|
| Heating the Sheet-Thermoforming Machinery |
| Published:2009-8-5 22:11:59 |
Formability is a key aspect of thermoforming.all thermoformable polymers have forming windows，defined by the lower，normal and upper forming temperatures(LFT，NFT and UFT)．As noted，the lower and upper forming temperatures fcirm the absolute boundaries on formability．Whether a specific polymer can be fclrmed into a specific shape at temperatures near these boundaries depends on：
·The sheet characteristics such as：
Intrinsic orientation，
Hot strength．
Sag tendencies，
Thermal sensitivity of the polymer，
Sheet geometry and
Thickness，
·Uniformity of heating．
·Depth of draw．
·General mold geometry，and
·Other mechanical aspects such as：
Transfer time，
Ambient air temperature，
Plugging geometry，
Plug rate，
Plug temperature，etc．
Despite these limitations，the upper and lower forming temperatures are useful in defining the nature of the temperature within the sheet．The upper forming temperature relates to the sheet surface．For example，if the upper forming temperature is set because the polymer is prone to blistering or color change above this temperature，the sheet surface temperature must never exceed this value during forming．If the lower forming temperature is set because the polymer is too stiff to be formed or because it forms microcracks below this temperature．the centerline temperature must exceed this temperature before the sheet can be formed．The forming temperature range is shown as a time-independent band .The superimposition of the time．dependent local temperatures for the ideal case where the surface temperature reaches the upper forming temperature at the same time the centerline temperature reaches the lower forming temperature．And the average sheet temperature iust equals the normal forming temperature at the same time．
AS an example of the interplay between the sheet characteristics and the time-de-pendent energy input to the sheet．consider the following examples：
Thin-Gage Sheet
As noted，when the sheet is very thin，energy input to the sheet controls．If the sheet is dramatically reduced in thickness，the temperature profiles are obtained．Although this profile is entirely acceptable，it is apparent that the rate of heating can be increased substantially without affecting the formability of the polymer.
Heavy-Gage Sheet
When the sheet is very thick，conduction from the sheet surface to the centerline
controls．If the sheet is dramatically increased in thickness，the temperature profiles. To get the temperatures back into the forming window, the rate of hesting must be decreased.
Changing Polymer Characteristics Without Changing Sheet Thickness
If the new polymer has a broader forming window than the old polymer，the rate of heating can be increased without affecting the formability characte ristics．If the new polymer has a narrower forming window，the heating rate must be decreased as seen in schematic ．If the new polymer has a higher thermal conductivity than the old polymer．as is the case when filled polymers are used，the rate of heating can be increased without affecting the formability characteristics．If the new polymer has a lower thermal conductivity．as may be the case when foamed polymers are used．the heating rate must be decreased．
Changing Other Aspects of the Forming Process
If the new mold req uires greater depth of draw than the old one，the sheet may need to be formed at higher temperatures than before．As a result．the forming window may need to be narrowed or the lower forming temperature value increased．The result is that the rate of energy input may need to be reduced and the time to the forming condition extended．If the sheet requires prestretching，time between exiting the oven and completing the stretching may be longer than with straight forming．As a result．the sheet may need to be heated to higher temperatures than before．The rate of heating may need to be reduced and the time to the forming condition extended to increase the average temperature without exceeding the upper forming temperature．The interaction of forming temperatures with heating cycle times for ABS and HDPE．
|
|
Chinese 
Spanish
Portuguese
Arabic
Russian