Patent Application
20090014918
None.
1. Field of the Invention
The invention relates generally to the forming of heat-cured work parts inside of a compression forming mold with the aid of microwaves, and more particularly toward methods and apparatus for forming in-mold cured brake pads under the combined influence of conductive and microwave heating modes.
2. Related Art
Many products are manufactured from a heat-curable material which, prior to curing, is formed in a die cavity. One example of the many different types of work parts which are manufactured in this manner may be found in brake pads and brake shoes such as used in the friction brake field of use. In the case of disc brake applications, it is well-known that a plate-like rotor rotates with the wheel of a motor vehicle. Friction pads, made of an abradable material, are held in a caliper on either side of the rotating disc. When an operator of the motor vehicle actuates the braking system, the brake pads are squeezed on opposite sides of the rotating disc, converting dynamic energy into heat which is rejected to the atmosphere. The abradable portion of the friction pads is typically made from a heat-cured material composition affixed to a rigid metallic backing plate.
Manufacturers of heat-cured work parts are always receptive to new methods for manufacturing their products more economically. Accordingly, there exists a strong interest to improve process productivity by reducing cycle time, that is the time required to produce a heat-cured work part. Generally speaking, two approaches have been pursued to achieve the goal of reduced cycle time in the manufacture of heat-cured work parts. One approach is to select or develop a chemical resin binder that is capable of curing more rapidly and/or at lower heat settings. The other approach is to accelerate the curing step by using more efficient heating methods.
Various prior art attempts have been directed toward the pursuit of more efficient heating methods. Traditionally, at least in the field of brake friction pad manufacture, the heat-curable resin intermixed with the other ingredients of the friction material was either cured in the forming press through conduction heating, or else transferred to a sintering oven after pressing for batch cure processing. The in-press curing method, which relied exclusively on conduction heating, was slow. Cycle times were quite long within the context of large-scale production environments. Conversely, the batch cure technique, in which batches of brake pads are cured in an oven after pressing, introduces an additional step in the manufacturing sequence and requires additional capital investment and processing management to properly execute.
More recently, there have been prior art attempts to accelerate curing through the use of more efficient heating methods by employing hybrid techniques which combine the traditional convection heating methods with microwave heating techniques. For example, U.S. Publication No. 2005/0184434 to R. Akopyan, discloses a combined microwave and conduction heating method of a polymer prior to its injection into a closed mold cavity. In another example, U.S. Pat. No. 5,576,358 assigned to AlliedSignal discloses an in-mold curing technique for brake pads which relies only on microwave heating modes. Other examples have also been proposed.
Accordingly, various prior art approaches aim to achieve more efficient heating of a heat-curable material, such as used in brake friction pads for example. More efficient heating techniques are attractive to manufacturers because traditional conductive heating techniques have proven inefficient to fully heat the core region of the work part due to the relatively low thermal conductivity of such materials. Furthermore, because molding temperatures cannot be too high so as to degrade the outside surfaces of the work part, lower conduction temperatures must be used which naturally increase the required cycle time.
Therefore, there exists a need for a more efficient hybrid heating method in which the traditional conductive heating technique is used but supplemented by microwaves as a secondary heating source. However, any such hybrid heating system will need to address the many issues which have frustrated prior art attempts involving microwaves, including the proper selection of the microwave source so that which is commercially available and relatively inexpensive, the need to excite only the core regions of the work part which are not rapidly heated through the conduction process (i.e., quick and even heat distribution throughout the work part), properly controlling the thermal heating process so as to avoid the well known problem of thermal runaway in microwave heating applications, and finally the practical challenge of integrating the supplemental microwave technique with existing machinery so as to reduce the need for large capital investments in new equipment.
The subject invention overcomes the disadvantages and shortcomings of prior art techniques, and addresses all of the necessary issues in implementing a hybrid heating source by providing a mold for forming an in situ, heat-cured work part under the combined influence of conductive and microwave heating modes. The mold of this invention comprises an internal die cavity defined by surrounding walls and a bottom. A press ram is movable within the confines of the walls toward the bottom for compressing the components of a heat-curable work part to shape inside the die cavity. The improvement comprises a generally lossless window directly exposed to the die cavity. The window transmits substantially all of the electromagnetic energy in a microwave-type wave into the die cavity while preventing the escape of heat-curable components from the die cavity during the ram pressing operation.
Thus, the subject mold permits microwaves to pass directly into the die cavity through the lossless window, thereby facilitating hybrid heating techniques and achieving the goal of reducing work part cycle time. This invention is ideally suited for manufacturing brake friction pad and shoe components, however other work parts and material types can be substituted with favorable results.
According to a second aspect of the invention, a method is provided for preventing thermal runaway in a work part made from a heat-curable material during microwave heating. According to this aspect of the invention, the method comprises the steps of preparing a heat-curable material consisting essentially of organic, inorganic and metallic solids suspended in a heat-curable resin binder. The method includes loading the heat-curable material into a die cavity having side walls forming a defined work part shape. The method goes on to include the step of conforming the heat-curable material to the shape of the die cavity under the press of a ram. Then, the heat-curable material is exposed to microwaves cycling within a defined frequency range during the conforming step. The improvement is found in the preparing step which includes adding an effective quantity of titania to the heat-curable material so as to reduce or eliminate the tendency to thermal runaway by moving the microwave absorption band of the heat-curable material out of the defined frequency range of the microwaves produced during the exposing step as a function of the temperature of the heat-curable material. In other words, the addition of titania to the heat-curable material has the natural effect of shifting the microwave absorption band out of the frequency range of the microwaves as the temperature of the heat-curable material increases. Therefore, thermal runaway is prevented automatically, whereby continued exposure of the heat-curable material to microwaves will not continue to increase its temperature because the microwave absorption band of the material has been shifted. Thus, the heat-curable material becomes self-regulating in terms of its ability to be excited by the microwaves.
Yet another aspect of the invention is defined as a method for forming an in-mold cured brake pad. According to this aspect of the invention, the method comprises the steps of preparing a friction material compound consisting essentially of organic, inorganic and metallic solids suspended in a heat-curable resin binder. The friction material is loaded into a die cavity having side walls forming a defined work part shape. The friction materials conformed to the shape of the die cavity under the press of a ram. Once conformed to shape, the friction material is exposed to microwaves. According to this aspect of the invention, the improvement comprises a step of passing the microwaves through a generally lossless window in the side wall of the die cavity during the exposing step. Thus, substantially all of the electromagnetic energy in the microwaves is transmitted into the die cavity through the generally lossless window. However, the window also prevents the escape of friction material are from the die cavity during the conforming step, so that the friction material can be shaped under the press of the ram.
These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:
Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, an exemplary braking system for a motor vehicle is generally shown at 12. This system 12 includes a rotor 14 for a disc brake of a vehicle which rotates together with a wheel (not shown) and has a pair of opposed friction surfaces against which brake pads, generally indicated at 16, are brought into contact to arrest rotation of the wheel. The brake pads 16 are held in a cuff-like caliper 18 which may be actuated through hydraulics, electricity, mechanical linkages, or other known methods.
Throughout the remaining discussion of the subject invention, reference will be made to one preferred application for this invention which is the manufacture of brake pads 16. However, the novel methods and apparatus can be applied to other fields of use wherein heat-curable material is formed and cured to shape inside of a mold. In other words, the invention is not limited to the manufacture of brake pads 16.
As perhaps best shown in
Referring now to
Typically, but not necessarily, the back plate 22 will be loaded into the die cavity 26 as a loose piece. In the illustrative embodiment of this invention, the bottom 30 is shown including a recess 36 for holding the back plate 22. To facilitate loading and unloading in this example, the die cavity bottom 30 is independently movable relative to the side walls 28.
The specific ingredients for the heat-curable components 34 will vary depending upon intended application. As stated previously, in the situation of friction materials 20 for brake pads 16, the heat-curable components 34 are likely to include organic, inorganic and metallic solids suspended in a heat-curable resin binder. However, in other (particularly non-brake-related) applications, the selected components 34 may vary widely. As for this specific material composition of a friction material 20, these ingredients are usually considered to be proprietary formulations which are closely guarded by each manufacturer. The novel features of this invention can be used with many, if not all, of the known formulations. Generally stated, there may be more than 17 different specific ingredients which are varied depending upon the situation, processing conditions, customer demands and other factors. Generally stated, the ingredients required to produce friction material 20 for a brake pad 16 may be selected from the following materials:
In order to reduce cycle time for in-mold cured work parts, the conductive heat mechanism supplied by the heating elements 38 is supplemented with a microwave heating mode. This is perhaps best depicted in
The output of the magnetron 40 is operatively associated with a transition wave guide 44 for directing the microwave-type wave 42 into the die cavity 26. More specifically, the transition wave guide 44 leads to a generally lossless window 46 that is directly exposed to the internal die cavity 26. The window 46 enables substantially all of the transmission of electromagnetic energy in the microwave-type wave 42 to enter the die cavity 26 while preventing the escape of heat-curable components 34 from the die cavity 26 during the ram 32 pressing operation. In other words, the window 46 and the sidewalls 28 together define and contain the components 34 as they are pressed and squeezed into shape by the press ram 32. However, the generally lossless window 46 forms that portion of the otherwise solid sidewalls 28 to which to transition wave guide 44 is attached so that microwaves 42 can pass directly into the compressed friction material without being reflected or otherwise diminished by the mold sidewalls 28. Those of skill in the art will envision other placements for the window 46, such as through the bottom 30 or perhaps through the ram 32. In this preferred embodiment of the invention, however, the window 46 is located in a relatively stationary sidewall 28, due to the fact that both the ram 32 and bottom 30 are moving elements during the loading, pressing and unloading steps of the molding sequence.
As illustrated in
In the production of some heat-curable materials such as friction materials 20 for brake pads 16, it has been found that the normal composition of ingredients is not readily responsive to microwave heating within the standard band frequency (2.4-2.5 GHz) produced by the low cost magnetrons 40 developed for home oven applications. In other words, it may be desirable to “tune” the composition of the components 34 to increase their receptivity to the microwaves 42 during the curing operation. It has been found that the addition of a suitable tuning agent to the ingredient composition has the effect of shifting the absorption band of the friction material components 34 into the frequency range of the microwaves 44 emitted by the low cost magnetron 40.
Not only does the addition of a suitable tuning agent help “tune” the friction material components 34 so as to be more readily receptive to the microwave heating step, it has also been found that the addition of the tuning agent produces an especially desirable self-regulating quality which reduces or eliminates the tendency to thermal runaway. Thermal runaway has been defined as the abrupt and localized rise in temperature which is usually caused by a material which becomes more receptive to microwave heating as its temperature increases. This phenomenon has been experienced by most people in home oven heating situations, where a food item heated inside a microwave oven exhibits localized areas which have been over-heated while other areas of the food item are under-heated. This phenomenon can be due, in part, to the tendency for thermal runaway in some materials subject to microwave heating.
The applicant has found that the material titania can serve as a suitable tuning agent. The addition of titania to the ingredients used to form the components 34 has the effect of moving the microwave absorption band of the heat-curdle material out of the defined frequency range of the microwaves produced during the exposing step as the temperature of the components 34 increases. This is perhaps best illustrated in
Although the effective quantity of titania will vary from one application to another, it has been found, at least within the specific field of manufacturing brake pads 16, that an effective quantity of titania will include mixing more than zero but less than about 6 volume percent of titania to the ingredients comprising the heat curable material components 34. In this mixing step, it is generally considered advisable to randomly disperse the titania throughout the heat-curable material components 34 in a thorough (i.e., homogeneous) mixing operation. Titania is likely just one of a wide range of materials that can be used as suitable tuning agents. For example, the mixed valence oxides (e.g., Fe3O4, Co2O3, CuO, NiO), the sulfide semiconductors (e.g., PbS, FeS2, CuFeS2), the various forms of carbon (e.g., lampblack, graphite, carbon fiber), and many other similar materials are all easily heated by microwaves and may possess the desired coupling characteristics to form suitable tuning agents.
Accordingly, the subject invention represents a more efficient hybrid heating method, in which microwaves 42 are used as a secondary heating source, together with the traditional conductive heating such as provided by conductive heating elements 38. The application overcomes many of the issues commonly encountered such as proper selection of the microwave source (magnetron 40), efficient coupling of the magnetron 40 into the loaded cavity 26 via a transition wave guide 42, proper mode shaping via the addition of titania or other suitable tuning agent, excitation of the needed zones (i.e., core region of the die cavity 26) to ensure proper heat distribution, adequate thermal control via the addition of titania, and easy integration of the concept to existing manufacturing equipment.
The subject invention thus implements a hybrid heating method to provide more uniform heating within a work part, and at the same time to retain the benefit of traditional conductive heating. The traditional conductive heating is advantageous as a thermal management technique due to its decreased sensitivity to environmental changes. The microwave application techniques of this invention achieve highly predictable and single-mode-dominant heating within the die cavity 26. Thus, the heating mode can be effectively excited by a standard microwave 42 from a low-cost magnetron 40 via its introduction through a generally lossless window 46 from a standard wave guide 44. The lossless window 46 can effectively help shape the heating mode, and thereby quickly heat the cold core region of the compressed components 34 resulting from the slow propagation of thermal conduction through the mold 24 elements. The selection of alumina as a composition for the window 46 meets both the thermal and mechanical requirements as the closure for a mold cavity 26.
Because of the somewhat specialized press machine requirements for making brake pads 16, wherein both the ram 32 and the bottom 30 move during compression, the applicants have found that application of the microwave 42 through the sidewall 28 of the mold 24 is most convenient. However, other arrangements may of course be used, especially in non-brake pad applications of this invention. The addition of the tuning agent titania (or other suitable substance) into the ingredients for the heat-curable components 34 solves numerous unexpected problems. For example, the peak absorption frequency of a given component 34 formula may be above the standard band for industrial heating (2.45 GHz). This peak frequency shifts downward when temperature increases. Consequently, the microwave heating is not effective at low temperature and has the potential to incur thermal runaway at high temperature. Titania has been found to tune the high absorption band of the modified formula into the standard band (
The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims.
