Method of forming sheet material

Claim 1

A method of forming a sheet of thermoplastic material into an article having a desired final shape, which comprises the steps of: providing a movable form block having a shape conforming to the desired final shape of the article and having a predetermined planform in a plane normal to its axis of movement; establishing a forming zone; placing a heat-softened flat sheet of thermoplastic material in said forming zone in a plane normal to the axis of movement of the form block; slidably gripping the entire marginal portions of said sheet along a periphery closely conforming to and in registry with the planform of the form block; initially applying a differential air pressure to the central portion of said sheet to slidably force it out of the gripping plane and cause local flow in various directions within the plane of said sheet to form it into a dome shape away from the form block prior to forming contact by it; moving the form block along its axis of movement through the forming zone into contact with said sheet; slidably forcing said sheet with the form block further out of the gripping plane and causing further local flow in various directions within the plane of said sheet to cause it to conform to the contour of the form block; producing a completely airtight enclosure in conjunction with said sheet arranged over said periphery and on the side of the form block when it is at its extreme axial position of movement in forming said sheet; gradually reducing said air pressure as the form block is moved along its axis of movement to its extreme axial position to produce said enclosure; evacuating said enclosure to produce a vacuum condition which draws said sheet tightly against the form block for very accurate shaping; maintaining said sheet in heat-softened condition throughout the forming operation; cooling said sheet while it is held in its final form until it takes a substantially permanent Set; and retracting the form block along its axis of movement.

Claim 2

A method of forming a sheet of thermoplastic material into an article having a desired final shape, which comprises the steps of: providing a movable form block having a shape conforming to the desired final shape of the article and having a predetermined planform in a plane normal to its axis of movement; establishing a forming zone; placing a heat-softened flat sheet of thermoplastic material in said forming zone in a plane normal to the axis of movement of the form block; slidably gripping the entire marginal portions of said sheet along a periphery closely conforming to and in registry with the planform of the form block; initially applying a differential air pressure to the central portion of said sheet to slidably force it out of the gripping plane and cause local flow in various directions within the plane of said sheet to form it into a dome shape away from the form block prior to forming contact by it; moving the form block along its axis of movement through the forming zone into contact with said sheet; slidably forcing said sheet with the form block further out of the gripping plane and causing further local flow in various directions within the plane of said sheet to cause it to conform to the contour of the form block; producing a completely airtight enclosure in conjunction with said sheet arranged over said periphery and on the side of the form block when it is at its extreme axial position of movement in forming said sheet; gradually reducing said air pressure as the form block is moved along its axis of movement to its extreme axial position to produce said enclosure; evacuating said enclosure to produce a vacuum condition which draws said sheet tightly against the form block for very accurate shaping; maintaining said sheet in heat-softened condition throughout the forming operation; cooling said sheet while it is held in its final form until it takes a substantially permanent Set; and retracting the form block along its axis of movement.