METHOD FOR MANUFACTURING LONG FIBER REINFORCED THERMOPLASTIC RESIN MOLDING MATERIAL

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION The present invention relates generally to the manufacture of long fiber reinforced thermoplastic resin molding material and more particularly to a method for this purpose.

BACKGROUND OF THE INVENTION

Methods and apparatus for manufacturing long fiber reinforced thermoplastic resin molding material are well known in the art. JP-A- 10-309756, JP-A- 10-315341 and JP-A- 2003-305779 are documents exemplary of state of the art methods and apparatus for this purpose. The prior art methods disclosed in these documents may be generally described as comprising a series of continuous steps including (a) the feeding of a continuous fiber material into an impregnation die filled with molten thermoplastic resin, (b) the pultruding of the continuous fiber material impregnated with thermoplastic resin from a pultrusion hole of the impregnation die and (c) the cutting of the resulting rod shaped product into pellets of desired length after cooling.

Usually, a nozzle is attached to the pultrusion hole of the impregnation die. The shape of the nozzle and the size of the hole or aperture in the nozzle are selected to perform several functions including the removal of excessive molten thermoplastic resin so that the desired amount of resin is impregnated into the pultruded rod shaped product and the formation of the pultruded rod shaped product so that its cross section becomes the desired shape. In effect, the design of the nozzle largely determines the efficiency and extent of molten thermoplastic resin impregnated into the continuous fiber material during processing.

^• JP-A-1 1-042639 discloses a method of preventing molten thermoplastic resin from trickling from a rod shaped pultruded product. This is done by setting a processing value calculated using a certain formula relating to the length and cross-sectional area of the hole in a nozzle. JP-A-2001-088223 discloses a nozzle having a cone portion and a linear portion of a particular size and shape designed to prevent cut filaments from accumulating near the nozzle as fuzz-balls that could potentially interfere with the efficient manufacture of the pelleted product.

JP-A-05-050432 discloses the concept of providing a removable nozzle on an impregnation die.

JP-A-08-090659 discloses a nozzle including a thinned down tip portion designed to improve the impregnation level of molten thermoplastic resin in the continuous fiber material.

While these prior references represent significant advances in the art, further improvements are still possible. Specifically, when seeking to improve productivity by increasing the speed of the pultrusion step,, problems such as the breaking of fibers at the nozzle or degradation of the impregnation level of the molten thermoplastic resin are still encountered. These problems are more likely to occur when the long fiber content ratio is high. For example, the problems are prominent when the long fiber content ratio is 60% in weight or more. If a molded product is made from long fiber reinforced thermoplastic resin molding material that has an insufficient level of impregnating thermoplastic resin into the continuous fiber material, mechanical properties and surface appearance of the molded product are degraded. The present invention provides a unique and heretofore unknown solution to these problems.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention as described herein, a method is provided for making long fiber reinforced thermoplastic resin pellets in an impregnation die including a processing chamber. The method comprises the steps of (a) filling the processing chamber with molten thermoplastic resin, (b) feeding at least one continuous fiber strand through the processing chamber, (c) pultruding the at least one continuous fiber strand impregnated with the molten thermoplastic resin and (d) cutting the at least one pultruded continuous fiber strand impregnated with the molten thermoplastic resin into pellets. The method is characterized by processing the pellets in accordance with a formula A = Q - L - N/S ² wherein:

A = a processing value < 5.0;

Q = total quantity in mm /sec of molten thermoplastic resin applied to said at least one continuous fiber strand and removed from said processing chamber during pultruding;

L = length in mm of said processing chamber in a continuous fiber strand feeding direction;

N = total number of continuous fiber strands impregnated with molten thermoplastic resin pultruded from said impregnation die; and

S = cross-sectional area in mm of said processing chamber in a direction perpendicular to said continuous fiber strand feeding direction. In one particularly useful embodiment the processing value A is between 0.5 and

3.5.

The method may further include fully partitioning the impregnation die in order to provide completely separate multiple processing chambers in a single impregnation die.

In still another embodiment the method includes partially partitioning the impregnation die in order to provide multiple processing chambers in a single impregnation die wherein the multiple processing chambers are in communication with one another.

In any method where the impregnation die includes multiple processing chambers, the method includes filling each of the multiple processing chambers with molten thermoplastic resin, feeding a continuous fiber strand through each of the chambers, pultruding the continuous fiber strands impregnated with molten thermoplastic resin and cutting the continuous fiber strands impregnated with the molten thermoplastic resin into pellets.

The method may further include the using of glass fiber strand as the continuous fiber strand. Alternatively, the method may include the using of carbon fiber strand as the continuous fiber strand. In addition the method may include using polyolefin resin as the molten thermoplastic resin. In another alternative embodiment the method may include polyamide resin as the molten thermoplastic resin.

Still further the method may include spreading multiple filaments of the continuous fiber strand in order to aid in the impregnation of the strand with molten thermoplastic resin.

In the following description^ there is shown and described several different embodiments of the invention, simply by way of illustration of some of the modes best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated herein and forming a part of the specification, illustrate several aspects of the present invention and together with the description serve to explain certain principles of the invention. In the drawings: Fig. 1 is a perspective view of an impregnation die of the present invention;

Fig. 2 is a perspective view similar to Fig. 1 but of an alternative embodiment incorporating partitions so that the impregnation die includes multiple processing chambers;

Fig. 3 is a partially schematical and cross-sectional view illustrating the impregnation die of Fig. 1;

Fig. 4 is a detailed, longitudinal cross-sectional view of the nozzle of the impregnation die of preferred embodiment of the present invention; and

Fig. 5 is a similar view of a prior art nozzle.

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Reference is now made to Figs. 1 and 3 illustrating an impregnation die 10 constructed in accordance with the teachings of the present invention. The impregnation die 10 includes a body or housing 12 constructed from an appropriate high strength material. The housing 12 includes an internal processing chamber 14. As illustrated in

Fig. 1, the impregnation die 10 includes a single processing chamber 14.

Referring to Fig. 3, the housing 12 includes a supply aperture 18 to allow the introduction of molten thermoplastic resin into the processing chamber 14. Additionally, the housing 12 includes an introduction aperture 20 for feeding continuous fiber strand 50 into the processing chamber 14. Further, the housing 12 includes a pultrusion aperture 22 opposite the introduction aperture 20.

An alternative embodiment of an impregnation die 10 is illustrated in Fig. 2. In this embodiment the impregnation die 10 includes a series of partitions 16 forming multiple processing chambers 14. The partitions 16 may each extend across the full length and width of the internal cavity of the housing 12 so that the processing chambers 14 are each completely separate. Alternatively, the partitions 16 may extend partially across the length and/or width of the internal cavity of the housing 12 so that the processing

chambers 14 are in communication with one another. In this embodiment, each of the multiple chambers 14 includes a supply aperture 18, an introduction aperture 20 and a pultrusion aperture 22.

A nozzle 24 is carried on the housing 12 (see Fig. 3). The nozzle 24 may be made from any appropriate material including, for example, brass or special steel. In the illustrated embodiment _^ the nozzle 24 overlies the pultrusion aperture 22 and communicates with the processing chamber 14 through the pultrusion aperture.

As illustrated in Fig. 3, the base end 26 of the nozzle 24 is received in a counter bore 28 formed in the housing 12 concentrically around the pultrusion aperture 22. A collar 30 engages the shoulder of the base end 26. The collar 30 may be fixed by screws or other fastening means (not shown) to the housing 12 in order to secure the nozzle 24 in position.

As best illustrated in Fig. 4, the nozzle 24 includes an opening 32 having an inlet end 34, an outlet end 36 and a central axis A. The inlet end 34 is in direct communication with the processing chamber 14 by means of the pultrusion aperture 22.

The opening 32 when described in detail is characterized by including a first tapered section 40 adjacent the inlet end 34, a first straight section 42 directly downstream from the first tapered section 40, a second tapered section 44 directly downstream from the first straight section 42 and a second straight section 46 directly downstream from the second tapered section 44 and adjacent to the outlet end 36. As illustrated in Fig. 4, the first tapered section 40 incorporates a curved taper (note the arcuate sidewall of the opening) while the second tapered section 44 incorporates a linear taper (note the straight sidewall of the opening). It should be appreciated, however, that the first tapered section 40 may incorporate a linear taper and the second tapered section 44 may incorporate a curved taper if desired. Thus, in one possible embodiment both the first and second tapered sections 40, 44 incorporate linear tapers. In another possible embodiment both the first and second tapered sections 40, 44 incorporate curved tapers. In still another possible embodiment the first tapered section 40 may incorporate a linear taper while the second tapered section 44 may incorporate a curved taper. In any of the possible embodiments, the first tapered section 40 converges toward the first straight section 42. Similarly, the second tapered section 44 converges toward the second straight section 46.

As further illustrated in Fig. 4, it should be appreciated that both the first straight section 42 and the second straight section 46 are symmetrically aligned and extend longitudinally along the central axis A of the opening 32.

The tip of the nozzle 24 typically extends outwardly from the mounting collar 30 by a length of from 5 to 20 mm. This distance helps to ensure that the product pultruded from the nozzle 24 becomes stable, thereby reducing cracking of the product and any fuzz created by fibers falling when the pultruded product is cut into pellets of a certain desired length.

The first tapered section 40 of the opening 32 has a length Ll. The first straight section 42 of the opening 32 has a length L2 and a diameter D2. The second tapered section of the opening 32 has a length L3. The second straight section of the opening 32 has a length L4 and a diameter D4. Typically the length Ll of the first tapered section 40 and the length L3 of the second tapered section 44 are between 0.5 and 5 mm respectively. In addition, the length L2 of the first straight section 42 is longer than the length L4 of the second straight section 46. The diameter D2 of the first straight section 42 is greater than the diameter D4 of the second straight section 46. Further, the first tapered section 40 has a diameter Dl at the first, upstream or inlet end and a diameter D2 at a second or downstream end wherein D2 = Dl/2. Still further, the ratio L4/D4 is typically between 1.4 to 3.4. The method of manufacturing long fiber reinforced thermoplastic resin molding material in pellet form will now be described in detail. The method includes the step of continuously filling the processing chamber 14 with a molten thermoplastic resin through the supply aperture 18. At least one supply aperture 18 is provided for each processing chamber 14 provided in the impregnation die 10. Any thermoplastic resin known to be useful in the production of long fiber reinforced thermoplastic resin pellets may be utilized including but not limited to polyolefin resins, polyamide resins and combinations thereof.

The method also includes the feeding of at least one continuous fiber strand 50 through the processing chamber 14. More specifically, the continuous fiber strand 50 is fed from a supply spool (not shown) across a guide member 48 and through the introduction aperture 20 into the processing chamber 14. The continuous fiber strand 50 is contacted with the molten thermoplastic resin in the processing chamber 14 before being pultruded and fully impregnated with the resin by passing through the pultrusion aperture

22 and nozzle 24. The continuous fiber strand 50 may be made from any appropriate

material including reinforcing material such as glass fiber strand and/or carbon fiber strand. Optional spreaders 54 of a type known in the art may be provided at spaced locations in the processing chamber 14 to spread the individual filaments of the continuous fiber strand 50 and aid in the impregnation process. The pultruded rod shaped product 56, comprising the continuous fiber strand 50 impregnated with the molten thermoplastic resin 51, is extruded through the nozzle 24 and then cut by a cutting device 52 into pellets 60 of a desired length.

The method of the invention is characterized by processing the pellets in accordance with the formula A = Q - L - N/S ² wherein: A = a processing value < 5.0;

Q = total quantity in mmVsec of molten thermoplastic resin applied to said at least one continuous fiber strand and removed from said processing chamber during pultruding;

L = length in mm of said processing chamber in a continuous fiber strand feeding direction;

N = total number of continuous fiber strands impregnated with molten thermoplastic resin pultruded from said impregnation die; and

S = cross-sectional area in mm of said processing chamber in a direction perpendicular to said continuous fiber strand feeding direction. Still more preferably the processing value A is provided between 0.5 and 3.5.

In an impregnation die 10 such as illustrated in Fig. 2 incorporating multiple processing chambers 14, pellets produced from continuous strand 50 passing through each of those individual chambers 14 are processed in accordance with the processing value A. In such case, the total value of the corresponding cross-sectional areas Sl, S2, S3, S4 of each of the chambers 14 is used as the cross-sectional area S of the impregnation die 10 when calculating the value A in the aforementioned formula.

The following examples will help illustrate the present invention. Experiment 1

Long fiber reinforced thermoplastic resin molding material in pellet form was obtained using an impregnation die in a shape shown in Fig. 2 with a length L and the cross-section area S (total value of the cross-section area of each processing chambers) as described in the table 1 , and setting the total number N of rod shaped product pultruded from the impregnation die to 4, and setting the speed of pultrusion of rod shaped product

so that the total quantity Q of molten thermoplastic resin removed from the impregnation die becomes as described in the table 1.

As for the fiber, glass fibers bundling 4,080 glass filaments of 16 μm in diameter were used. Polypropylene resin with 151 melt flow rate (MFR) as measured by the procedure set forth in ISO-1133 was used as thermoplastic resin. The rate of content of glass fiber of the obtained long fiber reinforced thermoplastic resin molding material was

50% by weight.

Long fiber reinforced thermoplastic resin molding material of examples 1 to 5 and comparative examples 1 to 3 were evaluated for the level of impregnation of polypropylene resin to glass fiber in the following method and the results are shown in the table 1.

Evaluation Method of the Level of Impregnation

10 g of long fiber reinforced thermoplastic resin molding material in pellet form were soaked in red water-based ink for one minute and taken out and rinsed in water and fluid was wiped. The lighter coloring pellets have the better level of impregnation of polypropylene resin to glass fiber because the ink penetrates in the part where polypropylene resin is not impregnated (i.e. tiny air gaps inside of the pellets). The level of impregnation was relatively evaluated by eye based on the depth of coloring.

As shown in Table 1, the pellets of the examples 1 to 5 which are long fiber reinforced thermoplastic resin molding material of the invention have a good level of impregnation of resin.

Table 1

Experiment 2

Long fiber reinforced thermoplastic resin molding material was manufactured using an impregnation die 10 as illustrated in Fig. 3 to which several nozzles in a shape

shown in Fig. 4 were attached. Each dimension of the nozzle is Dl=10mm, D2=5mm, D3=15mm, D4=2.2mm, D5=10mm, R(a curvature radius of the curved taper of the first tapered section)=5mm, Ll=2mm, L2=20mm, L3-2mm, L4=6mm, L4/D4 = 2.7 and its material is brass. As for the fiber, a glass fiber strand that draws together 17 pieces of a glass fiber, which was obtained by bundling 600 glass filaments with 13.5μm in diameter was used. Also, polypropylene resin was used as the thermoplastic resin.

The speed of pultruding rod shaped product was set to 15m/min. and the other conditions were set in that N=4pieces, Q=l,747mm ³ /sec, L=I, 000mm, S=2,360mm ² and A=I.3. Then, long fiber reinforced thermoplastic resin molding material with 70% by weight of the content ratio of glass fibers was manufactured. In this manufacturing process, the number of breakage of pultruded, rod shaped product around a nozzle was counted and converted to breakage frequency per nozzle in one day (24 hours). Breakage of the glass fiber caused by glass filaments comprising the glass fiber being broken partially and becoming fuzz and the fuzz being accumulated in the nozzle was counted as a breakage of rod shaped product. The breakage frequency in this invention was 0.099 times per nozzle per day.

For comparison, the number of breakage of rod-shaped product was counted using a nozzle in a shape shown in Fig. 5 (prior art) under the same conditions and in the same method as above. Although the nozzle 60 has a shoulder 68, it has almost a cylindrical shape with the hole 65 whose cross-section is circular penetrating it. The hole 65 has a tapered part 66 and a parallel or straight part 67. Each dimension of the nozzle 60 is D6=9mm, D7=2.2mm, D8=15mm, D9=10mm, L6=25mm, L7=5mm. The breakage frequency in rod shaped product in this prior art was 0.77 times per nozzle per day.

As above stated, the breakage frequency of rod-shaped product of the present invention is much smaller than the one in the prior art. Therefore, productivity of long fiber reinforced thermoplastic resin molding material is higher in an impregnation die of this invention.

The foregoing description of the preferred embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the

invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled. The drawings and preferred embodiments do not and are not intended to limit the ordinary meaning of the claims in their fair and broad interpretation in any way.