Thermoform bubble

Finally, another equibiaxial deformation test is carried out by blowing a bubble and measuring the pressure required to blow the bubble and the size of the bubble during the test, as schematically depicted in Fig. 2.52. This test has been successfully used to measure extensional properties of polymer membranes for blow molding and thermoforming applications. Here, a sheet is clamped between two plates with circular holes and a pressure differential is introduced to deform it. The pressure applied and deformation of the sheet are monitored over time and related to extensional properties of the material. 

PVC is used in overwrap film, shrink film, shrink sleeving, thermoforming for all types of blister and bubble packs. Plasticised PVC IV bags are usually PP/Nylon overwrapped to reduce moisture loss. 

Kydex is primarily formed into sheets, to be thermoformed later. The sheets pickup moisture when exposed to humid atmosphere. If sheets contain 0.2% moisture or more, bubbles will result when the material is heated to the thermoforming temperature (205°C). Sheets can be dried in a circulating air oven at 70°C in 16 h (for thickness of 3 mm) or in 24 h (thickness of 6 mm). Drying can be avoided if the sheets are stored in a low humidity environment or thermoformed at 180°C or less. 

A blister pack consists of a rigid (cardboard or other) bottom liner and a sleeve of a transparent pol5Tner material. The sleeve can either reproduce the packed article contours (blister strip pack, Fig. 2.55b) or be of regular spherical shape (blister bubble pack. Fig. 2.55c). The sleeve is produced by thermoforming and is bonded to the bottom liner by welding. For the blister pack, commonly inhibited film materials and sheets up to 0.15-1.0 mm thick are used. Mainly small-size hardware and tools are packed by this method. The blister pack adds an attractive appearance to the goods, protects reliably against various effects and enables direct observation of the contents by customers. 

In a number of processes, air pressure is applied to a bubble of polymer melt to change its shape. We will examine one continuous process—blown film production—and two cyclic processes—blow moulding and thermoforming. All of them involve some melt extensional flow with a resultant thinning of the bubble, and at least one side of the polymer solidifies without the constraint of contact with a mould. 

Thermoforming the four stages in the pressure-bubble vacuum-snapback technique (after Modem Plastics Encyclopedia). 

Hand Layup (Contact Molding or Open Mold Process). The oldest and simplest technique is primarily manual. An open mold is waxed to lubricate it. The surface of the proposed product is applied to the mold, usually as an unreinforced gel coat or sometimes as a thermoformed plastic sheet. A layer of fabric is hand cut and hand laid into the mold. Catalyzed resin is poured over the fabric and worked into it with brush, roUer, and squeegee to eliminate air bubbles. Another layer of fabric is laid over this, often oriented in a specific direction to optimize properties. Again, catalyzed resin is poured over it and worked into it. The process is repeated as many times as required to build up the desired thickness and optimum orientation of the layers. If the cure reaction is inhibited by air, a wax may be dissolved in the resin it exudes to the surface and forms a barrier to exclude air and permit complete cure. The resin may be allowed to cure at room temperature, or the assembly may be heated to complete or hasten the cure reaction. 

Bubble forming n. A variant of sheet thermoforming, in which the plastic sheet is clamped in a frame suspended above a mold, heated, expanded into a blister shape with air pressure, then molded to its final shape by means of a descending plug applied to the blister and forcing it downward into the mold. Weismantal GF (1981) Paint handbook. McGraw-Hill Corporation Inc., New York. 

Free forming n. A variant of sheet thermoforming in which a bubble is blown into the clamped, heat-softened sheet, either by applying a vacuum to the side that will be convex or pressure to the underside. The method has been used most with cast-acrylic sheet for applications where the best possible optical properties are foremost, such as airplane canopies. 

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