Statistical analysis of product variability associated with continuous and cut sheet thermoforming operations

Extensive published data exist with regard to the variation of part weight and dimensions for products manufactured by injection molding and blow molding. Relatively little information is available with regard to thermoformed parts. Such data are of considerable practical and theoretical value in establishing realistic processing targets in commercial processing operations. Experimental data have been systematically collected over an extended time from three continuous roll-fed thermoforming processes, as well as a cut sheet forming process. Two of the continuous processes studied were high-volume production lines. One of the operations produced 16-ounce drinking cups from a blend of a styrene-butadiene block copolymer with polystyrene. The other commercial continuous process manufactured tubs from high-density polyethylene. The third process studied involved a pilot plant test former used to produce 10-ounce drinking cups from the styrene-butadiene/polystyrene blend material. All three continuous processes employ plug assist in conjunction with a multi-cavity mold. The measured parameters included the weight of individual parts, and the wall thickness measured at several different locations. The cut sheet forming process employed a plug assist with a single-cavity mold for the production of a scanner cover. The polymer used consisted of a blend of an acrylic and a poly(vinyl chloride) resin. The deformation and wall thickness of the scanner cover were measured at several different locations.