Abstract:
A process for mixing plastic resins to produce a material having properties similar to medium and high impact polystyrene after thermoforming. The process includes mixing crystal polystyrene and a styrenic thermoplastic elastomer to form a mixture for thermoforming. The process enables the substitution of crystal polystyrene for medium and high impact polystyrene in the production of items such as compact disc trays and the like.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to thermoforming processes, and relates specifically to a process for injection molding crystal polystyrene to form compact disc trays.  
         BACKGROUND OF THE INVENTION  
         [0002]    Compact discs are plastic circular discs which have recorded digital audio information on spiral tracks formed on a surface of the disc. The discs are typically stored in a thermoplastic molded protective enclosure having a base and a cover hinged to the base. The base and cover are usually manufactured out of a transparent material in order to provide for inserting materials to be viewed through the base and cover. The base receives an opaque, compact disc holding tray that releasably holds the disc in position. Accordingly, the compact disc is releasably fitted onto the tray, and the cover typically snap fits onto the base member in a closed position with the tray and disc contained therein.  
           [0003]    The trays are typically formed out of a molded thermoplastic material such as polystyrene. Most of the manufacturers of compact disc trays use either medium or high impact polystyrene to injection mold the trays. These materials are selected because of their strength and resistance to cracking. The raw materials for manufacture include plastic resins such as polystyrene which are typically compounded and formed into small pellets about an eighth of an inch long by an eighth of an inch wide. These “virgin” materials are then mixed in a hopper on the injection molding machine or batch blended and then placed in the hopper on the injection molding machine. The virgin materials are often mixed with regrind. Regrind is produced by grinding scrap materials such as rejects or skeletons into small pieces. These small pieces can be recycled into the product in small percentages in order to lower material costs.  
           [0004]    For the production of compact disc trays most manufacturers mix virgin medium or high impact polystyrene with medium or high impact polystyrene regrind or with crystal polystyrene regrind or virgin crystal polystyrene. As described above, the addition of the regrind reduces the material costs and provides an opportunity to recapture some of the scrap. The virgin crystal polystyrene is sometimes added because it is less expensive than medium and high impact polystyrene.  
           [0005]    Typically the percentage of regrind is kept small because it causes the molded product to become brittle. The same applies for crystal polystyrene. Usually regrind makes up anywhere from 5-20% of the mix. The medium and high impact polystyrene regrind does not significantly increase the brittleness of the part. However, the crystal polystyrene in regrind or virgin form will cause brittleness that can lead to part failure. It would be desirable to be able to use crystal polystyrene because of its lower cost to produce a material having the properties of high and medium impact polystyrene.  
           [0006]    Accordingly, what is needed is a process for injection molding crystal polystyrene to produce compact disc trays having similar material properties as trays that are produced with medium and high impact polystyrene.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention meets the above described need by providing a method of injection molding crystal polystyrene to form compact disc trays. The method includes mixing crystal polystyrene with a thermoplastic elastomer to form a resin mixture for injection molding plastic parts such as compact disc trays. The molded parts produced with this mixture provide an acceptable substitute for parts normally produced with medium and high impact polystyrene because the brittleness normally associated with crystal polystyrene is eliminated. The basic mixture of crystal polystyrene and thermoplastic elastomer can be supplemented with a color additive and/or regrind materials. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0008]    Compact discs are typically stored in a thermoplastic molded protective enclosure having a base and a cover hinged to the base. An example of a protective enclosure is shown and described in U.S. Pat. No. 5,727,680 which is incorporated herein by reference. The base and cover are usually manufactured out of a transparent material in order to provide for inserting materials to be viewed through the base and cover. The base receives an opaque, compact disc holding tray that releasably holds the disc in position. Accordingly, the compact disc is releasably fitted onto the tray, and the cover typically snap fits onto the base member in a closed position with the tray and disc contained therein.  
         [0009]    The compact disc storage tray typically has a circular array of inwardly extending cantilevered, resilient fingers, each finger having an upstanding portion extending substantially normal to the plate-like body of the tray and an inwardly extending portion disposed substantially parallel to the body of the tray. A shoulder is formed on each finger at the junction of the upstanding and inwardly extending finger portions. The shoulders of the fingers thus are in an annular array. The compact disc has a central opening, and when the disc is fitted on the tray the annular array of shoulders receives the disc central opening in a snug fitting relation. This snug fit resiliently bends the fingers inwardly toward one another so that the disc is firmly held on the tray. When the inwardly extending portions of the fingers are pushed downwardly, the disc can be removed from the tray and be ready for use.  
         [0010]    The disc storage tray and the fingers thereof are an integral structure molded of thermoplastic material. The fingers are subject to breakage when encountering impact forces such as during mechanized packaging of the trays, dropping of the package or stacking heavy loads on the package during mail handling and the like, as well as during placement and removal of the compact disc on the tray. For these reasons most manufacturers use medium and high impact polystyrene for injection molding compact disc trays. The brittleness normally associated with injection molded crystal polystyrene would lead to part failure.  
         [0011]    The method of the present invention involves mixing the following ingredients either in a blender or in a machine hopper and injection molding them to form a compact disc tray.  
         [0012]    The first ingredient is crystal polystyrene which is commercially available from several sources including the large compounders that prepare the compound and extrude it into “virgin” pellets for further processing by others. One suitable resin and source is FINA Polystyrene 535 available from Fina Oil and Chemical company in Dallas, Tex.  
         [0013]    Products thermoformed of crystal polystyrene are typically very brittle in comparison to medium and high impact polystyrene, and therefore are typically not used for applications such as compact disc trays. Crystal polystyrene comes in several forms including virgin, wide specification, regrind, and reprocessed or compounded (scrap that is made into pellets again). Historically, general purpose virgin crystal polystyrene is significantly cheaper than medium or high impact polystyrene. Also, wide specification, regrind, and reprocessed can be as much as 60% less expensive. Crystal polystyrene forms the largest component of the mixture and preferably ranges from approximately 75-95% by weight.  
         [0014]    The second ingredient is a styrenic thermoplastic elastomer. This material comes in several forms, but for the most part includes copolymers comprising Styrene, Isoprene, and Butadiene. The copolymers include triblocks such as S-B-S, S-I-S, S-EB-S, and S-EP-S; alternating block copolymers such as (S-I) n ; branched block copolymers such as (S-B) n x and (S-I) n x (where x represents a multifunctional junction point); and, triblock/diblock blends (e.g. S-B-S/S-B). One example of a suitable thermoplastic elastomer is FINAPRENE®, a Butadiene-Styrene block copolymer available from Fina Oil and Chemical Company in Dallas, Tex. The styrene thermoplastic elastomer preferably comprises from 2-15% by weight.  
         [0015]    The third ingredient is a color additive for use when the main resins are colorless. The color additives are available in pellet form from compounders. Color additives are typically very expensive and are added to the mixture in very small quantities. The color pellets can be added through a blending station or mixed in at the machine hopper. The color additive preferably comprises 2-6% of the mixture by weight.  
         [0016]    The last ingredient is crystal, medium or high impact polystyrene regrind and is optional. The regrind preferably comprises 5-20% of the mixture by weight.  
         [0017]    The method of the present invention includes the steps of providing a mixture containing approximately 75-95% crystal polystyrene, approximately 2-15% styrenic thermoplastic elastomer, approximately 2-6% color additive, and approximately 5-20% medium or high impact polystyrene regrind. The mixture is blended at a blending station or is blended in the machine hopper. Additional blending occurs during the melt, and the molded product ends up having properties that are similar to medium and high impact polystyrene. The mixture is injection molded to form a compact disc tray. However, other items that are typically formed by injection molding medium or high impact polystyrene can also be produced with this mixture.  
         [0018]    As an alternative, the color additive can be deleted from the mixture by having the base crystal polystyrene pre-colored. Also, the regrind is optional.  
         [0019]    The principal advantage of the present invention is a significant reduction in the material costs associated with injection molding compact disc trays.  
         [0020]    While the invention has been described in connection with certain preferred embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.