Abstract:
A drumhead mold and process for making a plastic drumhead. The mold is structured to heat the drumhead collar area and maintain the battering surface cool to provide a drumhead with improved performance characteristics.

Description:
FIELD OF THE INVENTION 
     The present invention relates to percussion musical instruments, and in particular drums. More specifically, the invention relates to a process for making a drum head membrane and the drum head membrane resulting from the process. 
     BACKGROUND OF THE INVENTION 
     A variety of musical instrument drums exist such as bass drums, tympani drums, kettle drums, snare drums, tom-toms, etc. Although the various drums differ from each other in application and sound all require a battering or striking surface secured over a hollow generally cylindrical structure. 
     The sound generated by the particular drum results from the fundamental frequency overtones and resonance all of which are, in part, a function of vibration decay or dampening time. 
     The vibration decay or dampening time is determined by the structural characteristics of the drum such as the size and weight of the drum, the nature of the battering surface and the tension imposed on the battering surface. 
     In general, the battering surface is made of animal skin or synthetic polymer sheets held in tension. Generally the tension is applied by means such as adjustable lugs or turnbuckles secured to peripheral collars integrally formed with the drum head membrane. 
     One common method for producing drum head membranes from polymer sheets is a thermoforming process in which sheets of a polymer such as polyester usually 0.003 to 0.010 inch thick are placed in a mold and are subjected to the simultaneous application of heat and pressure. The resultant membrane is a flat, essentially circular surface with an integrally formed depending periphery or collar. In the formation process, heat is applied to the mold to raise the temperature of the polymer to the range of 250° F. to facilitate deformation of the polymer to properly form the collar. 
     It has been found that the thermoforming process has detrimental effects on the drum head membrane. The heat causes the polyester to shrink and become brittle. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a process for the manufacture of polymer drumheads that avoids shrinkage and brittleness caused by existing processes. 
     It is a further object of the present invention to provide a process for the manufacture of drumheads and a resultant drumhead that is easier to tune, has a full fundamental tone and a wide range of upper partial harmonics. 
     To this end a new and improved mold and process is provided. The mold is comprised of a peripheral section in which heat is delivered in the range of 180° F.-230° F. and a central section in which a heat exchange system is located. Coolant, such as water, is delivered to the heat exchange system to maintain the temperature of the mold central surface exposed to the polymer in the range of 90° F.-130° F. 
     The process proceeds by inserting a polymer sheet between the mold sections, securing the mold sections together under pressure and applying heat to the mold. As heat is applied to the mold, coolant is circulated through the heat exchange system to maintain a relatively low temperature in the area of the membrane battering surface and at the same time deliver higher temperature heat to the peripheral collar area. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood when viewed with the following drawings wherein: 
     FIG. 1 is a side elevational view of the mold of the present invention shown in the open position; 
     FIG. 2 is a side elevational view of the mold of the present invention shown in the closed position; 
     FIG. 3 is a top plan view of the mold as shown in FIG. 1; 
     FIG. 4 is a sectional elevational view taken through line  4 — 4  of FIG. 3; 
     FIG. 5 is a sectional plan view taken through line  5 — 5  of FIG. 2; and 
     FIG. 6 is a perspective view of the assembled drumhead of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The subject invention has application in the manufacture of polymer percussion surfaces and will be described as applied to circular drum head battering surfaces on which a collar is formed to facilitate securement of the drumhead to a drum. 
     As seen in FIGS. 1 and 2 a mold  2  comprised of an upper male member or upper mold (cone)  4  and a lower female member or lower forming die (cup)  6  is provided to form a drum membrane shown in FIG. 6 having a battering surface  8  and a peripheral collar  10 . 
     As seen in FIG. 1, the male member or cone  4  is configured with an internal protruding circular surface  12  and an inclined continuous peripheral edge  14 . The peripheral edge  14  has an array of pins  13  formed thereon. The female member or cup  6  is provided with a centrally disposed dished recess  16 , best seen in FIG. 4, dimensioned essentially the same as the male protruding surface  12  and an inclined surface  15  configured to mate with the inclined peripheral edge  14  of the male mold member or cone  4 . The peripheral edge  15  of the female mold member or cup  6  is provided with an array of slots  17  aligned with the pins  13  formed on the peripheral edge  14  of the male mold member or cone  4 . The dished recess  16  is a flat plate  19  mounted on the rod  9  of the hydraulic piston (not shown). The flat plate  19  moves vertically while the inclined surface  15  is fixed. 
     The male mold member  4  is also provided with a heating element  11  seen in FIG. 4, and a cooling or heat exchange system shown as a serpentine passage  18  having a coolant inlet  20  and an outlet  22  formed in a cooling plate  21 . The heating element  11  is conventional and has long been used in processes for forming drumheads from polymers. In practice, the male mold member or cone  4  is mounted on a main mounting plate  5 . 
     Temperature sensors in the form of thermocouples  24  are strategically positioned around the periphery of the male mold member  4  to regulate the temperature of the mold in the area of the serpentine passage  18  serving to cool the central area of the mold  2 . In addition, a temperature sensor  25  in the form of a thermocouple is located in the male mold member  4  to insure a periphery temperature in the range of 180° F. to 230° F. 
     The female mold member  6  is provided with an annular heater  26  and sensors in the form of thermocouples  28 , best seen in FIG. 5, to facilitate temperature regulation of the mold  2  in the area of the mating inclined surfaces  14  and  15  that form the drumhead collar  10 . 
     In the process of the present invention, a circular piece of polymer  3  shown in FIG. 4, preferably MYLAR polyester sized to extend uniformly beyond the inclined peripheral surface  15  of the female mold member  6  is placed on the interior of the female mold member  6 . Thereafter, the male mold member  4  is brought into aligned engagement with the female mold member  6  to deform the polymer circular piece  3  to conform to the configuration of the mold  2 . The temperature in the collar area is maintained at 180° F.-230° F. during the molding process. The temperature of the female member  6  in the area of the collar is maintained in the range of 130°-200° F. During the molding process the temperature of the center section of the polymer circular sheet  3  is kept at 90° F.-130° F. by delivery of coolant to the cooling passage  18 . Practice shows that delivery of about one gallon per minute of water through the cooling passage  18  will maintain the proper temperature in the center of a polymer circular piece  3  having a diameter of fourteen inches. 
     The pressure imposed on the mold is external and in the range of 100 pounds of air pressure for a mold sized to produce drumheads for a fourteen inch drum. In the preferred embodiment, a hydraulic press  31  shown illustratively in FIG. 2, is used and cooling plate spacers  27  are provided to mount the cooling plate  23  on the male die member  4 . 
     The residence time of the circular polymer sheet in the mold  2  is from thirty to one hundred eighty seconds depending of the size of the drumhead to be formed. 
     During the molding portion of the cycle, the lower inner pressure plate  19  pushes against the fixed inner cooling plate with force in the range of 20-60 PSI. The plate  19  is driven by a pneumatic cylinder (not shown). Practice has shown that the positive application of force against the cooling plate by the pressure plate  19  maintains the central portion of the polyester film(s) that ultimately become the vibrating area or battering surface  8  of the drumhead in a condition wherein it is held perfectly flat and kept cool during the forming process. The result is that the battering surfaces  8  vibrate better than the prior art battering surfaces. 
     The polymer sheet removed from the mold  2  after the forming process of the present invention, as seen in FIG. 6, has a circular battering surface  8  and a depending collar  10 . The collar  10  is formed with dimples  7  formed by the pins  13  on the male mold member  4  and slots  17  on the female member  6  during the molding process. A conventional tensioning ring  30 , usually aluminum, is secured to the edge of the collar  10  by conventional adhesive such as solid resin  29  to form a completed drumhead  32 . 
     The drumhead  32  of present invention has shown markedly noticeable improved characteristics. In particular, the drumhead  32  provides ease of tuning over prior art polymer drumheads. Drumheads  32  manufactured with the process of the present invention can be tuned to a musical pitch that is lower than drumheads made with prior art processes. This is a very important feature for all drumheads and is particularly evident when heads are used on tom-toms, where it is common for musicians to try to tune these drums to a very low pitch, while maintaining a musical tone. 
     The molds and process of the present invention can be used to form drumheads for any size drum. However, the rate of coolant flow and residence times may vary with the size of the drumhead being formed and the size of the mold required to form a particular size drumhead. 
     The process of the present invention is provided to produce drumheads from a single piece of polymer or a plurality of polymer sheets secured together. One common embodiment of a drumhead is comprised of two identical polymer sheets secured together at the edge by adhesive.