Peter J. Mooney, Plastics Custom Research Services, Advance North Carolina 27006
Thermoforming is the process of heating and shaping plastic sheet into rigid containers, components of final assemblies, and stand-alone end-use parts. The value of all thermoformed parts produced in North America in 2003 exceeded US$10 billion. Traditionally, about ¾ of all thermoformed products are produced from sheet of 1.5 mm or less in thickness and are primarily rigid disposable packaging products. Most of the rest is produced from sheet of 3 mm or more in thickness and are primarily durable structural goods.
Thermoforming has benefited by its ability to fabricate thin-walled parts having large areas, using relatively inexpensive, single-sided aluminum tooling. Its deficiencies – variable wall thickness, the added cost of sheet and trim regrind, and extensive trimming and additional cost to reprocess the trim – are offset by the ability to economically produce low-volume, thick-walled parts or high-volume thin-walled parts.
The advances in thermoforming technology in the past decade have allowed the industry to grow at a rate that exceeded the growth rate of the plastics industry in general. However, this pattern has changed in the past few years. Newer advances in plastic materials, tooling, forming machinery, and auxiliary equipment are needed to regain earlier growth rate momentum.
This paper considers several emerging technologies such as forming composite sheet materials, surface decoration, and new material development. It also considers the effect of globalization on both thin-gauge and heavy-gauge domestic thermoformers.
“New” Technologies to Advance the Industry
As pontificated in Part I, many extant technologies have not been fully exploited. This section highlights some of those technologies that appear to provide thermoformers with future market advantage.
Forming Composite/Laminated Structures
Heavy-gauge thermoforming has very thoroughly mined the “pretty part” or “easy” applications, where the part is made of unreinforced plastic and is designed to be incorporated into or fastened onto a supporting structure. Formers now need to go beyond their current comfort zones to new materials and processing variants. There are two general types of formed structures – single-layer composite materials that are formed into non-appearance parts, and thermoformed “skins” or “shells” that are thermoformed, then backed with composite materials.
Single-Layer Composites. A military drone structure made of matched-mold glass-reinforced nylon composite was an early commercial application of a non-appearance single-layer structural product. The composite bumper structure for the recent BMW 5 vehicle is another single-layer composite application. The reinforcing medium is usually either woven or non-woven continuous glass mat. In general, matched tooling is required and the sheet must slip or slide into the mold to avoid substantial fiber breakage (1). Furthermore, the force needed to bend the composite into even gentle shapes is usually quite high. As a result, forming presses for such applications are more akin to compression molding presses than conventional thermoforming presses.
Most applications have focused on forming thick composite sheet (2). However, composite sheets having thicknesses less than 1.5 mm (0.060 inches) are now commercially available (3,4). Glass levels are typically 10% to 20% by weight, but they can be less, depending on the applications. The focus will be on structural applications where the parts have large surface areas but they must be thin-walled.
Laminated Structures. The plastics industry has had success commercializing multilayer structures where one of the layers is a high-performance composite and another layer is a cosmetic shell. The best example is found in the sanitaryware industry where spas, shower stalls, and tub surrounds are fabricated of thermoformed ABS sheet that are backed with spray-up chopped fiberglass-reinforced polyester resin (FRP). Automotive innovators such as DeLorean and Bricklin adopted similar techniques in the 1980s to produce exterior car parts. Today some models of the SMART car in Europe boast of laminated parts.
The resurgence of this technology is due in part to automated methods of handling the reinforcing layer. Robots apply the fiberglass- or filler-impregnated resin (often polyurethane) to the formed “skin” residing in the lower half of a matched mold press. Then the press is closed, expressing air and compressing, shaping, and fully reacting the reinforcing layer. Although the automotive industry was apparently the first to adopt this technology, the marine and farm equipment industries are actively pursuing it (5,6).
In-mold decoration is not a new concept. Paper labels with pressure-sensitive adhesive layers were developed for thin-gauge containers in the 1980s. And rotational molders have been pre-applying heat-activated decoration to mold surfaces for a decade or more. Recently the automotive industry has been considering paint film technology as a way of minimizing the economic cost and environmental hazards of conventional “wet” exterior surface painting (7).
Paint film can be either single-layered or multi-layered. Polycarbonate is the preferred single-layer paint film (8). Multi-layer films are usually structures on the order of 0.5 mm (0.020 inches) in thickness. The film consists of at least a high-gloss, weatherable and durable clear outer layer (e.g., a fluoropolymer), a pigmented color layer, and a supporting substrate (9). This film is laminated to a structural sheet. To maintain surface gloss, the laminated sheet is very carefully heated and formed, usually against a male mold. To prevent color wash, care must be taken to ensure that the film is not stretched. Although there have been a few successful applications, the high current film cost, the concern with reprocessing regrind, and the degree of difficulty forming the part are mitigating against rapid non-automotive market penetration.
Nanofillers and Nanofibers
Nanomaterials are substances having dimensions in the range of 1 to 100 nanometers (0.001 to 0.1 mm). There are at least three general categories of nanoparticles – carbon nanotubes, intercalcated platelet particles of clay, and near-spherical particles of silica. Carbon-based nanotubes and larger-diameter nanofibers are apparently destined for reinforcement of specialty plastics (10). Nanoclays, primarily intercalated montmorillonite clays, are touted for their reinforcing effects at very low weight fractions of 10% by weight or less (11). Nanosilicas are touted for their ability to increase polymer strength and stiffness without dramatically decreasing impact strength, because the particle sizes are below the Griffin crack initiation size (12). Polymer viscosities are not greatly affected even at loadings in excess of 40 wt-%.
It appears that nanoclay-filled polymers offer opportunities in thin-gauge part thermoforming where stiffness is now achieved only with increased thickness. Polyolefins have good chemical and high temperature resistance but they tend to be weak at elevated temperatures. They appear to be prime candidates for nanoclay fillers.
Nanosilicas are being considered for heavy-gauge part forming applications. To date, nanosilicas are best dispersed in prepolymers that are then polymerized. Cast PMMA is one example. Because the filler particles are so small, forming forces should be substantially more modest than those for equivalently loaded glass-fiber reinforced sheet. Improved mechanical strength can lead to substantial reduction in formed part wall thickness in many industrial parts. Moreover, down-gauging usually leads to improved cycle time and lower production cost. And because nanoparticle sizes [about 20 nm] are far below the wavelength of light [400-700 nm], highly filled cast acrylic sheet remains transparent.
Nanofillers are finding early application in low-viscosity thermosetting prepolymers. Although addition to higher-viscosity thermoplastic polymers is being intensely researched today, uniformity in particle dispersion and distribution through the polymer matrix and production cost remain major concerns. Nevertheless, the unique property improvements that might be achieved indicate that the thermoforming industry must continue to monitor this new technology.
In this section, we simply highlight some other technologies that might influence future thermoforming developments.
Porous mold materials. There are now two commercial types of porous mold materials – porous aluminum and porous ceramic. Porous aluminum is best used when vacuum or vent hole mark-offs are not acceptable on the formed parts. Open areas and pore sizes range from 8% and 5 μm (13) to 20% and 100 μm (14,15).
Porous ceramics, used for years as liquid and gas filters and high-temperature diffusion plates, can now be fabricated directly into mold structures. Open areas and pore sizes can be tailored to essentially the same characteristics as porous metal. As with porous metal, the ceramic is mixed with a volatile material such as a polymer. The slip is formed against the pattern and dried. It is then fired to vitrify the ceramic and volatilize the pore-forming material. Shrinkage is about 30% or about the same shrinkage level as porcelain. Although the porous ceramics tend to be fragile, they are usually tough enough to be used for a few hundred parts (16).
Newer Polymers. The earliest polymers – camphorated cellulose nitrate and viscose rayon – were based on biological materials. Today, oil-based polymers dominate the thermoforming material palette. However, biopolymers are finding new interest, particularly in rigid packaging applications where compostability and biodegradability are desired. Polylactic acid or PLA, invented by Wallace Carothers in 1932, patented by Dupont in 1954, and available today primarily from Cargill Dow, is the leading polymer in this area (17,18). PLA processes as a “stiff polystyrene”. Although it is currently more expensive than current packaging materials, its “earth friendliness” often outweighs the additional cost.
Biopolymers based on polyhydroxybutyrate (PHB) may also offer thermoforming opportunities. PHB is reported to be a rather brittle highly crystalline polymer with properties similar to those of polystyrene. When copolymerized with polyhydroxyvalerate (PHV), the polymer degradation rate at elevated temperature is greatly reduced (19). It is thought that these polymers are best suited for medical applications.
Polymers based on norbornene are now commercial (20). These cycloolefins are produced by reacting ethylene or propylene with cyclopentadiene. The polymers are amorphous with glass transition temperatures that can be adjusted from 30oC to 230oC by increasing the norbornene content. Commercial grades have norbornene concentrations of 40 to 60 mol-% and Tgs from 70oC to 170oC. They are FDA food contact-approved and steam-sterilizable. It is reported that cycloolefins process more like PVCs than polyolefins.
Although these materials are not yet major players in thermoforming, there appear to be many future packaging applications.
“Moldless” prototyping. Since the 1930s, heat has been used to produce generous bends in plastics (21). Strip heating was introduced during WWII and again the allowable bends were generous. Cut sheet was fabricated into sharp-edged shapes by gluing. The objective of making sharp bends without excessive gluing has always required accurate machining techniques. Computer-driven three-axis machines are now being used in conjunction with precise bending protocols and exacting gluing procedures to produce very elaborate structures directly from sheet (22). These allow designs to be very quickly reduced to prototypes or even a few functional products.
Thermoforming, being the art and engineering of fabricating functional plastic parts from sheet, is maturing into a viable, competitive technology in packaging and structural parts. The future of thermoforming depends on quickly adapting advances in composites, nanofillers, and other commercialized technologies. The global scene will undoubtedly dictate future business decisions regarding offshore production, consolidation, and diversification.
- Throne, J.L., Technology of Thermoforming, Hanser Verlag, Munich, 683 (1996).
- Azdel GMTTM, Azdel Inc., 25900 Telegraph Rd., Southfield MI 48034.
- PennFibre, 2434 Bristol Rd., Bensalem PA, 19020.
- VeriflexTM thermoset polymer, CRG Industries, 2750 Indian Ripple Rd., Dayton OH, 45440.
- DKI Form a.s., Rundforbivej 281, DK-2850 Naerum, Denmark.
- VEC LLC (formerly Virtual Engineered Composites Technology division of Genmar Holdings), 639 Keystone Rd., Greenville PA, 16125.
- Hilgendorf, J.S., “Automotive Exteriors – Evolving to No-Spray Paint?” Plastics Engineering, 60:9, 34, 37 (Sep 2004).
- LexanTM SLX polycarbonate, GE Advanced Materials, Southfield MI.
- Spain, P.L., et al, “Dry Paint Transfer-laminated Body Panels Having Deep-Draw High DOI Automotive Paint Coat,” U.S. Patent 5,916,643, assigned to Avery Dennison Corp., (29 Jun 1999).
- The NanotubeSite lists dozens of universities and research laboratories active in C60 carbon-based nanotubes (and other geometries). Insofar as can be determined, none of these sites provide applications. The NASA nanotube site index has “applications” as a topic, but the location is blank. One major university active in this area is Inorganic Chemistry Laboratory, University of Oxford, South Parks Rd., Oxford OX1 3QR, UK.
- Nanocor, 1500 W. Shure Dr., Arlington Hts., IL 60004.
- Hanse Chemie AG, Charlottenburger Str. 9, 21502 Geesthacht, Germany.
- MetaporTM and EsporTM, Portec, Ltd., Barbara-Reinhart-Str. 22, P.O. Box 3139, CH 8404, Winterthur, Switzerland.
- Pyramid Technologies, Inc., 467 Forrest Park Circle, Franklin TN 37064
- Porvair Technology, Inc., Clywedog Road South, Wrexham LL13 9KS, North Wales, UK.
- Mould D/ATLAS M 130 porous casting system, ALWA GmbH, Roentgenstrasse 1, DE 48599, Gronau, Germany. [Note: The air-permeable casting system will tolerate 90oC mold temperature. Shrinkage in the casting system is about 30%.]
- NatureworksTM, Cargill Dow LLC, P.O. Box 5830, MS114, Minneapolis MN 55440-5830
- Balkcom, M., B. Welt, and K. Berger, “Notes From the Packaging Laboratory: Polylactic Acid – An Exciting New Packaging Material,” Doc. ABE339, Agricultural and Biological Engineering Dept., Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL (Dec. 2002).
- PHB and PHB-PHV copolymers available from Goodfellow Corporation, 800 Lancaster Avenue, Berwyn PA 19312-1780.
- TopasR COC, Ticona, Div. Celanese AG, 86-90 Morris Ave., Summit, NJ, 07901.
- Lockrey, A.J., Plastics in the School and Home Workshop, Governor Publishing Corp., New York City, 74-75 (1937).
- Tool-Less Plastic Technologies, LLC, 11208 47th Ave. W., Suite B, Mukilteo WA 98275.