Twin-Sheet Thermoforming

Twin-sheet thermoforming. Twin-sheet thermoforming is generally used to produce large, flat, double-walled parts. Nearly all of the thermoplastics can, at least theoretically, be thermoformed. However, the bulk of parts made by this process are usually polystyrene or ABS. The high-est-temperature plastic thermoformed in any significant amount is polycarbonate. 

Leading the processing growth was the expansion of twin-sheet and pressure-formed plastic products. Fueled mostly by advances in mold technology, material developments, and thermoforming machinery capabilities, technology improvements in the form of machine controls have led to machine designs that are faster and more consistent than was previously possible. As an example advanced materials and... 

Thermoformed (Coextruded Plug-Assisted Twin-Sheet) Fuel Tanks Show Promise, MP, Apr. 2000. 

Hollow objects can be produced using twin-sheet thermoforming, in which two sheets of plastic are thermoformed and sealed together. This can be done sequentially, with one sheet formed first and then the other added and formed, or simultaneously, with both sheets formed and sealed together in a single step (Fig. 10.11). 

Figure 10.11 Twin-sheet thermoforming, simultaneous forming (reprinted with permission...

There is a largely untapped opportunity for wood-plastic composites [10] in packaging and material handling. This is evident from the fact there are nearly 600 million pallets made each year in North America and only about 50 million are plastic. One Canadian manufacturer, Dura-Skid, produces pallets by assembling lineals made from wood-filled HDPE. The main advantage of wood-plastic composites to the pallet manufacturer is the excellent stiffness to weight ratio afforded by wood filler. The greatest opportunity may be in structural foam and twin-sheet thermoformed pallets. 

Rotational molding or twin-sheet thermoforming (application dependent)... 

Processes for hollow shapes have a much simpler pecking order. The ideal shape for rotational molding is a sphere. The ideal shape for twin-sheet thermoforming is a flat panel. The selection of which process to begin with would be based on the shape of the particular part to be manufactured, although it should be noted that rotational molding can make parts with sections as thin as 1 in. Additional factors would be the size of the part (rotational can go larger) and the selected material, since each of these processes favors different materials. 

Rotational molding, blow molding, and twin-sheet thermoforming can also make large, double-walled parts which are relatively flat (such as pallets). Twin-sheet thermoforming is best suited to the flattest of such parts. 

Thermoforming, pressure thermoforming, and twin-sheet thermoforming... 

For thermosets, the hollow part processes are filament winding and pultrusion. These processes are limited in shape and structure. There are three hollow part processes for thermoplastics (1) rotational molding, (2) twin sheet thermoforming, and (3) blow molding. 

Blow molding, until the last few years, was the main plastic process utilized to produce a hollow object. In the past few years, other plastic processes, such as rotational molding and twin sheet thermoforming, have evolved with technical achievements and plastic raw materials improvements to where today, they can compete with blow molding for many uses as toys, gasoline tanks, holding tanks, etc. 

Thermoformer Lands Biggest Twin-Sheet Job. Plastics World, pp. 15-16, Jan. 1989. 

DeGrandpre, C., et al.. Twin Sheet thermoforming of a Fuel Tank with a Converted Blow Mold, National Research Council Canada, SPE-ANTEC 2003. 

Thick-sheet thermoforming can be used to produce hollow parts that are more difficult to fabricate using a blow molding process. In this method, referred to as twin-sheet thermoforming, two sheets are fed to the open mold so that the sheets remain separated from each other with a blow-pin placed between the two sheets. The mold is closed and the polyethylene is forced into the mold above and below the sheets with a combination of air pressure introduced through the blow-pin between the sheets, while at the same time drawing a vacuum between the sheets and the molding wall to remove air from the mold above and below the two sheets. 

Kiefel Technologies plug-assisted twin-sheet thermoformed fuel tank design, aimed at producing under 100,000-300,000 units/year, was initially developed for mid-size trucks. The fuel tank consists of two HDPE outer layers, two tie layers, and an EVOH intermediary barrier layer. ... 

Variation of wall thickness has a direct impact on the way in which rotationally molded parts are dimensioned. Since the wall section may vary and in many cases cannot accurately be predicted before a part is made, the part must be dimensioned to an external feature. If an internal dimension is required, then allowances for the upper tolerance of wall thickness variation must be made. Designers used to specifying injection molded parts with controlled surfaces both inside and outside of the part can find this difficult to deal with. However, extmsion blow-molding and twin sheet thermoforming share the same limitations and rotational molding actually maintains more uniform wall thicknesses than either one of these competitive processes. 

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