Thermoforming: Plug and Yoke 

Reshaping a heated and softened thermoplastic sheet using a plug and yolk involves placing the softened plastic sheet material over the hollow mould called a yoke and then using a plug to press the plastic sheet into it.  The plug is usually attached to a press such as a pneumatically operated cylinder or a screw operated fly-press.  The process has to be carried out quickly so that the plastic sheet material is pressed into the yoke while it is still soft.

The process has been modified in industry so that it can be carried out quickly and repeatedly to make products such as vending machine drinks cups, meat trays and other food packaging products in very large quantities. 

The process involves:

• feeding the thermoforming machine with plastic sheet material off a roll ;

• passing it through a heating stage;

• locating it in a press that will press the sheet material into the yoke;

• punching the formed product out of the sheet.