Method for preparing thermoplastic fiber and reinforced fiber composite board through spiral blending process and application of thermoplastic fiber and reinforced fiber composite board

Abstract

The invention relates to a method for preparing a thermoplastic fiber and reinforced fiber composite board through a spiral blending process and application of the thermoplastic fiber and reinforced fiber composite board. The method comprises the following steps: pretreating thermoplastic fibers and reinforced fibers, drying and dewatering the two fibers, and splitting the two fibers into uniform tows through a beam splitter; spirally winding and twisting the two fibers to form a spiral fiber bundle; the spiral fiber bundles are made into composite woven cloth through a two-dimensional weaving machine, and the spiral fiber bundles are arranged in a staggered mode to form an in-plane multidirectional enhanced network; stacking a plurality of layers of composite woven cloth according to a preset angle, performing hot press molding, and performing gradient cooling to form the fiber cloth reinforced thermoplastic composite board. According to the method, the two kinds of fiber filaments are spirally wound and twisted, local gathering is avoided, the fiber filaments are woven into the cloth, the fibers are distributed at multiple angles, the in-plane anisotropy is low, the spiral structure absorbs impact energy, the impact strength is improved, and the method can be widely applied to the fields of aerospace, automobile lightweight, building structures and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of reinforced composite manufacturing, and in particular to a method for preparing a thermoplastic fiber and reinforced fiber composite plate through a spiral blending process and application thereof. Background Art

[0002] Different from traditional short fiber reinforced composites and long fiber composites, the reinforcing fibers in continuous fiber composites are continuous, and the fiber retention length is consistent with the product size, so the mechanical properties are further improved. Continuous fibers show a full-fold performance improvement on the overall material compared to non-continuous fibers. Among them, woven fiber cloth reinforced composite panels have the most excellent performance as high-performance structural parts, and are widely used in high-performance structural parts for land transportation such as automobiles and high-speed railways, aerospace structural parts, and high-performance military structural parts. Currently, most fiber cloth reinforced composite panel products on the market are thermosetting components, which have problems such as non-recyclability, high curing energy consumption, and long processing cycles. Fiber cloth reinforced thermoplastic composite panels are gradually becoming the hot products of the next generation of high-performance composite panels.

[0003] The key technology of fiber cloth reinforced thermoplastic composite panels lies in the production of their prepregs. Currently, the thermoplastic prepreg methods for fiber cloth include melt impregnation, solution impregnation, film lamination, and fiber blending. High-performance special engineering plastics generally have high melting temperatures, resulting in poor fiber wettability and weak interface bonding. Conventional melt impregnation processes are prone to damage to fibers and are difficult to adhere evenly; the solution impregnation method is not suitable for resins with good solvent resistance, and the use of large amounts of solvents is not conducive to industrial production; the resin in the film lamination method is difficult to infiltrate into the fiber gaps, but accumulates between two layers of fibers, resulting in a product that is too thick.

[0004] Fiber blending offers excellent composite effects and flexible fiber type design, making it a process technology with certain advantages in fiber cloth-reinforced thermoplastic composite panels. However, in existing fiber blending technologies, thermoplastic fibers and reinforcing fibers are only arranged in parallel or staggered patterns. The fiber distribution is easily affected by weaving tension, and "thermoplastic fiber-rich regions" or "reinforcement fiber-rich regions" may appear. Interfacial bonding depends on resin impregnation, significantly impacting shear strength. Furthermore, thermoplastic fibers in direct blending processes are often arranged in only a single direction, such as vertical, parallel, or twill. During hot pressing and melting, they exhibit a distinct molecular melt flow orientation, with an in-plane anisotropy ratio typically <0.5. Summary of the Invention

[0005] In view of this, a method for preparing a composite sheet of thermoplastic fibers and reinforcing fibers by a spiral blending process is provided, which has effective and uniform mixing, high efficiency, low cost and improved impact strength, and its application.

[0006] A method for preparing a composite sheet of thermoplastic fibers and reinforcing fibers by a spiral blending process comprises the following steps: Pretreatment of raw fiber; drying the thermoplastic fiber in an oven at 60-120℃ to remove moisture; separating the thermoplastic fiber and reinforcing fiber into uniform tows through a beam splitter; Spiral twisting: The thermoplastic fiber filaments and the reinforcing fiber filaments are spirally twisted at a predetermined twist to form a spiral fiber bundle. During the twisting process, the thermoplastic fiber is heated to above its glass transition temperature to soften it and closely adhere to the reinforcing fiber. Weaving into cloth; the spiral fiber bundles are made into a composite woven cloth through a two-dimensional weaving machine, and the spiral fiber bundles are staggered to form an in-plane multi-directional reinforcement network; Laying and hot pressing: stacking multiple layers of the composite woven fabric at a preset angle, placing them in a hot pressing mold, hot pressing them at a temperature 10-30°C higher than the processing temperature of the thermoplastic fiber, and forming a fiber cloth reinforced thermoplastic composite board after gradient cooling.

[0007] Preferably, the thermoplastic fiber is selected from at least one of nylon fiber, polyphenylene sulfide fiber, polyester fiber, polypropylene fiber, polyethylene fiber, polyacrylaldehyde fiber, polyphenylene ether fiber, polyimide fiber, polyetheretherketone fiber, and polysulfone fiber; the diameter of the thermoplastic fiber monofilament is 5-50um; the reinforcing fiber is selected from at least one of carbon fiber, glass fiber, basalt fiber, aramid fiber, polyphenylene benzobisoxazole fiber, polyaryl oxadiazole fiber, and ultra-high molecular weight polyethylene fiber.

[0008] Preferably, the diameter of the fiber bundles after being split by the beam splitter is less than 1 mm.

[0009] Preferably, the twist of the spiral fiber bundle during spiral twisting is 200-1200 TPM / m. In step S20, the spiral twisting is performed using a double twisting machine or a ring twisting machine equipped with a temperature control module to control the temperature to 90-150°C. During the spiral twisting, the tension of the reinforcing fiber is controlled to 5-60gf, and the tension of the thermoplastic fiber is controlled to 3-30gf.

[0010] Preferably, in the spiral twisting step, the mass ratio of the thermoplastic fiber filament to the reinforcing fiber filament is 0.5<1.3.

[0011] Preferably, in the step of weaving the cloth, the weaving method is one of plain weave, twill weave and satin weave.

[0012] Preferably, in the step of weaving the composite woven fabric, the thickness of the composite woven fabric is 0.05 mm to 0.5 mm; and in the layering process, the number of laminated layers of the composite woven fabric is 2 to 500.

[0013] Preferably, after hot pressing, the fiber cloth reinforced thermoplastic composite plate has a thickness of 0.5-10 mm.

[0014] Preferably, the multi-layer composite woven fabric is laid out at a predetermined lay-out difference angle so that the warp and weft threads are staggered, and then placed in a multi-layer hot and cold laminating machine, heated, preheated, and pressurized for 10 minutes to 1 hour, and then cooled and taken out to obtain a fiber cloth reinforced thermoplastic composite plate of a predetermined thickness; when laying the layers, the two layers of composite woven fabric are staggered by 30-90 degrees.

[0015] Furthermore, a method for preparing thermoplastic fiber and reinforced fiber composite panels by the spiral blending process as described above is provided for application in preparing fiber-reinforced composite panels, aerospace structural parts, vehicle lightweight structural parts, military high-performance structural parts or building structural parts.

[0016] The above method for preparing thermoplastic fiber and reinforced fiber composite sheet by spiral blending process and its application have at least the following beneficial effects: 1. The above method forms a spiral interlocking structure by spirally winding thermoplastic fibers and reinforcing fibers. There is physical entanglement between the fibers, and the two fibers are evenly distributed, avoiding local aggregation and achieving effective and uniform mixing. In contrast, in traditional mixing methods, thermoplastic fibers and reinforcing fibers are only arranged in parallel or staggered manner, without a spiral winding structure. The fiber distribution is easily affected by the braiding tension, and "thermoplastic fiber-rich areas" or "reinforcement fiber-rich areas" may appear.

[0017] 2. Reduced processing energy consumption: The long-term high-temperature impregnation step caused by the traditional melt impregnation process is eliminated, and the fiber bundle process, spiral twisting process, and weaving process are all low-energy consumption processes.

[0018] 3. After the spiral fiber bundles are woven into a cloth, the fibers are distributed at multiple angles, and the in-plane anisotropy is low. The differences in the test results of the tensile strength and mechanical indicators of the composite board in the warp, weft, and diagonal directions are all less than 10%.

[0019] 4. The spiral structure formed by this method absorbs impact energy, the crack propagation path is tortuous, and the impact resistance is improved.

[0020] 5. By spirally twisting reinforcing fibers and thermoplastic fibers, first compounding and then weaving them into cloth, and then using a hot lamination process, high-performance fiber cloth reinforced thermoplastic composite panels can be produced and prepared. The products can be widely used in aerospace, lightweight automobiles, building structures and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The present invention provides a flow chart of a method for preparing a composite plate of thermoplastic fibers and reinforced fibers by a spiral blending process.

[0022] Figure 2 This is a schematic diagram of the structure of a spiral fiber bundle after spiral twisting formed according to the method in the figure.

[0023] Marked in the figure: 1. thermoplastic fiber, 2. reinforcing fiber. DETAILED DESCRIPTION

[0024] The present invention will be described in detail below with reference to specific embodiments and accompanying drawings.

[0025] The embodiment of the present invention provides a method for preparing a thermoplastic fiber and reinforced fiber composite plate by a spiral blending process, such as Figure 1 As shown, the following steps are included: Step S10, pre-treating the raw fiber; drying the thermoplastic fiber in an oven at 60-120°C to remove moisture; and separating the thermoplastic fiber and the reinforcing fiber into uniform tows by a beam splitter; Step S20, spiral twisting: spirally twisting the thermoplastic fiber filaments and the reinforcing fiber filaments at a predetermined twist to form a spiral fiber bundle, and heating the thermoplastic fiber to above its glass transition temperature during the twisting process to soften it and tightly fit the reinforcing fiber; Step S30, weaving into cloth; the spiral fiber bundles are formed into a composite woven cloth by a two-dimensional weaving machine, wherein the spiral fiber bundles are staggered to form an in-plane multi-directional reinforcement network; Step S40, laying and hot pressing: stacking multiple layers of the composite woven fabric at a preset angle, placing them in a hot pressing mold, hot pressing them at a temperature 10-30°C higher than the processing temperature of the thermoplastic fiber, and forming them into a fiber cloth reinforced thermoplastic composite board after gradient cooling.

[0026] Preferably, in step S10, the thermoplastic fiber is selected from at least one of nylon fiber, polyphenylene sulfide fiber, polyester fiber, polypropylene fiber, polyethylene fiber, polyacrylaldehyde fiber, polyphenylene ether fiber, polyimide fiber, polyetheretherketone fiber, and polysulfone fiber; and the diameter of the thermoplastic fiber monofilament is 5-50 μm. The reinforcing fiber is selected from at least one of carbon fiber, glass fiber, basalt fiber, aramid fiber, poly(p-phenylene benzobisoxazole) fiber, poly(aryl oxadiazole) fiber, and ultra-high molecular weight polyethylene fiber. The thermoplastic fiber is preferably dried in an oven at 60-120°C for 6-15 hours to fully dry and remove moisture and avoid bubbles during hot pressing.

[0027] Preferably, the diameter of the fiber bundles after being split by the beam splitter is less than 1 mm, ensuring that the fiber bundles have good twistability, achieving tight spiral twisting of the fiber bundles, and preventing the decrease in fiber flexibility and ductility caused by excessive diameter.

[0028] In step S20, preferably, the twist of the spiral fiber bundle during spiral twisting is 200-1200 TPM / m, which can be either Z twist or S twist. Furthermore, in step S20, the spiral twisting can be performed using a double twisting machine or a ring twisting machine equipped with a temperature control module to control the temperature to 90-150°C. During spiral twisting, the equipment temperature is heated to above the glass transition temperature or softening temperature of the thermoplastic fiber to ensure the tightness of the spiral structure between the fibers. Furthermore, the tension of the reinforcing fiber is controlled to 5-60gf and the tension of the thermoplastic fiber is controlled to 3-30gf to avoid fiber breakage.

[0029] like Figure 2 FIG. 1 is a schematic diagram showing the structure of a spiral fiber bundle after spiral twisting formed according to the above steps. The thermoplastic fiber 1 and the reinforcing fiber 2 are cross-twisted into a bundle. After spiral twisting, the thermoplastic fiber 1 and the reinforcing fiber 2 can be effectively and evenly twisted into a bundle, thereby achieving uniform mixing of the two fibers.

[0030] Preferably, in the spiral twisting step, the mass ratio of the thermoplastic fiber filament to the reinforcing fiber filament is 0.5<1.3.

[0031] In step S30, the weaving method is preferably one of plain, twill, and satin. Furthermore, a multi-axial warp knitting machine can be used. In step S30, the composite woven fabric preferably has a thickness of 0.05 mm to 0.5 mm.

[0032] During the plying process of step S40, the number of laminated layers of the composite woven cloth is preferably 2-500 layers. Preferably, after hot pressing, the thickness of the fiber cloth reinforced thermoplastic composite plate is 0.5-10 mm. The multi-layer composite woven cloth is laid out at a predetermined ply difference angle so that the warp and weft lines are staggered, and then placed in a multi-layer hot and cold laminating machine, heated, preheated, and pressurized for 10 minutes to 1 hour, and then cooled and taken out to obtain a fiber cloth reinforced thermoplastic composite plate of predetermined thickness. The predetermined ply angle is preferably 30-90 degrees, that is, the two layers are staggered by 30-90 degrees. In specific applications, adjacent layers of woven cloth are optimally laid out at an angle difference of 45°, and the angle difference between adjacent layers should not be too large to reduce the shear stress between the layers and prevent delamination.

[0033] After the above steps, a composite sheet of thermoplastic fiber and reinforced fiber is obtained. The composite sheet prepared by the above method can be used in aerospace structural parts, vehicle lightweight structural parts, military high-performance structural parts or architectural structural parts.

[0034] The following uses multiple examples to illustrate the preparation method of the method for preparing a thermoplastic fiber and reinforced fiber composite board through a spiral blending process, as well as various aspects such as the performance of the prepared composite board.

[0035] Example 1 Polyphenylene sulfide fiber (TORCON™, 20 μm monofilament diameter) was dried in a 110°C oven for 8 hours. The polyphenylene sulfide fiber and 12K carbon fiber T700 fiber tows were then separated into uniform tows using a beam splitter. The average diameter of the polyphenylene sulfide fiber after the splitting process was 115 μm, and the average diameter of the carbon fiber tow was 221 μm.

[0036] PPS fibers and carbon fibers were twisted using a ring twister equipped with a temperature control module. The temperature was controlled at 150°C, the carbon fiber tow tension was adjusted to 35 gf, the PPS fiber tow tension was adjusted to 25 gf, and the twist was adjusted to 400 TPM / m.

[0037] The spiral fiber bundles are made into a composite plain woven fabric with a warp density of 12 strands / cm, a weft density of 10 strands / cm, and a thickness of 0.12 mm using a two-dimensional weaving machine. The spiral fiber bundles are staggered to form an in-plane multi-directional reinforcement network.

[0038] The composite woven fabric was cut into 450mm x 450mm pieces. Fifteen layers of the composite woven fabric were laid at a 90° ply angle, with the warp and weft threads interlaced. The composite fabric was then placed in a multi-layer hot and cold laminator. The temperature was raised to 285°C, preheated for 3 minutes, and maintained at 450 kgf / cm³ for 15 minutes. The press was then cooled at 8°C / min to produce a 1.22mm thick fiber cloth-reinforced thermoplastic composite panel. Testing revealed interlaminar shear strength of 136MPa, tensile strength of 714MPa, flexural modulus of 57GPa, and a long-term operating temperature of 220°C (UL746B).

[0039] Example 2 Polyphenylene sulfide fiber (TORCON™, 20 μm monofilament diameter) was dried in a 110°C oven for 8 hours. The polyphenylene sulfide fiber and 12K carbon fiber T700 fiber tows were then separated into uniform tows using a beam splitter. The average diameter of the polyphenylene sulfide fiber after the splitting process was 115 μm, and the average diameter of the carbon fiber tow was 221 μm.

[0040] The carbon fiber bundles and polyphenylene sulfide fiber bundles were mixed and woven using a two-dimensional braiding machine to produce a composite plain woven cloth with a thickness of 0.08 mm. The carbon fiber had a warp density of 10 fibers / cm and a weft density of 7 fibers / cm, while the polyphenylene sulfide fiber had a warp density of 12 fibers / cm and a weft density of 10 fibers / cm.

[0041] The composite woven fabric was cut into 450mm x 450mm sheets. Twenty layers of the composite woven fabric were laid at a 90° ply angle, with the warp and weft threads interlaced. The composite fabric was then placed in a multi-layer hot and cold laminator. The temperature was raised to 285°C, preheated for 3 minutes, and maintained at 450 kgf / cm³ for 15 minutes. The press was then cooled at 8°C / min to produce a 1.24mm thick fiber cloth-reinforced thermoplastic composite panel. Testing demonstrated an interlaminar shear strength of 102 MPa, a tensile strength of 650 MPa, a flexural modulus of 48 GPa, and a long-term operating temperature of 220°C (UL746B).

[0042] Example 3 Homemade PA6 fibers (BASF Ultramid® B27, 15 μm monofilament diameter) were dried in a 110°C oven for 12 hours. The PA6 fibers and 12K carbon fiber T700 fiber tows were then separated into uniform tows using a beam splitter. The average diameter of the PA6 fibers after the splitting process was 153 μm, and the average diameter of the carbon fiber tows was 236 μm.

[0043] PA6 fiber and carbon fiber are twisted using a ring twister equipped with a temperature control module. The temperature is controlled at 90°C, the carbon fiber tow tension is adjusted to 35gf, the PA6 fiber tow tension is adjusted to 25gf, and the twisting twist is 500 TPM / m.

[0044] The spiral fiber bundles are made into a composite woven fabric with a warp density of 12 fibers / cm, a weft density of 10 fibers / cm, and a thickness of 0.14 mm using a two-dimensional weaving machine. The spiral fiber bundles are staggered to form an in-plane multi-directional reinforcement network.

[0045] The composite woven fabric was cut into 450mm x 450mm pieces. 13 layers of the composite woven fabric were laid at a 90° ply angle, with the warp and weft threads interlaced. The composite fabric was then placed in a multi-layer hot and cold laminator. The press was heated to 240°C for 5 minutes, held at 450 kgf / cm³ for 15 minutes, and then cooled at 8°C / min. This resulted in a 1.25mm thick fiber cloth-reinforced thermoplastic composite panel. Testing revealed interlaminar shear strength of 123 MPa, tensile strength of 756 MPa, flexural modulus of 49 GPa, and a long-term operating temperature of 135°C (UL746B).

[0046] Example 4 Homemade PA6 fibers (BASF Ultramid® B27, 15 μm monofilament diameter) were dried in a 110°C oven for 12 hours. The PA6 fibers and 12K carbon fiber T700 fiber tows were then separated into uniform tows using a beam splitter. The average diameter of the PA6 fibers after the splitting process was 153 μm, and the average diameter of the carbon fiber tows was 236 μm.

[0047] The carbon fiber bundles and PA6 fiber bundles were mixed and plain woven using a two-dimensional braiding machine to produce a composite woven cloth with a thickness of 0.08 mm. The carbon fiber had a warp density of 10 fibers / cm and a weft density of 7 fibers / cm, while the PA6 fiber had a warp density of 12 fibers / cm and a weft density of 10 fibers / cm.

[0048] The composite woven fabric was cut into 450mm x 450mm pieces. Twenty layers of the composite woven fabric were laid at a 90° ply angle, with the warp and weft threads interlaced. The composite fabric was then placed in a multi-layer hot and cold laminator. The temperature was raised to 240°C, preheated for 5 minutes, and maintained at 450 kgf / cm³ for 15 minutes. The press was then cooled at 8°C / min to produce a 1.23mm thick fiber cloth-reinforced thermoplastic composite panel. Testing demonstrated an interlaminar shear strength of 89MPa, a tensile strength of 672MPa, a flexural modulus of 42GPa, and a long-term operating temperature of 135°C (UL746B).

[0049] It should be noted that the present invention is not limited to the above-mentioned embodiments. Based on the creative spirit of the present invention, those skilled in the art can also make other changes. These changes made based on the creative spirit of the present invention should be included in the scope of protection required by the present invention.

Claims

1. A method for preparing a thermoplastic fiber and reinforced fiber composite board by a spiral blending process, characterized in that: The following steps are involved: Pretreatment of raw fiber: drying the thermoplastic fiber in an oven at 60-120℃ to remove moisture; Thermoplastic fibers and reinforcing fibers are separated into uniform tows by a beam splitter; Spiral twisting: The thermoplastic fiber filaments and the reinforcing fiber filaments are spirally twisted at a predetermined twist to form a spiral fiber bundle. During the twisting process, the thermoplastic fiber is heated to above its glass transition temperature to soften it and closely adhere to the reinforcing fiber. Weaving into cloth; the spiral fiber bundles are made into a composite woven cloth through a two-dimensional weaving machine, and the spiral fiber bundles are staggered to form an in-plane multi-directional reinforcement network; Laying and hot pressing: stacking multiple layers of the composite woven fabric at a preset angle, placing them in a hot pressing mold, hot pressing them at a temperature 10-30°C higher than the processing temperature of the thermoplastic fiber, and forming a fiber cloth reinforced thermoplastic composite board after gradient cooling.

2. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, wherein: The thermoplastic fiber is selected from at least one of nylon fiber, polyphenylene sulfide fiber, polyester fiber, polypropylene fiber, polyethylene fiber, polyacrylaldehyde fiber, polyphenylene ether fiber, polyimide fiber, polyetheretherketone fiber, and polysulfone fiber; the diameter of the thermoplastic fiber monofilament is 5-50um; the reinforcing fiber is selected from at least one of carbon fiber, glass fiber, basalt fiber, aramid fiber, poly(p-phenylene benzobisoxazole) fiber, poly(aryl oxadiazole) fiber, and ultra-high molecular weight polyethylene fiber.

3. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, characterized in that: The diameters of the fiber bundles after being split by the beam splitter are all less than 1 mm.

4. The method for preparing a thermoplastic fiber and reinforcing fiber composite sheet by a spiral blending process according to claim 1, wherein: The twist of the spiral fiber bundle during spiral twisting is 200-1200 TPM / m. In step S20, the spiral twisting is performed using a double twisting machine or a ring twisting machine equipped with a temperature control module to control the temperature to 90-150°C. During the spiral twisting, the tension of the reinforcing fiber is controlled to 5-60gf and the tension of the thermoplastic fiber is controlled to 3-30gf.

5. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, wherein: In the spiral twisting step, the mass ratio of the thermoplastic fiber filament to the reinforcing fiber filament is 0.5<1.

3.

6. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, wherein: In the step of weaving the fabric, the weaving method is one of plain weave, twill weave and satin weave.

7. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, wherein: In the step of weaving the composite woven fabric, the thickness of the composite woven fabric is 0.05 mm to 0.5 mm; in the layering process, the number of laminated layers of the composite woven fabric is 2 to 500.

8. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, wherein: After hot pressing, the fiber cloth reinforced thermoplastic composite plate has a thickness of 0.5-10 mm.

9. The method for preparing a thermoplastic fiber and reinforcing fiber composite plate by a spiral blending process according to claim 1, wherein: The multi-layer composite woven fabric is laid out at a predetermined lay-up angle, with the warp and weft threads interlaced. The fabric is then placed in a multi-layer hot and cold laminating machine, heated, preheated, and pressurized for 10 minutes to an hour. The fabric is then cooled and taken out to obtain a fiber cloth reinforced thermoplastic composite plate of a predetermined thickness. During the laying process, the two layers of composite woven fabric are staggered by 30-90 degrees.

10. The method for preparing a thermoplastic fiber and reinforced fiber composite sheet by a spiral blending process according to any one of claims 1 to 9 is used to prepare fiber-reinforced composite sheets, aerospace structural parts, vehicle lightweight structural parts, military high-performance structural parts, or building structural parts.

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