Patent Publication Number: US-6210623-B1

Title: Method for blow down hydroforming sheet material

Description:
This is a continuation of application Ser. No. 08/833,716, filed Apr. 9, 1997, entitled “METHOD FOR BLOW DOWN HYDROFORMING SHEET MATERIAL”, to be issued on Aug. 3, 1999 as U.S. Pat. No. 5,932,167; which application is a divisional of application Ser. No. 08/545,970, filed Oct. 20, 1995, now U.S. Pat. No. 5,679,388, entitled “PROTECTED SEAL FOR BLOW DOWN PLATEN”; which application is a continuation of Ser. No. 08/105,033, filed Aug. 11, 1993, now U.S. Pat. No. 5,460,773, which applications describe inventions by the present inventors. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to platens for hydroforming finished plastic sheet material and, more particularly, to a seal used with a blow down platen. 
     2. Description of Related Art 
     The process of hydroforming has been practiced for many years. It includes the use of a flexible diaphragm disposed in a cavity of a platen for forcing sheet material to conform with the configuration of a mold. Hydraulic fluid, acting upon the diaphragm, causes the diaphragm to force the sheet material disposed intermediate the diaphragm and the mold to conform with the mold. Because substantial pressures, sometimes exceeding 5,000 psi, may be present, leakage of the hydraulic fluid about the edges of the diaphragm and onto to mold or sheet material to be formed may occur. In hydroforming operations involving a subsequent step in fabricating a product, cleaning is the norm and leakage of hydraulic fluid onto the product molded is of minor concern. When the product molded is to be a finished product, contamination of the product by leaking hydraulic fluid creates a significant problem of requiring an additional cleaning step and possibly compromising the product. To avoid the use of hydraulic fluid and the possibility of contamination of a molded product resulting from leakage of the hydraulic fluid, a hydroforming process known as a “blow down” process has been developed. In this process, the platen includes a cavity conforming in size and configuration with the mold and the sheet material to be molded. After the platen is placed against the table surrounding the mold to close the cavity, a source of high pressure air or gas is channeled into the cavity to create a high pressure environment within the cavity. The high pressure forces the sheet material to be formed against the mold and into conformance with the mold. 
     The pressure in the cavity of the platen may be maintained by a tight metal to metal fit between the surface of the platen circumscribing the cavity with the sheet material extending thereunder or with the underlying table. A seal of this type is a relatively poor seal and leakage of the air or gas from within the cavity is the norm. To enhance the seal around the cavity in the platen, an O-ring disposed in a groove in the platen has been used. Such an O-ring provides a better seal but the O-ring may easily become damaged due to creep upon application of pressure within the cavity. The integrity of the O-ring may also be compromised by wear upon the O-ring due to lateral movement of the platen relative to the underlying table or sheet material. While an O-ring is relatively inexpensive and easily replaceable, the lost downtime during replacement may be expensive because of the reduced production rate. As the sealing capability of the O-ring deteriorates, the pressures present within the cavity necessary to perform the hydroforming function may be inadequate to form properly the products. The resulting loss of products or shipment of defectively formed products may be costly in the short term and result in loss of business opportunities in the long term. 
     SUMMARY OF THE INVENTION 
     A platen for use in a blow down hydroforming process includes a cavity sized commensurate with the length and breadth of the sheet material to be formed in conformance with an underlying mold. A wall of low wear plastic material circumscribes the cavity to form an initial seal with the underlying table and about the cavity. An inflatable seal member is disposed in circumscribing relationship with the wall to form a pressure seal between the platen and the underlying table. A further wall of plastic material circumscribes the inflatable seal member. While the two walls provide a sealing function against escape of gas from within the cavity of the platen, they serve the primary function of preventing creep of the inflatable seal member and thereby prevent associated deterioration and destruction of the inflatable seal member. 
     It is therefore a primary object of the present invention to provide a pressurizable seal about a blow down cavity in a platen used in a hydroforming process. 
     Another object of the present invention is to provide a long lived inflatable seal member for use with a blow down platen of a hydroforming process. 
     Yet another object of the present invention is to provide sealing walls for preventing creep of an inflatable seal member. 
     Still another object of the present invention is to provide a blow down cavity in a platen used in a hydroforming process which accommodates use of various gases at temperatures other than ambient temperature. 
     A further object of the present invention is to provide a sealable blow down cavity for using gas pressure to form plastic sheet material in conformance with a mold. 
     A yet further object of the present invention is to provide a platen for using gases under pressure at elevated temperatures in a blow down hydroforming process to form thermoplastic sheet materials. 
     A still further object of the present invention is to provide a method for sealing the cavity in a platen used in a blow down hydroforming process. 
     These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described with greater specificity and clarity with reference to the following drawings, in which: 
     FIG. 1 illustrates a perspective view of the cavity and seals formed in the undersurface of a platen; 
     FIG. 2 is a partial cross sectional view taken along lines  2 — 2 , as shown in FIG. 1; 
     FIG. 3 is a partial cross sectional view illustrating placement of a platen upon table supported sheet material to be formed; and 
     FIG. 4 is a partial cross sectional view illustrating a product formed in response to pressure within a blow down cavity of a platen used in a hydroforming process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is illustrated a platen  10  having a centrally located cavity  12 ; this cavity includes an inlet  14  disposed in top surface  16 . The inlet is in fluid communication with a source of air or gas, such as nitrogen, under pressure. The pressure source is capable of providing a pressure of approximately 4000 psi; however, depending upon the nature of the material to be formed and the configuration of the mold for forming the material, the pressure may be less than 4000 psi or it may be significantly greater, such as 8000 psi or more. Cavity  12  is defined by top surface  16  and a surrounding wall  18  depending from the top surface. A wall member  20  of low wear plastic material, such as the material sold under the trademark DELRON, is located laterally adjacent wall  18 . A further wall member  22  is laterally displaced from wall member  20  to define a space therebetween. Wall member  22  is also of low wear plastic material, such as that sold under the trademark DELRON. An inflatable seal member  24  is disposed in the space intermediate wall members  20  and  22 . This inflatable seal member serves as a sealing element or seal in cooperation with the work table to maintain the pressure within cavity  12  during exercise of a hydroforming process. An exterior wall  26  defines the perimeter of platen  10 . Wall member  20  preferably extends somewhat below bottom surface  28  of platen  10 . With such positioning, the wall member will serve a sealing function upon contact with the underlying table or sheet material to be formed, which sheet material may be adjacent the table. Similarly, wall member  22  extends somewhat below bottom surface  28  for the same reasons. 
     Referring to FIG. 2, further details of platen  10  will be described. A source  30  of gas, such as air, nitrogen, etc. provides gas under pressure to inlet  14 , as representatively illustrated by arrow  32 . A heater  34  may be employed to heat the gas flowing from source  30  to inlet  14 , as representatively illustrated by line  36 . Alternatively, the heater may heat the gas prior to pressurization of the gas. A valve  38  regulates the flow of gas under pressure into and out of cavity  12 . 
     Inflatable seal member  24  is disposed intermediate wall members  20  and  22 . It includes a compartment  40  extending therealong for receiving hydraulic fluid under pressure. The hydraulic fluid is provided by a source  42  of hydraulic fluid under pressure. A valve  44  controls flow of hydraulic fluid through conduit  46  into a further conduit  48  disposed in platen  10 , as representatively depicted by arrow  50 . Conduit  48  is in fluid communication with compartment  40  through a slot, a plurality of slots or one or more passageways  52  in fluid communication with the cavity. Valve  44 , or an equivalent flow regulating mechanism, may be used to provide pressure relief for compartment  40  to depressurize the cavity. 
     Inflatable seal member  24  serves the purpose of forming a seal about the perimeter of cavity  12  in platen  10  to maintain the pressure within the cavity during the molding process. The inflatable seal member is of resilient flexible material to accommodate stretchability and elongation without damage to provide conformance with the surface against which a seal is to be made. Such surface may be the underlying table or the sheet material to be molded which lies upon such table. The force necessary for urging the inflatable seal member into sealing contact with the underlying surface is provided by source  42  of hydraulic fluid under pressure. The pressure within compartment  40  may range from a few hundred psi to more than 10,000 psi. 
     At the higher range of pressures, there is a great likelihood of oil seepage from compartment  40  between side walls  60  and  62  of inflatable seal member  24  and wall members  20  and  22  and on to the underlying table or the sheet material to be molded. Such seepage, if present, would contaminate the molded or formed sheet material and require subsequent cleaning. Since the cost for forming each sheet material is relatively modest, the additional costs associated with subsequent cleaning would create an unacceptable manufacturing cost increase. Furthermore, certain types of sheet material and products formed therefrom are not meant to be cleaned or they must not be contaminated due to an immediate subsequent manufacturing operation. To seal the inner and outer perimeters of inflatable seal member  24  and to prevent migration of the working surface of the inflatable seal member, wall members  20  and  22  are disposed on opposed sides. The material of the wall members is of harder and less compressible material than that of the inflatable seal member. Interface  64  between wall member  20  and the inflatable seal member and interface  66  between wall member  22  and the inflatable seal member is mutually conforming to eliminate any voids or passageways therethrough. Such effective seal may be formed by casting the inflatable seal member in the channel defined by wall members  20  and  22 . Compartment  40  is formed within the inflatable seal member by a lost wax process, as is well known in the hydroforming art. 
     The pressure within compartment  40  and acting laterally against wall members  20  and  22  forces the material of the inflatable seal member into absolute conformance with the corresponding surfaces of the wall members, as defined by interfaces  64  and  66 . Thereby, any seepage of oil or other hydraulic fluid intermediate inflatable seal member  24  and wall members  20  and  22  is precluded. 
     To minimize flow of a hydraulic fluid intermediate surface  68  of platen  10  and the inflatable sealing member, feet  70  and  72  extend inwardly from and in inscribing relationship with the interior perimeter of the inflatable seal member. The pressure within compartment  40  will bear against the interiorly exposed surfaces of feet  70  and  72  to urge the feet against surface  68  of platen  10 . Any perturbations or discontinuities in surface  68  will be filled by the material forming feet  70 ,  72  during the pouring operation of the inflatable seal member; thereby, the existence of any voids or channels at the junction between feet  70  and  72  and surface  68  are minimized. As a further precaution, an adhesive, mastic or chemical surface preparation, may be employed to secure feet  70  and  72  to surface  68 . 
     Referring to FIG. 3, there is shown the configuration of the apparatus in preparation for performing a blow down hydroforming process with platen  10 . Table  80  supports a mold  82 . This mold may be sized in general conformance with the perimeter of cavity  12  in platen  10 . Alternatively, as illustrated, a planar surface of the mold may extend laterally past wall member  22 . Sheet material  84  to be formed in conformance with mold  82  is placed upon the mold in registration therewith. As is usual, pins extending from the mold may cooperate with precisely located apertures in the sheet material to obtain registration between the sheet material and the mold. Other registration devices may also be employed. After mounting mold  82  upon table  80 , sheet material  84  is placed upon the mold. Platen  10  and table  80  are brought together in the conventional manner used in hydroforming processes, as represented by arrow  86 . As discussed above, wall members  20  and  22  extend somewhat below bottom surface  28  of platen  10  to urge these wall members into initial sealing engagement with the table directly or with sheet material  84  placed upon mold  82 , as illustrated. Such contact under pressure will establish a preliminary seal. The primary purpose of the resulting concentric seals disposed on opposed sides of inflatable seal member  24  is that of constraining lateral creep of the inflatable seal member during pressurization and depressurization of the inflatable seal member. Thereby, deterioration of the inflatable seal member which would result were creep present will be prevented. While wall members  20  and  22  provide a sealing function to assist in maintaining the pressure within cavity  12 , this benefit is primarily a secondary benefit. 
     After wall members  20  and  22  are brought into sealed engagement with the underlying surface by bringing platen  10  and table  80  together, valve  44  (see FIG. 2) is opened to permit flow of hydraulic fluid from source  40  of hydraulic fluid under pressure into compartment  40 , as representatively depicted by arrow  50  and arrows  90  and  92 . The resulting pressure rise within compartment  40  will cause downward movement of diaphragm portion  94  of the inflatable seal member into tight sealing engagement with the adjacent surface; this surface is sheet material  84 , as depicted in FIG. 3 but may be table  80 . The resulting seal will prevent leakage from within cavity  12  intermediate platen  10  and table  80 . As discussed above, the sealing engagement of wall members.  20  and  22  will prevent lateral creep of the material of inflatable seal member  24  and deterioration due to creep will be avoided. 
     After a perimeter seal about cavity  12  has been established by inflatable seal member  24 , cavity  12  is pressurized by actuating valve  38  depicted in FIG.  2 . Gas from source of gas  30  is conveyed, as depicted by arrow  32 , through inlet  14  into cavity  12 . The resulting pressure rise within cavity  12  will act upon sheet material  84  to force it downwardly into conformance with the configuration of mold  82  disposed adjacent therewith. 
     Depending upon a number of variables, including the thickness of sheet material  84 , its thermoplastic properties and the nature of the protrusions and indentations in mold  82 , conformance of the sheet material with the mold may be enhanced by raising above ambient the temperature of the sheet material. Through use of heater  34  (see FIG.  2 ), the gas injected through inlet  14  may be heated to a predetermined temperature or to a temperature within a predetermined range. The selection of such temperature or temperature range is primarily a function of the properties of the sheet material to be formed to enhance uniform conformation with the mold. The heated gas within cavity  12  will transfer heat to sheet material  84  and the latter will become more compliant with the underlying mold. 
     Theoretical analysis, supported by experience and certain experimentation, suggests that a pressure within cavity  12  on the order of 4000 psi will be adequate to force conformation of the sheet material to be molded with an underlying mold within a relatively short time interval. However, with the addition of heat to render the sheet material more malleable or formable, the pressure may be reduced. For certain sheet materials and/or mold configurations, higher pressures, with or without heating of the gas, may be entertained. To ensure an adequate and sufficiently leak free seal circumscribing cavity  12  between platen  10  and the underlying surface (table  80 ), it has been found that the pressure within inflatable seal member  24  should be approximately 25% greater than the pressure present within cavity  12  of platen  10 . 
     Hydraulic pumps and the like are presently available which can readily provide a pressure within the inflatable seal member on the order of 10,000 psi. Accordingly, the pressure within cavity  12  could be raised to 8,000 psi while still maintaining a 25% pressure differential present at the inflatable seal member. 
     After sheet material  84  has been formed, valve  38 , or other valve means, is actuated to relive the pressure within cavity  12  by either venting the gas if it is air, or by channeling the gas into a suitable tank. Alternatively, a pump may be employed to withdraw the gas within cavity  12  to reduce the pressure therein to approximately atmospheric pressure. Commensurate with or subsequent to reduction of pressure within cavity  12 , the pressure within inflatable seal member  24  is relieved by venting it to a tank or by pumping it out with a suitable pump. After the pressures within cavity  12  and inflatable seal member  24  have been relieved or brought to approximately atmospheric pressure, platen  10  and table  80  are separated to permit withdrawal of formed sheet material  84 . After withdrawal, the sheet material is replaced with further sheet material to be formed. The replaced sheet material is acted upon as depicted in FIGS. 2 and 3 and as described above to repeat the cycle of molding the sheet material into conformance with the underlying mold. 
     While the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications of structure, arrangement, proportions, elements, materials and components used in the practice of the invention which are particularly adapted for specific environments and operating requirements without departing from those principles.