Abstract:
A novel device and method for the injection and molding of a body of elastomeric composition within a mold cavity, around a core member and against the prepared surface of a test substrate, such as a ceramic or metallic disk, at the base of the mold cavity. The molded elastomeric composition is molded as an axial-symmetrical frustro-conical body which tapers down to a uniformly thin layer between the test substrate surface and the flat undersurface of the core member, and the core member supporting the molded elastomeric body bonded to the test substrate are separable from the mold cavity for attachment to a conventional tension test machine for measuring the interfacial bond strength between the elastomer and the prepared surface of the test substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a novel molding method and device for use in measuring the bonding strength of elastomeric compositions to various prepared surfaces. 
     The advent of complex and high density electronic systems for ground, airborne and space applications has required the parallel development of packaging technology including the use of elastomers to protect electronic system from vibration, shock, temperature extremes, and as a dielectric medium to prevent voltage breakdown between the internal components or to the containment vessel walls. Meeting these requirements depends upon maintaining a viable bond between the elastomer and substrates, components and containment walls. This bond strength depends on the elastomer and the physical and/or chemical preparation of the bonding surface. Therefore accurate quantification of this bond strength is critical to the development of these new systems to withstand the dynamic forces and thermal contraction which place significant tension stresses on the interface between the elastomer and the surface to which it is bonded. 
     2. State of the Art 
     Previously, lap shear tests have been relied upon to assess the bond strength of elastomers with various surfaces. However, the physics of the interface dictate that the relationship between the lap shear test results and the ability to withstand tensile forces at the surface is in itself a function of the surface/elastomer bond, resulting in the need to measure the tensile bond strength directly. Classical methods which measure the tensile bond strength successfully are accurate only when the bonding material is fairly rigid and low strains are induced during the measurement. In this case, most of the loading is uniaxial, with the average and local internal stress in the material very close, and the bond fails at substantially below the tensile limit of most rigid test materials. However, in the case of elastomers, if there is a significant thickness of material between the elastomer support structure and the test sample, at typical bond strengths obtainable, significant triaxial strains and stresses occur in the elastomer due to the stretching or necking down effects which cause rupture failures in the elastomer prior to reaching critical stresses at the interface. Thus valid results are not obtainable. With this background and in light of the limitations of the prior art, the present invention was conceived. 
     A variety of different devices are known for determining the bonding strength of various adhesives, coatings and paints for various surfaces by compressing a coated substrate and then measuring the bonding strength as the ultimate separation force that the bond can resist after the adhesive is set or cured. Reference is made to the following U.S. Pat. Nos. for their disclosure of such devices: 3,628,378; 3,821,892; 4,567,758; 4,586,371; 4,606,225; 4,893,513, 5,313,841; 5,361,639; and 5,649,447. 
     None of the aforementioned patents deal with or solve the problem of the test material developing significant triaxial stress risers, or necking down, under the effects of the tension forces generated during separation, and therefore the devices of these patents are unsatisfactory for the accurate measurement of the bonding strength of non-rigid, rubbery elastomers to prepared surfaces. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a novel self-aligning device and method for the injection molding and curing of a body of elastomeric composition within a mold cavity, around a core member having a flat undersurface, and against the prepared flat parallel surface of a test substrate, such as a ceramic disk, at the base of the mold cavity. A predetermined narrow space confines the elastomeric composition as a uniformly thin layer between the flat substrate surface and the parallel flat undersurface of the core member, and the core member supporting the molded, cured elastomeric body bonded to the substrate are separable from the apparatus for attachment to a conventional tension test machine for measuring the interfacial bond strength between the elastomer and the prepared surface of the test substrate. 
     The core member is a custom bolt, and the head of a standard bolt is then bonded to the undersurface of the test substrate by means of a strong adhesive to provide an axial assembly for attachment to the tension test machine. The tension test machine pulls the core/custom bolt and the standard bolt in opposite directions and provides a quantitative measurement of the force required to separate the elastomeric body from the prepared surface of the support, or ceramic disk, at the interface thereof, while the unique shape of the mass of the molded elastomeric body substantially reduces or eliminates triaxial strains and stresses and necking down or stretching of the thin elastomeric layer molded and cured against the surface of the test substrate, thereby preventing any internal rupture of the thin elastomer layer prior to separation at its interface with the test substrate. 
    
    
     THE DRAWINGS 
     FIG. 1 is a plan view of an apparatus according to one embodiment of the present invention; 
     FIG. 2 is a cross-section taken along the line  2 — 2  of FIG. 1; 
     FIG. 3 is a cross-section taken along the line  3 — 3  of FIG. 1, but illustrating the mold cavity filled with elastomeric composition in contact with the prepared surface of a test substrate; 
     FIG. 4 is a cross-section taken along the line  4 — 4  of FIG. 1; 
     FIG. 5 is a perspective view of the apparatus of FIG. 1 with the elements thereof shown in spaced relation for purposes of illustration, and 
     FIG. 6 is a side view of the core member/custom bolt with molded elastomeric body and test substrate bonded in axial relation to the head of a standard bolt, for attachment to a standard tension test machine. 
    
    
     DETAILED DESCRIPTION 
     The present apparatus includes a multi-piece mold assembly  10  comprising a top section  11 , a middle section  12 , a bottom section  13  and a custom bolt/core member  14  having a threaded upper end which is engaged by the threaded bore  15  of the top section  11  to the limit of an annular stop flange  21  to align and position the core member  14 . Sections  11  and  12  have central tubular openings to form a molding cavity  22  between their interior walls and the surface of the lower cylindrical core section  23 , including its narrowed neck portion  24  and its head portion  26  having a flat undersurface  25 . The upper surface of the bottom section  13  has a central recess or bore  27  for receiving a flat test substrate, such as a ceramic disk  28 , to be bonded to an elastomer molded within the cavity  22 . Disk  28  has parallel flat upper and lower surfaces. 
     The assembly  10  includes an opposed pair of alignment pins  19  which fit within alignment bores  20  in the sections  11 ,  12  and  13 , as shown by FIGS. 2 and 5, to facilitate the alignment of the bolt holes  16  for the reception of locking bolts (not shown) to secure the sections together during molding of the elastomeric composition injected through longitudinal filling ports  17  to fill the mold cavity  22  after the test disk  28  has been positioned within the bore  27 , as shown in FIG.  3 . 
     FIG. 3 illustrates the assembly  10  after curable liquid elastomeric composition  30  has been injected through the fill ports  17  to fill the mold cavity  22  and form a uniformly thin layer  29  between the flat undersurface  26  of the head  25  of the core member  23  and the parallel flat upper surface of the disk  28  which fills the bore  27  in the bottom section  13  of the mold. 
     As shown by FIGS. 4 and 5, the top section  11  of the mold assembly is provided with overflow ports  31  (internal) and overflow channels  32  (external) which control the level of the uncured liquid elastomeric composition admitted to the mold cavity  22  through the filling ports  17 , shown in FIG.  3 . As illustrated by FIG. 5, the custom bolt  14  threadably engages the bolt hole  15  within the top section  11  of the assembly and is screwed in until the annular stop flange  21  engages the undersurface of  11  adjacent the entry of the hole  15 , as shown in FIGS. 2,  3  and  4 , to align and fix the position of the assembled core member. 
     Next, a test substrate, such as a ceramic or other test disk  28  having a prepared flat test surface is inserted into the central recess or bore  27  in the upper surface of the bottom section  13 . Then the top, middle and bottom sections  11 ,  12  and  13 , respectively, are assembled by aligning the pins  19  of the middle section with the alignment openings  20  in the underside of the top section  11  and in the upperside of the bottom section  13  and pressing the sections together. Bolts (not shown) are inserted through the openings  16  in the three sections to fasten the sections together as an assembly. The assembly is filled with uncured liquid elastomer which flows into the mold cavity  22  to encase the core section  23 , neck section  24  and head portion  25  of the custom bolt  14  and form a thin uniform layer  29  between the flat undersurface  26  of the head portion  25  and the flat prepared upper surface of the test disk  28  contained within the central bore  27  of the bottom section  13 . 
     Preferably the surface of sections  11  and  12  forming the outer walls of the mold cavity  22  are previously sprayed with a mold release agent in order to facilitate the separation of the cured axial-symmetric elastomer body  30  from the walls of the mold cavity  22 . Curing is generally accomplished by heating the filled assembly to crosslink the elastomer and produce a solid rubbery mass which is strongly bonded to the core portion  23  of the custom bolt  14  and form a thin uniform layer  29  between the flat undersurface 26  of the head portion  25  and the flat prepared upper surface of the test disk  28  contained within the central bore  27  of the bottom section  13 . 
     Preferably the outer openings of the overflow ports  31  are sealed with plugs sprayed with mold release agent after the mold cavity  22  is filled and prior to the curing step to prevent leakage of the liquid elastomer. In many cases, the elastomer&#39;s un-cured mixture is very thick and viscous requiring significant flow channel width to reach and fill the cavity next to the test sample. This is the case for silicone rubbers, where passages on the order 0.030 inches are needed to achieve complete fill of the un-cured material. 
     After curing, the assembly  10  is disassembled by removing the fastening bolts from holes  16 , separating the sections  11 ,  12  and  13 , and unscrewing the custom bolt  14  carrying the cured solidified elastomer body  30  and bonded test disk  28 , as illustrated by FIG.  6 . 
     The flat side opposite the parallel prepared surf-ace of the disk  28  is then attached with strong adhesive to the flat head of a standard bolt  33  oriented in the opposing direction of the threaded custom bolt  14  as shown in FIG.  6 . This axial assembly is then placed in a conventional tension loading test machine to determine the interface bond strength of the cured elastomer with the test surface. The unique and novel shape of the potted portion of the assembly formed by the present invention allows application of tensile stresses close to the value of the elastomer itself by preventing formation of significant internal tri-axial risers in the elastomer as the tension forces are transferred through the test structure. 
     The cured elastomer body  30  on the core section  23  has an axial-symmetric frustro-conical shape and a frustrum  35  which tapers down to the lower outer circumferential edge of the cured elastomer layer  29 , as illustrated by FIG.  6 . This unique shape produces a radial outward stress component which effectively cancels inward necking and peel stresses under tensile loading and allows application of pure tensile stresses and measurement of the tensile bond strength at the interface of the layer  29  and the surface of the test disk  28 . 
     A convenient test sample consists of a round disk  28  of the test material, usually composed of ceramic or metal for electronic applications, which are on the order of 0.250 inches to 0.500 inches in diameter. With a minimum elastomer layer  29  in the range of 0.030 inches thick cured and bonded to one side of the test sample, an aspect ratio of sample diameter to elastomer thickness of 8.3 to 16.6 would be present. This would correspond to sample diameter to thickness of the bonded elastomer ranging from eight to sixteen to one. 
     If the elastomer body consisted simply of a cylinder of elastomer rubber 0.030 inches thick cast between the test sample with the prepared surface and an opposing bond surface, at typical bond strengths achievable of 50 to 130 percent of the elastomers modulus of elasticity, axial strains of 50 to 130 percent would also be reached. This would produce severe necking down of the material in the radial direction, causing significant peeling stresses, giving erroneous results. 
     However, according to the present invention, the unique shape of the annular mold cavity  22 , angled up and outward away from the top of the outer circumferential edge of the cured elastomer maintains a radial outward stress component which effectively cancels inward necking and peel stresses and allows application of pure tensile stresses and accurate measurement of the tensile bond strength at the interface. 
     Thus, the present invention consists of a uniquely shaped three piece mold apparatus, with the test sample disk inserted in a holding and positioning cavity located in the top surface of the lower section. The center section of the mold includes an axial-symmetric annular cavity which reduces in diameter in its lower part and expands to a larger diameter in its upper part. The center section cavity receives a custom bolt which is threaded into the top section of the three part mold, and positioning pins align the top and bottom mold sections to the center mold section allowing accurate positioning of the center core body of the custom bolt in the axial-symmetric cavity, with the tread allowing accurate vertical positioning and alignment of the center core. The elastomer is introduced through top fill holes, and all surfaces that will contact the elastomer are previously sprayed with mold release agent. After cure of the elastomer, the lower section  13  and center section  12  are removed and the center core section  23  having the molded elastomer body  30  attached to the test sample disk  28  is unscrewed from the upper mold section  11  of the assembly  10 . The opposite flat surface of the test sample disk  28  is then attached with strong adhesives to the head of a standard bolt  33  pointing opposite the direction of the custom bolt  14  and the unit placed in a conventional tension test machine for testing. The unique and novel frustro-conical shape of the potted portion or elastomer body  30  formed according to the invention allows application of the bonding tensile stresses close to the value of the elastomer itself by preventing formation of significant tri-axial stress risers in the thin elastomer layer  29  as the tension forces are transferred through the test structure. 
     The mold sections  11 ,  12  and  13  are formed from aluminum or other suitable mold material, and the test sample disk  28  is formed from ceramic or other material, the bonded surface of which is physically and/or chemically prepared in known manner to provide for high bond strength with the thin layer  29  of the elastomer, which is molded and cured as an axial-symmetrical mass  30 . 
     The custom bolt  14  and the standard bolt  33  are formed from hardened steel or other suitable metal. The custom bolt  14  has its core section  23  contoured with a narrowed neck section  24 , to provide a strong holding power for the cured molded elastomer mass  30 , and with a wider lower head section  25  having a flat undersurface  26 , parallel to the flat prepared upper surface of the test sample disk  28 , to provide a uniform narrow gap therebetween into which the liquid uncured elastomeric composition flows and cures to form a thin elastomer layer  29  of the mass  30  which is strongly bonded to both the core head  25  and the disk  28 . This configuration prevents the formation of significant triaxial stress risers in the elastomer layer  29  as the test machine applies tension forces in opposite directions until the elastomer layer is clearly separated from the prepared surface of the test sample disk  28  at a force measured by the tension test machine. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.