Thermoforming Machinery
As noted, there are two general thermoforming categorice. Typically, heavy-gage sheet is handled as discrete cut sections and the forming equipment are called cut- sheet thermoformers. Thin-gage sheet is handled in continuous rolls and the forming equipment is usually called roll-fed thermoformers. The equipment in both categories includes: Thermoforming Machinel Some form of sheet handling device,
l A way of moving the sheet from one station to another,
l A means of controlling the various elements that allow the sheet to be heated,formed and moved form station to sation,
l A sheet heating oven,
l A vacuum system,
l A forming press, and
l A forming part removal region.
In addition, the equipment many include:
l Some form of prestretching such as:
Preblowing or
Plug assist
l A pressure system
l A trimming press, and
l Some form of trim removal.
Certain guidelines pertain to both categories of forming equipment. Table 1.7 gives an overview for thermoforming equipment in general [20]. Some of these are summarized below. Cup Making Machine Heating Source
The various heating methods are detailed in Chapter 3. Sheet temperature should be controlled to within ±5℃ or ±10℉. During transfer to the forming station, the
sheet temperature drop should not exceed 5 to 10℃ or 10 to 20℉. Infrared heating is most popular today. The various heating methods include[21]:
l Simple nickel-chrome heating wires,
l Metal resistance rods, sometimes called calrods,
Table 1.7 General Soeifications for Thermoformers
|
Platen size, W×L(in×in or mm×mm)
Maximum depth of draw(in or mm)
Forming process (vacuum, pressure, matched, mold, plug against. Twin-sheet capability)
Platen power drive (penumatic, hydraulic, mechanical, electric)
Indexing power drive (penumatic, hydraulic, mechanical, electric)
Floor space(ft or m)
Heater type (metal rod, quartz, ceramic, radiant, gas, nichrome wire)
Heater controls (proportioning, percentage timers, timer controlled, zone controlled, programmable controlled, machine controlled)
Maximum heater output(kw/ft or kw/m)
Special features (purpose, type such as shuttle or rotary, number of stations, number of ovens,etc.) |
l Ceramic bricks or tile,
l Quartz heaters in rod, spril or square plate form,
l Direct gas-fired catalytic burners,
l Indirect gas-fired catalytic burners,
l Heat lamps,
l Quartz glass plates,
l Halogen bulbs, and
l Wire or rod heated metal plates.
Heater surface temperature is usually measured with thermocouples, thermistors or infrared pyrometric devices. Heating wires and resistance wires are inexpensive but oxidize rapidly and so lose heating effciency. Quartz heaters are quite efficient, can be turned on and off like bulbs but are quite expensive and fragile. Quartz is preferred for high temperature and “shaped heating”needs as described in Chapter 3. Printing Machine Sheet is also heated by direct contact with a hot metal plate (trapped sheet heating), by placing the sheet in a hot air oven (convection heating), or by passing it through a high-frequency electromagnetic filed (RF or microwave heating). In the last case, the plastic must zbsorb the high-frequency energy. PVC is heated by radio-frequency energy in the flow-molding embossing process. Other polymers must be doped with“lossy”substances such as inorganic hydrates or even carbon black. Automatic cup stacker There are certain elements that pertain to the ovens for all forming presses. For example;
l There must be a way of separating the sheet form the heater source at shut-down. Baffles and dampers are used for heavy-gage sheet and fly-open and
extracting shuttle are used for thin-gage sheet.
l There should be adequate means for rapid replacement of burned-out heater elements on both top and bottom heater banks. |
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