Sheets can be thermoformed using vacuum, compressed air/vacuum, or compressed air only. Radiant heaters for PLA in thermoforming production lines must be regulated to very low temperatures. Although preheating is not necessary, it is generally conducive to uniform heating of the sheet. Unlike PP, PLA does not have a large droop in the heating process. Care must be taken to use low processing temperatures.

    Polylactic acid (PLA) is a relatively new class of materials used in the packaging industry and is manufactured from renewable resources. There have been numerous studies on the function and thermoforming properties of PLA compared to other polymers.
     The properties of PLA make it suitable for a variety of thermoforming applications, especially cold beverage cups and fruit and vegetable dishes. To produce high molecular weight PLA, the industry uses a sustainable, solvent-free, environmentally friendly process. PLA can be produced entirely from maize. PLA can also be manufactured from other renewable resources, such as wheat, sugar beets or agricultural by-products containing natural plant sugars.
    First, the glucose is fermented to produce lactic acid, which is then converted into a cyclic dimer (lactide) by condensation and esterification. The dimer is converted to polylactic acid by catalytic ring-opening polymerization. There are two isomers of lactic acid: L-lactic acid (L-lactic acid) and D-lactic acid (D-lactic acid). The ratio of the two isomers in the polymer chain can be adjusted in the polymerization process.

       Physical property
    At room temperature, PLA is a highly transparent polymer with a glass transition temperature between 55 ° C and 65 ° C. The physical properties of PLA can be changed by material orientation, such as biaxially stretched sheets. PLA with a D-lactic acid content of less than 8% can become a semi-crystalline material if nucleating agent is added, annealed, or stretched in the stretching process. Higher crystallinity increases stiffness and heat resistance. The melting point of PLA increases with the decrease of D isomer content. The critical surface tension of PLA is 38mN/m(Dyne) between 130℃ and 175℃, so corona treatment has a long-term effect. So PLA is easy to print and metallize.
    compostability
   PLA can be composted, thus reducing waste disposal costs for suppliers. This refers to the case where waste is stored separately before collection. PLA is biodegraded by hydrolysis and disconnection of ester groups on the molecular chain. Lower molecular weight, lactic acid and low molecular weight oligomers are naturally metabolized by microorganisms to produce carbon dioxide and water.
    In environments with high atmospheric humidity and temperature (55-70 ° C), such as in active composting systems, PLA can be rapidly degraded. Low temperatures and low air humidity in storage and service conditions do not cause problems. At 60℃ and 80% relative humidity, PLA will completely degrade after 60 days.

       Processing of sheets
   Examine in detail the three packaging technologies used to process PLA: high quality sheet, blow molded and thermoformed food containers. Its performance includes high stiffness, high gloss and high transparency, folding and twisting.
   As with other polyesters, hydrolysis affects the physical properties of PLA. Therefore, PLA must be protected against moisture in the air
The effect is to dry until the residual moisture content is less than 200 ppm before extrusion. PLA is easy to extrude under conditions slightly below the recommended melting temperature of PE. These data were measured on the traditional PE co-extrusion line.
    Extensive tests have been conducted to determine the thermoforming properties of PLA and compared with other polymers such as polystyrene (PS), polypropylene (PP) and amorphous polyethylene terephthalate (A-PET). The material properties, thermoforming capacity, sheet transport capacity, puncture behavior, stackability, winding performance, unstacking performance of stacked finished products, and PLA behavior in processing machinery were investigated. In order to study these properties, a large number of experiments have been carried out on various thermoforming production lines. Sheets are produced into rolls on all production lines.
   The sheet is produced from Cargill Dow's Nature Works PLA pellet 2002D with a small amount of blue masterbatch and an anti-stick agent. An extruder with a two-stage multifunctional PS/PET screw (90mm diameter) is used. The sheet width is between 500-735mm, and the sheet thickness is between 0.5-1.2mm.