The present invention relates in general to plastic autoclave enclosures which are used for the receipt and storage of medical and dental devices and equipment.  More particularly the present invention relates to the manufacture of plastic autoclave enclosures where the enclosure as well as the flow through apertures for sterilant are fabricated in part by vacuum forming. 
In the design of autoclave cassettes and enclosures there are a number of options for suitable materials.  While high temperature resistance is obviously one important property of the selected material, there are other material properties of importance.  The machineability or shapeability of the material is a factor as are weight, strength and durability.  Questions about the handling ease and availability of the material as well as how the material will stand up after years of use, may need to be answered. 
Currently most autoclave cassettes and enclosures are fabricated out of metal (aluminum or stainless steel) or plastic using some high temperature thermosetting or thermoforming material which can be injection molded or vacuum formed.  Plastic autoclave enclosures offer advantages in weight reduction and to some extent greater shaping and styling freedom.  Plastic autoclave enclosures generally offer a shorter delivery time than metal enclosures and may have a lower cost.  Possible drawbacks of plastic as compared to metal include strength, rigidity and durability. 
Another concern with plastic autoclave enclosures is the ability to provide a large number of steam or sterilant apertures without either weakening the enclosure or incurring an unacceptable manufacturing cost as the sterilant apertures are typically drilled holes.  When the sterilant apertures are drilled holes there is a drilling operation which must be set up and performed after the plastic enclosure is molded or formed.  High temperature plastics require a careful drilling operation which is time consuming.  With straight drilled holes the panel of plastic material into which the holes are drilled is weakened and thus there is a practical limitation as to the number and size of holes which can be drilled per unit area. 
A variety of plastic autoclave enclosures are known to exist and the following patent listing is believed to provide a representative sampling of earlier plastic enclosures:
______________________________________ U.S. Pat.  No.  Patentee Issue Date ______________________________________ 5,098,676 Brooks, Jr. March 24, 1992 4,191,291 Brown March 4, 1980 4,798,292 Hauze Jan. 17, 1989 ______________________________________
In the present invention the plastic autoclave enclosure is created by vacuum forming and the steam or sterilant apertures are created as part of the vacuum-forming step that creates the enclosure.  What is initially created by the vacuum-forming operation are closed bosses which are then machined to create apertures.  In looking at the tray portion of the enclosure these aperture bosses are recessed (concave) relative to the interior and extend below the bottom surface of the base panel of the enclosure tray portion.  If the aperture bosses are formed in the lid portion of the enclosure then they are recessed relative to the exterior and extend inwardly into the interior of the enclosure- Since these bosses are closed there are no open apertures created immediately after the vacuum-forming operation.  However, a post-forming milling or planing operation cuts off the closed end of each aperture boss thereby opening up each boss into a cleanly formed aperture.  In one embodiment a mechanical assist is provided by the male side of the vacuum-forming die in order to create tighter corners and radii for the aperture bosses.  In another embodiment a pressure box is used and air pressure is introduced into a sealed chamber over the softened plastic.  This allows the part to be formed with much greater definition and detail. 
A related benefit of the present invention and of the mechanical assist or pressure box as contrasted to straight drilled holes is the ability to create virtually any size and shape or style of sterilant aperture.  Specialized aperture shapes may be used to stylize a case in order to create an appearance that denotes a certain manufacturer's product.  A further benefit afforded by the present invention is to create a higher percentage of open area and thus a greater capacity for passage of sterilant, and a faster (shorter) sterilizing time cycle.