METHODS AND APPARATUS FOR BLOW MOLDING USING MICROGRAINED PATTERNS

Abstract: A method and apparatus for blow molding articles having a smooth surface finish by using a mold having a surface provided with a micrograined pattern that facilitates the formation of venting passages with the surface of the parison. One embodiment of the micrograined pattern is a series of substantially circular projections (50) arranged in rows to form a network of interconnected channels (52). This network of channels (52) is connected to vent holes (56) formed in the surface of the mold.

Description: METHODS AND APPARATUS FOR BLOW MOLDING USING MICROGRAINED PATTERNS BACKGROUND OF THE INVENTION The present invention relates to molding processes and apparatus and to blow molded articles. Specifically, the present invention relates to blow molding processes and apparatus which improve the venting of trapped air from molded material and which improve the surface finish on molded articles, such as automobile fascias, bumpers or other body components. Fundamentally, blow molding is a manufacturing technique that involves the use of compressed gas to expand material, usually a thermoplastic, outward against the inner walls of a surrounding, split-cavity mold. In extrusion blow molding methods and apparatus, molding is accomplished by first extruding a preselected length of material, for example, heat-softened thermoplastic, into a tubular preform, also called a parison. The parison is then surrounded, and usually pinched at one or more locations, by the split-cavity mold. A blow pin is then inserted into the parison and compressed gas is injected through the blow pin to inflate the parison, much like a balloon, against the sides of the mold. Typically, low gas pressures are used. Contact of the thermoplastic material with the mold causes the material to cool and harden in the desired shape. The split-cavity mold is then separated and the molded article is removed. In the manufacture of blow molded articles such as automobile body components, it is often desirable to provide a very smooth, aesthetically pleasing surface finish. However, it is difficult to obtain smooth, high quality surface finishes on products molded using conventional blow molding techniques without further surface finishing, such as sanding and painting. This difficulty arises, in part, fi-om the presence of air or other gas at or near the surface of the molded article. Trapped air or gas may cause variations or defects in the surface finish of the molded product. Blow molding is especially susceptible to the undesireable effects of trapped air because of the use of compressed gases in the molding process and because of the displacement of air from the mold due to the expansion of the parison. Conventional blow molding techniques have addressed this problem by providing vent holes at spaced intervals on the mold surface to facilitate venting of trapped air. However, vent holes provide for venting of trapped air only in local regions on the surface of the molded product; some trapped air may not reach vent holes during the molding process. To provide more uniform venting, it is known to utilize porous metal molds. However, porous molds are prone to clogging after repeated use and require additional effort to maintain. Thus, there remains a need for blow molding processes and apparatus which provide uniform and sufficient venting of trapped air and high quality surface finishes, yet which do not require significant maintenance effort. SUMMARY OF THE INVENTION The aforementioned problems are eliminated and desired advantages are realized by the present invention, which contemplates new and improved processes and apparatus for blow molding by providing mold surfaces with micrograined patterns to improve venting and surface finish. The micrograined patterns are preferably in the form of a pattern of projections which define between them a network of interconnected channels. The projections are of a sufficient height that they engage the outer surface of the fully-expanded parison. The channels are of a sufficient depth that, when the parison engages the projections, the channels form venting passages with the parison surface to permit trapped air to escape and migrate through vent holes which communicate with the channels. The dimensions of the projections are selected to provide for the formation of venting passages while preventing the formation of patterns in the surface of the molded product. In one aspect, the invention may be defined as an apparatus for molding articles, the apparatus comprising a mold having a mold surface provided with a micrograined pattern which facilitates the formation of venting passages with the surface of an expanded parison. More particularly, the invention may be defined as an apparatus for blow molding articles in which the mold surface is provided with a micrograined pattern that includes projections of a substantially circular or parallelogram shape, the projections defining a network of interconnected channels which form venting passages with the surface of the expanded parison. In another aspect, the invention may be defined as a process for blow molding articles, the process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison forms venting passages with the micrograined pattern. More particularly, the invention may be defined as a process for blow molding articles, the process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern, the pattern including projections of a substantially circular or parallelogram shape defining a network of interconnected channels; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison engages the projections and forms venting passages with the channels. In yet another aspect, the invention may be defined as a blow-molded article, formed by a process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison foims venting passages with the micrograined pattern. More particularly, the invention may be defined as a blow-molded article, formed by a process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern, the pattern including projections of a substantially circular or parallelogram shape defining a network of interconnected channels; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison engages the projections and forms venting passages with the channels. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings which are incorporated into and fonn a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting. In the drawings, in which like numbers refer to like parts throughout: FIG. 1 is an illustration of a blow molding process and apparatus according to a preferred embodiment of the present invention; FIG. 2 is an isometric of a blow mold surface according to another preferred embodiment of the present invention; FIG. 3 A is a top view of a blow mold surface according to another preferred embodiment of the present invention; FIG. 3B is a cross-section of the surface taken along line 3B-3B of FIG. 3 A; FIG. 4A is a top view of a blow mold surface according to another preferred embodiment of the present invention; and FIG. 4B is a cross-section of the surface taken along line 4B-4B of FIG. 4A. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG 1A, an extrusion blow molding apparatus 8, accoidmg to the present invention, includes a split-cavity mold 10 compπsed of two movable half portions With the half portions separated, a tubulai parison 12 of a preselected length of extruded matenal, typically a thermoplastic such as polyethylene or polypiopylene, exits fiom an extiusion die 14 into the space between the half portions Referring to FIG IB, aftei the paπson 12 is extiuded to the appiopπate length, the half portions of the mold 10 are brought together to define a mold cavity 16 The parison 12 is pinched at top and bottom ends between the half portions A gas injection device 18, including a blow pin 20 that extends thiough a hole (not shown) m mold 10, introduces compiessed gas into the parison cavity 22, thereby expanding the parison 12 outward against the inner surface 24 of the mold 10 As shown in FIG 1C, the half portions are then separated to permit removal of the blow molded article 26 FIG ID is a magnified view of a portion of FIG IB showing the interaction between the outer surface 30 of the expanding parison 12 and the inner surface 24 of the mold 10 Mold surface 24 is provided with a series of vent holes 32 (two shown) spaced at regular intervals to permit the escape of trapped gas from the mold as the parison 12 expands outward in the direction of the arrows in FIG ID The blow molding process described above is characteristic of conventional blow molding techniques and may be used m conjunction with the features of the present invention, which l elates particularly to the mold surface Refeπing now to FIG 2, which is an isometric of a section of a mold surface according to a prefened embodiment of the invention, the mold surface 24 is provided with a micrograined pattern having a seπes of raised projections 40 which define a network of channels 42 therebetween In FIG 2, the channels 42 are shown parallel to one another It will be understood that the channels 42 may be interconnected by cross-channels (not shown) which extend in a generally orthogonal direction to the extent of the channels illustrated The channels 42 communicate with vent holes (not shown in FIG 2) to permit an to vent out of the mold In accordance with the present invention, the spacing of the peaks of raised projections 40 and the depth of channels 42 aie selected to maximize venting capabilities while pi eventing the formation of visibly detectable patterns on the suiface of the molded article A depth range of between 10 and 40 microns is preferred FIG. 3A is a top magnified view of a mold surface pattern according to another preferred embodiment of the present invention. The micrograined pattern is in the form of a series of circular projections 50 arranged in rows, with the centers of circular projections of adjacent rows being offset. The circular projections 50 define a network of interconnected channels 52 which communicate with vent holes 56 (one shown). The channels 52 are of an irregular shape and varying width, as defined by the circular periphery of the projections 50. Referring to the cross- section shown in FIG. 3B, the circular projections 50 have a flat upper surface 58 and rounded edges 59. The bottom of channels 52 is of a generally rounded shape, defining an undulating mold surface 54. The circular periphery of the projections 50 form the channels 52 of varying width and non-linear pathway from points on the mold to the vent holes 56. The offset centers of the circular projections 50 reduce the likelihood that noticeable micrograined patterns will appear in the finished molded article. FIG. 4A is a top magnified view of a mold surface pattern according to another preferred embodiment of the present invention. Here, projections 60 are shaped generally as a parallelogram and define a network of channels 62 which communicate with vent holes 66 (one shown). Referring to the cross-section shown in FIG. 4B, the projections 60 have a flat upper surface 68 and rounded edges 69. The bottoms of channels 62 are of a generally rounded shape. The generally flat peripheral surfaces of the projections 60 form interconnected linear channels 62 which provide for efficient evacuation of escaping air from the mold. Those of ordinary skill will recognize that the aforementioned surface patterns with defined channels provide for the formation of venting passages with the surface of the expanded parison during molding. The surface patterns may be formed by conventional mold surface preparation techniques, such as by etching or laser machining. It will also be appreciated by those of ordinary skill, that dimensions of the mold surface pattern are selected to provide for the formation of venting passages with the surface of the expanded parison. Thus, the dimensions will depend on a number of factors, including the fluid and shrinkage characteristics of the molded material and the pressure applied to the parison during molding. For example, the depth of the projections for a molding process at a given pressure is preferably less than the depth of the projections for a molding process at a higher pressure because higher molding pressures will tend to force the material deeper into the channels and eliminate the venting passages that would otherwise be formed.

Claims: CLAIMS 1. An apparatus for blow-moldmg articles having a smooth surface finish, the apparatus comprising a mold having a mold surface provided with a micrograined pattern that facilitates the formation of venting passages with the surface of a parison. 2. The apparatus of claim 1, wherein the micrograined pattern includes projections that define a network of interconnected channels, the channels being of sufficient depth to facilitate the formation of the venting passages with the parison. 3. The apparatus of claim 2, wherein the projections are of a substantially circular shape. 4. The apparatus of claim 3, wherein the projections are of a substantially parallelogram shape. 5. The apparatus of claim 1 , wherein the micrograined pattern includes a series of substantially circular projections arranged in rows to define a network of interconnected channels. 6. The apparatus of claim 5, wherein the centers of the substantially circular projections of adjacent rows are offset. 7. The apparatus of claim 5, wherein the channels are in fluid communication with vent holes formed in the mold. 8. The apparatus of claim 5, wherein the circular projections have a substantially flat upper surface and rounded edges. 9. The apparatus of claim 5, wherein the depth of the channels is from 10 to 40 microns. 10. The apparatus of claim 1, wherein the micrograined pattern includes a series of substantially parallelogram-shaped projections airanged in rows to define a network of interconnected channels. 11. The apparatus of claim 10, wherein the centers of the substantially parallelogram - shaped projections of adjacent rows are offset. 12. The apparatus of claim 10, wherein the channels are in fluid communication with vent holes formed in the mold. 13. The apparatus of claim 10, wherein the circular projections have a substantially flat upper surface and rounded edges. 14. The apparatus of claim 10, wherein the depth of the channels is from 10 to 40 microns. 15. A method of blow molding articles, the method comprising the steps of: a) providing a mold having a mold surface including a micrograined pattern; b) providing a parison of material to be expanded outward against the mold surface; and c) expanding the parison outward against the mold surface such that an outer surface of the parison forms venting passages with the micrograined pattern. 16. The method of claim 15, wherein the step of providing a mold surface further comprises the step of providing projections defining a network of interconnected channels. 17. The method of claim 16, wherein the projections are substantially circular in shape. 18. The method of claim 16, wherein the projections are substantially parallelogram in shape.

Inventors: PORTER MARSHALL RAY, PORTER, MARSHALL, RAY
Applicants: CONIX CORP, PORTER MARSHALL RAY, CONIX CORPORATION, PORTER, MARSHALL, RAY
Application year: 1999
Main classification: B29C33
IPC codes: B29C33, B29C49