Portage's Griep says his firm's tools often have as many as eight cooling-control zones.

RELATED ARTICLE: First-Hand Experience

These twin-sheet processors were interviewed for this article:

Arbor Craft Corp., Plymouth, Mich.

Cadillac Products Corp., Sterling Heights, Mich.

Grimm Brothers Plastics Corp., Wapello, Iowa

Kintz Plastics Inc., Howes Cave, N.Y.

Menasha Corp., Watertown, Wis.

PolyJohn Enterprises, Whiting, Ind.

Shuert Industries Inc., Sterling Heights, Mich.

Solar Plastics, Tampa, Fla.

Specialty Manufacturing Inc., San Diego 

Spencer Industries Inc., Dale, Ind.

TriEnda Corp., Portage, Wis.