Chapter 8 – Grind/Lap of Ceramics with UV-Bonded Diamond Wheels
• Qiuyun Huang*,
• Lei Guo*,
• Ioan D. Marinescu†
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doi:10.1016/B978-1-4557-7858-4.00010-8
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Abstract
In conventional lapping process, the lapping efficiency is limited due to the abrasion mechanism that is illustrated as three-body abrasion. In order to improve the efficiency and surface finish, a fine grinding or grind/lap process is proposed and studied in this research. First of all, the material properties of the ultraviolet-curable resin and mixture are investigated and the optimum combination of resin and diamond abrasive was selected. Besides, kinematics of the grind/lap was analyzed to study its influence on the performance of the resin bonded wheel. Moreover, based on experimental results analysis, the mechanism for grind/lap process was proposed, and it is considered as a combination of conventional grinding and lapping process. At last, two ways to improve the performance of wheel were carried out and the results were compared.
Keywords
• Ultraviolet curable resin;
• kinematics of fine grinding;
• grind/lap
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8.1. Introduction
Abrasive processes have been employed in industry manufacture for more than a century and the origin could be traced back to Neolithic times. Grinding, lapping, and polishing were significantly developed in twentieth century, and the application of machining technology for achieving high quality and efficiency increased rapidly. Compared to grinding, lapping and polishing are similar processes by which fine surface finish, high-dimensional accuracy, flatness, and minimal subsurface damage can be obtained. These techniques have been used in the optical lens, semiconductor, and electronics industry on a wide range of material from silicon, glass, and ceramic to metal and their alloys. Lapping, as one of the most important techniques in surface finishing, has become more and more important in the ceramic industry [1].
The traditional lapping procedure is based on a slurry process, and the abrasive grains are freely moved in the process. The major problem for conventional a lapping process is to get a uniform slurry distribution between the lapping pad and the workpiece. During the lapping process, the lapping pad becomes smoother and the pores fill with chips and pad material. Transportation of the slurry to and debris from the workpiece surface will be weakened, which is called glazing phenomenon. As a result, the material removal rate and machining efficiency will drop. In addition, slurry handling and disposal is problem that should be considered for conventional lapping [2].
With huge development and remarkable progress in material science, the modern machining technology needs to achieve high quality and efficiency. Therefore, a new lapping process called fixed-abrasive lapping appeared recently [3]. Fixed-abrasive lapping, also known as fine grinding or grind/lap, was first proposed by Gatzen [4]. Subsequently, in research by Choi and colleagues, thermocurable fixed-abrasive pad processes were used to achieve a submicron-level surface roughness [5]. Tomita and colleagues studied development of new bonding materials for fixed abrasives of grinding stone and proposed a new process by using a double-side lapping machine with a grinding stone [6].
Recently, researchers investigated the use of ultraviolet-curable resin in fabrication of abrasive tools. Since 1968, when a German made the ultraviolet-curable resin commercialized for the first time [7], ultraviolet-cured rapid prototyping technology has developed quickly and become one of the most advanced techniques in the manufacturing industry. Tanaka and colleagues proposed a method to develop a grinding wheel with cured resin, while others studied different abrasive tools using ultraviolet-curable resin [8], [9], [10] and [11].
8.2. UV bonding techniques
UV Curable Resin
The UV curing technology used in manufacturing can be traced back to the early 1950s. American engineers applied the technology of light solidifying to make a typography board with UV-curing resin. In the past 20 years, researchers have developed photo-manufacturing technology in various applications, and with the application of UV-curing resin in the fields of rapid prototyping, molding material, medical treatment, and micro manufacturing. Various kinds of UV-curing resins with different features have been developed.
In this research, two types of UV-curable resins and their mixtures will be evaluated according to the mechanical properties of the abrasive tool. The first type of adhesive is an epoxy-based material. While some people use the term epoxy generically to refer to all high-performance engineering resins, epoxy has a quite specific meaning within the adhesive world. The second is an acrylic-based UV-curable resin that we will exam because it differs from epoxy-based resins.
Epoxy resin is one of the stiffest plastic materials. In the UV-curing process, epoxy resins use a catalytic curing mechanism. The catalyst is a by-product from the reaction of the photoinitiator to UV light. One consequence of this is that UV-curing epoxy resins exhibit a special capability. Material that is not directly exposed to UV light will cure eventually, which may be promising for the uncompleted curing problem concerned with the abrasive’s resistance to UV light. Another advantage to using epoxy-based resins is they are easily modified by mixing them with different additives, which may create great potential for meeting the special needs of abrasive tool making.
Acrylic resin, the most wildly used UV-curable resin, results from an entirely different chemistry and a different type of photoinitiator than epoxies. Curing of acrylic resins is a free radical mechanism. The free radicals are produced by the photoinitiator when it is exposed to ultraviolet light. However, the free radicals are consumed in the adhesive curing process, so acrylic resins can only cure where UV light is delivered. At least one of the components being bonded must be UV-transparent to some degree. Modification of properties in acrylic resins is more often conducted at the chemical level, through changes in formulation or combination with other base resins. A wide range of properties can be utilized, including impact resistance, surface insensitivity, environmental resistance, and others.
In fact, few kinds of pure UV-curable resins are used as bonding agents because high-composite characteristics are required. However, the theory of composite material indicates that it is possible to develop such materials.
The early stage of rapid prototyping technology is mostly used to manufacture prototypes for the quick verification of designs or prototypes with a low range of functionality. These prototypes give a first impression of a part’s properties. Fully functional prototypes with the whole range of a part’s properties cannot be built with the UV-curing process because of its limited material properties. However, many opportunities to reinforce resins exist, one of which is to fill the resin with powders such as ceramics, which shows great promise. Research has shown that different powder-filled UV-curable resins can theoretically be used to manufacture highly loadable parts and tools. In general, the stiffness, wearing resistance, and thermal and chemical resistance of the composite are higher than those of the pure resin.
Mechanism of UV Bonding
Two components are present in the UV-curing resin. One is the basic oligomer and the other is the photoinitiator. The key characteristic of the photoinitiator is that it will not react with the resin by itself; the photoinitiator must absorb ultraviolet light before anything can happen (i.e. change of physical properties). When the UV light is delivered, the photoinitiator will undergo a chemical reaction and produce some by-products that cause the adhesive to harden. Figure 8.1 shows the principle of the reaction.
Figure 8.1.
Reaction of UV curing process.
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For the photoinitiator to react correctly, it must be exposed to light of the correct wavelength and of sufficient intensity. Otherwise, the chemical reaction will not happen, or it may not happen completely, resulting in poor or inconsistent adhesive performance. Compared with the thermosetting resin, this chemical reaction converts the resin from a liquid to a solid as a result of an increase in molecular weight, without the volatilizing. From this principle, the use of UV-curing resin has many benefits to product manufacturers.
1.
Short process of solidifying: It needs only a few seconds or minutes to finish the process of solidifying.
2.
Process consistency and flexibility without large field and equipment.
3.
Reduced environmental considerations: Almost no organic dissolvent results from the use of UV-curing resin.
4.
Less energy consumed: In comparison to the heat-solidifying process, this process could save about 90% of energy.
These advantages suggest that in the near future UV-curing resin will replace thermosetting resin and become the new bonding agent in the manufacture of thinner oligomer abrasive tools.
Mechanical Bonding
The bonding strength of abrasives is related to the efficiency of cutting tools and life of abrasive tools. The bonding mechanism of the UV-curing process is different from the conventional thermoset process, and many theories have been developed to explain the process of bonding in adhesive structures. Individually, each of these theories is inadequate to describe the complete process of bonding in most situations. However, each theory contributes an understanding of the overall process of bonding and, therefore, is important. Figure 8.2 illustrates the five predominant mechanisms of adhesion.
Figure 8.2.
Adhesion mechanisms.