Thin-gauge thermoforming

Albert KA et al (1993) Acrylic modified polypropylene for thin-gauge thermoforming improved processing properties and economics. J Plast Film Sheeting 9 293-305, October... 

Fig. 2. Schematic of small thin-gauge thermoforming machine (Kiefel GmbH, Freilassing, Germany). Used with permission of Hanser Publications.

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. 

A typical thin-gauge thermoformer is shown in Fig. 16.12. The roll is placed on a roll stand that usually has passive breaking or a roll speed governor to prevent uncontrolled unspooling. It may also be equipped with an end-of-roll alarm and rapid roll changeover capabilities. 

Figure 16.12 Schematic of a small thin-gauge thermoforming machine with major forming elements identified.

As with thin-gauge thermoformers, machine criteria include ... 

Rim rolling is an important method of stiffening thin-walled thin-gauge thermoformed cups. Cups are indexed through a device that heats and presses the trimmed rim of the cut against a curling screw (Fig. 16.33). The rolled rim dramatically increases the hoop strength of the cup and provides a smooth surface for the user. 

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. 

There are two main types of thermoforming, (1) vacuum forming and (2) pressure forming. In vacuum forming, the plastic sheet is heated until it becomes soft and pliable, the heater is removed and the mold is rapidly evacuated. The reduction in pressure on the mold side allows deformation of the sheet, by atmospheric pressure, into the mold cavity. On contacting the mold, the sheet cools rapidly and retains the shape of the mold. The force available to deform the sheet is limited to that provided at atmospheric pressure and this type of thermoforming is thus limited to thin gauge, low-modulus polymers. 

Sag bands or metal rods that mn length of the thin-gauge oven have been used for polymers that sag excessively when heated. Some heavy-gauge shuttle thermoforming machines are equipped with ovens that completely seal the sheet between the heaters. These ovens are equipped with vacuum and compressed air, so that the sheet is lifted when it begins to sag. 

Fig. 16. U.S. plastic materials consumption for thin-gauge (2.1 million metric tons) and heavy-gauge (682,000 t) thermoforming (32). PE = Polyethylene ABS = acrylonitrile-butadiene-styrene PS = polystyrene PP = polyproplyene PMMA = poly(methyl methylacrylate) PVC = poly(vinyl Chloride) PET = poly(ethylene terephthalate). Redrawn and used with permission of SPE Thermoforming Division.

Many thermoformed parts contain undercuts. Detents and interlocking lugs are frequently used in thin-gauge packages with integral lids. Undercuts in... 

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

If the beginning sheet is less than about 1.5 mm in thickness, it is called thin-gauge sheet. It is usually extruded in roll form, with the rolls being 1-1.5 m in diameter. The rolls may weigh as much as 1800 kg and may contain as much as 3000 m of sheet. The sheet is fed continuously into thermoformers that are often called roll-fed thermoformers. 

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

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