Recently, Profile Plastics has been winning major thermoforming awards on their thin-gauge twin-sheet thermoformed surgeon’s helmet. Although this is certainly not the first thin-gauge twin-sheet thermoformed part, it certainly has raised the bar in this area. This technical note discusses the nature of the process and some of the reasons why this technology has not blossomed in the manner of heavy-gauge twin-sheet forming. Before we get started on this discourse, let’s simplify the language. Whenever we need a shorthand phrase, let’s call thin-gauge twin-sheet forming just “thin-twin” forming.
First we need some historical information. J. Harry DuBois noted that the very first twin-sheet forming took place in the late 1800s. The products were baby rattles, teething rings, mirror cases and hairbrush backs. The company was Hyatt Brothers. And the cellulose nitrate or pyroxylin sheet was skived or sliced into very thin sheets from blocks. In other words, the very earliest twin-sheet forming was thin-twin forming. In the 1930s, table tennis (“ping pong”) balls were twin-sheet formed from Dupont Surlyn ionomer. And also in the 1930s, liquid containers were twin-sheet formed from the revolutionary new but very expensive polyethylene.
So, why did it take until the beginning of the twenty-first century to rediscover thin-twin forming? Well, it really didn’t. Let’s look at the current thermoforming market for a brief moment. First roughly three-quarters of all thermoformed products are formed from thin sheet. And nearly all of these are formed for the packaging industry. Although being disposable, a product such as the surgeon’s helmet is obviously not a package. This means that the currently developed applications for non-packaging hollow parts are apparently quite limited. This does not, however, mean that the potential markets are limited.
Does this mean that there are no markets for thin-twin packages? Consider this. In the early 1980s, Dairy Queen prototyped a twin-sheet pint PS ice cream tub for carryout. While the technical problems were mostly solved, the project was shelved in favor of expanded polystyrene foam. Ultimately, this container, too, was phased out when half-gallon containers were successfully marketed. In the early 1980s, Nestle developed a process for multi-layer forming of preforms for subsequent stretch blow molding of wide-mouth tea jars. The final product did not preserve the freshness and moisture resistance of glass. In the late 1990s, Kiefel developed a machine for thin-twin forming one-liter bottles of PP for a French dairy. The product was never marketed, the dairy preferring to continue using their flexible milk-in-a-bag container. About this same time, Kiefel developed a machine for fabricating thin-twin PS microwavable rice bowls for the Japanese market. The success of this development is unknown. I was told at the Kunststoffe 2001 show in Dusseldorf that, in the mid-1980s, OMV had developed technology to produce a twin-sheet PS “hot cup” for a Canadian company. The cup was designed to compete with expanded polystyrene foam, but the project lost out to a paper cup with a cardboard collar. Think Starbucks.
There are several ways to form a thin-twin product. If the product is needed in limited quantities, a “double-ender” shuttle press is used. In this method, a cut sheet is loaded into each of two clamp frames. The frames are then shuttled into their respective ovens at each end of the load/mold/unload station. When the sheets are at forming temperature, the frames are moved from the ovens to the molding station. The mold frame contains both mold halves, which are sequenced into the hot sheets. Typically, both mold halves are female, although this is not a requisite for the forming step. The sheets are formed independently of each other. Then the mold halves are brought together to form the seal area. Much mold design effort is devoted to achieving adequate seal, since in many cases, the seal area must be liquid-tight. Multiple-cavity molds can be used if market size warrants.
If the product quantities are large, the shuttle concept is usually not advocated. Special roll-fed machines are needed, in which the two sheets are brought into the heating ovens on separate pin-chain rails. If a nearly conventional top-and-bottom heating oven is used, the heating time must be increased by a factor of about four. Usually, however, a special oven, employing top-and-bottom heaters for the parallel sheets, is needed. These ovens are usually considerably “thicker” than conventional thin-gauge thermoforming ovens.
The sheets are conveyed between the two mold halves and simultaneously formed in the manner described above for the shuttle press. The mold halves are then brought together to form the seal. The sheets containing the formed products are then released from the top pin-chain and conveyed to the trimming press. It is believed that Kiefel used a technique similar to this to produce its PP liter bottles six-up or nine-up.
So it appears that thin-twin forming is doable and has, in fact, been developed for several applications. It is now time to discuss the reasons why it may not be the “next breakthrough” in thin-gauge thermoforming. First consider the technical limitations. First, the process is most suited to hollow products, where both mold halves are female cavities. This doesn’t mean that one half of the mold cannot be male or that the final part cannot have kiss-offs. It just means that the sheet thickness of the seal area is far better controlled with female cavities than with male molds. And since seal area integrity is of major concern, female molds are preferred.
A second technical limitation deals with sheet stretching. So far, essentially all the twin-sheet products evaluated have shallow draws. This is because that with simultaneous forming, there seems to be no practical way of plugging the sheet into the female cavities. This doesn’t mean it can’t be done. It just means that reliable technology is not in place to do it with current machine designs. Is it possible to develop a simultaneous technique that includes an active plugging step, maybe by separating the mold halves and shuttling the plug plate into the area between them? Probably, but see my comments at the end of the next paragraph.
In heavy-gauge twin-sheet forming, the plugging problem is bypassed by using sequential forming. In other words, the bottom sheet is formed first, using the top mold as a plug. Then the top sheet is formed against the top mold. Then the two sheets are mated. Is this sequential method possible with thin-twin forming? Insofar as I can tell, it hasn’t been successful to date. But keep in mind that anything is possible in thermoforming, so long as there is a suitably large money pit.
So, to this point, we’ve learned that thin-twin forming is not only possible but also that it has been technically practiced successfully in portions of three centuries. Now the question is: “Why hasn’t it been used in more applications?” After all, consider all the “hollow parts” that we see in a few minute supermarket or hardware store tour.
There are two major areas of thin-gauge rigid and semi-rigid containers that apparently compete with thin-twin products. First, consider lidded containers. There are two generic types – those with separate lids and those with integral lids. Deli containers are typical of the first type. Point-of-purchase products such as nuts and bolts are usually sold in the second type of container. Integral-lid containers are the dandies of the industry today. No lost lids. No lids that don’t fit. Containers can be easily made pilfer-proof. Containers can be closed mechanically. The forming technologies for integral-lid containers are well known. Most of the innovations focus on designing and redesigning the hinge and on methods of closure. In the latter case, the design considers whether the container must be opened more than one time by the user or whether the container is primarily a protection for the contents. The fast-food salad container is an example of the former. The point-of-purchase container protecting a portable CD player is an example of the latter. There are all manner of closures, including interference-fit buttons and bars, detents, staples and heat-sealed edges.
These containers are probably not candidates for thin-twin forming. In fact, there is strong evidence that the single-sheet forming technique used to make an integral-lid container can be used to produce a very suitable thin-twin container, by simply thermally welding the two halves together, then trimming off the integral hinge!
The second area of thin-gauge containers that compete with thin-twin containers is the so-called “nested container.” A nested container is simply two nearly identical thermoformed sheets, usually in the form of five-sided boxes. Cavities are formed in the centers of these five-sided boxes in a fashion allowing the product to nest between the two boxes. The boxes are designed so that the draft angles allow them to tightly nest around the product. The entire assembly is then either stapled or taped shut, or inserted into a cardboard sleeve. The nesting technique is used for containers that require very deep draw or must hold products are very bulky. Although this type of container is a faux thin-twin, it is doubtful that it is a strong candidate for thin-twin technology. The reason? What’s done now is cheap and easy. And it works just fine, thank you very much.
Oh, and by the way, it is not a difficult stretch to consider simply single-sheet forming the inside and outside shells of a deep-draw container in separate conventional forming machines that employ conventional plug assists, then feeding the sheets containing the formed shells into a third machine, where the shells are nested, then thermally welded and trimmed in a single step. Decoupling the two stretching processes from the welding step yields better quality control of the forming process.
So, should this prevent us from scurrying around for other thin-twin challenges? I mean, certainly if the surgeon’s helmet can engender all those awards, there certainly should be dozens of other applications just as spectacular. The importance of the surgeon’s helmet lies in the fact that it fulfills two very important aspects of innovation. First, the size of the market is relatively limited, thereby allowing adaptation of essentially existing technologies. And second, the market might be considered “mission-oriented,” in that competition was minimal and experimentation and prototyping could proceed apace to meet the unmet needs in the marketplace. Launching a thin-twin drink cup program, on the other hand, would simply not have the same advantages or luxuries, if you will. And perhaps that is why the previous thin-twin products – ice cream tub, milk bottle, “hot cup” – did not make it to the market.
Does this mean then that there will never be a thin-twin ice cream tub or milk bottle or hot cup? Of course not! It seems to me that these products represent the vanguard of high-volume thin-twin products. And that they may have been before their time from a marketing or technical viewpoint. So while it is important that we continue to scurry about seeking out the next “surgeon’s helmet,” we must also realize that major market penetration of thin-twin containers must come from high-volume products. And to penetrate these markets, efforts must be continually made to improve on or invent new methods of fabricating them.