Heating the Sheet

Sheets of thermoplastic material may be hot-formed into a variety of smooth shapes by heating the sheet in an oven to a temperature at which it becomes pliable. The sheet is then bent over a form. In the laboratory this method is... 

Thermoforming a material for a unit change in temperature. Values for plastics range from 0.01 to 0.2 mil/in. Any process of forming thermoplastic sheet which consists of heating the sheet and pulling it down onto a mould surface. 

The web, now moving at a line speed proportional to the stretch imparted by the MDO unit, enters the TDO unit. The TDO unit has a large enclosed oven that uniformly heats the sheet for the transverse stretch. The transverse stretch is accomplished by the use of film clips (Figure 11.3). These clips are attached to a chain-drive system on each side of the web. Each chain-clip assembly clamps the web upon entering the oven and stretches (tenters) the sheet because of the increasing width between the chains along the machine direction (Figure 11.4). On exit of the TDO unit, the web has now been oriented in both the machine... 

Hence, the overall cycle times is dominated by the heating time, if it is necessary to heat the sheet over the mould. 

The first approach (Fig. 6a) uses a laser to heat the sheet, just before it contacts the roller, allowing localised and controllable heating of the sheet (Klocke and Wehrmeister 2003). This approach has been applied in conventional spinning however, it is also applicable in shear... 

Orientation-release stress n. The internal stress remaining in a plastic sheet after orientation, which can be relieved by reheating the sheet to a temperature above that which it was oriented. This stress is measured by heating the sheet and determining the force per unit cross-sectional area exerted by the sheet as it attempts to revert to its pre-orientation dimensions. 

Sheet material is clamped at its edge and heated. The sheet expands and becomes less rigid, and may sag under gravity due to the temperature increase. 

Postdipping. Dip the developed chromatogram in the PMA for about 5 sec. Remove the sheet, allow the excess PMA to drain on a paper towel, and then air dry. Heat the sheet in an oven at 110°C for 2-5 min to detect the lipids. 

The pressure, time, and temperature variations that can exist will affect the final part dimensions. Of these factors, evenness in heating the sheet before forming is usually the most important control. An allowance must also be made for postforming shrinkage (as previously reviewed see especially Chapter 7). Molds should be designed oversize so that when shrinkage is complete the part dimensions will be correct to within the design tolerances. 

Low-density thermoplastic foam cannot be heated to a forming temperature appropriate for the plastic without excessive cell collapse and poor product quality. Inadequate heating yields low secondary expansion and products that do not replicate the mold cavities with just vacuum. Pressure forming will collapse the foam cell structure. As a result, foam sheet is usually heated in roll-fed machines on traditional pin-chain rails. The ovens are usually extended in length and have heaters that gradually heat the sheet to temperatures where the increasing internal gas... 

The mould, which is partially hollow underneath and drilled through with a number of fine holes, is placed over the air outlet. The plastics sheet to be formed is placed over the open top of the box and clamped down by the frame, thus sealing off the box and making it an airtight compartment (Fig. 2a). The heater panel is then placed over the plastics sheet, at a distance of 5-6 in (127-152 mm) in order to heat the sheet as uniformly as possible (Fig. 2b). 

Thermoplastic sheet is converted to a wide variety of finished articles by processes known generically as sheet forming or thermoforming. Although the details vary considerably, these processes all involve heating the sheet above its softening point and forcing it to conform to a cooled mold (Fig. 19.9). 

A sheet of (3.8 mm thick, 10 cm x 12 cm) amorphous PET is heated on both sides by a bank of quartz lamps (eight Fostoria T-3 quartz lamps per bank) rated at 500 watts with a peak filament temperature of 2250 °C at 120 volts. Formulate the equations and boundary and initial conditions which must be solved to find the time to heat the sheet up to... 

FIGURE 10.13 Forming in a matched metal die system. After heating, the sheet is formed by the application of mechanical pressure to the upper part of the mold. 

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