FIELD OF THE INVENTION
The present invention relates to a process of thermoplastic pultrusion. In particular, the invention relates to a process of impregnating continuous fiber bundles with a thermoplastic melt from an extruder. More particularly, the invention involves radial impregnation die (RID) which provides a method for impregnating reinforcing fibers in the form of fiber bundles so that individual fibers are surrounded by bonding thermoplastic resin. These impregnated fibers then become feedstock pellets for long fiber reinforced thermplastics (LFRT). BACKGROUND OF THE INVENTION
Advanced composite structures are often manufactured from various preformed material. Thermosets have been the more predominant plastic resins in prepregs, but thermoplastic resins attracted attention in the early 1980s for the ability to be formed by heat with low pressure. Thermoplastic materials exhibit high impact strength, high environmental resistance and often have a high toughness and are recyclable. Fiber reinforced thermoplastic structural components are most commonly manufactured from long fiber thermoplastic (LIT) granulates (pellets), glass mat thermoplastics (GMT) sheets, or pultruded sections. LFT granulates typically consist of glass fiber bundles encapsulated with a thermoplastic through a cable coating or pultrusion and/or commingling process. The problem with the commingling process is that the matrix should also be made in the form of fiber. Melt impregnation is one of the simple process by which any thermoplastic polymer can be melted and coated on the fiber bundles. In order to fully utilize the stiffness and strength of the reinforcing fiber, it is necessary to bring the polymer matrix and the reinforcement fiber together with homogeneous wetting. _But due to the high viscous nature of thermoplastic material (compared to thermosets), the impregnation quality is hampered and even becomes more pronounced if the pulling speed of the fiber bundle is further increased. Therefore, there is a need for the design of a die (which can be fitted to any extruder for fast & continuous production process) to facilitate the effective impregnation of thermoplastic melts on fiber bundles at low capital cost. This will enhance the percolation of LFRT technology in industry. In an effort to improve the wetting of continuous fiber bundles with thermoplastic melt, several techniques have been adopted earlier in prior arts and are given below.
US 6558146 Bl discloses an in-line compounding and extrusion deposition compression molding apparatus and method for producing a fiber-remtorced molded structural component. Apparatus consist of an extruder device having an internal cavity for producing a polymer melt. "Since the melt extrudes in the deposition die head, no pressure has been generated and therefore the quality of impregnation gets hampered. DE 089784 discloses a device for impregnating roll of plastic wovens of fiberglass reinforced plastic with a resin. Roll is provided with a

compartment of atleast one radial opening and with means that seal off the roll at both ends. A "feeding pipe linked with the compartment is also provided. The process of impregnating continuous fiber bundle with thermoplastic melt is not effective.
US 6251206 Bl discloses an apparatus for impregnating a reinforcing fiber bundle with a molten resin. The fiber bundle is subjected to opening bv bringing it into contact with opening pins. Auxiliary pin is positioned within a specific region relative to the opening pin. Apparatus permits the production of a continuous fiber reinforced thermoplastic resin composite material and limits in spreading out of fiber bundles.
US 6228432 Bl discloses a method and apparatus for production of continuous composite materials from unidirectional reinforcing fibers and from thermoplastics. Bundle of parallel continuous reinforcing fibers are being preheated and impregnated with continuous thermoplastic melt. The apparatus is not suitable for brittle fibers like glass fibers.
US 5798068 discloses a process for making fiber reinforced material. The fiber bundle slides over an arcuate support surface in the line of arc ad the molten or liquid polymer is injected into the bundle through a plurality of slots in the surface, each slot being transverse to the bundle. It is very difficult for the brittle fibers to take the path of arcuate support surface in this process and possibility of attrition reduces the quality of impregnation at relatively high production speed.
US 5766357 discloses an apparatus for impregnation for use in a fiber placement system. Resin impregnates the fiber roving as they pass through a manifold. Manifold includes grooves for the fibers to travel, a reservoir to receive the resin and channels for the resm to flow to the individual fibers for impregnation. The apparatus is not suitable for low capital investments.
US 5639410 discloses an apparatus and method for manufacturing a resin structure reinforced with long fibers. Continuous fiber bundle impregnated with a resin melt while the fiber bundle is being drawn continuously. The continuous resin impregnated fiber bundle is shaped to have a cross section of the intended final product. Shaped resin-impregnated fiber bundle is accomplished by a plurality of rolls, each having grooves formed on the outer periphery.
US 5176775 discloses a method and apparatus for forming thermoplastic composite materials. Advancing heated fiber is directed to move in one direction and while a flow of heated thermoplastic material is directed to move in a direction flow of thermoplastic material promote wetting and impregnation of the fiber by the thermoplastic material.
US 5158806 discloses a method for impregnating fiber bundles with molten or liquid
-resin in manufacturing a fiber reinforced material, which comprises one or more fiber bundles,
wherein each fiber is surrounded by matrix resin by impregnating a continuous web of one or

more fiber bundles with molten or liquid resin and solidifying the resin by cooling or chemical reaction. During the impregnation the molten or liquid resin material is subjected to forces by - bringing is between two closely nearby surfaces, which are moving relationship to each other.
In addition to the above-cited patents, a reference can be made to the published documents as herein after mentioned. Nygard and Gustafson have studied about the efficiency of different melt impregnation methods for producing continuous glass fiber polypropylene tapes. The selected methods used were pin assisted method, crosshead impregnation die using different vibration techniques and a slit die. The degree of melt impregnation was studied qualitatively with optical microscopy and quantitatively by opacity measurements. With slit die it was possible to achieve high fiber volume fraction and was easy to control the fiber-matrix content.
Weustint and Keulen designed an impregnation device which has powered rolls of convex and tapered shape. The authors developed a mathematical model of the impregnation ~ device for the analysis of geodesic filament trajectories over convex rolls or arbitrary geometry and location. Impregnation device is more suitable for thcrmoset materials and feasibility of fixing this device to an extruder is limited. OBJECTS OF THE INVENTION
The main object of the invention is to provide a process of thermoplastic pultrusion. A further object is to provide a process of impregnating continuous fiber bundles with a thermoplastic melt from an extruder.
In another object, a radial impregnation die is provided for use in impregnating reinforcing fibers in the form of fiber bundles so that individual fibers are surrounded by bonding thermoplastic resin. SUMMARY OF THE INVENTION
The process of the present invention allows effective impregnation of materials; for instance it ensures that the individual fibers in the fiber bundle are encircled by polymer. It is therefore capable of producing more uniform materials than the prior art. The prior art problem of impregnating fiber bundles with thermoplastic melt or liquid polymer has now been substantially solved by the present invention and the above-described disadvantages are substantially overcome. In addition, the process can be used to make fiber reinforced polymeric resin materials at relatively high production speed and at low capital costs.
The invention is based on the fact that some pseudoplatic materials being subjected to
shear stresses experience a decrease in viscosity. This phenomena, known as shear thinning is
common for various emulsions, dispersions, suspensions and other materials including melts of
.thermoplastic resins and solutions of thermoset resins. The amount of this shear thinning can be

altered with the level of shear stress applied to the material. At low stress, a small degree of thinning is achieved, but at intermediate shear stresses the degree of shear thinning increases dramatically, while at very high shear stresses the degree of thinning reverts to low levels as the material approaches minimum viscosity. A typical range of viscosity for thermoplastic resin materials is 100-103 Pas. In the impregnation of reinforcing fibers viscosity range of 1-10 Pas is ideal, a common magnitude of 100 Pas and in some cases 1000 Pas can be acceptable. Typical shear rate ranges for thermoplastics are l0'-l04 Sec 1.
The present invention involves manufacturing a radial impregnation die which is joined to a commercially available extruder that provides the impregnation of continuous fiber bundles with a thermoplastic melt. The die according to the invention comprises four different zones as shown in the Figure 1.
Zone 1 is the inlet zone which facilitates the feeding of fiber bundles into impregnation zone. Zone 2 is the melt zone, where the molten thermoplastic melt comes out of the extruder screw and reaches impregnation zone. Zone 3, the impregnation zone, the thermoplastic melt from the zone 2 is diverted into two paths and it passes through the pins which has radial elliptical holes. Figure 1 has two pins in the impregnation zone as a part of experiment. The fiber bundle from zone 1 slides over the circumference of two pins in the line of arc, and gets impregnated with the molten melt injected from the radial elliptical holes in the pins. Zone 4 is the outlet zone in which the impregnated strands are extruded. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is the schematic diagram of radial impregnation die Fig. 2 is the detailed sketch of impregnation zone Fig. 3 is detailed sketch of impregnation pin DETAILED DESCRIPTION OF THE INVENTION
The present invention is described with reference to the following example, which is given by way of illustration and should not be construed to limit the scope of the present invention. Figure 1 explains four different zones of the die. The web of fiber bundle is supplied from a creel through a preheating chamber of zone 1. Zone 1 is the inlet zone which facilitates the feeding of fiber bundles into impregnation zone. Zone 1 has a tapered section of 1-2 or more with inner diameter for a determined length which takes care of the melt pressure and shear rate. Zone 1 includes a step of advancing the fiber downward through the passageway while directing the flow of thermoplastic material in an upward counter-current relation to the advancing fiber. This zone also includes the step of heating to expose the fiber to polymer temperature to have good impregnation with thermoplastic molten melt.

With this method of invention it is possible to manufacture any fiber reinforced material, in which the kind of fibers is in the form of bundles. The fibers can be of any kind, bondable bv a matrix resin. Such products are for example granules for injection molding, compression molding or other processes. Suitable fibers for the invention are manmade or artificial continuous filament such as an organic fiber, glass fibers, carbon fiber or metallic fiber and should withstand the melt temperature. Typically such fibers are supplied by fiber suppliers in the form of a roving, tow or band and may be more or less twisted depending on the specific material and the manner of handling prior to introduction into the processes of this invention. Typically glass fiber bundles contain thousands of individual fibers with a diameter of 10-17 [xm.
Zone 2 is the melt zone, where the molten thermoplastic melt comes out of the extruder screw and reaches zone 3. Zone 2 has a converging tapered section which enhances the melt velocity into the impregnation zone. This zone includes the step of heating to maintain the thermoplastic material in the molten state. The resin material to be used for bonding fiber reinforced products can be a thermoplastic resin, which is impregnated into fiber bundles as melt. Suitable thermoplastic resins are among others olefin homogeneous polymers and copolymers, vinyl chloride homogeneous polymers and copolymers, polyesters, polyamides or copolyamides, polycarbonates, thermoplastic polymer blends and alloys or in general any other thermoplastics which show decreased viscosity by shearing action.
Zone 3, as shown in figure 2 is the impregnation zone, the thermoplastic melt from the zone 2 is diverted into two or more paths and it passes through the pins which has 3 or more radial elliptical slots. Figure 2 has three pins in the impregnation zone as a part of experiment. Zone 3 should have an inside diameter and length of dimension not less than the screw diameter. Inside diameter of this zone can be of rectangular, elliptical, circular or polygonal in shape. The space provided in zone 3 should have the space enough to accommodate the features mentioned here. The fiber bundle from the zone 1 slides over the circumference of pins in the line of arc, and gets impregnated with the molten melt injected from the radial slots in the pins as shown in _ figure 3. The pins may be of circular or elliptical or polygonal in shape. The pins should have 4 or more parallel grooves in-line with the fiber passage with a distance of minimum gap in order to branch out the fiber bundle. The hollow pin should have atleast 3 traverse slots in only one or both the half of the circumference of the pin. The fibers are allowed to slide on the groove surface provided on the circumference of the two pins in an 'S' curve shaped profile, thereby maintaining the tension of the fiber bundle in the impregnation chamber and resulting in the increase of residence time of fibers in the melt. The melt coming out of the slots provided in the pins impregnates the fibers on both sides in the radial direction.

Zone 4 is the outlet zone and provides the impregnated strand to be pulled with a constant speed in the downward direction through a converging tapered section. This tapered section enhances the compaction of melt into the fiber bundle for better impregnation. There is small land in this zone which governs the final diameter of the extrudate and gives a sizing effect for uniform and consistent melt impregnated fiber bundle.
The continuous fiber bundle passes through zone 1 and experiences shear forces arising between the advancing heated fiber and directed flow of thermoplastic material. This promotes wetting of the fiber by further impregnation of fiber bv the thermoplastic material. The thermoplastic molten material from zone 2 reaches zone 3 with high velocity. In zone 3 the melt passes through the pins and comes out through the slots in the radial direction. Fiber bundle from zone 1 slides over the circumference of the pins and takes 'S' shaped path in-order to increase the residence time of the continuous fiber with the thermoplastic melt.

claim:
1. A radial impregnation die for use in a pultmsion process comprising a plurality of areas provided therein, comprising in turn a first inlet area provided with a preheating chamber to receive a fiber bundle web from a creel, said first area being connected to a second area through a passage means, said second area being provided with a tapered section to enhance melt velocity- into an impregnation zone and provided with heating means therefore and an oudet means communicating with a third area comprising said impregnation zone and provided in turn with a plurality of paths for melt flow, said third area having a plurality of pins with three or more radial elliptical slots, said third area communicating with an outlet means comprising the fourth area.
2. A radial impregnation die as claimed in claim 1 wherein the first area has a tapered section of 1 -2 or more with inner diameter for a determined length which takes care of the melt pressure and shear rate.
3. A radial impregnation die as claimed in claim 1 or 2 wherein said second area has a converging tapered section which enhances the melt velocity into the impregnation zone, and is connected to said impregnation zone through an extruder screw.
4. A radial impregnation die as claimed in any preceding claim wherein said impregnation zone has an inside diameter and length of dimension not less than the screw diameter, and wherein the inside diameter of said impregnation area is rectangular, elliptical, circular or polygonal.
5. A die as claimed in claim 1 and 4 wherein said pins are circular or elliptical or polygonal in shape and are optionally provided with four or more parallel grooves in-line with the fiber passage with a distance of minimum gap in order to branch out the fiber bundle, said hollow pin having at least three traverse slots in only one or both the half of the circumference of the pin.
6. A die as claimed in any preceding claim wherein said oudet area is provided with a land to control the final diameter of the extrudate and gives a sizing effect for uniform and consistent melt impregnated fiber bundle.
7. A process of impregnating continuous fiber bundles with a thermoplastic melt from an extruder using the radial impregnation die of any preceding claim comprising feeding said web of fiber bundles into a first area in said die from a creel into a preheating zone provided with a first heating means, advancing said fiber downward through a passage means into a melt enhancement area while directing a flow of thermoplastic material in an upward counter-current relation to the advancing fiber, while optionally heating to expose the fiber to polymer temperature to have good impregnation with thermoplastic

molten melt, the heating being carried out to maintain the thermoplastic material in the molten state, advancing said fiber into an impregnation area wherein said thermoplastic melt is diverted into two or more paths such that said melt passes through pins having three or more radial elliptical slots, wherein the fiber bundle from the first area slides over the circumference of pins in the line of arc, and gets impregnated with the molten melt injected from the radial slots in the pins, said pins having four or more parallel grooves in-line with the fiber passage with a distance of minimum gap in order to branch out the fiber bundle and said hollow pin having at least three transverse slots in onlv one or both the half of the circumference of the pin, the fibers sliding over the groove surface provided the circumference of the two pins in an 'S' curve shaped profile, thereby maintaining the tension of the fiber bundle in the impregnation chamber and resulting in the increase of residence time of fibers in the melt, the melt coming out of the slots provided in the pins impregnating the fibers on both sides in the radial direction, said impregnated fibers being outlet through an outlet area provided with means to pull the impregnated strands at constant speed in a downward direction through a converging tapered section to enhance compaction of melt in the fiber bundle.
8. A method as claimed in claim 7 wherein said fibers comprise fibers bondable bv a matrix resin and selected from the group consisting of organic fiber, glass fibers, carbon fiber or metallic fiber.
9. A method as claimed in claim 7 or 8 wherein said resin material used for bonding the fiber reinforced products is a thermoplastic resin selected from the group consisting of olefin homogeneous polymers and copolymers, vinyl chloride homogeneous polymers and copolymers, polvesters, polyamides or copolyamides, polycarbonates, thermoplastic polymer blends and alloys or in general any other thermoplastics which show decreased viscosity by shearing action.
10. A method as claimed in any of claims 7 to 9 wherein said continuous fiber bundle passing through said first area in said die is subjected to shear forces arising between the advancing heated fiber and directed flow of thermoplastic material which in turn promotes wetting of the fiber by further impregnation of fiber by the thermoplastic material.