Thick-gauge thermoforming

Figure 16.18 Schematic of thick-gauge thermoforming machine with two ovens thereby allowing one sheet to be heated for each forming cycle thus doubling the throughput rate of a single oven machine. (Note this machine, often called a double-ender, can be equipped with a second mold to produce twin-sheet thick-gauge products).

The cut sheet may be manually inserted into the clamp frame of the thick-gauge thermoformer, but vacuum pick-up and placement devices are more common for long production runs. Pneumatic or mechanical tables are used to Uft very heavy or large sheets. Pneumatic clamp frame closures are usually... 

The thermoforming mold is usually single-surfaced. Aluminum is the preferred material for longer production runs for both thin- and thick-gauge thermoformers. The mold serves several purposes and has several requirements. 

Thermoforming is commonly used to fabricate disposable cups, containers, lids, trays and other items used in the food, medical and retail packaging applications. Thick gauge polyethylene sheets may be used to thermoform very large parts such as refrigerator panels or liners. 

A film/sheet thickness of less than 1.5 mm (0.060 inches or 60 mils) is referred to as thin-gauge thermoforming and is usually used in rigid or disposable packaging applications. At this thickness level, the polyethylene may be transferred to the molding cavity from a roll or pre-cut sheet. Thicker gauges above 1.5 mm (up to about 0.250 inches) are handled as sheets which are pre-cut to fit a particular molding application. 

If the beginning sheet is greater than about 3 mm in thickness, it is called thick-gauge sheet. It is usually extruded and cut or guillotined into discrete pieces that are stacked on pallets. These pieces are then fed, either manually or automatically, into thermoformers that are called cut-sheet machines. 

It is estimated that in 2007, the North American thermoforming market was about 2700 million kg with an estimated value of about US 13,000 million. Approximately 20% of this market was industrial products, or products that have structural characteristics [2]. The top five industrial product markets are transportation, recreation, electrical/electroiuc, medical, and storage and duimage. In 2007, in North America, there were about 250 thick-gauge compa-lues, running 2,800 machines, and employing 14,000 people. 

As noted, there are two general categories of machinery thin-gauge or roll-fed thermoformers and thick-gauge or cut sheet thermoformers. 

Figure 16.16 Schematic of typical thick-gauge shuttle thermoforming press detailing major components.

Figure 16.17 Schematic of three-station thick-gauge rotary thermoforming press detailing major components (Note Four-station heavy-gauge rotary thermoforming presses have a preheating station prior to the primary oven).

Very few thick-gauge products are trimmed from the web in the thermoforming machine. Off-Une trimming methods include ... 

Figure 16.19 In-line extrusion-thermoforming line that can be used for both thin-gauge and thick-gauge products.

As thermoforming is a relatively low-pressure, single-surfaced process, undercuts are commonly designed into both thin- and thick-gauge products. 

Thermoforming is used to form products from multilayer sheets. Thick-gauge products include PVC-polymethyl methacrylate sanitary structures and polymethyl methacrylate- and fluoropolymer-capped ABS exterior transportation components. An example of a family of thin-gauge products is PS-ethylene vinyl alcohol-polyolefin barrier packaging. Heating and stretching of multilayer sheets often require trial and error to minimize interlayer delamination, excessive barrier film thinning, and incomplete replication of the mold cavity. 

Thin-gauge (thickness less than 0.060 in) thermoforming uses material supplied in a roll. It is the high-volume variety of this process and is generally associated with packaging. With the exception of disposables, such as cups and plates, it is not generally used for product manufacturing. 

Fig. 21. Side wall thickness variation during normal production—thin-gauge thermo-formed cup (40). Redrawn and used with permission of SPE Thermoforming Division.

Blister pack is one of the most common applications for rigid PVC thermoforming. Since virtually all blister pack is clear, resin clarity is often a criteria for resin selection. Although most blister pack gauge thickness ranges from 0.02 to 0.25 mm (0.0008 to 0.001 in), differences in resin clarity can sometimes be observed in these applications. 

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