According to recent news, a new inspecting method offers sheet manufacturers and thermoformers to more precisely analyze and quantify the thermoformability of materials. The Technoform-III thermoformability analyzer is known to be a revolutionary device because it closely simulates the thermoforming process under controllable conditions and is repeatable and easy to use. From small samples, it collects quantitative information that can be used to predict or compare forming behavior.

Consistent Sheet
The novel test method fills an important need because of the growing use of thermoforming for large parts and the lack of reliable tests to determine thermoformability. At present, 75% to 80% of the variability in thermoforming is the result of inconsistent sheet, the most obvious problem faced today by thermoformers. Depending on process conditions, output rates, amount of regrind, and storage conditions, the feedstock may vary in residual thermal stresses from lot to lot and even between sheets in the same lot. This creates a "blind spot" that converters generally overcome by on-line adjustment of the thermoforming process.

According to industry experts, today's formability tests do not accurately reflect the thermoforming process. Among the current methods are the hot tensile/creep, dynamic rheology, melt-tension, and sag tests. In the absence of more useful test methods, thermoformers have had to resort to full-scale thermoforming trials, which require expensive material, tooling, and machine time.

Mimics Actual Forming
The Technoform-III analyzer is a miniature thermoforming machine with heating, forming, and cooling functions. It uses a 6X6 in. square sheet sample of 20- to 140-mil thickness, preferably die-cut from actual extruded sheet. The sample is clamped between two insulated plates, forming a "tray" which is placed manually on the loading rail.

The software input menu includes selection of forming method (plug, vacuum, or plug-assisted vacuum), temperature, heating rate, heat-soak time, plug speed, cooling time, maximum plug force, and maximum draw depth. When the test cycle is actuated, the sample tray moves into the oven, where upper and lower infrared heaters heat the sample from both sides. Heater temperatures and distance between heaters and the sample can be adjusted independently. A noncontacting infrared probe continuously measures the sample's surface temperature. Upon achieving preset surface temperature or temperature and heat-soak time, the tray moves into the forming station within 1 sec. A plunger with a male plug is applied at a programmed speed and a load cell on top of the plug continuously measures the deformation force. When a preset maximum force or deformation distance is achieved, a fan cools the sample for a programmed time. The part is then removed manually from the tray and visually inspected. The test takes less than 30 sec.

Part Thickness Distribution
Validity of the thermoforming index as a measure of formability is shown by its high correlation with melt relaxation time - a measure of melt elasticity, in this case, the index equals the force required to achieve a 75-mm draw, a measure of melt strength. Both higher melt strength and higher relaxation time (lower melt elasticity) show easier formability. The force required to achieve a specified draw depth is termed the Thermoformability Index (TFI). A higher index at the desired process temperature indicates better thermoformability. A high index over a wider range of temperatures indicates better formability with a broader processing window.