Resin bonded grinding tools are manufactured via mixing, pressing and hardening at temperatures up to 200 °C (Fig. 3.5) [COLL88]. Resins consist normally of the two main components resin and hardener. Mixing both parts results in the reactive resin material. During hardening, the resin’s viscosity rises and a duroplastic material is generated.
3.1.2.1 Mixing and Molding
Mixing is often done in several steps, wet mixing, dry mixing, and final mixing (Fig. 3.5). In the wet mixing step, abrasive grits and liquid resins or furfural, a wetting agent, are combined [COLL88, GARZ00, p. 328 f.]. The abrasive grits will be coated so that the powder resins and fillers will cleave easier onto the grit surface. Wetting agents also improve the aggregation of grits. In addition, the abrasive tool can be handled in its raw, so called “green” state [TYRO03, A2, p. 6, COLL88].
mixture and reducing the clumping tendency [GARZ00, p. 326 f.]. Examples for additives are powdered additives, silica and derivatives [GARZ00, p. 327]. The mixing process is continued until a homogenous, pourable mixture emerges [COLL88]. Dust is a safety issue during handling of powder resins. Therefore, antidusting agents can be useful additives to minimize dust [GARZ00, p. 327]. Before pressing, body material such as glass cloth for cut-off wheels or aluminum bodies for superabrasive wheels can be imbedded [KREB06].
3.1.2.2 Pressing
The bond and grit mixture can be either cold pressed and hardened in a furnace or hot pressed and hardened on a press with a heating plate. Figure 3.6 shows an example setup for pressing an abrasive layer onto a grinding wheel body. Conventional wheels are either hot or cold pressed and hardened at 140-200 °C [COLL88, p. 896 f., MENA00]. Most superabrasive wheels and dense, low porosity wheels are produced by hot pressing at 160-175 °C [MENA00, GARZ00, p. 338]. In the case of superabrasive wheels with a diameter below 200 mm, the abrasive layer can be pressed directly onto the tool body [KLOC09]. Epoxy or urethane bonds are casted or joggled in molds where they are hardened at temperatures of 20-80 °C [KLOC05a, p. 63, COLL88].
Cold pressing is done on hydraulic presses with compression strength of 15-30 N/mm2 [GARZ00, p. 330, KLOC09, p. 54]. Pressing time ranges from 5-50 s and depends on the dimensions and shape of the abrasive tool, grit size, mixture plasticity and distribution [GARZ00, p. 330]. In hot pressing, the pressing times are determined to be about 30-60 s per millimeter of wheel thickness [GARZ00, p. 338].
The pressing process works either on a defined volume or with a defined pressure [COLL88]. The internal friction and friction with the mold walls lead to a deviation in particle density and therefore tool hardness (Fig. 3.7) [TYRO03b]. Density deviations can be overcome by two-anvil presses or superimposed oscillations in the pressing process, which was successfully proven for vitrified bonded tools [BEHR11].
Fig. 3.7 Schematic theoretical hardness deviation in a pressed grinding wheel [TYRO03b]
Direction of pressure