air oven heating

AIR OVEN HEATING

For most forming operations, the whole sheet of Chemcast® GP is heated before forming, usually in a hot air oven with forced circulation. The basic elements of such an oven are a chamber of sufficient size, insulated against heat losses and provided with a forced circulation of air and a means of heating.

Gas or electricity may be used to heat the oven. Gas ovens should be designed with heat exchangers so that the flue gases are not used as the actual heating medium. Scoops should be used to direct the heated air from the heating elements to the Chemcast® GP sheet. The sheet should be protected from radiation to eliminate hot spots.

When Planning an electrically heated oven, it is desirable to design the heating elements so they can be removed conveniently for servicing and replacement. It is also desirable to use a switch capable of converting the wiring from series to parallel rather than a simple onoff switch.

A seriesparallel switch reduces power to the elements when actuated by the thermo-regulator but does not switch the power off. This mode of operation provides more uniform oven temperature.

To prevent thermal damage to the sheet, maximum oven temperatures should be carefully controlled. Automatic temperature regulators must be provided. Temperature recording devices are desirable, although not absolutely essential. The accuracy of temperature controls should be checked occasionally by means of a maximum reading thermometer hung in the oven near the sheets.

It is extremely important to wire the heater elements and air circulating blowers on the same circuit. If these units are on separate circuits a potential fire hazard exists. Overheated elements can start a fire if there is no air circulation across them.

Forced circulation of air and suitable baffles are necessary to distribute the heat evenly throughout the oven so that each sheet will be heated uniformly over its entire surface and so that all sheets will be heated to the same temperature. Air velocities across the surface of the sheet should be in the range of 150 to 250 feet per minute. This is very important.

Ovens should be designed with narrow doors to prevent excessive heat loss while loading and removing the sheets. It is also desirable to have one door in one section of the oven sufficiently large to permit the introduction of large formed parts for re-heating and flattening if reforming is necessary.

Small sheets may be supported in trays lined with soft cloth or woven Fiberglas cloth or Fiberglas coated with sintered Teflon to protect the surface of the Plastic from scratching. Large Chemcast® GP sheets should be hung vertically from racks. Overhead racks for hanging Chemcast® GP sheets may be equipped with tracks and trolleys leading to forming equipment to facilitate handling of the sheet.

Air Operated Oven Clamps and Clamping Station

Figure 2 shows construction details for a clamp. ing station and air operated clamps for hanging Chemcast® GP sheets in forming ovens. This clamping device can reduce appreciably the man" power required for clamping and unclamping sheets preparatory to forming. It can also shorten the elapsed time required to transport the heated sheet from the oven to the forming equipment, thus minimizing the possibility of cold forming.

The clamping station and clamp design permit the operator to open all the clamps at once by simply opening a valve on the air line. After the sheet has been Placed in the clamps, the clamps are closed by closing the air valve and venting the air pressure in the cylinders. The air pressure and vent line control can be operated by foot pedal so that the fabricator's hands are free to take hold of the sheet.

For a permanent installation the clamping station can be welded to the trolley rail. The clamping station can also be used as a portable unit so that it can be moved to various locations and bolted to any section of trolley rail in the shop.

An automatic clamping station of this type is essential if much forming is to be done but less costly clamping equipment can be constructed using standard spring paper clips. The Chemcast® GP must be clamped and unclamped by hand. Spring paper clips such as Hunt No. 4 or equivalent are suitable.

Methods of Heating Acrylic Sheet