Method for forming a non-pneumatic tire

Abstract

The invention relates to a method for forming a non-pneumatic tire constructed from a thermoplastic elastomer that is pre-tensioned during the process of its formation. The pre-tensioning is provided by thermal means that are naturally available in the forming process. This creates an efficient manufacturing process without the complexity of adding pre-tensioning by mechanical means. The method for forming a non-pneumatic tire includes: injecting a tread portion (101) and an annular beam (102) having circumferential reinforcements into a mold that only supports a tread pattern on the outer diameter limits of the tread pattern; injecting a rim (104) extending radially inward from the annular beam (102); and forming an annular support (103) comprised of a thermoplastic elastomer extending radially inward from the annular beam (102). The thermoplastic elastomer is hardened, and the tire is removed from the mold.

Description

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 018,496, filed April 30, 2020. TECHNICAL FIELD

[0002] The present disclosure relates to non-pneumatic tires (NPTs) for on-road or off-road vehicles (e.g., cars, light and heavy trucks, all-terrain vehicles, zero-turn radius lawn mowers, military vehicles). In particular, it relates to off-road vehicles that can require higher speed and load capacity, and high damage tolerance. BACKGROUND

[0003] Non-pneumatic tires (NPTs) have advantages over pneumatic tires. Unlike pneumatic tires, NPTs are not pressure vessels. They do not fail due to loss of air pressure.

[0004] NPTs generate load through mechanical means (bending, tensioning, and / or compression of various design elements). On the other hand, pneumatic tires are pre-tensioned due to air pressure. Vertical load can be offset by reducing tension in pre-tensioning members (such as reinforcement layers). Such “release” of pre-tensioning can be more effective than causing structural deflection. These pre-tensioning layers can be thin and thus lightweight. As a result, load and speed capacity of pneumatic tires can be enhanced.

[0005] Prior art discloses methods of adding pre-tensioning to NPTs. For example, US 2019 / 0009613 (owned by the current applicant) discloses a spoke design and mechanical means of adding pre-tensioning to the spoke. As a result, load capacity and fatigue are improved. However, adding pre-tensioning via the disclosed mechanical means can be laborious.

[0006] NPTs can require inefficient and / or expensive manufacturing methods. An example of prior art involving NPT manufacturing is US 9,004,901 (owned by the current applicant). A method for forming a thermoset polyurethane spoke and bonding the spoke to a rubber tread and a central hub is disclosed. The mold is rotated, creating a centrifugal force that enables the mold to fill. Then, the liquid polyurethane cures and hardens, and the NPT can be de-molded. Operation of such equipment and procedures can be expensive and can require creation of new production procedures.

[0007] US 9,751,270 (owned by the current applicant) discloses a thermoplastic molding procedure for forming NPTs. Thermoplastic injection molding is a mature industry that can reduce NPT costs. However, there is no disclosure of pre-tensioning in this prior art.

[0008] The present disclosure bridges this gap by providing an NPT that includes a thermoplastic elastomer that is pre-tensioned during the process of forming the NPT. This pre-tensioning is provided by the thermal means that are naturally available in the forming process. Thus, the benefits of high efficiency manufacturing are combined with the efficiency of pre-tensioning without the complexity of adding pre-tensioning by mechanical means. SUMMARY

[0009] Aspects and advantages will be set forth in part in the following description, or can be obvious from the description, or can be learned through practice. The application has general application to vehicles that use tires. In particular, the application is especially suitable for off-road vehicles that can require high speed and high load and high energy absorption capability.

[0010] According to one aspect, a non-pneumatic tire is provided, comprising: a reinforced annular beam; an annular support extending radially inward from the annular beam, the support comprising a thermoplastic elastomer; and a rim extending radially inward from the annular beam. The annular support has a circumferential reinforcement such that an inner diameter surface does not flex more than 2 mm. Further, the annular support has a pre-strain of no less than 0.5% that results from thermal contraction after an injection molding operation.

[0011] According to one aspect, a non-pneumatic tire is provided, comprising: a reinforced annular beam; an annular support extending radially inward from the annular beam, the support comprising a thermoplastic elastomer; and a rim extending radially inward from the annular beam. The non-pneumatic tire has an initial tangent vertical stiffness that is at least twice a tangent vertical stiffness at a design load.

[0012] According to one aspect, a non-pneumatic tire is provided, comprising: a reinforced annular beam; an annular support extending radially inward from the annular beam, the support comprising a thermoplastic elastomer; and a rim extending radially inward from the annular beam. The reinforcement of the annular beam has a compressive strength of no less than 0.5 MPa, and the reinforcement is configured such that the annular beam is transversely isotropic in the RY plane.

[0013] According to one aspect, a process for forming an NPT is provided. The NPT has a tread portion having a tread pattern. A reinforced annular beam extends radially inward from the tread. The annular beam and the tread are formed in a first process. An annular support comprising a thermoplastic elastomer extends radially inward from the annular beam and is formed in a second process that includes thermoplastic injection molding. The second process includes a mold in which the annular beam and the tread are positioned. The mold only supports the tread pattern on an outer diameter limit.

[0014] These and other features, aspects, and advantages of the present application will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the application and, together with the description, serve to explain the principles. BRIEF DESCRIPTION OF DRAWINGS

[0015] With reference to the accompanying drawings, in which is shown embodiments of the application for the benefit of one skilled in the art, and including the best mode, the complete and detailed disclosure of which, characterized in that:

[0016] A detailed description of embodiments of the application will be provided below, by way of example only, with reference to the accompanying drawings, in which:

[0017] Figure 1 is an exemplary example of an NPT.

[0018] Figure 2 is an exemplary example of an NPT and a graph of load vs deflection for prior art NPTs.

[0019] Figure 3 is an exemplary example of a toroidal beam and a tread.

[0020] Figure 4a and Figure 4b is a Y-Z cross sectional view of a toroidal beam.

[0021] Figure 5 is a 2D FE model of a toroidal beam under pressure as seen in thermoplastic injection molding.

[0022] Figure 6 is a RY section view of a toroidal beam placed in a mold for thermoplastic injection molding.

[0023] Figure 7a and Figure 7b is an FEA simulation of the deflection of two toroidal beams under pressure as seen in thermoplastic injection molding.

[0024] Figure 8a and Figure 8b shows an FEA simulation of two NPTs as they are loaded onto an off-road obstacle.

[0025] Figure 9 shows the critical load of NPTs with different numbers of reinforcement layers.

[0026] Figure 10 shows a thermoplastic spoke, toroidal beam, and tread of an exemplary example of an NPT.

[0027] Figure 11a and Figure 11b shows the difference between spoke curve distance and linear distance between spoke ends.

[0028] Figure 12 is a cross-sectional view of an exemplary NPT showing the hub region.

[0029] Figure 13 FEA simulation of two NPTs loaded to 4000 N is shown.

[0030] Figure 14 Exemplary NPT loaded to 7000 N is shown.

[0031] Figure 15 is an FEA prediction of load vs. deflection for four NPTs.

[0032] Figure 16 is an FEA prediction of tangential perpendicular stiffness for three NPTs.

[0033] Figure 17 FEA prediction of contact face length vs. deflection for three NPTs is shown.

[0034] Figure 18 FEA prediction for a single spoke subjected to a critical buckling load is shown.

[0035] Figure 19 Exemplary gate location for an exemplary NPT is shown.

[0036] It should be expressly understood that the description and drawings are merely intended to illustrate the specific embodiments and are to be used for the help of understanding.

[0037] Terminology

[0038] Unless otherwise indicated, the following terms are defined as follows for the present disclosure, where material properties refer to those at ambient temperature:

[0039] “Hub” refers to any structure used to support a tire and capable of being attached to an axle.

[0040] When referring to a thermoplastic elastomer, “modulus” refers to the Young’s tensile modulus measured in accordance with ISO 527-1:2019.

[0041] When referring to a reinforcing cord or cable, “modulus” refers to the Young’s tensile modulus measured in accordance with ASTM D2969. The tensile modulus can be calculated as the secant modulus at 0.5% strain.

[0042] When referring to a reinforcing cord or cable, “compressive strength” refers to the compressive stress at failure under uniaxial compression, by ASTM D695. Journal of Composite MaterialsThe method disclosed in the article "Theoretical and experimental compressive strength of a glass fiber-vinyl ester pultruded composite" of the journal Composites Part A: Applied Science and Engineering, Volume 49, Issue 6, Pages 739-748, can be used to measure the compressive strength.

[0043] When referring to rubber, the "shear modulus" refers to the dynamic shear modulus measured according to ASTM D5992 - 96 (2018) at 10 HZ, 23 C and 2% strain. When referring to rubber, the "elongation modulus" refers to the Young's modulus measured according to ASTM D412.

[0044] The "design load" of a tire is the regular and expected operating load of the tire.

[0045] The "design contact length" is the contact length at the design load.

[0046] The measurement of the relationship between the vertical force and the deflection and the footprint of the tire can be performed according to SAE J2704. DETAILED DESCRIPTION

[0047] The present invention provides a non-pneumatic tire and a method of forming the non-pneumatic tire. For purposes of describing the present invention, reference will now be made to the detailed description and / or methods, one or more examples of which are illustrated in or by the drawings. Each example is provided by way of explanation of the invention and is not meant as a limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features or steps illustrated or described as part of one embodiment, can be used with another embodiment or step to yield yet another embodiment or method. Thus, it is intended that the present invention covers such modifications and variations as come within the scope or spirit of the appended claims and their equivalents.

[0048] Figure 1 An exemplary example of an NPT 100 is shown, with dimensions of 26 x 10-12. This is a common size for off-road applications. The tire defines a cylindrical coordinate system with a radial direction R, a circumferential direction Θ, and an axial direction Y. A convenient Cartesian coordinate system has X as the direction of travel of the tire, Y as the axial direction, and Z as the vertical direction.

[0049] The tire includes a tread portion 101, an annular beam 102 (which includes stiffeners in the circumferential direction), an annular support portion 103 (which includes thermoplastic elastomer), a rim portion 104, and a hub portion 105. In this embodiment, the annular support portion includes spokes extending in the radial direction, connecting the inner surface of the annular beam to the rim without intersecting any of the other spokes.

[0050] In this embodiment, the annular beam and the tread are formed in an initial molding operation. The annular beam, along with the hub portion 105, is placed in a mold for a secondary molding operation, in which thermoplastic injection is used to form the spokes.

[0051] Prior art constructions, such as the one described in US 9,004,901, use thermoset polyurethane to form the spokes. The current application discloses a method of forming spokes with thermoplastic injection of thermoplastic elastomer. Furthermore, this method can result in NPTs with pre-tensioned spokes. This pre-tensioning is due to thermal contraction of the spokes, as they are constrained at radially inward and outward limits by the rim and annular beam, respectively.

[0052] From a historical perspective, the inventors were first motivated by the superior economics of thermoplastic injection molding, as compared to rotational molding of thermoset polyurethane. A prototype of a research tire of size 24x12-12 was made, tested, and compared to a prior art tire including thermoset polyurethane spokes. This size of tire is commonly used in off-road vehicles, such as all-terrain vehicles (ATVs) and zero-turn radius (ZTR) lawn mowers. The results were surprising—better than expected. These positive surprises include:

[0053] 1. Tire rolling resistance was reduced by 20%, even though the same tread and annular beam portions were used for the research tire as for the prior art tire.

[0054] 2. A strong non-linear load vs. deflection curve was produced. The initial slope of the load vs. deflection curve of the current invention was higher compared to the prior art, even though the tangent slope at design load was essentially unchanged. This is shown in Figure 2 FIG. 2. Those skilled in the art of tire design will recognize the value of such performance. At a design load of 3000 N, the deflection of the example NPT was 4.5 mm (30%) less than the prior art tire. However, both NPTs had a tangent KZ of 160 N / mm at design load FZ = 3000 N. The example tire had a much higher initial KZ of 700 N / mm. The initial KZ was more than 4 times higher than the KZ at design load. Thus, for the example NPT, ride comfort was maintained while reducing deformation and flexing.

[0055] 3. The process for forming the thermoplastic web is relatively easy, even though significant problems have been anticipated. Thermoplastic injection molding requires high pressure. Pressures of 28 MPa (4000 psi) or more can occur at areas near the gate. Even at locations not near the gate, the pressure at the end of mold fill can be 100 psi (0.7 MPa), 200 psi (1.4 MPa), or more. For this reason, injection molding employs robust steel or aluminum molds, which are held together by several tons of clamp force. Rubber, even reinforced rubber, is not used as a surface to inject high pressure plastic into. However, Figure 1 The architecture of the NPT 100 apparently requires a large area of contact between the radially inward facing surface of the annular beam 102 and the annular support 103, which comprises a thermoplastic elastomer. The inventors had surmised that high injection pressures would distort the annular beam and the tread, resulting in poor control of the thickness of the thermoplastic elastomer. However, this was not the case. The process is stable and well controlled.

[0056] After careful study, the inventors identified a number of process and product attributes necessary to produce better results. Each of these process and product attributes is disclosed below.

[0057] Stiffened ring beam with specific stiffening characteristics

[0058] Figure 3 Exemplary embodiments of the tread 101 and the annular beam 102 are shown. They are one molded part, formed in an initial molding operation. In Figure 4a The annular beam is further illustrated in Figure 4b In this embodiment, the annular beam is reinforced with a circumferential reinforcement 301. In this embodiment, the reinforcement comprises 7 layers spaced radially apart. The beam comprises an elastic matrix material 302, which can comprise rubber. The spacing of the radial layers is approximately equal to the reinforcement pitch in the axial direction. Thus, this reinforcement forms a transversely isotropic composite material in the R-Y cross section of the annular beam. If the intercord spacing in the radial direction is within + / - 33% of the intercord spacing in the axial direction, the resulting composite material is approximately transversely isotropic.

[0059] The reinforcement can comprise steel cables or any suitable high stiffness and high strength material. In this exemplary embodiment, the reinforcement comprises continuous pultruded glass filaments. The inventors have found that this type of reinforcement can be particularly advantageous.

[0060] In Journal of Composite MaterialsThe article "Theoretical and experimental compressive strength of a glass fiber-vinylester pultruded composite" in the Journal of Composite Materials, Volume 49, Issue 6, Pages 739-748, discloses the properties of continuous pultruded glass fiber monofilaments. This reinforcement has high stiffness and strength in tension and compression. The properties of this reinforcement are advantageous for the exemplary NPT. These properties include:

[0061] • Young's modulus E = 40 GPa

[0062] High modulus is advantageous because the exemplary example toroidal beam acts as a pressure vessel during the thermoplastic injection molding process. High stiffness enables a more stable molding process.

[0063] • Compressive strength σc= 1.1 GPa

[0064] The off-road use of the exemplary example results in very high impact loads. Those skilled in the art of off-road tire design know that for transient events, dynamic loads can approach 4 times the design load. High compressive strength is a performance requirement.

[0065] • CTE = 5 x 10 -6 mm / mm-C

[0066] After the molding process, the toroidal beam anchors the radial outer end of the spoke. The low CTE of the circumferential reinforcement results in a toroidal beam that does not shrink after ejection, even at high molding temperatures. Thermal pre-strain is created in the thermoplastic spoke. Steel has a CTE = 12 x 10 -6 mm / mm-C, which is also an acceptably low value. By comparison, the thermoplastic elastomer used in the exemplary example has a CTE = 1.6 x 10 -4 mm / mm-C. When low CTE is combined with high modulus, a very effective structure results, enabling spoke pre-strain to occur.

[0067] The inventors performed 2D finite element analysis (FEA) to understand and quantify these effects.

[0068] Figure 5 is a structured 2D FEA of the molding simulation used to predict deflection in the toroidal beam and tread. In this 2D plane strain model, stiffness is expressed in units of width. Thus, calculations of reinforcement stiffness are normalized with respect to a cross-section that is 1 mm thick in the axial direction.

[0069] In a thermoplastic molding process, molten elastomer is injected into a mold, with the annular beam positioned in the mold. In an exemplary embodiment, the annular beam is positioned so that the radially inward surface 201 acts as the outer diameter limit of the mold cavity. The molding operation then forms at least a portion of the annular support 103 and at least a portion of the rim 104. The hub 105 can also be positioned in the same mold, and the molding operation can overmold the thermoplastic elastomer around the hub.

[0070] When the elastomer fills the mold and contacts the radially inward surface 201 of the annular beam, very high pressure acts on the surface 201. The annular beam includes a circumferential stiffener 301 and a matrix material 302. The tread 101 contacts the rigid mold surface 400 on the outer diameter limit 305 of the tread pattern. The tread groove wall 303 can deform freely. In other words, the presence of the tread pattern can create an air gap between the tread and the mold surface 400. Thus, the stiffness of the annular beam in the circumferential direction becomes very important.

[0071] To counter the pressure, the stiffener develops a tension T. This is necessary because the tread groove wall can deform. Since the tread can include rubber, the compressive stiffness of the tread will be small compared to the injection pressure. Due to the Poisson effect, the block will compress and bulge out. A stiff stiffener is necessary to mitigate this undesirable effect.

[0072] Figure 6 An exemplary position of the annular beam and the tread in the mold in the RY cross section is shown. The mold surface 401 is at the lateral limit of the annular beam and the tread. This cross section acts as a cutoff device, preventing molten elastomer from flowing out of the mold cavity and around the annular beam. In this exemplary embodiment, the mold surface 400 only constrains the outer diameter limit 305 of the tread. In other words, in combination Figure 5 , the mold surfaces 400 and 401 can be smooth and thus can be low cost. There is no need for support structures to exist for the tread groove bottom 304 or the tread pattern side 303.

[0073] Figure 7a and Figure 7b The deflection results (in mm) are shown for the case of P = 0.7 MPa (100 psi) in 7a. 7a shows the deflection for the case of no stiffener. If isotropic rubber with an elongation modulus = 7 MPa is used, the deflection of the radially inner surface is between 2.5 mm and 3.05 mm. This is too large. Those skilled in the art of thermoplastic injection molding will recognize that wall thicknesses are typically 3 mm. Having a defined mold surface with deflection as much as the desired wall thickness would create serious problems. In addition, there is a circumferential variation in thickness of about 0.5 mm.

[0074] In contrast, Figure 7bResults are shown with reinforcement. Deflection is about one fifth of the above. Inner surface deflection is now 0.50 mm to 0.62 mm. This level of deflection is controllable in the thermoplastic injection molding process.

[0075] Those skilled in the mechanical engineering arts will recognize that, Figure 5 Approximates the case of a pressure vessel with soft tire and high modulus reinforcement. Thus, the well-known formula relating reinforcement tension to internal pressure applies. Further use of the fundamental relationship between modulus, area, and strain can be added to create a relationship between the desired maximum allowable radial deflection and other design parameters. This is given below:

[0076]

[0077] where R = inner radius of the ring beam

[0078] P = injection pressure at the inner radius

[0079] W = ring beam width

[0080] A = total reinforcement cross section

[0081] E = reinforcement Young's modulus

[0082] e = reinforcement circumferential strain

[0083] and Re = radial deflection at the inner radius

[0084] The inventors have found that an injection pressure P = 1 MPa (146 psi) is representative of the pressure at the inner radius of the beam during the molding process. Further, the inventors have determined that an acceptable value for the radial deflection at the inner radius, Re, can be as low as 2 mm. Substituting these into equation (1):

[0085]

[0086] That is, the ring beam width times the square of the radius R divided by the product of the reinforcement Young's modulus and the total reinforcement cross section should be less than 2 cubic millimeters per Newton. For a representative pressure P = 1 MPa, equation (2) simply gives the radial deflection of the inner radius of the ring beam in mm. In some cases, an acceptable value can be as low as 2 mm. In some cases, it can be 1.5 mm. In other cases, it can be 1.0 mm or even lower.

[0087] Equation (2) is in units of Newtons and millimeters. Thus, stress is in units of megaPascals. Other unit systems should be converted to these units in order to apply equation (2).

[0088] The above analysis assumes that the reinforcement has a constant spacing in the axial direction for each of the reinforcement layers. It also assumes that the reinforcement layers extend across the lateral bounds of the toroidal beam. Designs that are inconsistent with the above are not outside the intent of this application. The above formula can still be used to approximate the deflection of the toroidal beam during the injection molding operation.

[0089] The following procedure can also be used to directly measure the toroidal beam deflection at the inner radius under injection pressure:

[0090] 1. Place the toroidal beam in a thermoplastic injection mold

[0091] 2. Measure the minimum radial distance from the mold entity used to form the toroidal support to the radially inward bounds of the toroidal beam

[0092] 3. Form the NPT by forming the toroidal support using a thermoplastic process

[0093] 4. Cut the NPT toroidal beam and toroidal support at the location of the minimum radial distance.

[0094] 5. Measure the radial thickness of the thermoplastic elastomer used to form the toroidal support.

[0095] 6. Subtract the in-mold distance from the actual thickness. This is the amount of deflection of the inner diameter surface of the toroidal band during the molding process.

[0096] As an exemplary example of equation (2), Figure 1 An NPT with a total reinforcement cross section A = 800 mm 2 . The reinforcement has a modulus = 40,000 N / mm 2 . Thus, EA = 3.2E+07 N. With an inner radius = 300 mm, and a width W = 250 mm, R∈ = 0.7 mm.

[0097] According to prior art US 7,201,194, 2 reinforcement layers are disclosed. Using the disclosed values G = 5 MPa, and E’film / G = 1000: 1, the total EA = 2.5E+06 N. Using R = 330 mm, as disclosed, and W = 250 mm, R∈ = 10.5 mm. This is too high, and would not be stiff enough for injection molding.

[0098] While a high efficiency production process requires a high tensile hoop stiffness, product performance requires a high hoop compressive strength. The inventors have found that a multi-layer reinforcement also improves load carrying capacity under off-road conditions where very high impacts against objects with reverse curvature are often generated. This was also investigated with 2D plane strain FEA.

[0099] Figure 8aNPT with 2 layers of glass filaments reinforcement in a toroidal beam are shown. These layers are separated by an isotropic rubber layer. The NPT is loaded onto a surface with a reverse curvature, the radius of which is equal to the radius of the NPT. At 14,500 N, the reinforcement layer at the outer diameter limit of the toroidal beam buckles. It becomes unstable. This can lead to failure in the rubber matrix or in the reinforcement itself.

[0100] Figure 8b NPT with 3 layers of reinforcement are shown. Now, there are 2 adjacent layers at the outer diameter limit of the toroidal beam. At a load of 17,000 N, the structure remains stable.

[0101] The "critical load" is the load at which buckling of the cords starts to occur. In Figure 9 The critical load is shown in the article referenced above in the Journal of Composites versus the number of reinforcement layers. For 6 layers, the critical load is predicted to be 27,500 N. This represents a multiplication of the performance of 2 layers. The mechanism of this improvement can involve the creation of a transversely isotropic reinforced toroidal beam.

[0102] In the article referenced above in the Journal of Composites, the critical buckling compressive stress of a unidirectional composite is given by:

[0103]

[0104] where σ c = critical buckling stress

[0105] G 12 = shear modulus in the plane of the applied stress

[0106] G m = matrix shear modulus

[0107] Vf = fiber (or cord) volume fraction

[0108] The unidirectional composite is assumed to be transversely isotropic. When the composite is not transversely isotropic, the critical buckling stress decreases. Thus, Figure 4b The reinforcement pattern shown in the article referenced above in the Journal of Composites is exemplary. Thus, the inventors have found that the formulas that are generally applied to so-called classical composites (fibers of carbon or glass, with a thermoset resin, such as vinyl ester) also describe the cord-rubber composites.

[0109] It is not obvious to the person skilled in the art of tire design to consider that a cord-rubber composite can behave like a classical composite of high modulus resin and fibers. The excellent prototype performance in the tests, as well as these modeling results, show that this is indeed the case.

[0110] The inventors have found that performance is best when the intercord distance in the radial direction is within 33% of the intercord distance in the axial direction. In some cases, this can be 25%, and in some cases, 10%, and in some cases even less.

[0111] Furthermore, the inventors have found that, according to equation (3), the critical buckling stress needs to be at least 5 MPa; in some cases, at least 8 MPa, and in other cases even higher.

[0112] The required cord volume fraction can be at least 0.15; in other cases, 0.25; in other cases, 0.35, and in other cases, even higher. The rubber matrix shear modulus can be at least 3 MPa, in some cases 5 MPa, and in some cases even higher.

[0113] Even without considering macroscopic buckling, the outer diameter reinforcement layer is subjected to high compressive stresses. Therefore, the cords themselves need to have high compressive strength. A journal article discloses a method of measuring the compressive strength of a single cord or cable reinforcement. Using this method, a continuous pultruded glass filament can have a compressive strength of 1.1 GPa. The inventors have found that exemplary NPTs benefit from a reinforcement having a compressive strength of at least 0.5 MPa; in other cases, 0.8 MPa; and in the most demanding uses, over 1.1 GPa. The same procedure described in the journal article can be used to measure the compressive strength of any cord or cable.

[0114] Creating pre-tensioned non-inflatable structures

[0115] Figure 10 is an exemplary NPT. The annular support includes thermoplastic elastomer spokes 103 that extend in the radial direction, not intersecting other spokes in the wheel. The outer diameter limit adheres to the inner diameter limit 201 of the annular beam. A uniform thickness 401 of thermoplastic elastomer adheres to the inner diameter surface of the annular beam in an injection molding process. The inner diameter limit adheres to the outer diameter limit of the rim portion 104, which can include thermoplastic elastomer. The rim can be thicker and more rigid than the spokes. Therefore, this creates a rigid two-end fixed boundary for the spokes.

[0116] Figure 11a An R-T cross section of an exemplary NPT as configured in a mold is shown, prior to ejection. The spokes 103 extend from a radially outer limit B (joined to the annular beam 102) to a radially inner limit A (joined to the rim 102). The curved distance Lc is the length of the spokes prior to ejection. In this representative example, the curved distance Lc = R a, where the angle a is in radians. The distance L is the linear distance between point A and point B.

[0117] When the spokes cool after being demolded, thermal contraction occurs and the unstressed length decreases. The degree of tensile pre-strain in the cold spokes relates to the amount of thermal contraction and the difference between the spoke curve distance and the linear distance. This can be approximated as follows:

[0118]

[0119] where: S PS is the spoke pre-strain in percent.

[0120] CTE is the coefficient of linear thermal expansion

[0121] T H is the spoke temperature in the mold

[0122] T C is the ambient temperature

[0123] L C is the spoke curve length in the mold

[0124] L is the distance between the spoke radial limits

[0125] The associated spoke pre-tension is given by:

[0126]

[0127] where: S PT is the spoke pre-tension

[0128] E S is the spoke modulus

[0129] A S is the spoke cross-sectional area

[0130] Figure 11b The post-molding geometry of an exemplary NPT is shown Figure 10 a. Due to the circumferential stiffeners of the annular beam 102, the inner radius R B of the annular beam is relatively constant by using low CTE stiffeners. Similarly, the inner radius R R of the rim 104 is relatively constant or can even be slightly smaller. Exemplary examples of rim designs will be shown in the following sections.

[0131] The inventors have found that a spoke pre-strain S PS of at least 0.5% is needed to give the previously disclosed advantageous load vs. deflection performance; in some cases, a pre-strain of at least 1.0% is better; in other cases, a pre-strain of at least 1.5% is beneficial, and in other cases, even more.

[0132] The actual spoke pre-strain can vary depending on equation (4) depending on other construction factors such as the CTE of the reinforcement. The true pre-strain can be measured by:

[0133] • Measure the actual spoke length from point A to point B (outer diameter limit and inner diameter limit of the spoke) after molding.

[0134] • Mark the points used to measure this distance.

[0135] • Cut the spoke from the NPT.

[0136] • Measure the unstressed spoke length after it has been removed from the NPT

[0137] If the spoke is in tension in the molded tire, the unstressed length will be smaller. The amount it is smaller divided by the length before removal can give the percent pre-strain of the tire.

[0138] For in-mold spoke geometry Figure 11a The simple geometry used for in-mold spoke geometry

[0139] Figure 12 An example NPT R-Y cross section is shown. The rim material 104 can comprise a thermoplastic elastomer that is formed in the same operation as the spokes. In addition, the hub 105 can have a portion 501 that extends in the axial direction. In some cases, the hub can extend 15% over the axial width of the rim; in other cases, the hub can extend 30%; and in other cases, the hub can extend more than 50% over the axial width.

[0140] With a steel hub design that extends more than 50% over the axial width of the rim, the rim R B Can not change during the molding operation. Steel has a high modulus and low CTE that constrains the rim. If the hub extends 15% or less over the axial width of the rim, the rim R B Can decrease as the tire cools. Since a large portion of the rim comprises a thermoplastic elastomer, R B Decreases due to the high CTE. This can increase the amount of pre-strain in the spokes to more than the value given in equation (5).

[0141] This example rim design specifically embodies a hub overmolded by the same thermoplastic elastomer used to form the spokes and the rim. It can be low cost, but is very effective in providing anchoring for the lower end of the spokes.

[0142] High modulus spoke with high buckling load

[0143] The advantages of spoke pre-strain can be amplified with other design parameters. For example, the inventors have found that spoke modulus and spoke thickness can greatly accentuate positive performance. These were also investigated with 2D FEA, along with the effects of thermal contraction. The cumulative effects of these design parameters are surprising. The "tension-based" NPT can be transformed into a hybrid NPT that supports load through a combination of pre-tension, compression, and tension.

[0144] Tire size 33x10-15 is commonly used for larger utility task vehicles (UTVs). Four versions shown below were modeled in this size. Each version has the same toroidal beam. The only differences relate to the design parameters in the table below.

[0145] Version Spoke pre-strain (%) Spoke thickness (mm) Spoke modulus (MPa) V1 0 4 45 V2 1 4 45 V3 1 5 90 V4 1 6 90

[0146] Figure 13 The deformed geometry of V1 and V2 is shown. The only difference between them is spoke pre-strain. At the design load of 4000N, 1% spoke pre-strain apparently results in V2 having less deflection and less spoke deformation than V1.

[0147] Figure 14 Version V4 is shown at 4000N and 7000N. At 4000N, the deflection is minimal - the spoke deformation is very small. At the load of 7000N, the deflection is less than the reference version V1 at the load of 4000N. Although the spoke pre-strain has not changed from version V2 to V4, the spoke pretension has increased. The spoke modulus and spoke thickness have increased, which results in higher spoke pretension, as given in equation (5).

[0148] Figure 15 FEA predictions of load vs. deflection for V1, V2, V3, and V4 are shown.

[0149] • From V1 to V2, the effect of spoke pre-strain alone can be seen.

[0150] • From V2 to V3, the effect of increasing both spoke modulus and thickness can be seen.

[0151] • From V3 to V4, the effect of further increasing spoke thickness can be seen.

[0152] In a cumulative manner, pre-strain, modulus, and thickness can greatly increase the load carrying capacity of the example NPT. For example, V4 has less than 10mm deflection at the design load of 4000N, while the reference version V1 requires 30mm deflection.

[0153] The primary effect of the design lever described above is to increase the initial tangent KZ. At the design load of 4000N, the spoke pretension is 0.5% of the design load. The spoke pretension is proportional to the initial tangent KZ, as given in equation (5). Figure 15The relationship of tangent KZ to deflection is shown in FIG. 1 for each of V1, V2 and V3. At low deflection, the tangents KZ of tires V2 and V3 are much higher than V1. At larger deflection, the tangents KZ become closer.

[0154] The inventors have found that the tangent KZ at zero deflection can be at least 2 times the tangent KZ at the deflection that produces the design load. In other cases, this can be at least 3 times; in other cases, this can be 4 times, and in other cases, even more.

[0155] From Figure 14 It is apparent that even at very small deflection, the example NPT can produce a long contact patch. In Figure 17 The relationship of contact patch length to deflection is shown in FIG. 1 for each of V1, V2 and V3. V1 requires 31 mm of deflection to give the design load FZ = 4000 N. At that deflection, V1 produces a contact patch length of 135 mm. Thus, the contact length divided by the vertical deflection is 4.3. At the same load, V2 requires 22 mm of deflection and a contact length of 125 mm. This yields a ratio of 5.7. V3 requires 11 mm of deflection and a contact length of 110 mm, a ratio of 10.0.

[0156] Those skilled in the art of tire design will recognize the benefit of producing a long contact patch at very small deflection. Traction, especially off-road traction, is strongly related to contact patch length. The example NPT can combine low deflection with a long contact patch. Thus, low rolling resistance and improved durability can be combined with high traction performance.

[0157] The inventors have found that the ratio of tire deflection at design load to contact patch length at design load can be as high as 5. In other cases, it can be as high as 7; in other cases, it can be as high as 10, and in other cases, even higher.

[0158] After experimentation and modeling, the inventors have understood that even a moderately thick spoke can contribute greatly to load carrying capacity. Those skilled in the art of mechanical engineering are familiar with the Euler critical load for a compression strut.

[0159] The Euler critical buckling load formula for a compression strut that is fixed at both ends is:

[0160]

[0161] For the case of a rectangular spoke cross section as a compression strut:

[0162]

[0163] where: F C is the buckling load

[0164] E is spoke modulus

[0165] I is spoke moment of inertia in the R-T plane

[0166] w is spoke width

[0167] t is spoke thickness

[0168] L is the distance from the intersection of the spoke and the annular beam to the intersection with the rim

[0169] The inventors performed 3D FEA of spoke structures. Figure 18 The deformed geometry of a representative model is shown. The FEA results agree very well with Euler's formula. For example, given a spoke length of 100 mm, a width of 200 mm, and a modulus of 100 MPa:

[0170] Thickness F C FEA]]> F C Euler criticality 2.5 mm 111N 102N 3.5 286 280 4.5 591 594

[0171] The results for a 4.5 mm thick spoke are quite surprising. Buckling requires almost 600 N of compressive force. Referring to Figure 13 and Figure 14 , several spokes buckle radially inward from the contact face. Thus, the 600 N buckling force per spoke is not negligible compared to the design load of 4000 N.

[0172] The inventors have found that the spoke buckling load can thus be used as a design parameter in the development of exemplary NPTs that carry both tensioned loads (tire top around, tensioned spokes away from the contact area) and compressed spokes (spokes in the contact area). This is one of the effects seen in Figure 14 , where V4 has spokes with a higher buckling load.

[0173] The inventors have found that exemplary NPTs can have spokes with a critical buckling load that is at least 10% of the design load; in other cases, at least 15% of the design load; in other cases, at least 20% of the design load, and in other cases, even higher.

[0174] To reduce the injection pressure exerted by the thermoplastic elastomer on the radially inward surface of the annular beam, the inventors have found it advantageous to position the injection gate near the rim portion of the exemplary NPT. This is shown in Figure 19 , where the injection gate 600 is located at or near a portion of the rim. Exemplary NPTs can have thermoplastic elastomer that includes at least a portion of the rim. In such cases, the elastomer can first form a portion of the rim before forming the annular support.

[0175] Mold flow analysis (MFA) has shown that the injection pressure decreases as the thermoplastic material flows from the rim portion through the annular support, which can include radially oriented spokes. When the material reaches the radially inward facing surface of the annular beam, the pressure can decrease to 2 MPa (290 psi), or even as low as 1 MPa (140 psi), even though the injection pressure near the gate 600 can be higher.

[0176] Particular additional elements are not described or shown that can be needed for the operation of some embodiments, as it is assumed that they are within the ability of one of ordinary skill in the art. Also, particular embodiments can function without, can lack, and / or can function with any elements not specifically disclosed herein.

[0177] In some examples of embodiments, any feature of any embodiment discussed herein can be combined with any feature of any other embodiment discussed herein.

[0178] While various embodiments and examples have been presented, they have been presented by way of description only and are not intended to be limiting. Various modifications and enhancements will occur to those of ordinary skill in the art.

[0179] As used herein, the term "method" or "process" refers to one or more steps that can be performed in a different order than shown without departing from the scope of the invention. The order of any steps is exemplary and is not intended to limit the methods described herein to any particular order, nor is it intended to exclude adding steps, omitting steps, repeating steps, or performing steps simultaneously.

[0180] Every document cited herein, including any cross referenced or related patent or application, is hereby incorporated by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any application disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such application. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall control.

Claims

1. A method for forming a non-pneumatic tire, the method comprising: The tread portion and the annular beam are injected into a mold. The tread portion includes a tread pattern, and the annular beam extends radially inward from the tread portion and includes circumferential reinforcements. The mold supports the tread pattern only at the outer diameter limit of the tread pattern. Injected into the rim extending radially inward from the annular beam; as well as A ring-shaped support is formed, the ring-shaped support being composed of a thermoplastic elastomer extending radially inward from the ring beam.

2. The method according to claim 1, wherein the annular support is formed by thermoplastic injection molding.

3. The method of claim 2, wherein the non-pneumatic tire further comprises a rim extending radially inward from the annular support.

4. The method of claim 3, wherein the annular support includes spokes extending in the radial direction to connect the annular beam to the rim.

5. The method of claim 4, wherein the rim comprises the thermoplastic elastomer.

6. The method according to any one of claims 1 to 5, wherein the gating gate is located at the radially inward limit of the annular support.

7. The method according to any one of claims 1 to 5, wherein the gating gate is located at the radially inward limit of the rim.

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