METHOD OF FORMING A PREFORM USING A POWDER BINDER

Abstract: A process for producing a preform (40) includes the deposition of powder binder (25) onto chopped glass fibers (19) that are on a preform screen (14). The powder binder and chopped glass fibers are then heated to soften the particles of binder and make them stick to the glass fibers to form the preform.

Description: METHOD OF FORMING A PREFORM USING A POWDER BINDER TECHNICAL FIELD This invention relates generally to directed fiber preforming and, more particularly, to an improved process for forming a glass fiber preform. BACKGROUND OF THE INVENTION Glass fibers have been commonly incorporated in thermoplastic molded objects and other cured plastics for added strength and durability. The glass fiber is introduced in a mold where resinous plastic is then injected such that the glass fibers become imbedded into the final formed object. It has been found advantageous to form a preform of the final object out of glass fibers and place the preform into the mold. The glass fibers are often chopped and blown onto a preform screen. Immediately following the fiber placement, a liquid binder agent is sprayed on and allowed to cure which sets the fibers in place. The use of a liquid binder coats substantially the entire chopped glass fiber which can prevent matrix resin in the molded part from fully wetting-out the fibers in the preform. This lack of penetration of the matrix resin into the fiberglass strand within the preform for the finished molded part restricts its mechanical strength and integrity. What is needed is a method for adding a binder to the preform fibers that prevents substantial coating of the fibers so that the preform is more supple during handling thus improving its integrity and such that the later added matrix resin in the subsequent molding process more fully wets out the fibers of the preform. SUMMARY OF THE INVENTION In accordance with one aspect of the inven¬ tion, chopped fibrous material, such as chopped glass fibers, are deposited on a preform screen. The fibers may be retained on the screen by a vacuum draw there¬ through. A solid particulate binder material, commonly referred to as a powder, is deposited onto the deposited chopped fibers. The particulate binder are retained on the deposited glass fibers. The particles of binder may be retained by static electrical charges on either the binder particles or glass fibers or both. The binder particles can be softened while on the deposited fibers such that the fibers are adhered to the fibers independently of the electro-static charge. The softening of the binder particles also allows fibers to adhere to other fibers via the binder particles. The softening of the binder particles may be by applying heat to a binder made from a thermoplastic polyester material such that upon softening it becomes tacky and sticks to the adjacent glass fibers. The particles are then re-solidified to form a cured preform. The preform is then removed from the preform screen. BRIEF DESCRIPTION OF THE DRAWINGS Reference is now made to accompanying drawings in which: Figure 1 is side elevational view of a direct fiber apparatus used in a method according to the invention; and Figure 2 is an enlarged and schematic view of the glass fibers and particulate binder deposited onto the preform screen shown in figure 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to figure 1, a preform screen assembly 10 includes a mounting table 12, a foraminous preform screen 14 and. a suction fan 15. The screen 14 has a plurality of holes 27 and its contour conforms to one surface of the desired preform shape. The screen 14 is mounted about its periphery to the table 12 which in turn has appropriate duct work 13 for housing the suction fan 15. The suction fan when actuated draws a vacuum through the screen 14. A nozzle control system generally indicated as 20 is pointed toward the preform screen 14. The system includes a blower nozzle 18 for deposition of blown glass fibers 19 and at least one nozzle 21 for the spraying of dry binder powder 25. The glass fibers used may be chopped from a commercially available roving such as one sold under the brand name PPG-5542. The fiber nozzle 18 is the distal end of a flexible tube 26 that has its other end 27 mounted onto a rigid delivery tube 28 communicating from a chop¬ per/blower assembly 30. The tubes 26 and 28 and chop¬ per/blower assembly 30 are mounted on a gantry 32 that can move the nozzles left, right, up, and down relative to the screen 14. The chopped glass fibers 19 from nozzle 18 are directed onto the preform screen 14. The chopped glass fibers may be chopped between 1" and 4" in length depending on the application. Simultaneously or im edi- ately after the deposition of the fibers, the powder binder is blown onto the preform screen 14. The binder may be Reichhold's Atlac 32-626 and can be blown by a Volstatic Coatings LTD's Solidspray-90 spray station in combination with their SPG-1351 spray gun onto the preform screen at a rate of approximately 2.14 g/s. Other particulate thermoplastic polyester binders may be alternately used. The particulate sizes of the powder binder may range from mesh size 35 to 200 which corre¬ sponds to .00122" to .0021". The binder powder is retained on the glass fibers through static electrical charges. The glass fibers are retained on the preform screen by the use of a vacuum draw through the screen produced by suction fan 15. As shown in figure 2, the particles and glass fibers form a porous mat 50 with many interstices 52 between the fibers and particles. The amount of binder by weight is about 2-6% of the glass fibers. After the appropriate amount of glass fiber and powder binder is applied onto the preform screen 14, heat is applied to the preform 40. In one case, heat lamps 43 may be positioned about the preform screen 14 to direct heat onto the preform 40. The screen assembly 10 may be moved on a multi-station line such that the heat is applied at a different station than the station that deposits the glass fibers. The heat may be applied by heat lamps 43, heated air, or other conventional heating methods. The heat softens the particles of the binder such that each particle becomes tacky and sticks to any glass fibers with which it is in contact. After sufficient softening, the binder and glass fibers on the preform screen 14 are cooled such that the binder is set, i.e. cured, to form a one-piece preform 40. The preform is then allowed to cool by use of chilled air or deactuation of heat lamps 43. As shown in figure 2, the glass fibers are not coated with binder and as such, preform release from the screen is significantly easier compared to preforms made with sprayed liquid binders. Furthermore, preform integrity is improved. The strength derived from adhesion with particles exceeds that from coated fibers touching each other. Furthermore, coated fibers prevent matrix resin in the molded part from fully wetting-out the fibrous strands which are composed of thousands of filaments. By not coating the fibrous strands, nothing interferes with wet-out. Each fiber can be penetrated by the matrix resin to fully wet-out all the filaments in any subsequent molding process. Other variations and modifications are possi- ble without departing from the scope and spirit of the invention as defined in the appended claims.

Claims: CLAIMS The embodiments in which an exclusive property or privilege is claimed are defined as follow. 1. A process of manufacturing a preform comprising the steps of; depositing chopped fibrous strands onto a preform base member; depositing solid particulate binder material made from solid particles onto said deposited chopped fibrous strands; softening said particles of particulate binder retained on said fibrous to adhere the chopped fibrous strands to the particles and to other fibrous strands; re-solidifying said particles such that said chopped fibrous strands adhered to said particles form a preform. 2. A process for manufacturing a preform as defined in claim 1 wherein the depositing of solid particulate binder includes using electrostatic charge on said particles to retain said particulate binder on said fibrous strands. 3. A process for manufacturing a preform as defined in claim 2 wherein said particulate binder is blown onto said fibrous strands. 4. A process for manufacturing a preform as defined claim 1 wherein said particulate binder is made from a thermoplastic polyester material. 5. A process for manufacturing a preform as defined in claim 4 wherein said softening of said particles includes heating said particulate binder. 6. A process for manufacturing a preform as defined in claim 5 wherein said fibrous strands are made from glass fibers. 7. A process for manufacturing a preform as defined in claim 1 wherein said softening of said particles includes heating said particulate binder. 8. A process for manufacturing a preform as defined in claim 1 wherein said fibrous strands are made from glass fibers.

Inventors: HAMES ROBERT, KELMAN JOSH, HAMES, ROBERT, KELMAN, JOSH
Applicants: DAVIDSON TEXTRON INC, DAVIDSON TEXTRON INC.
Application year: 1993
Main classification: B29C41
IPC codes: B29C41, B29C70