The technology of thermoforming is rapidly changing．Old ideas about process limitations are being challenged daily In 1992，one machinery manufacturer said that：
“More innovations have been made in thermoforming machinery in the last hall-dozen years than in all the years before”．
As noted below，machinery innovations include:
         Improved and more reliable ways of clamping sheet，
　　　Improved heaters，
　　　Improved heat distribution patterns，
　　　Adaptation of infrared sensors for monitoring and controlling sheet residence time in ovens，
　　　Improved clamping systems，
　　　New plug assist materials，
　　　Better control of stretching foFees and pressures，
　　　Improved trim dies，and
　　　Many new mold materials and mold making techniques．
Of course，innovations are not iust restricted to machinery．The past decade has seen extensive efforts to produce uniform-property extruded sheet，newer polymers with superior sag resistance and oxidative resistance，regrind monitoring techmques and computer programs to predict time-dependent sheet temperature and local wall thickness．However，there are certain guidelines about the thermolormmg process that are relatively generic．For example，thermoforming has become economically important since it Ofiers processing advantages over competitive processes such as：
       Accumulator blow molding
       Injection blow molding，
       Rotational molding，and
       Injection molding．
Relatively low forming pressures are needed and SO mold costs are low and products of relatively large size are fabricated economically．Parts with very small thickness-to-area ratio are fabricated．For thin-walled products。fabrication time is very short，making the process economical for products requiring high multiplication factors．
For a few thick-walled parts，molds are made of wood，plaster or other easily shaped，relatively inexpensive materials．Mold fabrication time and thus lead time are very short．Thermoforming is the method most usually selected for prototype and display products to be made of other processes．
Gage Typical thermoforming steps are
  Clamping．
  Heating．
  Shaping．
  Cooling．and
  Trimming．
The general process of thermoforming is loosely separated by sheet thickness or gage．There are two broad categories thin-gage and heavy-gage．Recently，subcategories have been introduced to further classify the process．
  Thin-gage thermoforming means that the sheet thickness is less than about 0．060 in or 1．5 mm．This category is further classified into：
Film forming，where the sheet thickness is less than about 0．0 1 0 in or O．25 him．Thin sheet forming，where the sheet thickness is between about 0．1 0 in or 0．25 mm and 0．060 in or 1．5 mm．Heavy-gage lhermqlst'ming means the sheet thickness is greater than about 0．1 20 in or 3 mm．This category is further classified into：
Heavy sheet forming，where the sheet  3 mm and about 0．400 in or 10 mm．Plate forming，where the sheet thickness iS between about 0．1 20 in or is greater than about 0．400 in or 1 0 mm
There is a gray area between thin-gage f≤0．060 in or 1．5 mml and heavy-gage(≥0．120 in or 3 mm)．In some instances，sheet in this mid—gage range of 0．060 in to 0．120 in or 1．5 mm to 3．0 mm behaves as if it iS thin-gage．and in others．as if it iS Heavy-gage．This gray area is most apparent during handling of the sheet，heating the sheet and trimming the part from the web． Polystyrene and polyolefin foam sheet thicknesses are usual ly greater than 0．1 20 ifl or 3 mm but these foams are usually treated as thin—gage sheet  stock．
Clamping of Thin-Gage Sheet
Thin’gage sheet is usually supplied to the former in rolls．The majority of packaging applications such as blister pack．Form-fill-seal packaging．foam sheet forming and biaxially oriented forming uses thin-gage sheet．Formers that use roll sheet stock are called，oll-fed,d or continuous sheet formers．Clamping is by parallel continuous 1oop chain-fed pins or pin-chains that pierce the sheet at 1 in or 25 mm intervals at about 1 in or 25 mm from each edge．For high-temperature  forming of CPET or PP．the rails are shielded from the sheet heating source or are actively cooled．The edges of the sheet and the plastic between the formed product are trimmed，reground and reextruded into sheet for forming．