Why thermoforming is looking better than ever.

The ability not only to imitate injection molding's crisp design, but also to get parts to market months faster, is making customers sit up and take notice of today's pressure-forming skills.

From the crisp detail, sharp corners and surface texture on the front, you often can't tell anymore whether electronic equipment and appliance housings are pressure-thermoformed or injection molded. If you turn the parts over, you'll see telltale ribs, gate or ejector marks on the injection molded part. And on the pressure-formed part you'll see that the edge shape is CNC-routed, not smoothly molded. But these sharply detailed, new thermoformed parts will no longer have any thin corners that you can see light through and have to be patched. And they won't need to be painted. Today they are used right from the mold, with no defects to hide.

Heavy-gauge formers say they're experiencing strong growth in these "high-cosmetic" injection molding lookalikes. "That's the future of this business," says Clifford Robinette, owner of Techniform Industries Inc. in Fremont, Ohio. "There'll be a lot of people competing to be the last drape former."

Recently, a competition was held at the Society of Plastics Engineers' Thermoforming Div. annual meeting in Midland, Mich., for who did the "snappiest" impression of injection molding. The crowd of such "molded-look" parts dwarfed all the other contestants. Of 35 parts displayed in both thin- and heavy-gauge categories at the SPE meeting, half were heavy-gauge injection molding lookalikes. "We've had a lot more growth in the last two years in pressure forming than vacuum forming," says v.p./general manager Thomas Feid of Orbit Plastics Corp., Salisbury, Mass. Orbit says it does twice as much pressure forming now as vacuum forming, where two years ago the reverse was true. "Capabilities in pressure forming aren't new, but credibility for the process is," says sales manager Jack Schrieffer of Arrem Plastics Inc. in Addison, Ill., a leader in the molded-look field and winner of three SPE awards this year. "Ten years ago, product designers' fear of failure was so great that they would spend five times more for tried-and-true injection mold tooling rather than take a chance. Now we've shown that pressure forming is an effective substitute."

The reasons for this change are partly improved skills in pressure forming and partly that shorter time to market is worth more to manufacturers today than ever before.

There's no doubt that pressure-forming skills have improved. Processors today are achieving unprecedented levels of dimensional precision and repeatability: |+ or -~0.003 in. on small parts and |+ or -~0.005 in. on large ones. Part of the secret is using CAD technology to create these parts, along with "concurrent engineering" relationships among designers, formers and toolmakers.

Today's sophisticated thermoform tooling can involve complex slides, undercuts and other moving sections. Parts may be molded with high air pressure or even metal-to-metal "coining" for areas with close tolerances. Large flat parts may cool for up to 24 hr in special annealing fixtures to equalize stresses so that parts lie flat. Typically, after the clamp flange is removed by a manual saw cut, some parts are put into specially built female fixtures, perhaps held with vacuum, and trimmed with three-to six-axis CNC routers.

FASTER TO MARKET

For low part volumes, pressure forming easily beats injection molding to market. At volumes as low as a few hundred parts, pressure forming is in high demand because it offers finished parts in one-fourth to one-fifth the lead time necessary for injection molding. In highly specialized medical equipment, for instance, a customer might only anticipate selling 500 units/yr--an ideal run for pressure forming. A typical order might also be only a few hundred pieces for demonstrations, a test launch, trade show, seminar, or delivery to "beta-test" sites for further development.

Even for up to 5000 parts/yr, pressure forming can be nearly cost competitive on a unit basis especially for large parts, some formers say. At more than 5000 parts/yr, injection molding generally wins because of the time needed for trimming pressure-formed parts. In rare instances, thermoforming can still compete in larger runs. Tooling costs for a clear lighting lens with a prismatic pattern are about $20,000 for pressure forming and would be $100,000-$110,000 for injection molding, Arrem's Schrieffer estimates. The thermoformed lens cost about $10 apiece for 5000-15,000 parts/yr, vs. about $7-$9 for an injection molded cover for 20,000 units/yr (not including tool costs). In general, forming a part can take 2-3 min, annealing 8-10 min, further cooling 24 hr, and trimming 5-10 min.

Pressure forming offers an advantage of flexibility to a customer with a new high-tech product, even when the product is ultimately expected to sell 100,000 units. Thermoforming can be used to produce the initial consoles, allowing incremental changes to be easily made to the model before committing to injection tooling. A change like moving a CRT monitor within a console is easy with pressure forming since all you have to do is move an insert in the tool or change the programming on the CNC router to cut a hole differently. With injection molding, you'd need a whole new tool.

Also, on parts bigger than 4 sq ft, thermoforming has a cost advantage over injection molding, pressure formers say. Tooling cost for a large ABS console (56 in. wide x 26 in. high x 24 in. deep) pressure formed by Arrem for a blood-chemistry analyzer might have been $750,000 to $1 million with injection molding, but only 20% of that for pressure forming, says Arrem.

Lastly, thermoforming can make lighter-weight parts than injection molding. This can be both a cost saver and a design advantage. In a 24-in. prismatic lens formed by Arrem, thinner walls transmit more light.

USING CAD ALL THE WAY

Freetech Plastics Corp. in Fremont, Calif., says it was one of the first thermoformers to design parts with CAD/CAM, as far back as 1982. Freetech started with simple geometric programs on an Apple II PC, graduated to 2-D software for a Macintosh, and two years ago went to a full 3-D program (called Quick-Surf) that has an IBM-compatible program, Surf CAM. Freetech says 80-90% of its work now comes already prepared as CAD files. Arrem says its use of CAD has mushroomed from very little three years ago to 40-50% of its projects today.

Arrem's blood-analyzer console is a tour-de-force of design ingenuity, with 28 separate panels formed on 18 female molds that would have been "impossible to design" without CAD, Arrem says. (The analyzer won top honors at the SPE meeting, receiving both the Chairman's Award for criteria such as innovation, forming and trimming difficulty and the People's Choice voted by the show's 400 attendees.)

The part makes extensive use of formed undercuts to mount the separate parts and mask joint lines. Surface texture is also critical because the parts aren't painted. Twenty separate fixtures were built to hold the parts for trimming and drilling by a six-axis CNC router. Keyhole slots are made by raising ribs in the formed sheet with a male protrusion in the mold, then milling the ribs off. The blood analyzer took six months from concept to finished product. Despite the unit's complexity, Arrem reportedly met tolerances of |+ or -~0.010 in.

CONCURRENT ENGINEERING WINS

With a "mission to get the product to market as soon as possible," Orbit Plastics produced a cover for a new wireless local-area-network (LAN) transceiver for Windata Inc. of Northboro, Mass., says Orbit's Feid. The customer was first in its field with a unique product for wireless modem communication, so it didn't want to be beaten to market by waiting for injection molded covers, which would be required for high-volume commercial sales. But thermoformed transceiver covers had to look absolutely commercial, not like mock-ups.

The solution was three-way concurrent engineering of part design to be sure nearly identical covers could be made by both thermoforming and injection molding. CAD allowed engineering data to be shared among designer, thermoformer and injection molder, and both pressure forming and injection tools were made from CAD files. Orbit delivered the first 30 thermoformed parts in under four weeks from the initial meetings with Windata, and another 100 parts two weeks later. The first injection molded parts took 21 weeks.

COMPLEX, MOVING TOOLS

Hydraulic or pneumatic tooling articulation is being used to form undercuts and grooves with new complexity. Up to 2-in.-deep undercuts aren't uncommon today. Previously, mechanically actuated tooling segments made only small movements to form undercuts of up to 0.5 in. on bearings or rollers. Such tooling undercuts were also traditionally made only at right angles so as not to obstruct withdrawal of a bar into a tool. Today's undercuts are trickier and may create channels around multiple obtuse angles. A side panel for a laser printer and collator, pressure formed by Arrem, retracts tooling segments on slides that move into a different plane to allow room for other slides. Or on another part with a very large (26 in. long x 3 in. deep) undercut, the whole sidewall of the tool retracted with hydraulic cylinders to remove the part.

Sometimes metal-to-metal molding or "coining" is also used in tools. Arrem's clear lighting lens has several coined ridges on the circumference so the cover can catch onto the lighting fixture.

Big lifters are also used to remove some large, deep-draw parts from the tool, like an exercise-cycle body formed by Profile Plastics Corp., Northbrook, Ill. "In the last few years, all of us certainly have had to get more sophisticated in using slides and 'pullaways' |portions of the tool that move so you can remove a part~," says David Bowllan, group leader for injection molding at Eastman Kodak Co. in Rochester, N.Y., which forms a molded-look housing for its photo CD system. "Nobody chooses to use slides and pull-aways if they can avoid them, because they're expensive. But quite often you're required to use them in order to make the part."

HEAT PROFILES FOR GREATER DETAIL

Pressure formers increasingly use computer programs to analyze drawdown into the mold, wall thinning, optimum heat profile, and other processing factors during the design stage. Two semi-commercial programs for this purpose are being developed by AC Technology, Ithaca, N.Y., and IDES Inc., Laramie, Wyo. AC Technology is commercializing a forming-analysis program initially developed by GE Plastics in Pittsfield, Mass., which includes some finite-element mesh (FEM) modeling. It is expected to be released late next year.

The IDES program, called TF1, is an offshoot of R&D done at the University of Wyoming. It is a planar program with no thickness dimension in its analysis, unlike the AC Technology program. TF1 floppy discs and manuals cost $95.00 and are intended primarily as a training aid. They've been commercially available for several years. Since neither program is able to address moving mold parts or any but the simplest tool shapes, they aren't useful in determining the feasibility of more complex parts.

The key to forming is getting the entire cross section of a sheet up to forming temperature, processors say. If sheet is still cool on the inside, that adds stress and spoils the sag. "At that point, beginners turn the heat up, which just distorts or discolors the sheet," one former explains. "Instead, they need to turn the heat down and leave the sheet in the oven longer."

Pressure formers are also experimenting with techniques like chilling the surface of hot sheet with air or a cooled mold, which is said to increase sag resistance or "hot strength" of the web. Arrem says it can change surface finish and appearance by boosting air pressure while forming at lower-than-normal temperature. "It's somewhat unusual," says Arrem's Schrieffer.

FILLING OUT THE CORNERS

Pushing hot plastic sheet into sharp corners without thinning is a perennial problem in pressure forming. Today, skilled formers use what are variously called "articulated helpers," "pushers," or "moving plug assists." Such devices date back 25 years, though controlling their movement and temperature is more sophisticated today.

Profile Plastics uses articulated plug assists that make two sequential movements in addition to the up/down movement of the tool itself. Sometimes the whole sheet is prestretched by blowing into a "billow" before the vacuum and plug assist are used. Billow forming is an old vacuum forming technique. "But it's unusual in pressure forming," says Profile's president and owner, Stephen Murrill. His company combined billow and pressure forming to make a medical heat-exchanger cover displayed at the SPE meeting.

Freetech often vacuum forms over a male tool and then pressure forms into a female to distribute material evenly. Arrem uses a two-sided, dual-tool pressure system it patented in the mid-80s for better material distribution into deep-draw parts. Arrem forms first over a male tool (either cast aluminum, made off the milled female tool, or syntactic foam). It then pressure forms into the milled female tool to imprint surface texture and detail. Molding first against the male tool chills the sheet enough to improve hot strength, Arrem says.

Temperature control becomes more exacting when plug assists are used, says Profile's Murrill, because the plug cools the sheet slightly. If the sheet is too hot when the plug touches it, it chills on the plug and thinning occurs around the plug. "The best plug material is syntactic foam of hollow glass spheres in an epoxy matrix, which has very low thermal conductivity," Murrill says. (W.R. Grace & Co. is one supplier of such material.) "If the nominal processing window for a part is 350-375 F, the temperature in the corners with plugs has to be right in the middle of that range. You'd have to maintain 355-365 F."

Heat may also be unevenly applied to help the plug assist to move more material toward one end of the tool. "Even a few thousandths of an inch make a difference in the integrity of a corner," Arrem's Schrieffer says. Freetech uses several articulating plugs on a demanding part for a piece of scientific test equipment. The part has 5-in. vertical walls with a 0.5-in. undercut the whole way around and no draft at all. So the articulating plugs actually stretch material from the middle of the sheet (which gets cut out anyway) into the corners.

TURNING UP THE PRESSURE

Pressure-forming machines are typically rated for 50-psi forming-air pressure, and many smaller molded-look parts are formed at well below machine capability. Orbit's transceiver cover, for instance, was molded at around 30 psi. But some formers substantially exceed these pressure levels, using custom-built machines with heavy posts and locking bayonets. For example, a custom-built Brown R244T pressure former (with twin-sheet capability) at Profile Plastics has six bayonets locking a 6 x 8 ft forming area with 3 ft depth of draw. Profile says it uses up to 120 psi to produce very large parts with high-definition details. One example is a 4.5 ft x 2.5 ft x 8 in. exercise machine that won an SPE prize last year.

Arrem says it forms at 100-120 psi, with a minimum of 50 psi sustained over the entire platen. Otherwise, formers say most parts are formed at 50-80 psi or less.

Machine frames, clamps, chain rails and hydraulics are all beefier on a pressure-forming machine (which typically costs three to four times more than a vacuum former). But the really high pressures (100 psi) require custom-built machines with even heavier post construction. Both tools and platens need special lock-up features to hold the pressure. "We're set up to sustain pressure throughout the forming cycle--anywhere from one to 10 minutes. And we can go up to 400 psi for some very high-tensile-strength materials like glass-filled polycarbonate," says Arrem's Schrieffer.

Aluminum molds, whether cast or machined, can take such pressure. Epoxy molds with sprayed-aluminum coatings aren't recommended for pressures much above 30 psi, Schrieffer adds.

Instead of higher pressure, some formers increase vacuum. "We do a lot of special things with venting to make sure we have very high vacuum. Good venting practices give an extra 20% of crispness," says Freetech owner Richard Freeman, who explains that this is achieved by eliminating a tiny cushion of air otherwise trapped behind the part.

MORE TOOLING TRICKS

Portage Casting and Mold Inc. in Portage, Wis., recently found that surface quality of pressure-formed parts can be improved by shot-peening the aluminum tool casting to eliminate porosity.

Using machined rather than cast aluminum (which is the industry standard) eliminates porosity altogether, says Freeman of Freetech, which uses CNC machining for all its own tools made from solid aluminum. Freeman says machining makes sharper surface definition and more complex venting possible. Tooling is even CNC machined directly from customers' CAD files, as in the case of a large computer file-server door displayed at the SPE meeting.

Pressure-formed tool cost is 10%-30% of injection molded tool cost. CNC-machined tools are said to be slightly more expensive than cast tools, but the real cost differential is in how many secondary mechanical movements the tool does. Freeman says his tool cost "is very predictably 10%-15% of injection mold cost." Arrem and Profile say a more representative cost figure for complex tools is 20%-30% of an injection mold tool.

COMPLEX POST-TRIMMING

Secondary fixturing and robotic trimming and drilling contribute greatly to the improved repeatability and tighter tolerances achieved in today's pressure forming. ABS, the most common material in molded-look pressure forming, is tricky to trim with precision because of its high thermal-expansion coefficient. A 40|degrees~ F difference in ambient temperature can make a 0.070-in. dimensional change in a large part like Arrem's 48-in.-wide cardiac catheterization unit (which won an SPE thick-gauge forming prize this year). Arrem controls ABS expansion by strictly monitoring ambient temperature with a thermometer at each CNC milling machine. Profile bought its first CNC router 12 years ago and now has seven, including two bought this year, to trim parts from its eight pressure-forming machines.

Trimming is a production bottleneck that can take up to twice as long as forming, so a former running at full capacity could keep two routers busy depending on the part. A part that requires one pass from a router head might have a 10-min trim cycle; two passes could take 15 min, though sometimes parts can be doubled up and trimmed two at a time.

One leading pressure former hopes to increase trimming efficiency with a new state-of-the-art CNC trimmer from Shoda Iron Works Ltd. of Japan (represented in the U.S. by Computer Router Services Inc., Arlington, Wash.). The trimmer can be programmed to change tool bits automatically from a magazine of 16 bits and cutters. The Shoda model costs around $350,000--twice as much as most other routers--but helps increase efficiency since it can pick up a small drill bit for a small job and switch to a big cutter for big jobs. With a standard single-head router, the operator must change bits manually for different-size holes, or else the machine must use one bit--sized for the smallest hole--throughout the part. Fixtures to hold parts during routing are generally built in-house and add 10%-20% to tooling costs, processors say.

Acrylic Modifier Improves PP Sheet Extrusion & Thermoforming

A new acrylic-based modifier from Rohm and Haas Co., Philadelphia, is said to go farther than ever before toward making polypropylene competitive with HDPE, ABS and even HIPS as a thermoforming material. Aimed primarily at PP resin producers and major compounders, new Paraloid PM-800 modifier reportedly answers all sheet extruders' and thermoformers' needs--including cost--with no attendant sacrifices. The modifier just became commercially available this month, though it has been in development for at least two years and in use by some PP resin suppliers for about a year. For example, Rexene Products Co., Dallas, uses the modifier in its 9000M and 6000M series (See PT, Oct. '90, p. 13; Aug. '91, p. 71; Oct. '91, p. 76).

Rohm and Haas has been pursuing the goal of improving PP formability with acrylic modifiers for almost 10 years. An earlier all-acrylic product suffered from excessive plate-out problems. The new entry, however, is a patented acrylic/PP graft copolymer, which is said to be completely compatible. It is used at 3-5% by weight in either homo- or copolymers.

The modifier's benefits have been demonstrated in over 50 commercial-scale trials of sheet extrusion and forming. Almost 40 different heavy-gauge parts have been formed by more than 25 companies, as well as 11 different thin-gauge parts by nine processors.

According to Rohm and Haas, these trials have demonstrated improved formability without the tradeoff in extrudability that reportedly has been experienced with some "improved-thermoforming" grades of PP. That's because Paraloid PM-800 increases the PP melt viscosity at low shear rates without increasing the viscosity at higher shear rates. It also greatly reduces surging in the extruder, which improves sheet gauge uniformity and consistency of orientation--both of which contribute to improved forming. What's more, the sheet reportedly shows reduced neck-in, lower edge color, and better gloss. The modifier also contributes to faster fusion (melting) in the extruder, leading to 5-6% higher output rates.

When it comes to thermoforming, the benefits are even more numerous, says the company. The acrylic-modified PP heats to forming temperature faster under infrared heating. And it also serves as a nucleant, speeding crystallization so parts can be ejected from the mold faster.

Other advantages are increased melt strength and elasticity for improved sag resistance and drawability without webbing. Overall, a larger forming window and reduced reject rates are said to result. Acrylic-modified PP allows production of some heavy-gauge parts that cannot be formed repeatably in unmodified PP, according to Rohm and Haas.

As if all that weren't enough, company data show that modified PP has higher tensile strength, flex modulus, and HDT, with little or no change in impact strength.

The productivity advantages in sheet extrusion and forming should more than offset the approximately 11-12|cents~/lb that the modifier adds to raw-material costs. (Paraloid PM-800 costs around $2.70/lb.) What's more, the higher stiffness and HDT conferred by the modifier, as well as more uniform material distribution in parts, are said to permit reduced sheet thickness. This not only lowers materials cost, but further increases the savings from faster forming cycles.

Rohm and Haas has applied for FDA clearance, which it hopes to receive in the near future.