Abstract:
A method for suppression of weld line weakening by virtually eliminating weld lines is disclosed. The process involves the near explosive injection of a metered amount of foamed thermoplastic molding material into an enclosing compartment formed by partly closing a compression type mold followed immediately by compression molding of the well mixed foamed material. The process finds particular utility in the manufacture of very long fiber reinforced parts. Variations on mold design and clamping cycles yield a variety of parts having foamed regions surrounded by more dense un-foamed regions. The method is particularly well adapted to the manufacture of large, very long fiber reinforced, foam-centered, hard surfaced, light weight, rigid parts.

Description:
SUMMARY OF THE INVENTION 
     The present invention relates generally to plastic injection molding techniques and more specifically to such techniques which reduce or eliminate weld lines and their weakness problems. 
     Weld lines are the “lines” where flow fronts of injected heat softened plastic materials meet in the filling of pre-closed molds. These fronts are cooled by flowing over cool mold surfaces and therefore bond less well and give a weakened inter-flow-front surface bonding. 
     Since there is no mixing, interlocking, interleaving or knitting of the flow front materials, this weakening effect is particularly apparent when fiber reinforced thermoplastic materials are molded. The reinforcing fibers enhance the part&#39;s strength everywhere except at the weld lines where the fibers fail to intermesh. 
     In U.K. Patent 2,170,142 attempts to avoid weld lines or seams by providing pistons positioned about a die cavity which operate in synchronism to pump the plastic back and forth generally perpendicular to the direction of extrusion so that as a melt passes through the die cavity the pistons operate causing the extrudate to move circumferentially or transversely about the die cavity thereby eliminating seams, voids and other defects. Special heating of the runners would be required for some part configurations. The piston configuration needs to be tailored to a particular part configuration and large gates along with extensive trimming of scrap would be frequently required. For many part configurations, the process appears to be relatively slow. 
     The present invention may advantageously utilize molding materials such as disclosed in my copending application Ser. No. 08/333,504, entitled FOLDED FIBER FILLED MOLDING MATERIAL, filed on even date herewith, the entire disclosure of which is specifically incorporated herein by reference; however, other materials may be utilized in the practice of the present invention. 
     Among the several objects of the present invention may be noted the reduction or elimination of weak regions in fiber reinforced injection molded plastic parts; the avoidance of the above noted prior art drawbacks; the provision of a technique for forming foamed plastic parts having selected low density regions; and the provision of a process of operating an injection molding machine to manufacture, in a mold cavity, a substantially weld line free plastic part. These as well as other objects and advantageous features of the present invention will be in part apparent and in part pointed out hereinafter. 
     In general, after injection molding of a prior part, a compression type mold is partly pre-closed sufficiently to form an enclosing compartment. The enclosing compartment is then explosively filled with a molding material followed by rapid compaction of the material within the enclosing compartment. By “explosively filling” is meant a highly turbulent flow through open gates and runners (if used) under high enough injection pressures to turbulently fill the mold very quickly, e.g., in from a fraction of a second to under five seconds and providing fill and compaction time short enough to permit satisfactory part finish before cooling causes blemishes. Preferably, the compaction immediately follows mold fill. 
     Also in general, a new molding method for virtually eliminating weld lines and flow orientation in molded plastic parts utilizes an almost explosively rapid turbulent injection fill of a partly closed compression-type mold with a suitable amount of foamed plastic followed by compression molding of the foamed material to the desired density. 
     Still further in general and in one form of the invention, a foamed plastic part having selected low density regions is made by injection molding the part with a uniform density and subsequently reducing the density in the selected regions. The subsequent reduction may be effected by selectively heating the selected regions or by selectively reducing the pressure to which the part is subjected during, or immediately after, a compression molding step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a somewhat schematic illustration of an injection molding machine, cavity defining mold, and mold closing press ram illustrating the present invention in one form; 
     FIG. 2 shows a portion of the apparatus of FIG. 1 closed to compress the plastic material therein; 
     FIG. 3 shows a portion of the apparatus of FIG. 1 opened to eject a completed part after the plastic material has solidified; 
     FIG. 4 shows an illustrative part as ejected from the mold cavity of FIG. 3; 
     FIG. 5 shows a portion of the apparatus of FIG. 1 which has been modified to produce a foam-centered, hard-surfaced part; 
     FIG. 6 shows an illustrative part as produced by the modified mold of FIG. 5; 
     FIG. 7 is a somewhat schematic illustration of an injection molding machine, cavity defining mold, and mold closing press ram similar to FIG. 1, but illustrating the present invention in another form; 
     FIG. 8 shows a portion of the apparatus of FIG. 7 as material is being injected into the mold cavity; 
     FIG. 9 shows a portion of the apparatus of FIG. 7 closed to compress the plastic material therein; 
     FIG. 10 shows a variation on producing a foam-centered, hard-surfaced part similar to FIG. 6; 
     FIG. 11 is a top view of a part similar to the part of FIGS. 4,  6  and  10  illustrating the plastic flow pattern and creation of weld lines characteristic of the prior art; and 
     FIGS. 12-14 illustrate the sequential positions of the mold halves in practicing the present invention with a two cavity center-gated mold for producing illustrative foam cups. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawing. 
     The examples set out herein illustrate a preferred embodiment of the invention in one form thereof and such examples are not to be construed as limiting the scope of the disclosure or the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, foamable thermoplastic pellets are fed to the hopper  11  and from there are supplied to a reciproscrew or similar injection molding machine  13  having a hydraulic forcing cylinder  15  and heated barrel  17 . When valve  19  is opened, a measured quantity of pressurized, melt-softened, premixed foam material is introduced into the cavity of a sidegated mold  23 . Mold  23  has a pair of cores  25  and  27  for forming a pair of openings  29  and  31  in the illustrative part  33  of FIGS. 4,  10  and  11 . With conventional injection molding techniques, the mold  23  would be closed to the position shown in FIG.  2  and then the plastic injected. Under these conditions, the flow of plastic is along the arrowed lines of FIG.  11  and where those lines meet, as at  35  and  37 , weld lines are formed. As noted earlier, these lines represent weak regions of the part where breakage is more likely. To eliminate these weld lines, the foamable plastic is injected almost explosively rapidly into the cavity  43  with the mold only partly closed as shown in FIG.  1 . After the foamable material is injected and the valve  19  closed, the press ram  39  forces the mold portion  41  downwardly from the position of FIG. 1 to that of FIG. 2 thereby compressing the foamed material within the cavity  43  causing intimate mixing of the material fronts as they meet and virtual elimination of the weld lines  35  and  37 . Thus, a measured quantity of foamable plastic material which preferably contains fibrous reinforcement is injected into the cavity  43  and then the volume of the cavity along with the volume of the material therein is reduced by closing the mold from the position of FIG. 1 to that of FIG.  2 . The volume of the cavity may be reduced and the foamed material compressed sufficiently to produce an essentially foam-free part, or reduced a lesser controlled amount so as to produce a foam-centered, hard-surfaced part. A volume reduction to, on the order of, at least one-half the initial volume has been found preferable for producing foam-free fiber reinforced parts. A reduction to one-fifth, for example, has been found suitable for many applications. The material is then allowed to solidify. The ram  39  then raises mold portion  41  separating the mold halves so that the part  33  may be ejected as indicated by arrow  45  in FIG.  3 . 
     It is possible to take further advantage of the presence of the foamable material to produce a foam-centered, hard-surfaced part by enlarging the volume of the mold cavity relieving the compressive force on the part during (or after partial) solidification. This is accomplished by allowing an initial solidification of the outer surface of the part prior to enlarging the cavity and then allowing the remaining solidification of the part interior subsequent to enlarging the cavity. FIG. 5 illustrates such an enlargement of only a portion of the cavity causing re-foaming of the central region of the part as shown by the enlargement at  47  in FIG.  6 . There are two parts  41   a  and  41   b  to the top portion  41  of the mold of FIG.  5 . These parts are spring biased so that as the ram  39  raises, only the central part  41   a  moves upward. This enlarges only the central portion of the mold cavity. Further raising of the ram  39  will then raise part  41   b  opening the mold for part ejection. Thus, FIG. 5 illustrates an intermediate step between those of FIGS. 2 and 3 for producing the slightly modified part of FIG.  6 . FIG. 5 shows a mold cavity enclosed by three relatively movable parts. The initial mold cavity volume reduction is effected by moving the two parts  41   a  and  41   b  downward relative to the third part while the subsequent enlargement is effected by moving part  41   a  only relative to the other parts. 
     Since part  33  cools from the surface inwardly, the central region retains a measure of re-foamability. Hence, a part having a foamed portion may be made by the technique shown in FIG.  10 . Here heat shown at  49  is utilized to reheat a portion of the part after it has been allowed to solidify and ejected from the mold. 
     In FIGS. 7-9, a further variation on the present inventive technique which enhances material mixing at the flow fronts and increases the rate at which material can be injected into the cavity is shown. This modification also eliminates the need for any mold cavity air escape vents. In FIG. 1, the mold parts were initially only partly closed, while in FIG. 7, those parts are completely closed at the beginning of an injection cycle. Thus as the initial step, the volume of the cavity is reduced while the cavity is void of material. In FIGS. 7 and 8, valve  19  is opened and, at the same time, ram  39  is actuated to move upwardly thereby expanding the volume of the cavity while injecting the measured quantity of foamable plastic material. In essence, ram motion tends to suck material into the cavity. Of course, some provision is be made for allowing air to escape from the mold as it is pre-closed after molding a prior part. Rapid mold opening just prior to or in synchronism with valve  19  opening creates a vacuum which sucks in material and assists in mold fill. A preferred method for controlling and measuring, reproducably, the amount of injected material is control of reciproscrew or other ram movement during mold injection fill. When ram is moved full stroke forward each cycle, the reciproscrew travel rearward accurately determines the volume of unfoamed, but foamable melt, if the valve  19  is closed and there is enough “ram forward pressure” to prevent foam formation until valve  19  is opened. In one preferred form, the time rate of change of cavity volume is substantially the same as the rate at which material is being injected so that the cavity expands in synchronism with the injection of the material. When the cavity if filled to the preferred volume, the valve  19  is closed and the ram  38  lowered to compress the cavity to the volume shown in FIG.  9 . FIG. 9 corresponds to FIG. 2 from which processing continues as discussed earlier. 
     FIGS. 12-14 show a three plate mold where the middle runner plate  21  provides the cavity expansion as depicted in FIG.  12 . These figures illustrate an adaptation of somewhat conventional center gating to form “flashless” parts such as the illustrated foamed cups. Conventional compression molding techniques have pinch zones which result in flash on the parts. In FIG. 12, a measured quantity of foamable plastic material is injected into the cavity by way of the sprue bushing  51 , and runners  53 . As the upper press platen moves downwardly, the runner  53  is cut from the sprue  59  and the volume of the cavity along with the volume of the material therein is reduced until the cavity and part are, in cross-section, as shown in FIG.  13 . The press platens are then separated and the knock-outs  61 ,  63  and  65  are actuated to eject the cups and sprue  59 . Upward travel of the compression compartment plate  21  is limited by the rods  55  and  57 . 
     As an alternative, subsequent to the ejection of completed cups, the mold halves  67  and  69  may be closed to the position of FIG.  13  and then opened in synchronism with, and to aid in, the injection of plastic into the cavity much the same as discussed in connection with FIGS. 7-9. The sequence of events would then be FIG. 13 with the cavity empty, FIG. 12 filling the cavity, FIG. 13 compressing the material, and FIG. 14 ejecting the completed parts. 
     The method of operation of the invention should now be clear. A part is formed by first injecting a predetermined volume of foamable plastic material into a part forming mold and then compressing the material thereby reducing the volume of plastic in the mold. The compressing is maintained until at least a portion of the material has solidified. Preferably, the part is fiber reinforced and the fibers are at least ⅜ inch in length. 
     In summary, the invention has a number of advantages over known prior art and provides material intermixing and compaction interleaving of most weld lines and flow orientation thereby preventing or greatly diminishing weakening in those areas. 
     From the foregoing, it is now apparent that a novel arrangement has been disclosed meeting the objects and advantageous features set out hereinbefore as well as others. While the invention finds particular utility with long (⅜″ to ½″) and very long (½″ to, e.g., 2½″) fiber reinforced foamable thermoplastic materials used in molding large parts, weld line improvement is not restricted to either fiber reinforced materials or to thermoplastic materials. Thus, numerous modifications as to the precise materials, shapes, configurations and details may be made by those having ordinary skill in the art without departing from the spirit of the invention or the scope thereof as set out by the claims which follow.