FULL-FORM FLAT-KNITTED HELMET SHELL PREFORM, PREPARATION METHOD, AND HELMET SHELL THEREOF

Abstract

Disclosed are s full-form flat-knitted helmet shell preform, the preparation method, and the helmet shell, which belong to the field of helmet material technology. Two different knitting directions of the helmet shell preforms are prepared by means of longitudinal knitting and transverse knitting combined with partial knitting respectively; Meanwhile, reinforced yarn is added during the knitting process of the preforms to obtain the transversely and longitudinally knitted helmet shell preforms with reinforced yarn. The fabric structure of the preforms disclosed in this invention is a flat-knitted three-dimensional fabric with reinforced yarns, which solves the problem of low tensile strength and high elongation of flat-knitted fabrics, and solves the poor impact resistance caused by poor bonding strength between layers of helmet shells to some extent. Moreover, the method of preparing helmet shells from preforms improves production efficiency, reduces material waste, and solves the problem of poor dimensional stability.

Claims

1. A preparation method of a full-form flat-knitted helmet shell preform, wherein the helmet shell preform is prepared in two different knitting directions, by respectively combining longitudinal knitting and transverse knitting with partial knitting; moreover, reinforced yarns are added during preform knitting to obtain a transversely knitted and longitudinally knitted helmet shell preform with reinforced yarns; The structure of the full-form flat-knitted helmet shell preform is a flat-knitted three-dimensional fabric structure with reinforced yarns, and the basic unit knitting process is as follows: two horizontal rows of interlock stitches are knitted on the front and back needle bed of the flat knitting machine, and then one horizontal row of non-buckling reinforcement yarns is knitted by inlay yarns, followed by one horizontal row of tuck stitch in the front and back needle bed of the flat knitting machine. After that, one horizontal row of non-buckling reinforcement yarns is knitted by inlay yarns. Finally, one horizontal row of tuck stitch is knitted on the front and back needle bed of the flat knitting machine; The method for preparing a helmet shell preform with reinforced yarns in a longitudinal knitting direction by combining longitudinal and partial knitting comprises the following steps: The basic unit of the flat-knitted three-dimensional fabric with reinforced yarn is knitted on the front and back needle bed of the flat knitting machine, and the knitting width is determined according to the arc length from the left ear to the right ear part of the helmet shell preform. When knitting the next basic unit of the flat-knitted three-dimensional fabric with reinforced yarns, the two ends of the knitting area are respectively partially knitted by means of narrowing 4-8 stitches for each basic unit of the flat-knitted three-dimensional fabric with reinforced yarns. After a certain number of narrowing stitches, 1-2 basic units of flat-knitted three-dimensional fabric with reinforced yarns are knitted with the initial knitting width, and then, when the number of narrowing stitches reaches a certain amount in the knitting area, a partial knitting is carried out at each end of the basic unit of a flat-knitted three-dimensional fabric with reinforced yarns by widening 4-8 stitches, until the knitting width reaches the initial knitting width, so that the basic unit of a longitudinally knitted helmet shell preform is obtained. According to the actual size of the arc length between the helmet shell from the forehead to the back of the head, 6-9 basic units of longitudinally knitted helmet shell preform are knitted to form a fully formed helmet shell preform knitted longitudinally; The method for preparing a helmet shell preform with reinforced yarns in a transverse knitting direction by combining transverse and partial knitting comprises the following steps: The basic unit of the flat-knitted three-dimensional fabric with reinforced yarn is knitted on the front and back needle bed of the flat knitting machine, and the knitting width is determined according to the arc length from the forehead to the back of the head of the helmet shell preform. When knitting the next basic unit of the flat-knitted three-dimensional fabric with reinforced yarns, the two ends of the knitting area are respectively partially knitted by means of narrowing 6-12 stitches for each basic unit of the flat-knitted three-dimensional fabric with reinforced yarns. After a certain number of narrowing stitches, 1-2 basic units of flat-knitted three-dimensional fabric with reinforced yarns are knitted with the initial knitting width, and then, when the number of narrowing stitches reaches a certain amount in the knitting area, a partial knitting is carried out at each end of the basic unit of a flat-knitted three-dimensional fabric with reinforced yarns by widening 6-12 stitches, until the knitting width reaches the initial knitting width, so that the basic unit of a transversely knitted helmet shell preform is obtained. According to the actual size of the arc length from the left ear to the right ear part of the helmet shell preform, 6-9 basic units of transversely knitted helmet shell preform are knitted to form a fully formed helmet shell preform knitted transversely; The yarns used in the method include at least one member selected from the group consisting of high tenacity polyester, aramid or ultra-high molecular weight polyethylene (UHMWPE).

2. The transverse and longitudinal knitting helmet shell preforms with reinforced yarns obtained by the method according to claim 1.

3. A helmet shell, which is prepared by compounding the transversely and longitudinally knitted helmet shell preforms with reinforced yarns according to claim 2 with the resin.

4. A method for preparing the helmet shell according to claim 3, including the following steps: The helmet shell preforms with reinforced yarns knitted transversely and longitudinally are laid in a helmet mold in a perpendicular manner; after which the resin solution is injected into the helmet molds, cured, and demolded to obtain the helmet shell.

5. A method for preparing the helmet shell according to claim 4, wherein the perpendicular manner of laying needs to ensure that the reinforced yarns in the transversely and longitudinally knitted helmet shell preforms are perpendicular to each other.

6. A method for preparing the helmet shell according to claim 4, wherein the resin solution is injected by a vacuum-assisted resin injection method, and the resin solution is injected into the mold using a vacuum pump, with a vacuum degree of 0.08 to 0.1 MPa.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0034] FIG. 1 is a schematic diagram of the knitting technology of a basic unit of a flat-knitted three-dimensional fabric structure with reinforced yarns.

[0035] FIG. 2 is a schematic diagram of the knitting technology of a full-form flat-knitted helmet shell preform knitted longitudinally.

[0036] FIG. 3 is a schematic diagram of the knitting technology of a full-form flat-knitted helmet shell preform knitted transversely.

[0037] FIG. 4 is a top view of a full-form flat-knitted helmet shell preform knitted longitudinally.

[0038] FIG. 5 is a top view of a full-form flat-knitted helmet shell preform knitted transversely.

[0039] FIG. 6 is a top view of a full-form flat-knitted helmet shell preform.

DETAILED DESCRIPTION OF THE INVENTION

[0040] In order to make the above purposes, features and advantages of the invention clearer and easily understood, the specific embodiments of the invention will be further described below in conjunction with specific drawings.

[0041] Many specific details are expounded in the following description to provide a comprehensive understanding of the invention, but the invention can also be implemented in other ways different from those described here. Those skilled in the art can make similar modifications without departing from the concept of the invention, so the invention should not be limited by the specific embodiments disclosed below.

Embodiment 1

[0042] This embodiment provides a preparation method of a full-form flat-knitted helmet shell preform, wherein the helmet shell preform is prepared in two different knitting directions, by respectively combining longitudinal knitting and transverse knitting with partial knitting; moreover, reinforced yarns are added during preform knitting to obtain a transversely knitted and longitudinally knitted helmet shell preform with reinforced yarns.

[0043] The longitudinal knitting specifically includes:

[0044] When knitting on a flat knitting machine, the knitting width is the arc length from the left ear to the right ear of the helmet shell, and the knitting width is related to the fineness of the yarn and the transverse density of the fabric; the knitting direction follows the helmet shell from the forehead to the back of the head or from the back of the head to the forehead.

[0045] The transverse knitting specifically includes:

[0046] When knitting on a flat knitting machine, the knitting width is the arc length from the forehead to the back of the head of the helmet shell, and the knitting width is related to the fineness of the yarn and the transverse density of the fabric; the knitting direction follows the helmet shell from the left ear to the right ear or from the right ear to the left ear.

[0047] The structure of the full-form flat-knitted helmet shell preform is a flat-knitted three-dimensional fabric structure with reinforced yarns. The three-dimensional fabric structure is formed by interlocking and tucking stitches. During the knitting process, non-buckling reinforcement yarns are added as inlay yarns to the inside of the three-dimensional fabric, resulting in a flat-knitted three-dimensional fabric structure with reinforced yarns. Adopting UHMWPE fibers to knit the full-form flat-knitted helmet shell preforms. Adopting 600D UHMWPE fibers to knit interlock and tuck stitches, while 1000D UHMWPE fibers as inlay yarns (reinforced yarns). The knitting technology of the basic unit is shown in FIG. 1: F represents the front needle bed, B represents the back needle bed. [0048] represents the knitting needle of needle bed [0049] .fwdarw. and ? represent the movement direction of yarn guide, respectively represents the movement from left-to-right and right-to-left. [0050] represents the stitch knitted on the the front needle bed [0051] represents the stitch knitted on the the back needle bed [0052] represents the tuck stitch knitted on the the front needle bed [0053] represents the tuck stitch knitted on the the back needle bed [0054] represents the yarn guide

[0055] The specific knitting process is as follows:

[0056] Two horizontal rows of interlock stitches are knitted on the front and back needle bed of the computerized flat knitting machine using yarn guide 1, after which one horizontal row of non-buckling reinforcement yarns is knitted with inlay yarns using yarn guide 2, and then one horizontal row of tuck stitch is knitted in the front and back needle bed of the computerized flat knitting machine using yarn guide 1, and then one horizontal row of non-buckling reinforcement yarns are knitted with inlay yarn using yarn guide 2. Finally, one horizontal row of tuck stitch is knitted on the front and back needle bed of the flat knitting machine to form the basic unit of a flat-knitted three-dimensional fabric with reinforced yarns.

[0057] The longitudinal knitting can realize the knitting of the helmet shell preform from the front to the back; The method for preparing a helmet shell preform with reinforced yarns in a longitudinal knitted direction by combining longitudinal and partial knitting is shown in FIG. 2, specifically as follows:

[0058] The basic unit of the flat-knitted three-dimensional fabric with reinforced yarns is knitted on the front and back needle bed of the flat knitting machine with the initial knitting width of 280 stitches. Knitting two horizontal rows of the basic units of the flat-knitted three-dimensional fabric with reinforced yarns, and then the two ends of the knitting area are respectively partially knitted by means of narrowing 6 stitches. After 15 times of narrowing, when the width reaches 100 stitches, the basic unit of the flat-knitted three-dimensional fabric with reinforced yarns is knitted for 2 rows with the initial knitting width of 280 stitches, and then, at each end of the knitting area with the knitting width of 100 stitches, a partial knitting is carried out by widening 6 stitches of the basic unit of a flat-knitted three-dimensional fabric with reinforced yarns. Until the knitting width reaches 280 stitches, the basic unit of a longitudinally knitted helmet shell preform is formed. After that, 6 basic units of the longitudinally knitted helmet shell preforms are cyclically knitted to complete a helmet shell preform in the longitudinal direction, as shown in FIG. 4.

[0059] The transverse knitting can realize the knitting of the helmet shell preform from the left to the right; The method for preparing a helmet shell preform with reinforced yarns in a transverse knitting direction by combining transverse and partial knitting is shown in FIG. 3, specifically as follows:

[0060] The basic unit of the flat-knitted three-dimensional fabric with reinforced yarns is knitted on the front and back needle bed of the flat knitting machine with the initial knitting width of 360 stitches. Knitting two horizontal rows of the basic units of the flat-knitted three-dimensional fabric with reinforced yarns, and then the two ends of the knitting area are respectively partially knitted by means of narrowing 10 stitches. After 10 times of narrowing, when the width reaches 160 stitches, the basic unit of the flat-knitted three-dimensional fabric with reinforced yarns is knitted for 2 rows with the initial knitting width of 360 stitches, and then, at each end of the knitting area with the knitting width of 160 stitches, a partial knitting is carried out by widening 10 stitches of the basic unit of a flat-knitted three-dimensional fabric with reinforced yarns. Until the knitting width reaches 360 stitches, the basic unit of a transversely knitted helmet shell preform is formed. After that, 6 basic units of the transversely knitted helmet shell preforms are cyclically knitted to complete a helmet shell preform in the transverse direction, as shown in FIG. 4.

[0061] After the helmet shell preforms finished knitting, they are placed at room temperature for 72 hours, and the average thickness of one piece of longitudinal or transverse knitting helmet shell preform is 2.5 cm, which is measured on the YG141 fabric thickness tester according to the standard of GB/3820 1999 (the determination of the thickness of textiles and textile products); Fabric surface density is 821 g/m.sup.2, fabric longitudinal density is 40 rows/5 cm, fabric transverse density is 28 rows/5 cm; Refer to the standard of GB/T 3923-2013 (Textile fabricsTensile propertiesPart 1: Test of tensile strength and elongation at break (strip method)), the tensile performance of helmet shell preform was tested on MTS universal strength tester. The test results show that the transverse tensile strength is 5 times higher than the flat-knitted three-dimensional fabric without reinforced yarns, and transverse elongation at break reduced by 92%, which greatly improved the transverse tensile mechanical properties of the flat-knitted fabric, thus improve the tensile mechanical properties of the flat-knitted fabric.

[0062] Knit two horizontal rows with the initial knitting width after the completion of the partial knitting with narrowing and widening of the helmet shell preform, which can reduce or eliminate the holes caused by the partial knitting with narrowing and widening, thus solving the problem of uneven surface density of the helmet shell preforms, and improving the impact resistance of the helmet shell in the narrowing and widening part.

[0063] As shown in FIG. 6, the full-form flat-knitted helmet shell preform is formed by laying two integrated preforms formed by longitudinal and transverse knitting in a perpendicular manner, which can make the non-buckling reinforcement yarns in the helmet shell preform with two different knitted directions approximately cross arranged in 90 degrees, and greatly improved the mechanical properties of the full-form flat-knitted helmet shell preform in different directions.

[0064] During the preparation of the preforms, the lack of reinforced yarns will lead to the large extension and low fabric surface density of the full-form flat-knitted helmet shell preforms. Moreover, the helmet shells obtained after compounding the preforms with resin have poor rigidity and large shape variable after impact, which cannot meet the requirements of impact resistance performance of the helmet shell. And by adding reinforced yarns as inlay yarns in the full-form flat-knitted helmet shell preforms can improve the tensile strength, reduce the extensibility and increase the surface density of the fabrics. The tensile performance test shows that the tensile strength of the full-form flat-knitted helmet shell preform with reinforced yarns can be increased by 5 times and the transverse elongation at break can be reduced by 92%.

[0065] During the preparation of the preforms, the desired shape of the helmet shell is knitted by using the partial knitting technology of flat knitting, and two horizontal rows of fabric with the initial knitting width of the preforms are knitted in the middle of the narrowing and widening areas, thus reducing or eliminating the holes produced at the narrowing and widening places (the holes will cause uneven surface density of the helmet shell preforms, and the prepared helmet shell will have poor impact resistance around the hole part), enhancing the mechanical properties of the connection points of narrowing and widening areas, improving the surface density uniformity of the helmet shell preforms, and thus improving the impact resistance of the helmet shell at this position.

Embodiment 2

[0066] This embodiment provides a preparation method of the helmet shell, including the following steps:

[0067] Laying the helmet shell preforms with reinforced yarns knitted transversely and longitudinally in a helmet mold in a perpendicular manner; then injecting the resin solution into the helmet molds, curing, and demolding to obtain the helmet shell.

[0068] The details are as follows: [0069] (1) First, wiping the releasing agent in the helmet mold evenly, after which the longitudinally and the transversely knitted helmet shell preforms obtained in embodiment 1 are laid in the helmet mold in a perpendicular manner; [0070] (2) At room temperature of 26? C., epoxy resin and curing agent were evenly mixed with a mass ratio of 2:1 to obtain resin solution. The resin is purchased from Changzhou Hualike New Material Co., LTD. The resin type is epoxy resin A02 and the curing agent is B02. [0071] (3) The resin solution prepared in step (2) is injected into the helmet mold by the vacuum assisted resin injection process to make it fully in contact with the helmet shell prefabricated parts. After the resin solution injection is completed, the helmet mold is solidified at room temperature for 24 hours, and then it is placed in the oven at 60? C. for secondary curing for 2 hours. After cooling to room temperature, the full-form flat-knitted helmet shell was obtained by demolding.

[0072] After testing, the helmet shell obtained mass is 480 g, fiber volume content is 46%.

Comparative Example 1

[0073] Replace the preform in embodiment 2 with carbon fiber plain fabric, and refer to the method in the reference (D. Thomas Campbell, David R. Cramer. Hybrid thermoplastic composite ballistic helmet fabrication study[J]. Advancement of Materials&Process Engineering, 2008, 32(3):135-146) to cut and layer the carbon fiber plain fabric to prepare the carbon fiber plain fabric helmet shell prefabricated part. By cutting and changing the number of layers, the surface density of the carbon fiber plain fabric helmet shell preform part is similar to that of the preform in embodiment 2. The rest is consistent with Embodiment 2 to obtain the helmet shell.

[0074] The impact resistance performance of the helmet shell prepared by Embodiment 2 and Comparative Example 1 (parameters are shown in Table 1) is tested. The test results are as follows:

TABLE-US-00001 TABLE 1 Parameters of the helmet shell prepared in Embodiment 2 and Comparative Example 1 Raw Layers Area density of Example material of fabric the preform(g/m.sup.2) Embodiment 2 UHMWPEs 2 1642 Comparative Carbon 7 1680 Example 1 fibers

TABLE-US-00002 TABLE 2 Testing results of the helmet shells prepared by Embodiment 2 and Comparative Example 1 Thick- Area Fiber volume Example ness(mm) density(g/m.sup.2) content (%) Mass(g) Embodiment 2 3 3422 46 480 Comparative 2.8 3308 44 468 Example 1

[0075] Referring to the standard ASTM D7136, the impact resistance of two kinds of helmet shells was tested on an Instron Dynatup 9250 drop weight impact tester, where the total weight of the drop hammer was 7.78 kg, the end of the punch was a hemispherical structure with a diameter of 12.7 mm, and the impact energy was 20J. After the impact was completed, the carbon fiber helmet shell in comparative example 1 showed penetrating damage in the impact area, while the full-form flat-knitted helmet shell in embodiment 2 only had pits with a depth of about 4 mm.