Bonded sheet molding composite automotive components without surface pre-treatment

Abstract

An automotive assembly comprises a sheet molding composite (SMC) element comprising an SMC sheet body shaped into a generally smooth functional section and at least one generally planar bonding section. The bonding section comprises a textured surface impressed as a result of molding the SMC sheet body in a compression molding tool having a corresponding textured pattern. The textured surface has a predetermined surface roughness for increasing the effective surface area of the bonding section relative to a non-textured surface. The predetermined surface roughness is in a range from 0.25 R.sub.a (m) to 8.5 R.sub.a (m). A second element has a secondary bonding surface adjoining the bonding section of the SMC element. An adhesive joins the secondary bonding surface to the bonding section without requiring any pre-treatment of the bonding section.

Claims

1. A method of making an automotive assembly, comprising the steps of: preparing a compression mold having a smooth functional portion and a bonding pad portion, wherein the compression mold has a textured surface defining the bonding pad portion, wherein the textured surface has a textured pattern of generally parallel scratches to provide a predetermined surface roughness for increasing the effective surface area of the bonding pad portion relative to a non-textured surface, wherein the predetermined surface roughness is in a range from 0.35 R.sub.a (m) to 3.6 R.sub.a (m) in a direction parallel with the scratches and is in a range from 0.25 R.sub.a (m) to 5.64 R.sub.a (m) in a direction transverse with the scratches, and wherein the smooth functional section has a respective surface roughness less than 0.25 R.sub.a (m); inserting an SMC sheet body into the compression mold to shape the SMC sheet body into a generally smooth functional section within the functional portion of the compression mold and at least one generally planar bonding section within the bonding pad portion of the compression mold; removing the shaped SMC sheet body from the compression mold; aligning the bonding section with a second assembly element having a secondary bonding surface; and bonding the bonding section and the secondary bonding surface with an adhesive.

2. The method of claim 1 wherein the second assembly element is made by the method further comprising the steps of: preparing a second compression mold having a second functional portion and a second bonding pad portion, wherein the second compression mold has a second textured surface defining the second bonding pad portion, and wherein the second textured surface has a second textured pattern of generally parallel scratches to provide a second predetermined surface roughness for increasing the effective surface area of the second bonding pad portion relative to a non-textured surface, wherein the second predetermined surface roughness is in a range from 0.35 R.sub.a (m) to 3.6 R.sub.a (m) in a direction parallel with the scratches and is in a range from 0.25 R.sub.a (m) to 5.64 R.sub.a (m) in a direction transverse with the scratches, and wherein the second functional portion has a respective surface roughness less than 0.25 R.sub.a (m); inserting a second SMC sheet body into the second compression mold to shape the second SMC sheet body into a second generally smooth functional section within the second functional portion of the second compression mold and into the secondary bonding surface within the second bonding pad portion of the second compression mold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of an SMC body.

(2) FIGS. 2A and 2B are cross sections of an SMC body in a compression mold before and after compression, respectively.

(3) FIGS. 3A and 3B are end views of a structural assembly wherein at least one element of the assembly is formed from an SMC body and has a bonding section with a textured surface according to the present invention.

(4) FIG. 4 is a perspective view of an SMC structural element.

(5) FIG. 5 is a flowchart showing conventional processing of an SMC element prior to adhesive bonding.

(6) FIG. 6 is a flowchart showing an improved method of the present invention for adhesively bonding an SMC element without conventional pre-treatment.

(7) FIG. 7 is a perspective view of a portion of a compression mold according to one embodiment of the present invention.

(8) FIG. 8 is a plan view of one embodiment of a textured surface according to one embodiment of the present invention.

(9) FIG. 9 is a plan view of one embodiment of a textured surface according to another embodiment of the present invention.

(10) FIG. 10 is a cross section through an adhesive bond joining an SMC element with a second element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(11) Referring now to FIG. 1, an SMC body 10 is comprised of a resin paste 11 between a pair of films 12 and 13. A plurality of fibers 14 are embedded in resin paste 11.

(12) An SMC body 20 may be inserted into a cavity in a compression molding tool having a lower portion 21 and an upper portion 22 configured to provide a mold cavity in the shape of a desired final SMC element. Lower tooling die 21 has bonding pad portions 23 and 24 and upper tooling die 22 has bonding pad sections 25 and 26, which are all conventionally given a smooth surface. Dies 21 and 22 are brought together to compress SMC body 20 while heat is applied in order to compression mold an SMC element 28 as shown in FIG. 2B.

(13) As shown in FIGS. 3A and 3B, SMC element 28 has bonding sections 30 and 31 for bonding with a second element 32 in an automotive assembly. Bonding section 30 and 31 were conventionally pre-treated for better adherence of an adhesive 33 for joining elements 28 and 32 as shown in FIG. 3B.

(14) FIG. 4 shows bonding sections 30 and 31 on SMC element 28 intended to receive the adhesive. Bonding surfaces 30 and 31 are compression molded and, therefore, had a smooth surface in the prior art due to a desire to provide a long lifetime for the compression molding die tools that interface with the SMC material. In order to deal with the abrasive tendencies of the SMC material, and due to the desire to obtain a smooth, blemish-free surface on all Class A portions of compression molded elements, all tooling surfaces have conventionally been polished smooth and then coated with chrome. Due to the smooth surfaces of bonding sections 30 and 31, pretreatment was necessary when using adhesive bonding.

(15) One conventional prior art manufacturing method is shown in FIG. 5 wherein an SMC part is compression molded with a bonding pad in step 40. The bonding pad on the SMC part is then pre-treated in step 41 in order to roughen or activate the surface. In step 42, the SMC part and a second part to which it is being bonded are placed in a bonding fixture. Adhesive is applied in step 43, and the parts are compressed and the adhesive is allowed to cure in step 44. The surface treatment such as scuff sanding, flame treatment, and plasma treatment that have been routinely required to achieve acceptable bond performance were time consuming, labor intensive, and resulted in additional costs for the manufacturing of SMC components.

(16) The present invention enhances SMC bonding by incorporating a textured tool surface for the bonding pads to obtain superior bonding adhesion without the need for pre-treatments. By deliberately introducing a predetermined surface roughness created by the mold at the bonding pad areas, an increased area of the bond surface is created which in turn provides increased joint strength. In one embodiment of a method of the present invention shown in FIG. 6, a texture is applied to a mold tooling die at the bonding pad surface in step 45. An SMC part is compression molded in step 46. In step 47, adhesive is applied to the bonding pad without pre-treatment. Thereafter, the parts to be bonded are compressed and the adhesive is cured in a conventional manner. The second component can be made of any desired material including a second SMC element with bonding pads similarly textured for ensuring excellent joint strength.

(17) FIG. 7 shows a tooling die 51 having bonding pad portions 52 and 53 and a functional portion 54. Portions 52 and 53 have a predetermined surface roughness for increasing the effective surface area relative to a non-textured surface. Preferably, the predetermined surface roughness is in a range from 0.25 R.sub.a (m) to 8.5 R.sub.a (m). Textured surfaces 52 and 53 impress a corresponding texture into the SMC sheet body during compression molding so that the corresponding generally planar bonding sections on the SMC element are produced with the desired surface roughness, leading to enhanced joint strength when adhesively bonded.

(18) Various textures can be employed to achieve the desired surface roughness. In one example, a multiplicity of lineal scratches is employed as shown in FIG. 8. Such scratches can be formed on the molding die by etching or machining to create a surface similar to what would result from scratching with sandpaper of various grits as reflected in the following table.

(19) TABLE-US-00001 Sample Grit Direction R.sub.a (m) 1 400 Parallel 0.35 2 400 Transverse 0.25 3 240 Parallel 0.84 4 240 Transverse 0.72 5 120 Parallel 1.47 6 120 Transverse 1.48 7 80 Parallel 3.6 8 80 Transverse 5.64
Direction refers to whether the roughness measurement (e.g., using a profilometer) is taken in a direction parallel or transverse with the direction of the scratches. Samples 1 and 2 correspond to altering of the tooling surface in a manner equivalent to a 400 grit sandpaper in a direction parallel and transverse to the measurement direction, respectively. The resulting surface roughness for die samples 1 and 2 produce a value of R.sub.a of 0.35 and 0.25, respectively. As known in the art, R.sub.a is the arithmetic average of vertical deviations in a roughness profile.

(20) In samples 3 and 4, a texture corresponding to a 240 grit sandpaper produces roughness R.sub.a with values of 0.84 and 0.72, respectively. Altering the tooling surface equivalent to sandpaper grits of 120 and 80 result in the surface roughness as shown in the table. Using a texture that is coarser than 80 may be undesirable because the corresponding surface roughness would tend to expose internal fibers. A finer texture than 400 would be undesirable since insufficient surface roughness would be created and no significant increase in mechanical joint strength would be obtained. The most preferable range in predetermined surface roughness lies from about 0.35 R.sub.a to about 3.6 R.sub.a (m).

(21) FIG. 9 shows an alternative texture wherein any common stipple pattern with a grain exhibiting the desired surface roughness may be employed. Any stipple pattern could be selected provided that the roughness R.sub.a of the stipple pattern falls in the range from 0.25 R.sub.a (m) to 8.5 R.sub.a (m).

(22) FIG. 10 shows a first SMC element 55 and a second SMC element 56 with textured surfaces 57 and 58 having the predetermined surface roughness. An adhesive 60 has been applied and cured between textured surfaces 57 and 58. The respective surface areas of each SMC element that are in contact with adhesive 60 are increased relative to a non-textured surface to thereby increase joint strength. The surface texture is applied in a manner that avoids disruption of the internal fibers which could otherwise lead to delamination if fibers extended out from the SMC surface.