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Thermoforming was re shaped when com panies like OMV Group in Italy and Brown Machine in Beaverton, Mich., developed positive-air-pressure thermoforming systems that gave parts improved detail and structural integrity along with an injection molded appearance. Until that time, vacuum forming was the only option.
While vacuum forming used only atmospheric pressure (14.7 psi) to force molten sheet into the contours of the mold, pressure forming used compressed air (20 to 50 psi) to do the same. Pressure-formed parts had sharper edges and detail similar to injection molded parts, plus closer tolerances, better material distribution, and less residual stress. Another advantage of applying pressure on the sheet was better heat transfer and shorter cycle times.
OMV originally developed pressure forming for its own processing use in 1963 and started to sell equipment in the early 1970s. In Italy, OMV produced rectangular fruit containers made of PS and PVC. The process was used to make PS drinking cups by the late 1960s.
In the U.S., Brown’s Model 821, introduced in 1963, incorporated pressure-forming capability. The first applications were PS drinking cups and cleanser containers.
In 1975, resin maker Shell Chemical and machine maker Illig of Germany teamed up to develop an easier way to thermoform PP via pressure forming. This was during a time when food and drug packagers were demanding stronger and higher quality plastics packaging at lower weight and cost. Previously, PP lacked sufficient melt strength for good formability. It also was relatively slow to melt and to solidify because of its high crystallinity. The solid-phase PP pressure-forming process licensed by Shell and Illig ran the material colder (below its crystalline melting point) and at a higher pressure (100 psi).
Pressure forming became a popular process as manufacturers of industrial parts increasingly sought better quality, improved surface appearance, and aesthetics. Over time, a pressure box became ubiquitous on new thermoforming machinery. And pressure forming became a key weapon in the competition with injection molding.