Processes

Choosing The proper forming parametres

Sheet storage
To maintain the properties of Stat-Rite sheet, Noveonwraps and seals the rolls in heavy polyethylene with adesiccant to help prevent damage and moisture pick-upduring shipping or storage. Handling guidelines follow:

• Rolls should be used within 6 months after receipt.
• Do not remove the wrapping until you are ready to use the roll.
• Rolls should be opened only in a controlled humidity andtemperature environment.
• Rolls should be used as soon as possible after removal ofprotective wrapping
• Stat-Rite sheet should be stored under controlledtemperature of 60°F to 80°F and low humidity conditions.
• Sheets stored for any length of time should be pre-driedbefore forming.

Forming temperature
Sheet temperature should be determined with aninfrared pyrometer. For best results, the infrared deviceshould be mounted through the oven wall. Unlike theirhost polymers, Stat-Rite alloys show little sheet sag attheir optimum forming temperature. Heat transfer inthermoforming depends on heater radiation, air convec-tion, and conduction through the plastic. With infraredtemperature measurement and heating cycle time control,sheet temperature can be controlled to within +/- 10°F or +/- 5°C.

For thin-gauge thermoforming, where the sheet thicknessis typically less than 0.060" (1.5 mm) conduction throughthe sheet is usually less important than radiation and convection to the sheet surface. Sheets thicker than about0.010" (0.25 mm) should usually be heated on both sides.

When determining the best forming temperatureremember:

• Electrical properties will be affected by excessive heating.
• Maximum thermoforming temperature for Stat-Rite is 374°F(190°C) for acrylic, 338°F (170°C) for PETG (see Table 1).
• Forming at lower sheet temperatures yields the best hotstrength, minimum spot thinning, and shorter forming andcooling cycle times.
• Forming at higher sheet temperatures yields lower internalstress levels, better mold surface replication, deeper draws,longer cooling times, higher formed part shrinkage, morenonuniform part wall thickness, and vacuum hole nipples.
• Plug assist forming often produces improved parts.
• The heater temperatures should be selected to meet thedesired heating time and overall cycle time.

Heating Time
The time required to heat a sheet to its proper formingtemperature depends on sheet thickness, surface finish,material color, heater temperature, and the type ofheaters used. Generally, excessive sheet heating leadsto color shift, discoloration, surface blistering, delamina-tion, and loss of both physical and electrical properties.For thin-gauge thermoforming, the time to form andcool the sheet against the mold surface must equalthe time to heat the sheet to the forming temperature. Cooling begins the instant the sheet is transferred fromthe oven to the forming station. It is recommended thatthe average sheet temperature drop not exceed 10°F(5°C) during this transfer. Thus, transfer time should beonly a few seconds

Heating source
SInfrared heating elements are the most commonly usedheating source. Ceramic and quartz tube heaters arereplacing older metal rod heaters, since they are farmore energy efficient and more easily controlled. Theintensity of the heating source, usually given in Watt/in2or kW/m2, is usually controlled by the power and thefraction of time the heater is on. Ovens should bedesigned to provide even heat distribution over theentire sheet surface. In certain instances, screens orheat shields can be used to shadow local areas on thesheet to aid in improving wall thickness distribution

Cooling time
The formed part should be cooled to a temperaturebelow its distortion or set temperature. Cooling timesdepend on mold temperature, mold material heat transferproperties, coolant type, part wall thickness, part design,sheet temperature and ambient temperature.

Vacuum
A good vacuum system with the capacity to quicklyevacuate the volume of the mold is essential. A goodrule of thumb is that the volume of the vacuum tankshould be at least four times the volume of the moldcavity. And the vacuum developed by the vacuum pumpshould be 28.5 inches of mercury or 35 Torr.

Mold design
Machined or cast aluminum molds are recommended forcommercial Stat-Rite thermoforming. Water coolingchannels are recommended for mold temperature uniformity and cooling cycle control. Highly polishedmolds are not needed or recommended for vacuum forming. Matte part surface is achieved by grit blastingor chemical etching. In certain instances, polyfluorocarbon-impregnated aluminum surfaces are used to allow thesheet to locally slide during forming. This can yield apart with more uniform wall thickness.Syntactic foam and epoxy are recommended for plugs.For deep draw parts, plugs should be bull-nosed andpolyfluorocarbon-coated, to provide local slip and moreuniform wall thickness.