Ram-Based Methods for Forming Thermoplastic Article Having Smooth Edges

Abstract

The disclosure relates to forming shaped thermoplastic articles having smooth peripheries. Many thermoplastic articles have sharp edges formed upon molding or cutting the article from a feedstock sheet. Such sharp edges can damage thin plastic films or flesh which they contact, and smoothing the edges is desirable. Described herein are methods of forming a smooth periphery for such sharp-edged articles by rolling over the sharp edge. The smoothing operation is performed by forming a deflectable flange including a bend region separated from the potentially sharp peripheral edge by a spacer, deflecting a portion of the deflectable flange, and softening at least one bent portion of the deflectable flange to yield a smooth periphery upon cooling. The deflection can include curling the spacer at or near the peripheral edge.

What is claimed is: 
     
         1 . A method of making a container having a smooth periphery, the method comprising
 thermoforming a thermoplastic sheet to yield a precursor article having a shaped body including a concave compartment with a non-circular orifice surrounded by a rim, the rim comprising
 an extension that peripherally surrounds and extends distally away from the orifice, 
 a smoothly-curved bend region
 that peripherally surrounds the extension, 
 that, at its proximal extent, connects with the extension, 
 that, between its proximal and distal extents, forms an angle (A) having a magnitude from 75 to 120 degrees, and 
 that, at its distal extent, connects with 
 
 a spacer that peripherally surrounds the bend region; 
   cutting the sheet distally about the rim to substantially sever the precursor article from the sheet, whereby a peripheral edge is borne by the spacer peripherally about the precursor article;   heating at least a portion of the rim to at least the glass transition temperature (GTT) of the thermoplastic;   impinging a ram against the spacer to deflect the spacer toward the extension, whereby the magnitude of A decreases; and   thereafter cooling the heated portion of the rim below the GTT of the thermoplastic while the spacer remains deflected to yield the container having a smooth periphery.   
     
     
         2 . The method of  claim 1 , wherein the spacer includes a peripheral flange which bears the peripheral edge and which is connected with the remainder of the spacer by an elbow. 
     
     
         3 . The method of  claim 1 , wherein the heated portion of the rim is cooled below the GTT of the thermoplastic while the spacer remains deflected sufficiently that the peripheral edge is deflected away from the periphery of the container about its entire periphery. 
     
     
         4 . The method of  claim 1 , wherein the heated portion of the rim is cooled below the GTT of the thermoplastic while the spacer remains deflected sufficiently the peripheral edge is beneath the extension and behind the spacer and bend region about the entire periphery of the container. 
     
     
         5 . The method of  claim 1 , wherein the heated portion of the rim is cooled below the GTT of the thermoplastic while the spacer remains deflected sufficiently around the entire periphery of the container that the peripheral edge cannot be touched by a human fingertip that is swiped along the gap between the deflected peripheral flange and the body. 
     
     
         6 . The method of  claim 1 , wherein the heated portion of the rim is cooled below the GTT of the thermoplastic while the spacer remains deflected sufficiently that the dimension (d) of the nearest approach of the peripheral edge to the body is at least ½ the distance (D) from the body of the peripheral-most extent of the rim of the container. 
     
     
         7 . The method of  claim 1 , wherein the heated portion of the rim is cooled below the GTT of the thermoplastic while the spacer remains deflected sufficiently that the ratio d/D is not greater than ¼. 
     
     
         8 . The method of  claim 1 , wherein the heated portion of the rim is cooled below the GTT of the thermoplastic while the spacer remains deflected sufficiently that the ratio d/D is not less than ¾. 
     
     
         9 . The method of  claim 1 , wherein the magnitude of an offset angle (OA), defined as the angle between the plane of the spacer and the plane of the extension, is greater than 90 degrees after cooling the heated portion of the rim. 
     
     
         10 . The method of  claim 1 , wherein the magnitude of OA is at least 105 degrees after cooling the heated portion of the rim. 
     
     
         11 . The method of  claim 1 , wherein the magnitude of OA is at least 120 degrees after cooling the heated portion of the rim. 
     
     
         12 . The method of  claim 1 , wherein the magnitude of OA is at least 135 degrees after cooling the heated portion of the rim. 
     
     
         13 . The method of  claim 1 , wherein the magnitude of OA is at least 180 degrees after cooling the heated portion of the rim. 
     
     
         14 . The method of  claim 1 , wherein the magnitude of OA is at least 270 degrees after cooling the heated portion of the rim. 
     
     
         15 . The method of  claim 1 , wherein the bend region is heated above the GTT of the thermoplastic and wherein the magnitude of A is less than 90 degrees after cooling the bend region. 
     
     
         16 . The method of  claim 1 , wherein at least one portion of the spacer is heated above the GTT of the thermoplastic and wherein the magnitude of OA is greater than 90 degrees after cooling the spacer. 
     
     
         17 . The method of  claim 1 , wherein at least the peripheral edge of the sheet is heated above the GTT of the thermoplastic and wherein the magnitude of OA is greater than 90 degrees after cooling the peripheral edge. 
     
     
         18 . The method of  claim 17 , wherein the ram is heated and includes an upper surface which conforms against the peripheral edge, wherein the ram heats portions of the spacer above the GTT of the thermoplastic and impinges against the spacer in a plurality of discrete, incremental advances, whereby multiple portions of the spacer soften sequentially. 
     
     
         19 . The method of  claim 17 , wherein the ram is heated and includes an upper surface which conforms against the peripheral edge, wherein the ram heats portions of the spacer above the GTT of the thermoplastic and impinges against the spacer in a single smooth motion, whereby multiple portions of the spacer soften sequentially. 
     
     
         20 . The method of  claim 1 , wherein the ram includes an upper surface having a J-shaped profile. 
     
     
         21 . The method of  claim 1 , wherein the ram includes an upper surface having a U-shaped profile. 
     
     
         22 . The method of  claim 1 , wherein the ram is heated and wherein the heating and impinging steps are performed simultaneously and completely about the periphery of the precursor article using the heated ram. 
     
     
         23 . The method of  claim 1 , wherein each of the bend region and at least the peripheral edge of the sheet is heated above the GTT of the thermoplastic and wherein the magnitude of OA is greater than 90 degrees after cooling the peripheral edge. 
     
     
         24 . The method of  claim 1 , wherein the rim is sandwiched between an upper body and the ram and wherein the upper body urges the spacer toward the ram during impingement of the ram against the spacer. 
     
     
         25 . The method of  claim 1 , wherein the extension is substantially planar, having an upper face and a lower face, and wherein each of the spacer and the body extends away from the lower face of the extension. 
     
     
         26 . The method of  claim 1 , wherein the container is a rounded rectangular tray having a substantially planar rim surrounding the concave compartment. 
     
     
         27 . The method of  claim 1 , wherein the portion of the rim is heated to at least the GTT of the thermoplastic during thermoforming, thereafter cooled below the GTT, and thereafter heated to at least the GTT prior to impinging the ram against the spacer. 
     
     
         28 . The method of  claim 1 , wherein a plug is disposed within the compartment while the ram is impinged against the spacer. 
     
     
         29 . The method of  claim 28 , wherein the plug substantially fills the compartment and overlaps at least a portion of the rim. 
     
     
         30 . The method of  claim 1 , wherein the thermoplastic is a polyethylene terephthalate.