WO2006028449A1

WO2006028449A1 - Method for curing a tire

Info

Publication number: WO2006028449A1
Application number: PCT/US2004/028727
Authority: WO
Inventors: Douglas I. Greenwell; Michael Luneau; Donald Faulkner
Original assignee: Michelin Recherche et Technique SA Switzerland; Societe de Technologie Michelin SAS
Current assignee: Michelin Recherche et Technique SA Switzerland; Societe de Technologie Michelin SAS
Priority date: 2004-09-03
Filing date: 2004-09-03
Publication date: 2006-03-16
Anticipated expiration: 2007-03-03
Also published as: KR20070106966A; CN101432115A; CN101432115B

Abstract

A method is presented for the use of three-dimensional finite element analysis (3-D FEA) and thermocouple tests in determining the state of cure (SOC) for each zone of the tire as well as detailed features in the tire tread. From this detailed knowledge of these cold zones of the tire, different tread apertures are proposed to enhance heat transfer into these zones in order to provide a more optimum cure. A concept of heat transfer pins is developed as an efficient and practical means of accomplishing this optimized cure state. In addition, these new tread geometries are evaluated with respect to some basic tire tread performance parameters to understand their impact on the performance of the tire. Using 3-D FEA, these new combinations of tread apertures are analyzed and predictions of a more uniform SOC for all zones of the tire and a reduced cure time. Reductions in cure time of 15 % are demonstrated.

Description

Method for Curing a Tire

Field of the Invention

The present invention is in the field of rubber curing, more particularly in the field of curing pneumatic tires.

Background of the Invention

During vulcanization a rubber compound changes from a relatively weak, viscoelastic liquid to a relatively strong viscoelastic solid. Rubber articles, such as pneumatic tires, for years have been vulcanized or cured in a press wherein heat is applied both externally and internally of the tire mold for a certain length of time to effect the chemical vulcanization reaction in the tire. These presses, which are well known in the art, generally employ separable mold halves or parts (including segmented mold parts) with center shaping and curing mechanisms utilizing bladders into which shaping, heating and cooling fluids or media are introduced for shaping, molding and curing the tires which are composed of complex compounds of rubber, process accelerators, carbon black and other materials that are cross-linked in the presence of sulfur during the curing process. The aforesaid tire shaping and curing presses typically are controlled by a mechanical timer which cycles the presses through various steps during which the tire is shaped, heated and in some processes cooled prior to unloading from the press. During the curing process the tire is subjected to high pressure and high temperature for a preset period of time intended to ensure sufficient cure of the tire to allow it to be removed from the mold without deleterious effect. The cure process usually continues to completion outside the press. Rubber chemists are faced with the problem of predicting the time period within which each particular type of rubber compound will be satisfactorily cured and once such a time period is established, the tire is heated for that precise period. The extent of cure also could be affected by the composition and aging history of the uncured tire as well as variance in geometry from tire to tire. While such time control has been used to cure millions of tires, because of the varying properties of the rubber even within the same compound, some tires are slightly overcured while others are slightly undercured. Neither undercuring nor overcuring is desirable with respect to the quality of the end product. Undercuring can result in less than optimal endurance and tensile strength. Overcuring is undesirable because production time on the valuable capital vulcanizing machinery is thereby wasted, and production efficiency is reduced. It is well known that overcure of many tire and other rubber compounds results in reversion. Reversion degrades physical properties such as modulus, fatigue life, and the like. This effect is directly observed as a disadvantageous decrease in modulus relative to the optimum value.

Various curing presses and methods have been proposed, including: U.S. Patent Nos. 4,344,142; 4,044,600; 4,022,555; 3,819,915; 3,718,721; 3,659,974; 3,649,729; 3,102,425; 4,371,483; 4,517,146; 4,608,218; 4,779,206; 4,517,146; 4,819,177; 4,861,253; 2,128,417; 2,173,588; 3,443,280; 3,489,833; 3,632,712; 5,055,245;5,680,315; 3,397,583; 5,784,283; 6,478,991; 4,422,987; 4,027,543; RE 22188; 2,066,265; 1,477,879; 4,044,600; 4,568,259; 6,402,489; 2,031,560; 2,047,858; 1,718,631; 1,733,064; 1,604,451; 1,604,452; 5,221,379; 5,198,049; 2,627,888; 603,154. However, none has proven entirely satisfactory.

Thus, the rubber industry is faced with an issue of producing a better quality tire in a commercially satisfactory time period. A more "optimized cure" insures superior tire compound properties.

Summary of the Invention

The present invention is a method of curing a tire including a tread, carcass, and sidewall, comprising the steps of: (a) placing the tire inside a tire mold;

(b) applying heat to the tire;

(c) removing the tire from the tire mold; wherein if the sidewall has been cured to between about .90 and 1.40 alpha, the tread has been cured to at least .90 alpha. Another embodiment of the present invention is a method of curing a tread block of a tire tread, comprising the steps of:

(a) inserting a heat transfer element into the tread block;

(b) applying heat to the tread block;

(c) removing the heat transfer element from the tread block; wherein the cure state of any portion of the tread block is between about .85 alpha and .99 alpha.

A further embodiment of the invention is a method of optimally curing a tire comprising: (a) a carcass which comprises sidewalls and a crown;

(b) a tread affixed to said crown, wherein said tread comprises tread blocks; wherein the method comprises inserting said tire into a tire mold comprising heat transfer pins or heat pipes fixed to the interior of the mold, wherein said heat transfer pins or heat pipes are inserted into portions of the tire effecting a more even transfer of heat from the mold to the tire.

A further embodiment of the invention is a method of making a tire mold for curing tires comprising the steps of:

(a) determining the heating rate of substantially all portions of the tire to be inserted thereto;

(b) determining which portions of the tire should receive additional heat during the curing of said tire in order to effect an optimal cure of those portions of the tire during the time of cure, and to not overcure other portions of the tire, and

(c) affixing heat transfer pins into the tire mold to intrude into those portions of the tire requiring additional heat during the curing of said tire.

A further embodiment of the invention is a method of reworking a tire mold comprising the addition of heat transfer pins to said mold, wherein said heat transfer pins are configured so as to provide for a more even curing of all portions of a tire to be inserted into said mold.

Brief Description of the Drawings

Figure 1 shows a section of a flat tread for recapping a tire.

Figure 2 shows a tread pattern for a tread for recapping a tire. Figure 3 shows relative cross-section positions of a flat tread for recapping a tire.

Figure 4 shows the cure state (alpha) as a function of time, for the tread shown in

Fig. 3.

Figure 5 shows the cure state as a function of tread depth, for the tread shown in Fig. 3. Figure 6 shows the cure time at alpha=0.9 as a function of tread depth, for the tread shown in Fig. 3.

Figure 7 shows the cure state in the middle of the tread of Fig. 3, taken as a horizontal slice.

Figure 8 shows the changes from Fig. 7 after the use of heat transfer pins. Figure 9 shows the cure state in a horizontal slice of a tread.

Figure 10 shows the tread of Fig. 9, after the use of 14mm heat transfer pins.

Figure 11 is a profile of truck tire shoulder showing the state of cure at 22 minutes.

Figure 12 shows an FEA mesh of the shoulder of a truck tire, and the heat profile of the shoulder of the truck tire. Figure 13 shows a heat transfer element according to the present invention.

Figure 14 shows a heat transfer element according to the present invention.

Figure 15 shows a heat transfer element according to the present invention.

Figure 16 shows a heat transfer element according to the present invention.

Figure 17 shows a heat transfer element according to the present invention. Figure 18 shows a heat transfer element according to the present invention.

Figure 19 shows a heat transfer element according to the present invention.

Figure 20 shows an FEA model of heat transfer pins protruding into a tire and in a tire mold. Figure 21 is a diagram showing portions of a tire.

Detailed Description of the Invention

The present invention is a method of curing a tire including a tread, carcass, and sidewall, comprising the steps of:

(a) placing the tire inside a tire mold;

(b) applying heat to the tire;

(c) removing the tire from the tire mold; wherein if the sidewall has been cured to between about .90 and 1.40 alpha, the tread has been cured to at least .90 alpha. In another embodiment of the invention, the tire mold comprises an interior face contacting the tire, and the interior face further comprises heat transfer elements protruding from the interior face, and heat is transferred from the heat transfer elements to the tire.

Another embodiment of the present invention is a method of curing a tread block of a tire tread, comprising the steps of:

(a) inserting a heat transfer element into the tread block;

(b) applying heat to the tread block;

(c) removing the heating element from the tread block; wherein the cure state of any portion of the tread block is between about .85 alpha and .99 alpha.

A further embodiment of the invention is a method of optimally curing a tire comprising:

(a) a carcass which comprises sidewalls and a crown; (b) a tread affixed to said crown, wherein said tread comprises tread blocks;

wherein the method comprises inserting said tire into a tire mold comprising heat transfer pins or heat pipes fixed to the interior of the mold, wherein said heat transfer pins or heat pipes are inserted into portions of the tire effecting a more even transfer of heat from the mold to the tire.

A further embodiment of the invention is a method of reworking a tire mold comprising the addition of heat transfer pins to said mold, wherein said heat transfer pins are configured so as to provide for a more even curing of all portions of a tire to be inserted into said mold. The present invention also is method of designing a mold for a tire, which tire comprises a tread, carcass and sidewall, and wherein the mold comprises an interior face to contact the tire, comprising the steps of:

(a) determining the points of least cure of the tire to be inserted into the mold; (b) placing heat transfer elements protruding from the interior face of the mold, to contact the tire; in order that if the points of least cure of the sidewall of the tire have been cured to between about .90 and 1.40 alpha, the points of least cure of the tread have been cured to at least .90 alpha. The present invention is also the tire mold produced thereby, and the tire cured by said mold. The present invention also comprises the use of finite element analysis to determine the points of least cure of the tire.

The process of curing a pneumatic tire is to apply heat energy from the exterior mold parts and the internal curing bladder in order to drive the chemical process of vulcanization. A challenge for the tire industry is to provide a curing process that provides a uniform amount of energy to the inherently non-uniform cross section of a tire. Tire constituents that do not achieve the proper state of cure (SOC) may not meet design requirements. Therefore an optimized cure insures superior tire compound properties and an informed product quality. In the present invention, a method is presented for the use of three-dimensional finite element analysis (3-D FEA) and thermocouple tests in determining the state of cure (SOC) for each zone of the tire as well as detailed features in the tire tread. From this detailed knowledge of these cold zones of the tire, different tread apertures are proposed to enhance heat transfer into these zones in order to provide a more optimum cure. A concept of heat transfer pins is developed as an efficient and practical means of accomplishing this optimized cure state. In addition, these new tread geometries are evaluated with respect to some basic tire tread performance parameters to understand their impact on the performance of the tire. Using 3-D FEA, these new combinations of tread apertures are analyzed and predictions of a more uniform SOC for all zones of the tire and a reduced cure time. Reductions in cure time of 15% are demonstrated. Finite Element Analysis

According to the present invention, a determination is made of the points of least cure of the tire. In the sidewall the tire may be only a few millimeters thick, while in the summit a cross section thickness of over 50 millimeters is common. Therefore, uniform curing is a significant challenge. One conventional method of cure state determination is to build a tire, place thermocouples throughout the tire, and record the thermal profiles during the curing process and the cooling down period. Knowing the thermal profile, one can use reaction kinetics to determine the state of cure throughout the tire.

Finite Element analysis (FEA) consists of a computer model of a material or design that is subjected to external loads (i.e., structural, thermal, etc.) and analyzed for specific results. It is used in new product design, and existing product refinement. A company is thereby able to verify a proposed design will be able to perform to the client's specifications prior to manufacturing or construction. Modifying an existing product or structure is utilized to qualify the product or structure for a new service condition. FEA uses a complex system of points called nodes which make a grid called a mesh. This mesh is generated and contains the material and structural properties which determine how the structure will react to certain loading conditions. Heat Transfer analysis models the conductivity or thermal fluid dynamics of the material or structure (Figure 1). This may consist of a steady-state or transient transfer. Steady-state heat transfer refers to constant thermoproperties in the material that yield linear heat diffusion. See, e.g., Toth. W.J., et al., "Finite Element Evaluation of the State of Cure in a Tire," Tire Science and Technology, TSTCA, vol. 19, No. 4, Oct.-Dec, 1991, pp. 178-212, incorporated by reference.

Alpha

Alpha is given by the following equation: alpha = (time of curing in tire mold)/t99 where t99 is the time for completion of 99% of the cure as measured by torque as shown by a rheometer curve. ASTM D2084 and ISO 3417 describe how to measure cure times (time t₀ for the onset of cure, and time t99 for 99% completion of cure) for rubber compounds using an oscillating rheometer. These standards are incorporated by reference. As is known in the rubber industry, "cure" refers to the vulcanization of a rubber article. Vulcanization is the process of cross-linking elastomer molecules to make the bulk material harder, less soluble and more durable.

The invention may be further understood by reference to the following non- limiting examples.

Example 1

The present method was applied to the cure of a recapped tread band.

Figure 1 demonstrates a flat tread sandwiched between a flat plate (platen) and a sculptured mold, while Figure 2 gives an example of a sculptured tread pattern as a result of this molding process. Figure 3 depicts the relative cross-section locations that will be referred to. In defining the relative locations, the minimum cure-state location was first identified in the x-y plane. This position was then used as a basis for comparison in the z-direction (or through the thickness of the tread block). KmI refers to the first tread layer, while Km2 refers to the second tread layer. The kmlbot location refers to a position 2 mm from the first tread layer/platen interface. The kmlmid location refers to a position in the middle of the block thickness, while first tread layer/top refers to a location 2mm from the first tread layer/second tread layer interface.

Now, using these reference locations, Figure 4 demonstrates cure state, α, as a function of time through the block thickness. Clearly, next to the bottom platen, the tread rubber cures the quickest, while the rubber near the KM1/KM2 interface cures the slowest. In comparing the kmltop location for the base case and the pins case, we see

that the addition of pins reduces the time to cure (α = 0.90) by approximately three minutes. In this case the pins were approximately 2 mm in diameter.

Figure 5 shows the variation of the state of cure through the KMl thickness at the end of the cure. The more flat the curve, the more even state of cure through the block thickness. The figure demonstrates that the addition of pins greatly increases the evenness of cure through the KMl thickness.

Figure 6, similarly, shows the variation of time necessary to cure to α = 0.90 for

different depths in KMl. The addition of pins reduces the time to cure (α = 0.90) by

about 3 minutes at the top of the KMl (i.e., the KM1/KM2 interface).

Finally, Figures 7 and 8 show iso-contours (in the x-y plane at the KM1/KM2 interface) of cure state at the end of cure. Without pins the state-of-cure ranges from 0.61 - 0.99 (Figure 7), while the addition of pins reduces the range to 0.87 - 0.99 (Figure 8).

Example 2 In Figure 9, to achieve cure in large blocks, the small blocks are three minutes

overcured. The reference case (no additional heat transfer features) results in α_mj_n (KMl)

= 0.73 and α_ctr bl_ock (KMl) = 0.93 after 26 minutes of curing. As evidenced in Figure 1 (iso-contours of cure state), the smaller center blocks are cured well before the larger shoulder blocks.

As shown by Figure 10, by use of the present invention, cure time is reduced by 3 minutes (12% reduction) and the cure state is uniform across the tread. When 14 mm straight heat transfer pins are added to the sculpture, the same cure time (26 minutes)

results in α_mj_n (KMl) = 0.93 and α_{ctr bl}ock (KMl) = 0.93. Two observations are made: Firstly, there is a more even distribution of cure state in the retread sculpture, and secondly, there is a 3 minute reduction of overcure in the sculpture center tread blocks.

Example 3

This example shows the reduction in cure time effected by the introduction of heat transfer elements into a tread block for a pneumatic truck tire. (Figure 11). For the reference case (without any features in the block),

cure time (DSP - time in the closed mold under pressure and heat) = 57 min. The cure of this tire is limited by the cure of the summit a. bead requires 39min. Therefore bead has + 18 min. overcure b. sidewall (NCl) 22min. Therefore sidewall has + 35 min. overcure

For the test cases, cure time reductions from .5 to 7.5 minutes were obtained. Therefore with the invention (depending on case chosen), cure time (DSP) = 49.5 to 56.5 min. Still summit limited a. bead overcure reduced by up to 42 % b. sidewall overcure reduced by up to 22%

Example 4

Figures 12-18 illustrate a basic tread block, and various heat transfer element configurations.

Example 5 <

The objective here is to determine where the limiting points of cure are located and to propose different tread geometries to better transfer energy into these limiting zones. At the same time the tire tread must continue to function as designed, providing the same physical properties to the overall tire design. A series of designs were modeled with some of the different cases presented here. Finally, comparisons will be made between the different designs and a "base case" with respect to some basic tire design requirements. The parameters examined include:

1. The amount of tread surface traction edges.

2. Rigidity of the tread block. This contribution was approximated by using a factor which related the reduction in rigidity to the depth of the feature. 3. Contact stresses generated at the tire contact patch with the road. There is a well- established correlation between the contact stress and the rate of tread wear.

4. The maximum thermal gradient in a tread component. A large thermal gradient provides for drastically varying properties in a single tire component. 5. A resistance to tread chunking and tearing.

By knowing where the cure limiting points are, we will place heat transfer features targeted at these cold zones to decrease the thermal gradient. These proposed tread feature additions will take into account the various tire performance parameters summarized above.

Features Evaluated

A large aluminum feature was added to the tire mold to transfer heat from the mold into the shoulder of the tire. Again, this feature is designed to transfer heat more efficiently to the cold zone on the breaker skim. This feature is full depth, 8 mm wide and protrudes approximately 10 mm into the side of the tread block (Fig. 19). The result from this model showed a cure time of 54 minutes or a reduction of 3 minutes. Heat Transfer Pins Next, a series of tests are presented using a comb-like configuration. The idea behind this geometry was to have long pins that pipe heat energy deep into the tread. At the same time, the rubber bridging between these pins helps maintains the rigidity of the block. Tests were conducted at varying heat transfer pin lengths of 7, 10, 14, 17, and 19 mm. The designer is motivated to keep the pins as short as possible, thus causing the least reduction in the rigidity of the tread block. In addition, there is a concern about the pins bending or breaking after repeated cycles at high temperatures. It was also decided to couple these comb-like features with the shallow mini-sipes as described in Fig. 20. The mini-sipes were added because the flow of rubber around these shallow features is well known and the aspect is improved with this type of feature (the comb-like features are actually hidden by the mini-sipes).

Summary of results

A variety of heat transfer apertures have been added to the tread on the truck tire in an attempt to reduce cure time and provide for a more uniform cure of the different tread sections.

As previously mentioned, the tire designer needs to understand the impact of these features on tire performance. This section will examine in more detail the impact of the different tread design cases on a few of the key tire performance parameters. A table of values is included in Table 1. Some of the different cases evaluated in this project are listed on the left of the table. These cases include various combinations of flowers, sipes, mini-sipes, wedges, and heat transfer pins. These apertures have been described in detail in preceding section.

Table 1 Summary of results from FEA modeling of different apertures

Across the top of the table the tire performance parameters are defined as follows:

Cure time in minutes - For the "base-case" (reference case) this is the actual cure time in minutes for the heating phase of vulcanization. A prototype tire has been modeled and thermocouple tires cured to validate its accuracy. Cool down time is not considered. For all other cases the results are from FEA modeling.

% Reduction contact patch @ new - Contact patch is defined as the amount of tread in direct contact with the road. For each heat transfer aperture that is added, the contact patch is reduced. A simple geometric measurement was made to determine the percent reduction for each case evaluated. All measurements were taken when the tire was new or at full tread depth. This parameter is used as an estimator of dry traction. In general, the more rubber on the road, the better the dry traction. Also, contact stresses and tread wear rates are inversely related to the contact patch area.

% Reduction contact patch @ - worn - This is the same measurement as above, but now estimated after the first 3 mm have been removed from the tire tread.

% Traction edge increase - Typically free edges which are perpendicular to the direction of rotation are good for traction in rain and snow. These "biting" edges can be made by the shallow apertures added to the tread geometry, and their effectiveness is usually evaluated when the tire is in a like-new state.

% Reduction of volume - The percent reduction in tread volume is a direct way of predicting the tread rigidity. For this project a simple approximation was used. It is intuitively obvious that the reduction of volume of the block at any section will have a more profound effect on rigidity as the depth into the block is increased. For the first 5 mm of tread depth there is a simple correlation between the reduction in rubber volume and the tread block rigidity. For the next 5 mm (6-10 mm depth), the loss of rigidity as a result of the reduction in volume is increased by a factor of 2. From 11-15 mm the rigidity is reduced by a factor of 4 and for 16 + mm by a factor of 8. In order to account for the fact that some of the features allow for "bridging" of the rubber blocks, another factor is added back to account for this addition of rigidity. Finally, a "% Reduction in rigidity" is developed as a comparator for the different cases.

Claims

We Claim:

1. A method of curing a tire including a tread, carcass, and sidewall, comprising the steps of:

(a) placing the tire inside a tire mold; (b) applying heat to the tire;

(c) removing the tire from the tire mold; wherein if the sidewall has been cured to between about .90 and 1.40 alpha, the tread has been cured to at least .90 alpha.

2. A method of curing a tread block of a tire tread, comprising the steps of:

(a) inserting a heat transfer element into the tread block;

(b) applying heat to the tread block;

(c) removing the heat transfer element from the tread block; such that the resulting cure state of any portion of the tread block is between about .85 alpha and .99 alpha.

3. The method of claim 1, wherein the tire mold comprises an interior face contacting the tire, and the interior face further comprises heat transfer elements protruding from the interior face, wherein the method further comprises the step wherein heat is transferred from the heat transfer elements to the tire.

4. A method of more optimally curing a tire comprising: (a) a carcass which comprises sidewalls and a crown;

(b) a tread affixed to said crown, wherein said tread comprises tread blocks;

wherein the method comprises inserting said tire into a tire mold comprising heat transfer pins fixed to the interior of the mold, wherein said heat transfer pins are inserted into portions of the tire effecting a more even transfer of heat from the mold to the tire.

5. The method of claim 4, wherein said heat transfer pins comprise heat pipes.

6. The method of claim 4, wherein alpha for the tread is between about .85 alpha and .99 alpha, and alpha for the sidewall is between about 1.0 alpha and 1.4 alpha.

7. A method of curing a tire comprising:

(a) inserting heat transfer pins into portions of the tire; (b) transferring heat to said portions of the tire by means of said heat transfer pins; and

(c) thereby effecting a more optimal curing of the tire.

8. A method of making a tire mold for curing tires comprising the steps of:

(a) determining the heating rate of substantially all portions of the tire to be inserted into the mold;

9. A method of reworking a tire mold comprising the addition of heat transfer elements to said mold, wherein said heat transfer elements are configured so as to provide for a more even curing of all portions of a tire to be inserted into said mold.

10. The method of claim 9, further comprising wherein the total time a tire is in the tire mold for curing can be reduced between 10 and 15 percent, while the reduction in contact patch when new is less than about 8 percent and the reduction in contact patch when worn is less than about 4 percent, the reduction in traction edges, and the reduction of tread rigidity is less than about 10 percent.

11. The method of claim 9, wherein said elements are selected from the group consisting of pins and heat pipes.

12. A method of designing a mold for a tire, which tire comprises a tread, carcass and sidewall, and wherein the mold comprises wherein the tire mold comprises an interior face to contact the tire, comprising the steps of:

(a) determining the points of least cure of the tire to be inserted into the mold; (b) placing heat transfer elements protruding from the interior face of the mold, to contact the tire; in order that if the points of least cure of the sidewall of the tire have been cured to between about .90 and 1.40 alpha, the points of least cure of the tread have been cured to at least .90 alpha.

13. The tire mold of claim 12.

14. A tire cured by the mold of claim 13.

15. The method of claim 12, further comprising the step wherein finite element analysis is used to determine the points of least cure of the tire.