Thermoforming stretch

These calculations can give a useful first approximation of the dimensions of a thermoformed part. However, they will not be strictly accurate because in a real situation, when the plastic sheet is being stretched down into the cold mould it will freeze off at whatever thickness it has reached when it touches the mould. 

Plastic memory is a phenomenon of TPs that has been stretched while hot beyond its heat distortion point to return to its original processed or molded form. Different plastics have varying degrees of this characteristic and degree of return is basically dependent on temperature (Chapter 4, Thermoforming, Memory). 

Derive the equation that describes the thickness distribution after thermoforming the constant diameter tubular geometry described in Fig. 6.75. Use the notation presented in the figure. Assume that the sheet stops stretching once it touches the mold wall. 

The parison is inflated fast, within seconds or less, at a predetermined rate such that it does not burst while expanding. It is a complex process that involves expansion of a nonuniform membrane-like element. Because the extension ratio is high (above 10), it is difficult to calculate the final thickness distribution. Naturally, much of the recent theoretical research on parison stretching and inflation (as in the case with thermoforming) focuses on FEM methods and the selection of the appropriate rheological constitutive models to predict parison shape, thickness, and temperature distribution during the inflation. 

With viscoelastic models used by an increasing number of researchers, time and temperature dependence, as well as strain hardening and nonisotropic properties of the deformed parison can, in principle, be accounted for. Kouba and Vlachopoulos (97) used the K-BKZ viscoelastic constitutive equation to model both thermoforming and parison membrane stretching using two-dimensional plate elements in three-dimensional space. Debbaut et al. (98,99) performed nonisothermal simulations using the Giesekus constitutive equation. 

Sheet can be produced by extrusion or calendering. For use in thermoforming, the sheet must be extremely uniform. For improved properties, the sheet may be biaxially stretch-oriented up to 300 percent. For large automatic production runs, it is fed continuously off a roll for short runs, large, and/or specialty jobs, it may be cut into individual sheets and fed one at a time. 

Different processes take advantage in applying orientation to gain certain properties in certain products. Major product lines include stretched blow molded bottles/containers (Chapter 6), thermoformed oriented containers (used for such products as fruits, vegetables, and baked goods) (Chapter 7), tapes, etc. 

The main markets for PLA are thermoformed trays and containers for food packaging and food service applications. Other developing areas include films and labels, injection stretch blow moulded bottles and jars, specialty cards and fibres. 

The main types of NatureWorks PLA that are available for packaging applications include general purpose film grades, extrusion coating, extrusion and thermoforming grades and injection stretch blow moulding. 

PLA is also used for thermoformed trays and containers. Injection stretch blow moulded bottles and jars for short-shelf-life applications that use cold-filling techniques for contents such as still water, fresh juices, dairy beverages and edible oil is also a potentially interesting market for PLA. 

---