Abstract:
Improved lifter ( 32 ) for injection molding apparatus and method of use thereof are disclosed. To inhibit the unsightly blemishes that may be formed in plastic parts wherein the lifter ( 32 ) comprises a leading edge ( 44 ) that is perpendicularly oriented relative to the longitudinal axis of the molded part, a lifter plate is provided wherein the leading edge ( 44 ) thereof is obliquely angled relative to the longitudinal axis of the molded part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is an application filed under 35 USC §371 of International Application PCT/US00/40555 filed Aug. 3, 2000. Priority benefit of U.S. Provisional Patent Application No. 60/149,127 filed Aug. 16, 1999 is hereby claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improved lifter structure and method of using that structure in the ejection of molded articles from an injection molding machine and the like. 
     BACKGROUND OF THE INVENTION 
     It has become increasingly popular to manufacture automobile and truck trim and body pans such as fenders, bumpers. rocker panels, body panels, doors, filler panels, wheel covers, dash boards, arm rests, chin spoiler and other parts via injection molding techniques. Moreover, to minimize the emission of VOC compounds and to improve upon the aesthetic appearance of many parts, plastic parts having a paint film laminate covering the show surface have been successfully utilized in auto and truck part manufacture. 
     Film covered plastic parts are normally formed via a process in which the paint film laminate is insert molded with the desired thermoplastic in such manner to fuse the film over the injection molded part. The resulting injection molded part is ready for assembly without subsequent painting. 
     The paint film laminate used in these insert molding techniques may comprise a backing sheet of about 0.020″ in thickness to which paint layers are adhered to form a colored side of the laminate. Typically, the backing sheet comprises an extruded thermoplastic sheet. 
     In such paint film laminates, a paint film, comprising a cast dried continuous paint coating is provided over the backing sheet. The paint film may consist of a monocoat, a clear coat over a base coat or a clear coat and a base coat with interposed print or design. The paint film, including base coat, clear coat and print or design, if desired, commonly ranges from about 0.5-4 mil. in thickness. 
     The laminated paint films are typically provided in a roll, unwound, then trimmed to a proper “preform” size and shape, ready for insertion into the injection mold. The preform is usually placed along the cavity side of the mold with the painted side thereof facing the mold cavity surface. In some instances, the preform may be placed along the core side of the mold. The mold is then clamped and the desired molten resin is injected into the mold cavity. Heat and pressure conditions in the mold partially melt the backing sheet and a melt bonding or fusion of the injected resin and film occur. Injection molds used for these processes are rear or edge gated so that the molten resin is directed along the backside of the film. 
     After the part has been cooled in the mold, and one of the mold cavity support surfaces is displaced from the other, the part is then ejected by the use of pneumatically or hydraulically actuated lifters and the like. The lifter cooperates with one side of the mold (usually that side contiguous with the plastic substrate) and ejects or “pops” the finished part out of the mold so that the molding production cycle can continue with the insertion of paint film laminate and plastic into the mold cavity. In many instances, the aesthetic appearance of the paint film laminate covered side of the part, normally used as the “show” side of the part, exhibits a surface blemish in the region in which the flowing plastic first impinges upon the lifter surface. 
     When elongated paint film covered parts are produced, such as in the case of rocker panels, chin spoilers, bumpers, etc., the blemish can take the form of an unsightly surface line or bump on the show side of paint film laminate covered side on the part, extending transversely to the longitudinal axis of the elongated part, causing a difference in gloss or light reflectance between neighboring sides of the paint film surface adjacent the bump. 
     SUMMARY OF THE INVENTION 
     The present invention is accordingly directed to methods and lifter apparatus for ejecting plastic parts and specifically paint film laminate covered plastic parts from a mold cavity that inhibit the formation of surface blemishes during the ejection step of the part in the molding sequence. 
     In conventional injection molding apparatuses and methods, the lifter comprises a plate like structure having a leading edge that is perpendicularly oriented relative to the longitudinal axis of the elongated part. After the part has been formed in the mold, one half of the mold is normally retracted and the lifter plate associated with the other mold section is actuated via hydraulic or pneumatic pistons to strike the part and eject it from the mold cavity. 
     The present invention pertains to methods of molding an automobile or truck plastic part wherein a mold cavity is formed by at least two mold portions. A charge of plastic is injected into the mold cavity along a upstream to downstream direction by a runner that is provided in one of the mold portions. After the part is molded in the mold cavity, one of the mold cavity portions is separated from the other to thereby open the mold cavity . The part is ejected from the mold cavity by contacting it with the lifter member having an angled edge portion located along an upstream boundary of the lifter and a trailing edge portion located downstream from the leading edge. 
     The method is particularly advantageous in the formation of plastic parts of the type wherein a paint film laminate is superposed over at least a portion of the substrate plastic and forms a film laminate covered show face of the part leaving a rear face comprising the exposed plastic surface. In such cases, the lifter strikes the rear face of the formed part. 
     It has been found that perceptible blemish formation on such paint film laminate covered plastic parts is minimized by providing a leading edge of the lifter that is disposed at an oblique angle relative to the longitudinal axis of the part. Preferably, this oblique angle is from about 30 to 60 ° and in the presently preferred embodiment, is about 45°. 
     In accordance with conventional co-molding of paint film laminate covered parts, the desired paint film laminate is provided along one surface of the mold cavity with the injected plastic entering the rear of the cavity and fusing with the paint film laminate. The parts formed are of generally elongated nature and have a longitudinal axis extending therethrough. During the molding operation, the charge of plastic material is injected into the mold cavity usually along the longitudinal axis of the part (and the mold) from an upstream to a downstream direction along the back of the paint film laminate. 
     After the mold portions have been separated. the rear side of the part (opposite from the paint film laminate show side) is struck by the lifter plate in accordance with the invention. The provision of the obliquely angled upstream or leading edge of the lifter has been shown to minimize perceptible blemish formation in the finished part. Although applicant is not to be bound to any particular theory of operation, it is thought that the shear forces that act along the flowing plastic material are substantially decreased by provision of this angled leading edge of the lifter compared to the prior art. In contradistinction. in prior art devices where the leading edge of the injector is oriented perpendicularly with regard to the longitudinal axis of the part, excessive shear of the plastic leads to a turbulent flow in the region proximate the leading edge of the lifter with the plastic thereby forming a perceptible bump in the paint film show face of the part. Either side of the bump reflects light differently, causing an undesirable gloss difference along the length of the part resulting in characterization of the part as a reject. 
     The apparatus in accordance with the invention thereby comprises a first mold part and a second mold part which, when in contiguous relation one to another, define a mold cavity there between. One of the mold parts is separated from the other via pistons or other linear movement actuators such as a ball screw or equivalent to open the mold cavity. The lifter in accordance with the invention is associated with one of the mold parts and is adapted to contact the plastic part after the first and second mold parts have been separated to eject the part from the mold cavity. 
    
    
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top schematic sectional view of an injection molding machine in which a paint film laminate covered pan may be made in accordance with the invention; 
     FIG. 2 is a schematic sectional view taken of the injection molding machine of FIG. 1, taken along the plane represented by the lines and arrows  2 — 2  of FIG. 1, showing a lifter structure in accordance with the invention; and 
     FIG. 3 is a view, similar to that shown in FIG. 2, of another embodiment of a lifter in accordance with the invention; 
     FIG. 4 is a partially cut-a-way perspective view of an elongated paint film laminate covered plastic part made by the method and the apparatus disclosed herein; 
     FIG. 5 is a schematic view in perspective of a prior art lifter structure showing plastic flow thereover; and 
     FIG. 6 is a schematic view in perspective of another lifter structure in accordance with the invention shown in conjunction with a less turbulent or laminar flow pattern. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 4 of the drawings, there is shown a paint film laminate covered part such as rocker panel  2  that is made in accordance with the invention. As shown, the part  2  comprises a paint film laminate  6 , superposed over plastic substrate  4 . The rocker panel  2  is an elongated part that is provided with a longitudinal axis  8  extending therethrough. The portion of the part covered by the paint film laminate is sometimes referred to as the “show” face. 
     Typically, the plastic, for the plastic substrate, is chosen from a host of various thermoplastic and thermoset plastic materials with more common plastic substrates comprising polyvinyl chloride, polyolefins, such as polypropylene, polyacrylates such as poly(methyl)methacrylate, polyvinyl acetate, polyamides, polyimides, polyesters, phenol-formaldehyde polymers, amino polymers, polyurethanes, etc. 
     The parts are made in accordance with conventional injection co-molding techniques wherein the paint film laminate is placed in the mold cavity with the charge of plastic material injected into the mold along the backside of the paint film laminate. 
     With regard to FIG. 1, there is shown a typical injection molding machine  10  with which the present. invention can be utilized. Here, the molding machine comprises a feed hopper  12  for pressurized injection of plastic material to be plasticized therein. The ejection molding machine comprises a molding cavity  25  that is formed between first mold portion  22  and second mold portion  24 , shown in abutting relation to form the desired mold cavity. 
     Plastic is fed into the feed hopper  12  and is injected into the mold cavity under pressure via feed screw  14  or the like and cooperating sprue  16  that communicates with runner  18  to deliver the pressurized charge of molten plastic to the cavity  25 . As shown, the injection of plastic material proceeds from the runner  18  to the downside end  26  of the cavity. 
     Pistons  28 ,  30  are operatively associated with the second mold portion  24  so that this mold portion may be reciprocated along rails  31 , 33  between a mold formation station and a station in which the mold cavity is retracted toward the top with regard to FIG. 1 into an open mold form. The artisan will appreciate the fact that many different arrangements can be used to displace one of the mold parts from the other. One example is shown in U.S. Pat. No. 5,122,051, incorporated by reference herein. The first mold portion  22  is stationary with a portion of the wall of the mold being formed via lifter  32  which is mounted for reciprocation by arms  34 ,  36  that are carried by ejector plate  100 . As shown, the ejector plate  100  is reciprocated by means of piston  102 ,  104 . 
     Upon operation, the required paint film laminate is first inserted into the mold along the portion of the cavity  25  defined by the contoured configuration of the second mold portion  24 . Then, a sufficient shot of plastic material is injected into the molding machine in an upstream to downstream direction from the sprue and runner  16 ,  18 , towards the downstream end of the cavity. As is conventional in the art, the injection of the plastic shot into the cavity may be assisted via pressurized gas flow and the like, and a predetermined amount of the desired plastic material, at desired pressure and temperature is fed into the mold cavity. After the molded part  2  is made in the mold cavity, the second mold portion  24  is retracted via the pistons  28 ,  30  to the open position. Then, pistons,  102 ,  104  are actuated to move ejector plate  100  and the lifter  32  carried thereby toward the top as shown in FIG. 1, to “knock” or eject the part  2  out of the mold cavity. 
     As previously mentioned, in some of the prior art devices, the leading edge of the lifter (the edge first impinging upon the plastic flowing from the upstream to downstream direction) provides a square surface that is pressed upon the plastic substrate during the injection cycle. This can cause an unsightly blemish line in the part manifesting itself especially along the side of the part covered by the paint film laminate  6 . This blemish line usually extends perpendicularly to the longitudinal axis  8  of the part  2  and presents a distinct change in gloss characteristics as can be seen from one side of the blemish line to the other. 
     In accordance with the invention, after the requisite part has been made, it is ejected from the mold cavity by the provision of a lifter structure as more clearly shown in FIG.  2 . The lifter  32 , in accordance with the invention, comprises a top edge  38  and bottom edge  40 . A trailing edge  42  (or downstream edge) is provided and the leading edge  44  is positioned at an oblique angle relative to the bottom edge  40  of the lifter or stated differently, relative to the longitudinal axis  51  of the mold. Provision of this angled leading edge  44  has, in preliminary studies, been shown to minimize the formation of a surface line or blemish that otherwise would form generally perpendicularly with regard to the longitudinal axis  8  of the part or of the mold. At present, it is preferred that the leading edge  44  provide an angle of about 30 to 60 degrees relative to the bottom edge  40  of the lifter. The preferred embodiment comprises an angled leading edge of about 45°. 
     FIG. 3 shows another lifter embodiment in accordance with the invention. Here, the leading edge provides an edge such that the top edge  38  of the ejector plate is longer than the bottom edge  40 . The opposite configuration is shown in FIG.  2 . 
     A typical prior art lifter structure is shown in FIG. 5 in simplified schematic form. Here, the surface  22  of the mold and the surface of the lifter are substantially conterminous, presenting a smooth unimpeded surface over which the plastic is to flow as it is injected into the mold. In practice, the leading edge  44  of the lifter may be slightly raised or lowered from the surface  22 . Plastic flow is shown by the vector arrows  110 ,  112  as it is injected from an upstream to downstream direction. 
     As previously mentioned, when the flow impinges upon the leading edge  44  of the lifter as shown in the FIG. 5 prior art device, an undesirable turbulent plastic flow is fored in the region  114 . This is especially true in those situations in which the surface of the lifter  32  is raised from or lower than surrounding mold surfaces. 
     In contrast, and with reference to FIG. 6, the obliquely angled leading edge  44  of the lifter in accordance with the invention promotes the formation of a laminar flow region  116  in the vicinity of the interface of that edge and its neighboring surface of the mold. This structural relationship has been shown to provide aesthetic benefit in the finished plastic parts as previously explained. 
     While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.