Transparency and density
    For beverage cups or food such as salad or vegetable packaging, the material is often required to have high transparency. Materials such as A-PET, biaxially drawn polystyrene (OPS) or styrene-butadiene-styrene (ABS) copolymers are often used in these thermoforming applications. PLA is only slightly less transparent than A-PET, but higher than PP and even OPS.
    The density of PLA is 1.25g/cm3, which is lower than that of A-PET(1.34g cm). However, the density of PS(1.05g/cm) and PP(0.91 g/cm) was much lower than that of PLA. The transportation of raw materials, semi-finished products and finished products will affect the selection of plastic materials.

      Mechanical property
   Stiffness of thermoformed products is very important for stability. Various plastics are generally compared by elastic modulus. Products made of high elastic modulus materials have higher stiffness. This reduces the wall thickness, thereby reducing weight and saving costs. By comparing the elastic modulus of various packaging materials, it is found that PS-GP(general polystyrene) has the highest elastic modulus in styrene materials. However, due to its brittleness, it is usually thermoformed by an in-line process, that is, the extruder is directly connected to the thermoforming machine.        At a pressure of 3500MPa, PLA has the highest elastic modulus among the polymers compared. Due to the high stiffness of PLA, the containers can be stacked automatically. However, in order to facilitate unloading, it is necessary to add anti-sticking agent in the sheet production process.

      Thermal behavior and shrinkage characteristics
    For thermoforming machines, an important characteristic is the softening temperature. The softening temperature of PP grade used in packaging is about 150 ° C, followed by A-PET, which is 86 ° C. The softening temperature of PS is 80℃. For PLA, the Veka softening temperature is 58℃.
    Polypropylene block copolymers have a wide temperature range. The operating temperature is 30℃ -100℃. The continuous temperatures of polystyrene and A-PET meet the requirements of most applications. For PLA, the maximum continuous service temperature is about 45℃. Therefore, PLA is not suitable for hot filling applications. However, PLA can be used at temperatures as low as -20 ° C. Due to its limited continuous use temperature range, PLA is suitable for use in everything from cups to soft drinks, fruit and vegetable dishes, as well as refrigerated and deep-frozen packaging.
    Semi-perimorphic plastics generally have a larger shrinkage rate than non-crystalline plastics. In this comparative study, PP also had the largest shrinkage (1.5-1.8%). The shrinkage of non-crystalline materials such as PS, A-PET, and PLA is about 0.5%. In this paper, A-PET and PLA are considered as amorphous materials because the tiny crystals they produce under typical extrusion and thermoforming conditions are negligible. It is also said that in some cases, PLA can also be processed with PS or A-PET molds.

       Thermoforming of PLA
   Thermoforming can be assisted by 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. Thermoformed products made of sheets can have excellent thermoformed patterns. PLA requires a longer cooling phase in the mold. Thermoformed drink cups can be rolled.
    The thermoforming temperature is measured on the sheet surface. Compared with other plastics, the thermoforming temperature of PLA is very low. PLA has a wide molding temperature range (80-110℃). The maximum temperature range of A-PET is between 100 ° C and about 120 ° C. PS-HI has the widest molding temperature range (120-160 ° C). In the process of thermoforming on the roll feeding machine, the maximum forming temperature range of PP is slightly lower than the melting temperature range of 150℃-165℃. In the initial stage of PLA production, it is recommended that the heater be set at a lower temperature and gradually rise to the processing temperature range.
    Tests were carried out on independently formed machines using strip die punching and on machines that completed forming and punching in one step (blanking type punching). Two punching processes are suitable for processing sheets. The tests were carried out on various machines (RDM, RV, RD and RDK series from IIlig), and for the temperature-controlled steel strip die, the die temperature was 120 ° C.