CA2255524A1 - Pultrusion die and method of manufacture - Google Patents

Abstract

An improved pultrusion die and method of manufacture provides improved product appearance while retaining required operation characteristics. The die includes a modified split-cavity or multi-piece die having a seamless insert positioned at an axial position at the point of surface replication to cause surface cure and detail reproduction to occur in a limited region where no die parting line is present.

Description

CA 022~24 1998-12-07 PULTRUSION DIE AND METHOD OF MANUFACTURE
This application claims the benefit under 35 U.S.C. 1 l9(e) of co-pending provisional application Serial No. 60/067,832 entitled "IMPROVED PULTRUSION DIE AND METHOD
5 OF MANUFACTURE" filed December 5, 1997, which is incorporated herein by reference.

FIELD OF THE INVENTION
The present invention relates to the p.epal~lion of an improved pultrusion die, and more particularly to a combination seamless die and split cavity die and method of manufacture that 10 provides improved product appearance while also retaining required operational characteristics.

BACKGROUND
Pultrusion dies of any geometry other than round are made of multiple pieces that are assembled to form a closed cavity through which resin impregnated fibers are drawn to produce 15 continuous lengths of thermoset reinforced plastic profiles. Pultrusion dies are typically 36 to 60 inches in length which precludes well known methods of creating seamless cavities, such as wire EDM, where work piece length limitations at present are about 16 inches. It is feasible to align several seamless pieces end-to-end with proper tolerance control. However, because of the occurrence of pultrusion processing problems which cause blockages in the die that require die~0 11ic~csç-nbly, the seamless die concept of substantial length segments is one that precludes selnbly for process maintenance. Accordingly, with the exception of round dies which can be gun-drilled to form a seamless die, all of the dies in use commercially are of split cavity design. The two approaches are shown in Figures 2A-2C.
The benefits of using a seamless die are substantial. The most significant benefit is the 25 elimination of a "parting line" or "witness line" on the finished profile which occurs despite the close tolerance match of die segments. This line can be very pronounced as die wear occurs making the finished profile objectionable aesthetically. (See Figure 3). A pultruded product having a visible parting line may be a very evident flaw that most likely is objectionable to company marketing personnel and the consumer. Because of the fabrication methods required for 30 making a complex profile die, the location of parting lines is often unavoidable despite the desire to orient them on non-visible surfaces. This is especially true for applications such as window lineals, bus or automotive parts and tool handles for example.
In production, as the parting line begins to wear, fiber under plocessing pressure exudes into the mating surface causing fiber breakage which may hll~ t ploces~ g as well as further CA 022~24 1998-12-07 abrasive die wear. The parting line condition is shown in Figure 4B in large scale. In order to reduce the wear tendency, the entire die length may be through-hardened or case-hardened which introduces additional problems in piece distortion and fracture toughness. In addition, hard chrome plating must be provided to achieve the surface lubricity and corrosion resistance necessary for long life and smooth profile processing. Despite best practices, such designs and precautions can not eliminate visible parting line marks on the profile.

SUMMARY OF THE INVENTION
The present invention seeks to achieve the product appearance only possible with a 10 seamless die while retaining the operational characteristics of the split cavity die. The approach is to position a seamless die insert at an axial position within the multi-piece die so as to cause the surface cure and detail reproduction to occur in a limited region where no die parting line is present.
This is accomplished by having a pultrusion die with an upper die block and a lower die 15 block defining a cavity, and a seamless insert with an axial hole carried by said upper and lower die blocks where the axial hole forms a portion of the cavity. When a continuous material of reinforcing fibers which has been impregnated with a thermosetting resin is drawn through the seamless insert, and the temperature within the insert is maintained so that the resin gels, a pultruded product without a parting line product is created.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of an al")al~lus for manufacturing a pultruded product.
Figure 2 is a cross-sectional view of various pultrusion die design al.pl~,acl1es.
25 Specifically, Figure 2A shows a cross-section of a seamless gun barrel die, Figure 2B shows a cross-section of an assembled split cavity tube die with mandrel, and Figure 2C shows a di.c~cs~tnbled split cavity tube die with mandrel.
Figure 3 is a prospective view of a pultruded product of the prior art showing a parting line.
Figure 4 is a cross-sectional view of a pultrusion die with a mandrel highlighting the area of the parting line and a close-up view of a parting line breakdown. Specifically, Figure 4A shows a cross-section of an assembled split cavity tube die with mandrel indicating where parting line break down occurs, and Figure 4B shows a detail of parting line break down.

CA 022~24 1998-12-07 Figure 5iS a graph of the process exotherm temperature versus die position for preparing a pultruded product of the present invention.
Figure 6 is a view of the seamless insert placed in the gel zone region. Specifically, Figure 6A is a cross-sectional view of a split cavity die with a seamless insert and a mandrel, Figure 6B is a side view of a split cavity die with a seamless insert.
Figure 7 is a solid model with the seamless insert placed in the gel zone.
Figure 8 is an exploded view of an insert and matching pocket.
Figure 9 is a prospective view of the die, insert, and pultruded product with the top half of the die removed.
Figure l O is an exploded view of the top and bottom halves of the pultrusion die removed thereby freeng the insert.
Figure 11 shows a seamless insert with a portion of pultruded product passing through the insert available for material extraction.
Figure 12 shows a disassembled pultrusion die with an insert showing a portion of solidified pultruded product passing through the insert.
Figure 13 is prospective view of a series of contiguous seamless inserts placed in a pultrusion die.
Figure 14 is prospective view of a series of intermittent seamless inserts placed in a pultrusion die.
Figure 15is a front view of a seamless insert having an internal axial hole with a narrow section and wide section.
Figure 16 is a side view of a seamless insert having an internal axial hole with a narrow section and wide section.
Figure 17 is a front view of a seamless insert having a straight internal axial hole and a variable external geometry.
Figure 18 is a side view of a seamless insert having a straight internal axial hole and a variable external geometry.
Figure 19 is a top view of a seamless insert having a straight internal axial hole and a variable external geometry.
Figure 20 is a graph of three dirr~l~.ll process exotherm tests showing the temperature within the die as material is pulled through the die at three dirr~.~.lt speeds.Figure 21is a cross section of a split cavity pultrusion die with a hollow mandrel.

CA 022~24 1998-12-07 DETAILED DESRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure l, a pultrusion appdldtus is generally designated by the numeral 11.
The appal~llus includes a plurality of creels or spools 12 from which reinforcing fibers 13 are supplied and drawn through guide 14 which guides and converges the reinforcing fibers 13. The reinforcing fibers 13 are pulled through a thermosetting resin bath 15. The reinforcing fibers 13 are passed around several redirect bars 16 which cause spreading of the fibers in the bath and provide for thorough impregnation of fibers in the bath and provide for thorough impregnation of each of the fibers with a liquid heat curable thermosetting resin contained within the resin bath 15. Various alternative techniques which are well known to the art can be employed to apply or 10 impregnate the fibers 13 with thermosetting resin. Such techniques include, but are not limited to, spraying, dipping, roll coating, brushing and the like. Alternatively, pre-impregnated fibers can also be used. Other known techniques which can be employed to impregnate the fibers with thermosetting resin include pressure-assisted i",p,egl1ation which is also often referred to as resin injection.
The resin impregnated reinforcing fibers 17 emerging from resin bath 15 are pulled through a forming guide system 18 which, for example, can be comprised of one or a plurality of machined plates, sheet metal guides or the like, which consolidates the resin impregnated fibers 17 into the approximate shape of the desired pultruded article. The consolidated mass of resin impregnated fibers 19 emerging from the forming guide system 18 is pulled through a pultrusion die 4 which is m~int~ined at an elevated temperature due to a heating means 21 and passes to a pulling means 22. As shown in Figure 2C, the die may be a split cavity die, having an upper die block 8, a lower die block 10, and, optionally, a mandrel 3. The mandrel 3 would be used to produce a hollow pultruded product. The upper die block 8 and the lower die block 10 can be attached to each other by various methods, including, but not limited to, socket head cap screws that pass through the upper die block 8 and into corresponding threaded bores on the lower die block 10. Other means, such as clamps, may also be used. As shown in Figure 2C, each die block has an axial groove 1 along its mating surface. When upper die block 8 and lower die block 10 are attached to each other to create the pultrusion die 4, the axial grooves 1 form an axial cavity. The path of the cavity extends through the length of the pultrusion die 4.
Conventional pultrusion processes are generally ope~led as steady state conditions, i.e.
temperature and other process parameters generally remain relatively constant at any location in the process stream. As shown in Figure 1, heating means 21 are provided to selectively heat the die 4. It should be understood that the heating means are preferably comprised of a plurality of individually controllable heating means, such as an electrical heater resistance strip, to facilitate CA 022~24 1998-12-07 optimal temperature control along the entire length of the die. Alternatively, other conventional controllable heating means can be provided such as external platen heaters, infrared heaters, cartridge heaters, quartz heaters, conduits or channels provided in the die parts for circulating a heated fluid, and the like. Sufficient heating means 21 are provided so that the temperature of 5 substantially the entire pultrusion die 4 can be rapidly heated to a desired axial temperature profile suitable for continuous steady state pultrusion processing.
The pulling means 22 which continuously extracts the cured article 23 from the die 4 may well be relatively uncomplicated. For example, one arrangement comprises a plurality of generally opposed upper and lower roller means, such as small rubber tires 25 oriented to engage 10 the horizontal surfaces of the cured article 23. A similar roller means 26 engages the vertical surfaces of the pultruded product. The speed of the roller means 25, 26 is synchronized with the application of heat to the die 4 to effect the cure of the thermosetting resin at the desired location within the die 4. After passing through the pulling means 22, the composite 23 is cut into articles of desired length by a cutting means 24 which can be generally any known means suitable for 15 cutting thermoset articles such as a circular saw, hand saw or the like.
During start up, the lei-lfo-~;ing fibers can be drawn by hand through guide 14, resin bath 15, forming guides 18, the spaced upper die block 8 and lower die block 10 of the pultrusion die 4 and to the pulling means 22. After the temperature within at least a portion of the pultrusion die 4 is raised to a temperature sufficient to effect curing of the heat curable thermosetting resin, the 20 material 23 is engaged by the pulling means 22 which can be any conventional means known for continuously drawing pultruded material through a pultrusion die.
Figures 2A, 2B, 2C, 4A and 4B illustrate the conventional pultrusion die design aproaches. The seamless gun barrel die 2 provides a seamless pultruded product, but, due to the potential for product blockage within the die, the production utility is sometimes compromised.
25 The split cavity tube die 4 of Figures 2B and 2C, due to its multiple piece construction, allows relative ease in clearance of product blockage and cle~ning but includes an undesirable parting line or seam 6 (Figures 3, 4B) where the top half 8 and the bottom half 10 of the die 4 meet. The seam 6 in turn imparts a rough or visible mark on to the pultruded product profile at the parting line replication region as the product passes through the die 4. The rough edge on the pultruded 30 product is undesirable to both the company marketing personnel and the consumer.
Figure 5 illustrates the cure process of a pultrusion product. The die temperature profile shown is typical wherein the front of the die is kept deliberately below the initiation temperature of the resin to prevent gellation outside the die entrance or in the radiused or tapered front end of the die. Directly after the material is compressed to its final shape, the temperature of the die is CA 022~24 1998-12-07 rapidly ramped to a temperature at which the thermally induced curing reaction can proceed.
Since the heat transfer occurs from the surface of the die cavity toward the center, the cross-linking behavior always begins at the die surface and transfers toward the center of the volume.
Since a parting line replication is a surface effect only and not dependent upon volumetric heat 5 transfer or curing rate, it is possible to identify and control the region in which surface gel or surface detail replication occurs.
By positioning a seamless insert 20 (Figures 6-ll) within a recess or pocket 33 in a bottom portion 32 of a modified split cavity die 30 at the point of surface replication, a part without a visible parting line can be produced despite the fact that further downstream from the 10 insert 20 the split cavity geometry is retained. This concept is shown in Figure 6 in drawing form and Figure 7 as 3D solid model.
In practice, it is reasonable to estimate the length of this effective insert 20 to be approximately 6 to 8 inches, however, lengths up to 12 inches may be used. Although any heat resistant metal or ceramic may be used, the insert 20 is preferably constructed of steel. It is well 15 within the capability of the wire EDM process to achieve accuracy in an internal seamless cavity of .0002" which is within the required tolerances of the pultrusion die. The challenge is to modify the conventional split cavity die 4 to accept the insert 20 with close tolerance match of the insert 20 to the assembled cavity. EDM fabrication can solve this problem. Conventional or RAM EDM can burn the pocket 33 into the conventional split cavity die 4 to a precise depth and 20 mating configuration. Alternate precision m~r~ining methods well known in the metal work industries may be employed to achieve the desired result.
The external geometry of the insert 20 can be wire cut as well to match the pocket geometry. The insert 20 can be secured in a manner which allows positive placement during insertion at time of assembly as shown in Figure 8. The dimensional match of the wire cut to 25 conventional cavity can be further assured by linear lapping or abrasive extrusion honing in an assembled state to reduce even minor edge mi.~m~tçl~.
Disassembly of a modified split cavity die 30 during cleaning if required would proceed by splitting the die 30 into top 31, insert 20, and bottom 32 component pieces as shown in Figures 8, 9 and 10. Any r.om~ining product that might be retained in the seamless portion of the die can 30 usually be cleared either by pulling the residual out of the seamless segment in the direction opposite to the pulling direction or in extreme cases of blockage by heating the insert 20 to a level above the decomposition temperature of the polymer which is around 700 degrees F. This temperature is well below the annealing or distortion temperature of the steel insert 20 and should have no permanent effect on the insert fit. In the case of a hollow profile, by extracting the CA 022~24 1998-12-07 mandrel 3 relative to the insert 20, the composite material will no longer be retained and will be freely removed from the cavity.
In addition, a further benefit is evident in that the insert 20 can be made of a hardened steel and with special surface treatment which enhances release while the remainder of the die 30 5 can be a conventional softer substrate with chrome plated surface. Even if the parting line 6 should break down preceding the insert 20, the material will not gel and replicate the surface of this section since the insert temperature at the leading edge can be m~int~ined below the activation le~ alule.
An additional benefit of the a~,ploach since it is based on a computer controlled 10 manufacturing technique, is the exact replication of replacement inserts when repair or replacement is eventually required.
It should be noted that the triangle-shaped cross-sectional die configuration described above is for illustrative purposes only. It is contemplated that that insert of the present invention may be shaped for use in any die cross-sectional geometry including, round, square, V-shaped or 15 any other die configurations, and placed at the point of surface replication. Similarly, it should be noted that the insert shape depicted in Figures 7- 11 is for illustrative purposes only. As shown on Figure 12, the insert may also be a hexahedron 35. The insert may be manufactured in any shape, so long as the insert encompasses an axial hole which can align with, and become a portion of, the die cavity and can m~int~in structural integrity during the pultrusion process. Finally, it 20 should be noted that the seamless insert may be mounted on either end of the split cavity die rather than being held within a pocket between the ends of the die.
As shown on Figure 20, the length of the gel zone of a thermosetting resin can be modified by increasing or decreasing the temperature of the die and the speed the material is pulled through the die. As shown by line A on Figure 20, if heat is added rapidly, the gel zone 25 occurs early in the die and lasts for a short length. However, if too much heat is added at once, the pultruded product is prone to have defects such as cracking. Conversely, as shown by line B
on Figure 20, if heat is added more slowly or speed is increased, the gel zone occurs later and is longer. However, given that the EDM process will produce seamless inserts of 6 to 8 inches, if heat is added too slowly or the speed is too fast, as in line C, the gel zone will be longer than the 30 seamless insert and the pultruded product may develop surface defects, such as a parting line, from the split cavity die. Therefore, it is desirable to determine what amount of heat added or what line speed will produce a gel zone shorter in length than the insert to be used and where this gel zone will occur. As shown on Figure 21, a die 60 or a hollow mandrel 61 can be modified to accept a series of thermocouples 62, 63 which may be used to determine the telllpelalule within CA 022~24 1998-12-07 the die cavity. Thermocouple 62 is depicted in a bore 64 in the die. Thermocouple 63 is depicted mounted in a hollow mandrel. Through a process of experimentation, varying die l~ Jelalule and pull speed while using materials and resins whose properties are known, the location and length of a gel zone within a specific die can be determined. The method of designing the split cavity 5 die would involve determining the location of the gel zone and positioning the seamless insert to be in the location of the gel zone. Alternatively, computer modeling of the heat transfer and the reaction of material can predict the optimum location for insert placement.
Once the location of the gel zone of a particular die is determined, the die assembly can be manufactured by producing an upper die block with an axial groove, a lower die block with an 10 axial groove that can be attached to the upper die block to form a die assembly with an axial cavity, cutting a pocket within said die assembly at the location of said gel zone, then producing a seamless insert, with an axial cavity, that matches said pocket. When assembled, the die will have the seamless insert carried within the upper and lower die blocks at the location of the gel zone.
Once the die and seamless insert are manufactured, the die assembly can be used to create a pultruded product that does not have a parting line. To accomplish this, a continuous material comprised of reinforcing fibers which are impregnated with a heat curable thermosetting resin is pulled through a cavity within a split cavity die 30 that has at least one seamless insert 20 and a heating means 21. Using the controllable heating means, the thermosetting process is controlled 20 so that the exterior surface of the pultruded product cures within the seamless insert. To do this, the temperature upstream of the seamless insert is m~int~ined below the initiation temperature of the thermosetting resin. The seamless insert is maintained at a temperature where thermally induced curing of the thermosetting resin will begin to occur and will cure the outer surface of the pultruded product. The portion of the pultrusion die 30 downstream of the seamless insert 20 is 25 maintained at a temperature sufficient to complete the curing process of the thermosetting resin so that the resin is completely set as the pultruded product exits the die 30.
In addition to the single insert assembly described above, the invention furthercontemplates a multiple seamless insert assembly. The seamless inserts may be mounted in a contiguous fashion, as shown on Figure 13, or may be spaced intermittently, as shown on Figure 30 14. As with the single seamless insert assembly, the multiple seamless insert assembly, shown in Figure 13, is based on a split cavity die. For clarity, only the lower die block 40 is shown. The lower die block 40 has an axial groove 44. The lower die block 40 has been modified with a pocket 45 that accepts a series of inserts 41, 42, 43. Each insert 41, 42, 43 has an axial hole and can be wire cut to fit precisely within the pocket 45. When installed in the lower die block 40 the , - CA 022~24 1998-12-07 inserts 41, 42, 43 are mounted immediately adjacent to each other and each insert's axial hole aligns with the other inserts' axial holes and with the die's axial groove 45. The upper die block, not shown, also has an axial groove and an insert pocket. The upper die block can be attached to the lower die block 40 to create a pultrusion die assembly having an axial cavity that extends 5 through the length of the die. The shape of the die cavity upstream and downstream of the inserts 41, 42, 43 corresponds with the shape of the axial hole in the inserts 41, 42, 43.
Another embodiment, shown in Figure 14, envisions the split cavity die having insert pockets cut intermittently in the die. As with Figure 13, Figure 14 shows only the lower die block 50 with intermittent pockets 53 and an axial groove 52. Again, the seamless inserts 51 would 10 each have an axial hole and can be wire cut to match the pockets' 53 geometry. When installed in the lower die block 50 each inserts' 51 axial hole aligns with the die's axial groove 52. An upper die block, not shown, having an axial grove and intermittent insert pockets, can then be attached to the lower die block 50 to create a pultrusion die assembly having an axial cavity that extends through the length of the die. The shape of the u~sll~,alll and downstream of the inserts 51 15 corresponds with the axial hole in the inserts 51.
Each seamless insert in either multiple-insert assembly can be made of a di~.'t;lll material with di~l~ properties, e.g., the first seamless insert could be made of hardened alloy steel, which is useful for its wear resistance in the high friction beginning phase of the pultrusion process, while a downstream insert could be made with a surface treated with Teflon impregnated 20 chrome or other known treatments which have a better release property and lower coefficient of friction. Each seamless insert could be approximately 6 to 8 inches long. Additionally, a mandrel may be used with either the continuous or intermittent configuration to produce hollow protruded products.
The axial hole of the first seamless insert in a series must be of the same cross-section as 25 the die cavity. When the first seamless insert is heated above the initiation temperature of the resin, the surface of the pultruded product will begin to solidify in the seamless insert. Once the pultruded product's outer shell is sufficiently hard the pultruded product's shape is set and other seamless inserts, downstream of the first seamless insert, do not have to maintain the same cross-section as the die. For example, as shown in Figures 15 and 16, a second seamless insert 55 may 30 have an axial hole 56 with a larger diameter or shape as the die's axial cavity at either or both ends of the insert, while having a cross-section the same size as the die within the insert 57.
Thus, a gap between the seamless insert and the product can be created. Given that, unless heated, any downstream insert segment that remains in contact with the pultruded product will act as a heat sink cooling the pultruded product. As shown on Figures 17-19, an in~ul~ting effect can be achieved by modifying the exterior dimension of the insert so as to i~ JL the heat transfer from the seamless insert to the die. This can be accomplished by m~int~ining an internal axial hole 65 with the same diameter or size as the cavity within the die, but reducing the surface area of the seamless insert that contacts the die. As shown in Figures 17-19, this can be done by 5 cutting a seamless insert where both ends of the insert match the pocket geometry 58, but the center portion of the seamless insert has a reduced exterior width 59. Thus, when installed, the surface of the center portion of the insert will not contact the die and heat transfer will be reduced. Such a gap would be useful in creating an in~ ting region for m~int~ining the pultruded product at a higher temperature thus assisting in completing the cure of the 10 thermosetting resin.
Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those of ordinary skill in the art without departing from the spirit and scope of the invention as defined by the following claims, including all equivalents thereof.

Claims (28)

1. A pultrusion die comprising:
an upper die block and a lower die block defining a cavity; and a seamless insert carried by said upper and lower die blocks so as to form a portion of said cavity.

2. The pultrusion die according to claim 1 wherein said upper die block and said lower die block define a pocket, and wherein said insert is carried within said pocket.

3. A pultrusion die comprising:
an upper die block;
a lower die block attached to said upper die block to form a die assembly having an axial cavity; and a seamless insert having an axial hole, said seamless insert carried by said die assembly so that said seamless insert axial hole forms a portion of said die assembly axial cavity.

4. The pultrusion die according to claim 3 wherein said die assembly has an upstream end and a downstream end, and where said die assembly has a pocket between said upstream end and downstream end for carrying said seamless insert.

5. The pultrusion die according to claim 3 further comprising a controllable heating means for heating said die assembly.

6. The pultrusion die according to claim 5 wherein the controllable heating means maintains the temperature within the die assembly cavity upstream of said insert below the initiation temperature of the heat curable resin, the temperature within the axial hole of the insert above the initiation temperature of said resin, and the temperature downstream of said insert at a sufficient temperature to completely cure said resin.

7. The pultrusion die according to claim 3, further comprising a means for pulling at least one strand of continuous material of reinforcing fibers which are impregnated with a heat curable resin through said axial cavity.

8. The pultrusion die according to claim 3 wherein said seamless insert is 6 to 12 inches long.

9. The pultrusion die according to claim 3 further comprising at least one mandrel positioned within and through said die assembly cavity.

10. The pultrusion die according to claim 9 wherein the die has a bore which carries at least one thermocouple for determining the temperature within the die cavity.

11 11. The pultrusion die according to claim 9 wherein the mandrel is hollow and is provided with at least one thermocouple for determining the temperature within the die cavity.

12. A pultrusion die comprising:
an upper die block;
a lower die block attached to said upper die block to form a die assembly having an axial cavity; and a plurality of seamless inserts each having an axial hole, said seamless inserts carried within said die assembly so that said seamless inserts axial holes form a portion of said die assembly axial cavity.

13. The pultrusion die according to claim 12, further comprising means for pulling a continuous material of reinforcing fibers which are impregnated with a heat curable resin through said axial cavity.

14. The pultrusion die according to claim 13 wherein said die assembly has an upstream end and a downstream end, and where die assembly has at least one pocket cut between said u~ a end and said downstream end for carrying said seamless inserts.

15. The pultrusion die according to claim 13 further comprising a controllable heating means for heating said die assembly.

16. The pultrusion die according to claim 15 wherein the controllable heating means m~int~in~ the temperature within the die assembly cavity upstream of said insert below the initiation temperature of the heat curable resin, the temperature within the axial hole of the first insert along the process stream above the initiation temperature of said resin, and the temperature downstream of said first insert at a sufficient temperature to completely cure said resin.

17. The pultrusion die according to claim 12 wherein said pocket is cut to a size sufficient to accommodate multiple seamless inserts and said seamless inserts are carried contiguously within said pocket.

18. The pultrusion die according to claim 12 wherein said pockets are cut intermittently in said die assembly, each pocket cut to a size sufficient to hold a single seamless insert, and said seamless inserts are carried within each said pocket.

19. The pultrusion die according to claim 12 further comprising at least one mandrel positioned within said die assembly cavity and extending through said seamless insert axial hole.

20. The pultrusion die according to claim 19 wherein the die has a bore which carries at least one thermocouple for determining the te...l,e-alu.e within the die cavity.

21. The pultrusion die according to claim 19 wherein the mandrel is hollow and is provided with at least one thermocouple for determining the temperature within the die cavity.

22. A method of designing a split cavity die with a seamless insert comprising:
determining the location within the die of the gel zone given a certain thermosetting resin product; and positioning a seamless insert within a split cavity die at the position of said gel zone.

23. A method of producing a split cavity die with a seamless insert, comprising: producing an upper die block with an axial groove;
producing a lower die block with an axial groove that can be attached to form a die assembly with an axial cavity;
determining the position within the cavity of the gel zone for a thermosetting resin product;
cutting a pocket within said die assembly at the location of said gel zone; and producing a seamless insert, with an axial cavity, that matches said pocket.

24. A method for producing a fiber-reinforced thermoset article, comprising:
pulling a continuous material impregnated with a heat curable thermosetting resin through a split cavity die having a seamless insert; and controlling the thermosetting process such that the gelling of the said material's exterior surface occurs within said seamless insert.

25. A method for producing a fiber-reinforced thermoset article, comprising:
pulling a continuous material comprised of reinforcing fibers which are impregnated with a heat curable thermosetting resin through a cavity within a die that has at least one seamless insert portion and at least one split cavity portion.
maintaining the temperature of said die downstream of said seamless insert below the initiation temperature of said thermosetting resin.
maintaining the seamless insert at a temperature where thermally induced curing of the thermosetting resin will begin to occur;
maintaining the portion of said die downstream of said seamless insert at a temperature sufficient to complete the curing process of the thermosetting resin.

26. The method of claim 25, wherein the temperature of said seamless insert is maintained between 150°F and 250°F.

27. The pultrusion die according to claim 2 wherein said insert is carried within said pocket such that said die block acts as a heat sink for said insert.

28. The pultrusion die according to claim 2 wherein said insert is carried within said pocket such that there is a gap between at least a portion of said insert and said die blocks.