Abstract:
A method for forming an angled plastic article of varying density whereby a blowing agent is added to a plastic material and the material is injected into a mold cavity of a mold unit. The interior walls of the mold cavity are maintained at a temperature sufficient to prevent the outer skin of the hardening plastic article from producing gas bubbles. The volume of the mold cavity is increased to allow bubbles to produce and uniformly distribute themselves throughout the mold cavity and then a reconversion phase is instituted whereby the desired density of the finished plastic article is achieved without voids or aberrations appearing in the surface of the plastic article. The mold unit is provided with extensible beveled edges which extend and retract as the mold cavity is enlarged and compressed. By this method, a resulting plastic article of varying density is produced having grooves running the length of the article. After the molding process is complete, an adhesive is applied to the grooves and the molded article is bent whereafter the adhesive maintains the article in the angled orientation. By this method, a complex structure having a smooth outer skin and a lightweight interior may be produced quickly and efficiently.

Description:
This application is a divisional of application Ser. No. 08/534,513, filed Sep. 27, 1995, now U.S. Pat. No. 5,972,259. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for forming an angled plastic article of varying density and, more particularly, to a method for molding and bending a plastic article to form a smooth dense outer shell and a rare interior. 
     Injection molding machines generally include a two-section mold unit wherein one of the mold sections is stationary and includes an end gate opening for the injection of mold material into a mold cavity formed by the two mold sections. The other mold section is generally movable between an open position away from the stationary mold section and a closed position wherein the two mold sections are in sealed contact engagement to form the mold cavity. Once the mold cavity has been formed, a screw or similar injection device is used to inject a plastic material into the mold cavity where the material is cured under pressure. Injection molding provides an efficient means for producing plastic articles both quickly and economically. 
     It is often desired to create a plastic article of a decreased density which reduces the amount of material required to make the part and which significantly decreases the weight of the part. One way of creating a plastic part of reduced density is through the use of a blowing agent. Blowing agents are well-known in the art with most blowing agents being heat-activated. When mixed with a plastic material under controlled conditions these blowing agents produce bubbles. If a blowing agent is evenly distributed throughout the plastic material the bubbles are generally trapped evenly throughout the plastic article as the article cures. These trapped bubbles form a cellular structure within the finished article. 
     While it is often desirable to have a cellular structure within a plastic article, this cellular structure is typically undesirable on the outer shell of a finished plastic article. Bubbles forming near the outer shell of a curing plastic article often result in unaesthetic holes appearing on the surface of the article. Such surface bubbles may also compromise the integrity of the surface by creating sink holes or weak points where the article is unable to sustain pressure. 
     As described in co-pending U.S. patent application Ser. No. 08/082,266, now U.S. Pat. No. 5,437,823 it is desirable to mold a plastic part with a uniform cellular structure on the interior, which allows the part to be molded lighter and with less material, while at the same time having a denser outer skin, which increases the aesthetic and structural aspects of the finished article. To mold a plastic article without surface defects, the co-pending application describes a process whereby a two-section mold unit is filled with a plastic injection material having a blowing agent incorporated therein. As the injection material contacts the cool sides of the mold unit, the blowing agent is deactivated and the material near the sides of the mold cavity hardens into a smooth exterior. The warm interior of the material, however, has a cellular structure caused by the heat activation of the blowing agent within the injection mixture. 
     Because an article produced by the above process produces a lightweight article with a smooth aesthetic exterior appearance, it would be desirable to produce large containers and other objects which could benefit from the weight decrease of the interior cellular structure. Given that this process requires that the mold units move in relationship to one another during the molding process, the process is very well suited for producing flat sheets of material. While it is possible to produce more complex shapes with the above process, the mold units for producing such articles would necessarily be larger and more complicated owing to the requirements of moving the mold sections in relationship to one another during the molding process. 
     Another difficulty associated with molding a plastic article with a rare cellular interior and a smooth plastic exterior is the difficulty in maintaining an exterior skin of a constant thickness around comers in the mold cavity. Typically, in a molding process, the increased cooling effect of the mold cavity at places where two sides of the mold cavity meet leads to a thicker skin being formed at that point. This added thickness not only increases the amount of material which must be added to the mold cavity, thereby increasing costs, but the increased thickness also adds additional weight to the finished plastic article. Additionally, because the necessity of moving the mold sections in relationship to one another, and due to the limitations of mold cavity shape in plastic injection processes, it has not been possible to mold certain shapes with a cellular interior and smooth exterior. It would therefore be desirable to mold a flat sheet which could then be transformed after molding into a more complex shape. 
     The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method for producing a reduced density angled plastic article without voids on or near the surface of the article. 
     Another object of the present invention is to provide a repeatable method for producing an angled plastic article with a dense outer skin and a rare uniform interior. 
     Still another object of the present invention is to provide an apparatus for reproducibly forming an angled plastic article with a dense outer skin and a rare uniform interior. 
     Yet another object of the present invention is to provide a method for forming an angled plastic article from a flat sheet. 
     Another object of the present invention is to provide a method for forming an angled plastic article having a cellular interior and a smooth exterior wherein the thickness of the smooth exterior remains substantially constant across the angled portion of the plastic article. 
     These and other objects of the invention will become apparent upon reference to the following specification, drawings, and claims. 
     By the present invention, it is proposed to overcome the difficulties encountered heretofore. To this end, a method of forming an angled plastic article having a cellular interior and a smooth exterior is provided whereby a plastic injection molded article is provided with an outer deformable plastic skin. First and second plastic cellular sections are molded integral with the outer deformable plastic skin a sufficient distance from one another to allow the plastic article to be bent between the two plastic cellular sections along the outer deformable plastic skin. The plastic article is angled along the outer deformable plastic skin between the two plastic cellular sections and modified to maintain the plastic article in this orientation. 
     Preferably, the plastic article is molded as a one-piece article with each plastic cellular section having: (1) a cellular interior contiguous with the outer deformable plastic skin; (2) a first side covered with an outer skin; and (3) a second side extending away from the outer deformable skin at an acute angle relative to the outer deformable skin. Before the plastic article is angled, the second side of the first plastic cellular section is treated with heat or an adhesive to attach to the second side of the second plastic cellular section. Accordingly, when the plastic article is angled, and the second sides of the first and second plastic cellular sections adhered to one another, an angled plastic article having a smooth outer skin and a cellular interior is formed from a readily molded flat sheet. 
     The flat sheet is preferably molded with a specially designed mold unit. In this mold unit, one of the mold sections is provided with a beveled core which is extensible relative to the mold section to which it is attached. During the molding process, as the two mold sections are moved in relationship to one another to form the cellular structure of the plastic article, the beveled core is extended and retracted to maintain the beveled portion of the core a constant distance from the opposing mold section throughout the molding process. This extensible/retractable beveled core allows a flat plastic article having a smooth exterior skin and a cellular interior to be molded with a groove which separates the plastic cellular sections and along which the finished plastic article may be angled. When hot melt welding or adhesive is applied to the groove and the article is angled along the groove, the article is maintained in the angled orientation. The angling produces a complex article having a cellular interior and smooth exterior. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation in partial cross-section showing the plastic injection molding machine of the present invention in the open position; 
     FIG. 2 is a perspective view of the movable mold section of FIG. 1 with the beveled core retracted; 
     FIG. 3 is a perspective view of the movable mold section of FIG. 1 with the beveled core extended; 
     FIG. 4 is a side elevation in partial cross-section showing the mold unit of FIG. 1 closed in its initial position and the beveled core retracted; 
     FIG. 5 is a side elevation in partial cross-section showing the mold unit of FIG. 1 closed in its initial position with the beveled core retracted and a plastic material injected into the mold cavity; 
     FIG. 6 is a side elevation in partial cross-section showing the mold unit of FIG. 1 in its intermediate position with the beveled core extended and the plastic material expanded to fill the mold cavity; 
     FIG. 7 is a side elevation in partial cross-section showing the mold unit of FIG. 1 in its final position with the beveled core retracted and a resultant plastic article formed therein; 
     FIG. 8 is a perspective view in partial cross-section showing the plastic article of FIG. 7 with the resultant grooves therein; and 
     FIG. 9 is a perspective view showing the plastic article of FIG. 7 maintained in an angled orientation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the Figures, a mold apparatus  10  is shown which includes an injection assembly  12  and a molding assembly  14  (FIG.  1 ). The molding assembly  14  includes a movable mold section  48  and a stationary mold section  62  which when clamped into sealed engagement with one another form a mold cavity  74  (FIG. 4) for the molding of a plastic article  78  (FIG.  7 ). The movable mold section  48  includes a pair of extensible and retractable beveled cores  52  (FIGS.  2  and  3 ). 
     A plastic injection material  68  is provided with a blowing agent and injected into the mold cavity  74  where the blowing agent begins to produce bubbles and expand the injection material  68  (FIG.  5 ). Due to the cooling effect of the mold sections  48  and  62 , the plastic injection material  68  in contact with mold sections  48  and  62  and beveled cores  52  does not expand. The cooling deactivates the blowing agent around the exterior of the plastic injection material  68 , thereby preventing the formation of bubbles in the cooled portion of the plastic injection material  68 . After a bubbleless skin  76  has formed along the edges of the mold cavity  74 , the mold sections  48  and  62  are moved apart slightly to allow interior bubbles formed by the blowing agent to expand (FIG.  6 ). As the mold sections  48  and  62  are moved apart, the beveled cores  52  are extended to maintain a constant distance between the beveled cores  52  and the stationary mold section  62 . After sufficient expansion of the bubbles has occurred, the mold sections  48  and  62  are moved toward one another, with the beveled cores  52  being retracted to maintain the predetermined distance between the beveled cores  52  and the stationary mold section  62  (FIG.  7 ). This compression step forces the bubbles into a uniform cellular structure. 
     After the injection material  68  has sufficiently hardened, the movable mold section  48  is moved away from the stationery mold section  62  back to the initial positions shown in FIG.  1  and the molded plastic article  78  is removed from the mold cavity  74 . The resulting molded plastic article  78  is formed with a pair of grooves  82  running the length of the plastic article  78  (FIG.  8 ). To form the finished article  96  of the present invention, a propane torch  90  is passed along the grooves  82  and the molded plastic article  78  is angled to bring sides  84  and  86  of the grooves  82  together until the heated sides  84  and  86  of the grooves  82  have hardened. Alternatively, a hot plate (not shown) can be used. In this way, an angled plastic article may be formed from a flat molded sheet (FIG.  9 ). While sides  84  and  86  are only shown on one of the grooves  82 , it is to be understood that the same would apply to both of the grooves  82 . 
     As shown in FIG. 1, the injection assembly includes a materials hopper  16 , an injection barrel  18 , and an injection nozzle  20  provided on a movable carriage  22 . The movable carriage  22  is positioned on a stationary frame  24  which allows the injection assembly  12  to move into and out of contact with the molding assembly  14 . 
     The molding assembly  14  is provided with a frame  26  on which rests a headplate  28  and a footplate  30  (FIG.  1 ). Four support rods  32 , only two of which are shown in FIG. 1, and bolts  34  secure the headplate  28  and footplate  30  to one another. Slidably connected to the support rods  28  is a slidable carriage  36 . Support blocks  38  are mounted to the comers of the carriage  36  to add extra support to the carriage  36  as it slides along the support bars  28 . 
     A hydraulic cylinder  40  is secured to the headplate  28  over a bore  42  passing through the headplate  28  (FIG.  1 ). Slidably positioned within the bore  42  is a ram  44  which is secured on one end to the hydraulic cylinder  40  and on the opposite end to a platen  46 . The platen  46  is secured to the slidable carriage  36  to evenly distribute the force of the ram  44  across the slidable carriage  36 . 
     Secured to the slidable carriage  36  on the side opposite the platen  46  is the movable mold section  48  provided with a molding surface  50  and the two beveled cores  52  (FIGS.  1  and  3 ). As the beveled cores  52  are substantially identical, the description will be limited to a single beveled core  52 . As shown in FIG. 3, the beveled core  52  is provided with a body  54  extending substantially the entire length of the molding surface  50  and with a beveled surface  56  which also extends substantially the entire length of the molding surface  50 . The movable mold section  48  is provided with a slot  58  cut across the length of the molding surface  50  having a width substantially equal to the width of the body  54  of the beveled core  52  and a depth equal to approximately one-half the depth of the movable mold section  48 . It should be noted that the beveled core  52  and slot  58  may be provided with various dimensions to accommodate various molding requirements. 
     As shown in FIG. 1, a hydraulic cylinder  58  is provided with a ram  60  which extends through the slidable carriage  36  and movable mold section  48  to connect with the beveled core  52 . Accordingly, as the hydraulic cylinder  58  is actuated, the ram  60  extends and retracts the beveled core  52  relative to the movable mold section  48 . Secured to the footplate  30  opposite to and in mating alignment with the movable mold section  48  is a stationary mold section  62  having a recess  64 . Preferably, a lamina  66  such as cloth, carpet, or a flexible wood veneer is placed within the recess  64  against the stationary mold section  62 . The footplate  30  is provided with a sprue  70  and the stationary mold section  62  is provided with a runner  72  to allow the plastic injection material  68  to pass from the injection assembly  12  into the molding assembly  14 . 
     A plastic injection material  68  is prepared by adding a blowing agent to a plastic material. Although several blowing agents are known in the art, in the preferred embodiment the blowing agent is bicarbonate of soda. Bicarbonate of soda decomposes when heated to produce a gas consisting of mainly nitrogen and carbon monoxide. In the preferred embodiment of the present invention, thirty percent by weight of bicarbonate of soda is added to polyurethane to produce the plastic injection material  68 . 
     Before the injection material  68  is injected into the mold cavity  74 , it is first plasticized to provide a flowable material which eventually hardens into the finished plastic article  78 . The following parameters affect the plasticization process: the type of raw plastic material to be plasticized; throat temperature; the temperature of injector heating zones; the size, length and type of screw; the rate at which the screw is turned; and the rate at which the screw is allowed to move back. Of these parameters, only the type of raw plastic material and screw dimensions are not directly controlled by software within a central processing unit (CPU) such as a personal computer. The throat temperature and temperature of injector heating zones are controlled by standard proportion integral derivative (PID) control algorithms. The software controls the rate at which the screw is turned so that the rate is directly proportional to the rate of oil produced by a variable vane hydraulic pump. The software directly controls the rate at which the oil is delivered by the variable vane pump by supplying two set points, a rate set point and a maximum pressure set point. The rate set point specifies the rate of oil flow requested while the maximum pressure set point specifies a maximum pressure limit. Consequently, since pressure is a function of rate of flow times a resistance, the maximum pressure set point will limit the rate of flow in the case when maximum pressure is obtained. Preferably, the rate at which the screw turns during the plasticization process is profiled. Profiling causes the screw to turn at different rates during the plasticized cycle. Software is written to specify the particular profile by supplying the required rate and the maximum pressure set points to the variable vane pump supplying a profile to flow of oil to the screw. 
     The rate at which the screw is allowed to move back is also controlled by software. The profiled turning of the screw during the plasticization process causes the plastic material to be metered to an area in front of the screw. The plastic material being metered forward causes a counter plastic pressure attempting to force the screw backward. The rate at which the screw moves backward, from the counter pressure of the plastic material being metered forward, is profiled by software. Profiling of the rate of movement backward allows for variations in the amount of time the plastic material is mixed by the screw, as well as profiling the amount of frictional heat created by the screw turning which in turn is induced into the plastic material. Software specifies this profile by supplying a back pressure set point to a variable hydraulic back pressure valve. 
     Like the plasticization process, the injection process is also controlled by software. The parameters associated with the injection process include the following: the type of raw plastic material to be injected; the temperature of the plastic material; the size, length and type of screw; the rate at which the plastic material is injected; the mold design; and the mold temperature. Preferably, both the temperature of the material and the rate at which the plastic material is injected are controlled by software. To control the temperature of material as the material enters the mold cavity  74  during injection, software controls several parameters. The heater bin temperature is manipulated during the plasticization process and frictional heat is controlled during both the plasticization and back pressure cycles. Additionally, frictional heat developed from the plastic material moving through the injection nozzle during the injection process is controlled to provide the optimum amount of heat into the material based upon the desired resulting plastic product. 
     Software allows the rate at which the plastic material is injected during the inject cycle to be profiled. Profiling causes the material to be injected at different rates during the inject cycle. Software specifies this profile by supplying the required rate and maximum pressure set points to the variable vane pump supplying a profile to flow of oil to the cylinders attached to the screw. A typical inject profile will begin at a slow inject rate with the pump set to allow for maximum pressure. As the mold cavity  74  fills, the rate proportionally increases to the optimum rate for the filling of the mold cavity  74 . When the mold cavity  74  is approximately ninety percent (90%) filled, the maximum pressure set point of the hydraulic pump is lowered to a value which prevents the injection pressure from overcoming the clamp pressure. The remainder of the material is injected into the mold cavity  74  allowing the pressure compensation characteristics of the pump to control the rate at which the material is injected. This type of profiling describes a manner in which the tool is approximately ninety percent (90%) filled using rate control with the last ten percent (10%) using pressure control. This is accomplished by the design of the hydraulic system and control software. 
     To begin the molding process, the plastic injection material  68  is placed within the materials hopper  16  of the injection assembly  12 . As the plastic injection material  68  is heated and plasticized within the injection assembly  12 , the hydraulic cylinder  40  is actuated to move the slidable carriage  36  and movable mold section  48  into sealed engagement with the stationary mold section  62 . As shown in FIG. 4, when the movable mold section  48  and stationary mold section  62  are in an initial molding orientation, a mold cavity  74  is formed between the mold sections. In this initial orientation, the beveled edge  56  of the beveled core  52  is positioned approximately 1.5 millimeters from the lamina  66  provided along the stationary mold section  62 . While the distance of the beveled edge  56  from the lamina  66  may be varied according to the desired characteristics of the finished plastic article, the distance is preferably equal to the thickness of the skin which will surround the finished plastic article. 
     After the movable mold section  48  has been moved into the initial orientation relative to the stationary mold section  62  to form the mold cavity  74  of an initial volume, the plastic injection material  68  is moved from the injection barrel  18  through the nozzle  20 , the sprue  70 , and the runner  72  into the mold cavity  74  (FIGS.  1  and  5 ). As the plastic injection material  68  enters the mold cavity, the plastic injection material  68  moves under the beveled edge  56  of the beveled core  52  to completely fill the mold cavity  74  as shown in FIG.  5 . 
     As the plastic injection material  68  enters the mold cavity  74 , the coolness of the movable mold section  48  and stationary mold section  62  prevent the blowing agent contained within the portion of the plastic injection material  68  contacting the movable mold section  48  and the lamina  66  from activating to produce gas bubbles (FIG.  5 ). This unactivated portion of the plastic injection material  68  forms a skin  76  which surrounds the plastic injection material  68  (FIG.  6 ). Once the skin  76  has formed, the hydraulic cylinder  40  is actuated to move the slidable carriage  76  and movable mold section  48  away from the stationary mold section  62  as shown in FIG.  6 . As the movable mold section  48  is moved away from the stationary mold section  62 , the hydraulic cylinder  58  is actuated to extend the ram  60  and beveled core  52  enough to maintain the distance between the lamina  66  and the beveled edge  56  of the beveled core  52 . 
     As the size of the mold cavity  74  is increased, the blowing agent within the plastic injection material  68  creates expanding gas bubbles which are distributed throughout the plastic injection material  68 . Once a sufficient number of gas bubbles have formed to produce the density desired in a molded plastic article  78 , the hydraulic cylinder  40  is actuated to move the slidable carriage  36  and movable mold section  48  toward the stationary mold section  62  to form a mold cavity  74  having a final volume less than the aforementioned intermediate volume. Preferably, the final volume equals the initial volume, but may of course be any suitable volume. During this compression stage, the hydraulic cylinder  58  is actuated to move the ram  60  to draw the beveled core  52  into the slot  58  of the movable mold section  48  to maintain the distance between the beveled edge  56  and the laminate  66 . This compression stage distributes the air bubbles formed by the blowing agent evenly across the molded plastic article  78  and allows the molded plastic article  78  to harden to its finished form. 
     As shown in FIGS. 8 and 9, the molded plastic article  78  is provided with a lightweight cellular interior  80  surrounded by the smooth exterior skin  76 . The molded plastic article  78  is also provided with two grooves  82 , each having a first wall  84  and a second wall  86 . In the preferred embodiment of the present invention, groove  82  is deep enough so that there is no cellular material  80  between the skin surrounding the groove  82  and the skin surrounding a back portion  88  of the molded plastic article  78 . Due to the absence of intervening cellular material  80  and the flexibility of the skin  76 , the molded plastic article  78  is capable of being angled along the groove  82 . In this manner, the groove  82  divides the molded plastic article  78  into two sections (see FIG.  8 ). Specifically, on one side of the groove  82  lies a first inner plastic cellular section  100 , and on the other side of the groove lies a second inner plastic cellular section  102 . Each of the inner plastic cellular sections  100 ,  102  consists of a lightweight cellular interior  80 , and an outer deformable skin  76  that surrounds the cellular interior  80  (see FIGS.  8 - 9 ), Additionally, since the groove  82  forms the boundary between the first and second inner plastic cellular sections  100 ,  102 , the first wall  84  forms a first side  104  of the first inner plastic cellular section  100 , and the second wall  86  forms a first side  106  of second inner plastic cellular section  102  (see FIG.  8 ). Further, because the outer skin  76  forms everywhere along the perimeter of the molded article  78 , the outer skin covers the first sides  104 ,  106  of each of the inner plastic cellular sections  100 ,  102 . The first sides  104 ,  106  of each of the inner plastic cellular sections  100 ,  102  lie between a second side  108 ,  110  and a third side  112 ,  114 . The first sides  104 ,  106  of each of the inner plastic cellular sections  100 ,  102  are angled such that an acute angle  116  is formed between the first sides  104 ,  106  and the second sides  108 ,  110  of each of the inner plastic cellular sections  100 ,  102 . An obtuse angle  118  is formed between the first sides  104 ,  106  and the third sides  112 ,  114  of each of the inner plastic cellular sections  100 ,  102  (See FIG.  8 ). To form a finished plastic article  96 , the skin  76  within the groove  82  is heated with a propane torch  90  or similar heating element to allow the first side  84  of the groove  82  to adhere to the second side of the groove  86  when the molded plastic article  78  is angled as shown in FIG.  9 . Since the outer skin  76  actually covers the first wall  84  of the groove  82  and the second wall  86  of the groove  82 , the outer skin  76  adheres to itself. An alternative method of maintaining the angled orientation of the molded plastic article  78  is to apply an adhesive material  92  such as an epoxy or similar adhesive to the second side  86  of the groove  82  before angling the molded plastic article  78  as shown in FIG.  9 . After the grooves  82  have been properly treated and waste laminating (not shown) has been removed from the molded plastic article  78 , the adhesion means used to adhere the first side  84  of the groove  82  to the second side  86  of the groove  82  is allowed to harden to provide the finished plastic article  96  shown in FIG.  9 . 
     The foregoing description and drawings merely explain and illustrate the invention and the invention is not limited thereto, except insofar as the claims are so limited, as those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention. For example, it is anticipated to be within the scope of the invention to eliminate the density reduction phase and to merely compress the reaction material after a sufficient number of gas bubbles have been formed to provide the desired density of the plastic article. Additionally, it is anticipated that any number and orientation of grooves may be provided in the molded plastic article to produce finished plastic articles of various dimensions. Furthermore, the maximum pressure set point of the hydraulic pump is lowered to a valve which prevents the injection pressure from overcoming the clamp pressure can be done when the mold cavity  74  is less than the preferred ninety percent (90%) filled. Also, other blowing agents such as azodicarbonamide can be used rather than the preferred bicarbonate of soda.