Patent Publication Number: US-2011074120-A1

Title: Rotary centrifuge seal with a phenolic overmold component and method of manufacture

Description:
CROSS REFERENCE 
     This application claims the benefit of U.S. Provisional Application No. 61/277,738, filed Sep. 28, 2009, and entitled ROTARY CENTRIFUGE SEAL WITH A PHENOLIC OVERMOLD COMPONENT AND METHOD OF MANUFACTURE, the content of which is incorporated herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to seals for a rotary centrifuge bowl used for blood processing such as illustrated in U.S. Pat. Nos. 4,300,717; 5,045,048 and 5,882,289. 
     The seal incorporates a static seal that is formed of a ring-shaped diaphragm of elastomeric material which is joined at its radially inward periphery to the outer periphery of a non-rotatable ring member which rests on top of a lower rotatable ceramic ring. The elastomer member is formed of silicone rubber and the non-rotatable ring member is normally molded of a phenolic containing graphite or Teflon™. 
     The silicone rubber ring-shaped diaphragm must be mechanically secured to the phenolic ring member. This requires the use of time consuming and expensive manual labor for assembly, and a common result is that the elastomeric diaphragm does not properly seat on the phenolic ring which causes the seal to leak. Failure of this seal connection can result in contamination of the blood being processed. 
     The carbon phenolic compound is generally a mineral and graphite filled phenolic molding compound developed for seal applications. It consists of a condensation resin of phenol and formaldehyde, with enough crosslinker to cure any novolac presence, where the crosslinker is a phenolic resol or especially where the crosslinker is hexanethylenetetramine, and from 10% to 70% carbon filler consisting of graphite or amorphous carbon or any mixture of the two, and from 0% to 50% other additives and fillers commonly used in industry, such as waxes, pigments, and minerals, including, but not limited to, clay, talc, hydrated alumina, and calcium carbonates, and organic fibers, including, but not limited to, wood, cotton, and processed cellulose, and mineral fibers, including, but not limited to, glass, processed mineral fiber, and ceramic fiber. The premolded carbon phenolic member is compression molded. 
     The thermoset carbon phenolic materials are considered to be inert in that once molded under the conditions of high compression and high heat, it cannot be ground and remolded. They are not affected by heat or cold and they are highly resistant to petrochemicals, acids, bases, and ultraviolet light. 
     The premolded silicone membrane is injection molded and is slippery, smooth and nonbonding and considered to be inert in that once molded it cannot be ground and remolded. The nonbonding factor is considered to be a favorable attribute in many applications. Previous to the present invention, in order to achieve a bond with a nonbonding injection moldable silicone, a bonding agent had to be added to the silicone, therefore increasing the cost of the material. Also, in order to bond one part to another part produced from nonbonding injection moldable silicone to a substrate, a secondary adhesive, primer or mechanical bond (inter-engaging protrusions) has to be incorporated into the process. In the current seals, the molded silicone membrane is attached by hand assembly to the compression molded carbon phenolic using the compressive, elastic properties of the silicone rubber to provide a mechanical seal. As previously explained, a secondary process is required to impose the molded silicone rubber or molded silicone part onto the carbon phenolic. 
     SUMMARY OF THE INVENTION 
     The present inventors have discovered an effective method of overmolding the silicone rubber membrane securely onto the carbon phenolic ring utilizing injection compression molding techniques. 
     The industry has and is of the opinion that the silicone rubber diaphragm and phenolic ring member must be mechanically joined as both components, the silicone rubber and the phenolic, are thermoset resins and have only been adequately joined with the use of primers, adhesive, bonding agents or mechanical means, as overmolding of the carbon phenolic has been perceived to be not possible due to the nature of the thermoset resins. Silicone by its very nature is very slippery and elements do not adhere to it. However, contrary to the beliefs in the industry, the present inventors have discovered and invented a method for overmolding the silicone rubber membrane on to the carbon or graphic phenolic ring member with a strong static bond that provides a seal which prevents the migration of liquids and the passage of air between the molded phenolic and the silicone, even though the two materials are dissimilar thermoset resins. 
     The design of the premolded phenolic features are critical, due to the brittle nature of the material. The phenolic part or element must withstand the clamp tonnage of the mold press and the mold without breaking, and also seal tightly to the mold cavity to prevent the silicone leaking around the premolded phenolic insert under injection pressures. The carbon phenolic premold is thus inserted into the injection compression mold with a precise fit, the mold is then partially closed and the silicone is then injected by runnerless injection techniques directly into the mold cavity, and the mold cavity is then completely closed to provide compression overmolding of silicone to the phenolic part with the application of heat (typically 450° F., more or less, which is a typical heat range utilized when overmolding products). 
     The strength of the bond between the carbon phenolic and the nonbinding injection molded silicone is such that a sharp implement or enough force to tear the silicone is required to break the bond. No hand assembly or secondary process such as application of primers, adhesives, bonding agents or mechanical bonding is required to apply the silicone to the phenolic part, thus providing unexpected results which solve or fill a long felt need in the industry, whereby overmolding silicone to graphite phenolic is provided while maintaining a molecular bonding of the two materials without using adhesives or primers. As previously mentioned, the design, form and mold dimensions of the phenolic part and a precise fit in the mold of the phenolic part are critical to the process as the premolded or preformed phenolic part is fragile and will otherwise break in the molding process. 
     The silicone features are designed to allow stripping the overmolded phenolic and silicone overmolded parts from the steel core of the mold without damage, while maintaining the critical seals in the molding operation. 
     The phenolic compound element is shaped in the form of a ring having external and internal peripheral surfaces, whereby during the injection compression molding process, the silicone rubber membrane is bonded to both the internal and external peripheral surfaces of the phenolic element for more secure bonding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages appear hereinafter in the following description and appended claims. The accompanying drawings show, for the purpose of exemplification, without limiting the scope of the invention or the appended claims, certain practical embodiments of the present invention wherein: 
         FIG. 1  is a top perspective view of the rotary centrifuge seal of the present invention; 
         FIG. 2  is a top view of the seal shown in  FIG. 1 ; 
         FIG. 3  is a view in front elevation of the seal shown in  FIG. 1 ; 
         FIG. 4  is a bottom perspective view of the seal shown in  FIG. 1 ; and 
         FIG. 5  is a view in vertical mid cross section of the seal of the present invention as seen along section line V-V of  FIG. 2  as received within a mold for illustrating the injection compression molding method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring to the drawings, the rotary centrifuge seal  10  of the present invention is comprised of a preformed annular phenolic compound element  11  which is molecularly bonded to a pliable silicone rubber membrane  12  by injection compression molding as illustrated in  FIG. 5  without the use of primers, adhesives, bonding agents or mechanical bonding. 
     The preform phenolic compound element  11  is typically a carbon phenolic compound, generally a mineral and graphite filled phenolic molding compound developed for seal applications. It consists of a condensation resin of phenol and formaldehyde, with enough crosslinker to cure any novolac present, where the crosslinker is a phenolic resol or especially where the crosslinker is hexamethylenetetramine, from 10% to 70% carbon filler consisting of graphite or amorphous carbon or any mixture of the two, and from 0% to 50% other additives and fillers commonly used in the industry, such as waxes, pigments and minerals, including, but not limited to, clay, talc, hydrated alumina, and calcium carbonate, and organic fibers, including, but not limited to, wood, cotton and processed cellulose, and mineral fibers, including, but not limited to, glass, processed mineral fiber, and ceramic fiber. This premolded carbon phenolic member is compression molded. 
     As is partially illustrated in  FIG. 5 , the phenolic element  11  is inserted into injection compression mold  13 , including base mold member  14  and upper mold member  15 , with a precise fit, which is necessary as the phenolic part is extremely brittle in nature and must withstand the clamp tonnage of the mold press and the mold  13  itself without breaking and also seal tightly to the mold cavity to prevent the silicone leaking around the premolded phenolic element  11  under injection pressure. Once phenolic element  11  has been inserted into the mold bottom  14  with precise fit, the mold is then partially closed and the silicone  12  is injected through injection ports  16  by runnerless injection techniques directly into the mold cavity  17 , and the mold cavity is then completely closed as illustrated in  FIG. 5  to provide compression overmolding of the silicone  12  to the phenolic element  11  with the application of heat. The applied heat is typically in the area of 450° F., more or less, as is generally practiced in the art of injection compression molding. 
     In order to provide a more secure bond between silicone  12  and the phenolic element  11 , the phenolic compound element is shaped in the form of a ring having external and internal peripheral surfaces  18  and  19  respectively, whereby the silicone rubber membrane  12  is bonded to both of these internal and external peripheral surfaces  18  and  19 .