Abstract:
A method is disclosed for blow molding polyethylene terephthalate preforms, such as injection molded parisons, to achieve exterior surface indentations on blown containers without using profiled mold cavity walls. In the method, a profile is molded or otherwise formed in the exterior wall of the preform, defining a preform wall portion having sections of different thicknesses. Then, the preform is blow molded under conditions so that the polyethylene terephthalate material strain hardens in order to cause the different wall thicknesses to expand and stretch proportionately. As a result, the blown containers formed by this method include a surface contour which is essentially proportionately shaped to the surface profile formed on the pre-existing preforms.

Description:
This is a continuation of application Ser. No. 789,483 filed Jul. 21, 1977, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates generally to a method of forming blown containers, and more particularly to a method of forming blown containers having exterior surface contours. 
     2. The Prior Art 
     Prior to the present invention, patterns, grooves, logos or other surface contours have been imparted to the exterior surface of blown thermoplastic containers in the final blow molding step by contours on the interior side walls of the blow mold cavity. Examples of bottles formed in this manner are shown in U.S. Pat. Nos. 3,403,804 and 3,397,724. 
     This procedure has been generally undesirable because of the expenses involved in machining or otherwise providing the contours on the mold walls and because the contoured molds can only be used for a particular job. Unfortunately, these problems have been heretofore unavoidable since no satisfactory alternative has been available. 
     The present invention overcomes these problems by applying a desired contour to a parison or preform, particularly of polyethylene terrphthalate material, and then proportionately enlarging that contour as the parison is blown into the final article within a standard blow mold cavity having smooth interior walls. 
     Of course, parisons and preforms have been provided with contours and varying wall thicknesses prior to the present invention. However, such contours have conventionally been provided either: to achieve an interior reinforcement in the final container, as shown by U.S. Pat. No. 3,114,932; to achieve a visual effect, as shown by U.S. Pat. No. 3,420,924; or to form a reinforced bottle portion, as shown by U.S. Pat. Nos. 3,294,883, 3,137,748 and 3,754,851. 
     Additionally, the prior art recognizes that polyethylene terephthalate is suitable for forming blown bottles, as shown by U.S. Pat. No. 3,733,309. 
     In short, however, the prior art does not recognize that profiling may be provided to a thermoplastic preform to achieve exterior contours on a blown container. Additionally, the prior art does not teach that portions of a preform having differential thicknesses may be proportionately reduced in thickness during a blow molding operation. 
     SUMMARY OF THE INVENTION 
     This invention relates primarily to a method of imparting exterior surface profiles or contours to a blown thermoplastic container, by imparting a proportionately shaped surface contour to a blowable preform and then expanding the contour proportionately with the other portions of the preform during the blow molding operation. The method is particularly suited for polyethylene terephthalate, because this material exhibits the characteristic of strain hardening, thus enabling thick and thin sections of a preform to expand and stretch proportionately and carry the preform profile over to the blown container. This method distinguishes significantly from prior art methods where the thin sections of a parison are expanded to a greater extent in the blowing operation, such that the thick and thin sections are expanded non-proportionately. 
     More specifically, the method includes closing the sections of a blow mold around a blowable preform of suitable thermoplastic material, such as polyethylene terephthalate. In accordance with the invention, the preform includes a profiled exterior surface region defined by exterior surface indentations forming preform wall sections which are thinner than the remaining wall sections. Then, in an essentially single operation, blow fluid is injected into the interior of the preform (a) to expand the preform outwardly, (b) to expand the thin wall sections of the preform essentially proportionately with the remaining wall sections of the preform, (c) to expand the profiled preform surface region into engagement with an essentially smooth interior mold wall surface and (e) to form a blown plastic container having an exterior surface indentation. Additionally, the preform may optionally be axially stretched just prior to or during the blowing operation. 
     When using polyethylene terephthalate material, the preform should be blown within a general range of specified conditions, so that the material will strain harden during the blow molding operation to achieve the proportionate expansion in the thin and thicker wall sections of the preform. These parameters and conditions may vary in relationship to one another, but generally fall in the following ranges. First, the preform should be at a temperature within the range of between about 75 degrees C. and 110 degrees C. depending upon the molecular weight (or inherent viscosity) of the material, with lower molecular weight materials generally requiring lower forming temperatures. Second, the material should be expanded at a rate of about one inch per inch per second; that is, the material should be expanded approximately one inch per second for every inch of starting length. Third, the blow molding operation should be carried out by a blow fluid at a pressure of between about 100 and 600 psi; and more preferably by initialling inflating the parison with fluid at a pressure of between about 150 and about 200 psi and then completing the blowing operation with fluid at a pressure of between about 350 and 550 psi. These parameters may vary slightly from the specified ranges, inasmuch as these ranges are derived from laboratory observations. 
     Accordingly, the present invention provides several significant advantages, including primarily the ability to use the same blow molds for several different distinctive bottle designs. This is of particular importance from a commercial standpoint because (1) it is now unnecessary to machine separate blow mols for each bottle design and (2) job change overs are not required. 
     Additionally, it has previously been difficult to achieve sharp details of grooves and contours in blow molded containers. In this invention, the details are present in the preform and are simply carried over into the container. 
     These and other advantages and meritorious features of the present invention will be more readily appreciated and understood from the following detailed description and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration, showing a profiled preform enclosed within a blow mold cavity; 
     FIG. 2 is a schematic illustration showing the preform blown to a container within the blow mold cavity; 
     FIG. 3 illustrates the blown container having an exterior surface indentation proportionately shaped to the profile which previously existed in the blowable preform; 
     FIGS. 4-7 are enlarged fragmentized views, illustrating the manner in which the preform expands into engagement with the mold walls in the region of the external surface indentation; and 
     FIGS. 8 and 9 are schematic illustrations depicting the proportionate expansions between the thin and thick regions of the preform. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now more particularly to the drawings, the overall method proposed by the present invention relates to the formation of a blown container such as that illustrated in FIG. 3 by reference numeral 10, and including an exterior intaglic profile or decoration 20. More generally, however, the method relates to the formation of a blown container having a thin wall section that is expanded proportionately with the thicker wall sections of a preform during a stretch-and-blow or blow molding operation. As previously discussed, this method is particularly suitable for polyethylene terephthalate material because of that material&#39;s ability to achieve extensive strain hardening during a blow molding operation. 
     Accordingly, this method offers significant utility in the formation of exterior surface profiles on blown containers, such as spirals, flutes, grooves, patterns, logos, indicia and other decoration. 
     Referring to FIG. 1, the method of this invention includes enclosing a blowable thermoplastic preform 30 within a blow mold cavity 40 defined by closed mold sections 42 and 44. It will be understood that the mold sections are opened and closed by suitable power means (not shown) such as hydraulic rams. Additionally, the blow molds include a finish-forming region 45 which forms a neck or finish 12 on the blown container. 
     The thermoplastic preform 30, when positioned within the mold cavity, includes an external surface intaglio contour or profile 32, which may be formed by various techniques, as desirable. For example, the surface indentation may be formed on the preform during an injection molding step or may be cut or otherwise formed in the preform prior to the blow molding step. 
     After the sections 42 and 44 have closed around the preform 30, a blow rod 50 having a fluid supply passageway 52 is axially inserted into the neck portion of the preform. Thereafter, blow fluid under pressure is injected into the interior of the preform 30 by way of passageway 52, thereby expanding the preform outwardly into contact with the mold cavity 40 and forming the blown body portion 14 of the container. This blow molding operation and the behavior of the thermoplastic material in the region of the exterior intaglio indentation 32 are illustrated particularly in FIGS. 4-7. 
     FIG. 4 illustrates the plastic preform just prior to striking the mold wall surface 40. As shown, the exterior indentation 32 has expanded essentially to the size of the intaglio surface decoration 20, with the material having necked down slightly at the juncture 31 between the preform thicknesses 30 and 32. FIG. 5 illustrates the plastic material at the moment when it strikes the smooth mold wall surface 40, with the plastic material having been stretched and expanded only slightly from its configuration of FIG. 4. FIG. 6 illustrates what is believed to occur when the full blowing pressure within the blown article 40 has developed, causing the plastic material in the region of the intaglio surface decoration 20 to be pressed against the mold wall surface 40. This blowing pressure may be relieved within a short time by venting the interior of the blown shape, permitting the plastic material in the region of the surface intaglio 20 to return essentially to its position of FIGS. 4 and 5, as shown in FIG. 7. It is believed that this inward retraction of the plastic material at the intaglio occurs for two reasons. First, the plastic material preferred for this invention, polyethylene terephthalate, strain hardens during the stretching operation under proper stretching conditions, and therefore has an elastic memory for its original position; and second, the intaglio has a greater surface area than the corresponding mold surface area against which it is blown, tending to discourage the material from being pressed into that region. 
     Upon completion of the blowing operation, the intaglio surface design 32 is stretched and expanded substantially proportionately with the other portions of the preform to form intaglio surface design 20, even though this preform portion is thinner than the remaining preform portions. It is believed that the exterior surface indentation is achieved in the present invention and that the proportional expansion occurs because polyethylene terephthalate material is susceptible to strain hardening during the blow molding operation. In order to achieve the strain hardening, however, the blow molding conditions should be maintained to within certain specified conditions. For example, the pressure of the blowing fluid should be within the range of from about 100 to 600 psi. Most preferably, the parison is expanded in response to an initial lower pressure and then to a final high pressure in order to achieve good bottle characteristics, particularly clarity and uniform expansion. The initial low pressure is applied for about one-half second or more and is in the range of about 150 to 200 psi. The final high pressure is in the range of about 350 to 550 psi. These pressures may be regulated by suitable regulator controls and flow valves (not shown) which do not form a part of the present invention. 
     Depending upon the particular molecular weight (or inherent viscosity) of the material, the parison should be thermally conditioned to have a temperature broadly within the range of between about 75 degrees C. and 110 degrees C. This temperature range is particularly suitable for polyethylene terephthalate having an inherent viscosity of between about 0.5 and about 0.9 with the lower inherent viscosity materials generally requiring lower temperatures. Additionally, the expansion rate for the material should preferably be about one inch per inch per second; that is, the material should be stretched about one inch per second for every inch of starting length. Of course, these parameters and conditions may vary in different environments and different operations. 
     In FIGS. 8 and 9, the proportionate relationship between the corresponding preform and final container portions is schematically emphasized. By way of explanation, X 0  indicates the thickness of the thin preform portion corresponding to the surface indentation 32; Y 0  indicates the thickness of the preform wall 30; and Z 0  indicates the length of the thin preform wall portion. Likewise, X 1 , refers to the thickness of the thin container portion, corresponding to the surface indentation 20; Y 1  indicates the thickness of the blown container wall 14 adjacent the surface indentation; and Z 1  indicates the length of the thin container wall portion. Although not shown, W 0  and W 1  are intended to respectively designate the widths of the thin preform wall portion and of the thin container wall portion. Accordingly, Z 0  W 0  and Z 1  W 1  represents the respective surface areas of the thin preform wall portion and the thin container wall portion. 
     In accordance with the present invention, the ratios between the corresponding thicknesses of the preform and container are essentially equivalent. That is: ##EQU1## Additionally, the ratio of the thin preform portion to the thick preform portion is essentially equal to the ratio of the corresponding containers portions with respect to one another. That is: ##EQU2## 
     Further, the ratios between the corresponding thicknesses of the preform and container are essentially equivalent to the ratio of the final indentation surface area after stretching to the original indentation surface area. That is: ##EQU3## 
     It is to be understood that the foregoing disclosure is exemplary in nature, rather than limiting. For example, the invention has been disclosed primarily in reference to polyethylene terephthalate material. However, the invention is equally adaptable to similar types of thermoplastic materials which are susceptible to proportionately expanding different wall thickness portions during stretching, such as in a blow molding operation. 
     Additionally, the blow molding operation may include an axial stretching step in order (1) to proportionately expand the preform and exterior indentation in the axial and radial directions and (2) to biaxially orient the material. Such an operation may be preformed by either stretching the preform prior to or during the introduction of blow fluid into the preform, as is known in the art, reference being made to U.S. Pat. No. 3,865,530, incorporated by reference.