D13. dual sheet forming (twin sheet forming) - is a method for producing hollow objects from two plastic sheets. Fig. 4D13 illustrates one method for achieving such a result. Two sheets are fed to the machine together, slightly spaced apart. Both are clamped and heated and moved between two halves of a mold. The process requires longer heating time than that required for a single sheet. This is sometimes compensated for by using a rotary table system that includes two heating stations so that each sheet goes through two heating cycles before it is formed. After the sheets are placed for forming, an inflation pin enters the space between them and the mold closes. Air pressure is introduced between the sheets and vacuums are drawn from the two opposing mold cavities. Pressure on one side and vacuum on the other causes the sheets to press against the walls of the two mold cavities. The pressure of the mold closure also bonds the softened plastic sheets together. The formed sheets and the joint between them cool and harden. The inflation pin is withdrawn, the mold opens, and a hollow part is ejected or removed.

Sometimes, the bottom sheet is formed first and an insert is placed on it before the other half of the part is formed and assembled. Another variation of the process introduces urethane foam instead of air pressure between the sheets. The foam adheres to both sheets, to make a strong sandwich construction. Foam-filled boat hulls are produced with this method.

Dual sheet forming is used extensively in Europe to produce such items as phone booth roofs and gaming table tops from PVC or ABS sheet. Shipping pallets and other dunnage products are produced from high-density polyethylene heated to above its melt temperature.

Fig. 4D13 Dual sheet (twin sheet) forming: a) Two heated plastic sheets are placed between the halves of the die. A thin air-inflation tube is also positioned between the die halves. b) The die closes, clamping the heated sheets and the inflation tube. c) Compressed air is pumped into the space between the sheets, driving the sheets against the die cavities. d) Vacuums draw the sheets tightly against the die walls. The air-inflation tube is withdrawn. Pressure of the die halves seals the two sheets together. e) The formed sheets cool, creating a sealed hollow component. The dies open and the finished part is removed.

D14. solid phase pressure forming - is a technique used with several thermoforming processes rather than a particular forming method. It was first used in the thermoforming of thin gauge sheet polypropylene homopolymer. Some sheets tended to split when thermoformed at the temperature that would normally be indicated for the operation. Instead, an approach using a slightly lower temperature for the material was developed. The temperature was such that the material would, technically, still be in the solid phase. Higher forming pressures were used. This approach is still in use although new copolymer polypropylene formulations are thermoformable at higher temperatures. The technique is applicable to various crystalline plastics as well as polypropylene. Because of the reduced formability at the temperatures used, an increased pressure of 50 to 100 psi (345 to 690 kPa) is required. Vacuum forming does not provide sufficient pressure. Careful monitoring and control of the heating process is also necessary. The major application is the forming of smaller food cups such as those used for single portion servings.