reference is now made to fig1 , which is a simplified pictorial illustration of a dental crown formed of acetal homopolymer resin and to fig2 , which is a sectional illustration of the dental crown of fig1 , taken along lines ii - ii in fig1 . as seen in fig1 and 2 , there is provided in accordance with a preferred embodiment of the present invention an injection molded dental crown 10 formed of an acetal homopolymer resin . a preferred material for the crown is acetal homopolymer resin ( delrin ®) which is commercially available from dupont . as can be readily seen in fig1 and 2 , the dental crown 10 is formed with a generally conventionally tooth shaped top surface 12 and depending side surfaces 14 at least one of which defines an undercut 16 . preferably , the depending side surfaces 14 are flexible . crown 10 may readily be mounted , by conventional methods , such as through the use of dental cement in the mouth of a patient , typically a child , as part of treatment of primary teeth and permanent molars having extensive carious lesions . it is a particular feature of the invention that crown 10 is of a color which generally matches that of the patient &# 39 ; s teeth . the crown of the present invention is characterized by high tensile strength , high impact resistance and stiffness , excellent fatigue endurance and resistance to moisture , excellent dimensional stability and sufficient resilience and resistance to creep . it has the natural appearance of a vital tooth . reference is now made to fig3 a , 3 b and 3 c , which illustrate the operation of an apparatus for manufacturing a dental crown from acetal homopolymer resin in accordance with a preferred embodiment of the present invention . as seen in fig3 a , 3 b and 3 c , the crown 10 is molded in a mold cavity 20 which is defined by a top mold element 22 , a bottom mold element 24 and an ejector 26 . the ejector 26 forms part of an internal mold element 32 . fig3 a shows the stage of molding when the top mold element 22 lies in tight engagement with the bottom mold element 24 and the ejector 26 . the dental crown 10 , which is fabricated on the ejector 26 , is formed by the injection of acetal homnpolymer resin material from a source of acetal homopolymer resin ( not shown ) into the mold cavity 20 , via a channel 30 cut in the top mold element 22 . fig3 b shows an initial release stage wherein the bottom mold element 24 is separated from the top mold element 22 , thus permitting removal of the molded crown 10 from cavity 20 . fig3 c shows an ejection stage wherein ejector 26 , driven by a piston 28 moves upwardly relative to bottom mold element 24 and pushes crown 10 out of cavity 20 . due to the resilience of the depending side surfaces 14 , the action of the ejector 26 is able to disengage the internal mold element 32 from the crown 10 notwithstanding the presence of undercut 16 . it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove . rather the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove as well as variations and modifications which would occur to persons skilled in the art upon reading the specification and which are not in the prior art .
the present invention provides a dental crown formed of a thermoplastic polymer material , the crown comprising : a tooth shaped top surface ; and flexible side surfaces , at least one of which includes an inwardly directed bottom portion .