Abrasive chips for surface treatments in rotary and vibratory apparatus

Abstract: Abrasive chips obtained by extrusion or moulding comprise a thermoplastic resin mixture and abrasive material. Regenerated polyethylene terephtalate (PET) as thermoplastic resin, and powder from recovered abrasive grinding wheels as abrasive material, are particularly suitable to this aim. Such formulations allow important savings in raw matters and enable obtaining two subsequent steps of strong removal and polishing, respectively, in the same working cycle and in the same apparatus by using the same abrasive chips.

Description: The present invention relates to abrasive chips as per the preamble of claim 1, an example of such a chip is disclosed in US-A-3 684 466. More particularly, the invention concerns abrasive particles or chips used in vibratory finishing apparatus or in rotary tumbling barrels for deburring, burnishing, edge chamfering and smoothing parts or pieces being worked. The abrasives used at present for surface finishing in rotary barrels and in vibratory apparatus are chips, of different shapes and sizes, comprising a binder and abrasive material. Two different kinds of such chips are at present manufactured: with a ceramic binder or with a thermosetting resin (polyester and urea-formaldehyde resins). Ceramic binder chips are used for deburring and for treating hard materials; thermosetting resin chips are used on softer materials and for finishing. US-A-3 684 466 discloses an abrasive chip obtained by curing a thermosettable organic polymer in which a certain quantity of granulated or pulverized thermoplastic polymers have been dispersed. According to US-A-3 684 466 the thermoplastic polymer attributes a plasticizing effect to the relatively rigid structure of the reticulated cross-linked thermosetting resin. With both kinds of chips, working times are very long and a same abrasive cannot be used for both rough machining and finishing. Moreover, due to their stiffness, such chips do not allow using high intensity vibrations and working great amounts of material relative to the amount of the abrasive load included. It is an object of the invention to provide abrasive chips for use in vibratory apparatus and in rotary barrels for surface finishing, with improved efficiency and shorter working cycles than possible at present, as per claim 1. A further object of the invention is to allow using regenerated plastic materials and abrasive products for producing the abrasive chips. The above and other objects are achieved by means of an abrasive chip made in accordance with the present invention, as claimed in the appended claims. The abrasive chips made in accordance with the invention may be employed in the vibratory apparatus and the rotary barrels conventionally used for such kind of working. The above and other objects of the invention will become more apparent from the following description of a preferred embodiment. The abrasive chip may comprise a binder made of a high-resistance thermoplastic polymer, belonging to the category of the "building materials" or "technopolymers" (e. g. highly crystalline PET), and high-performance abrasive grains or particles (e. g. corundum and silicon carbide). A typical formulation is a mixture containing 25% PET and 75% corundum. For particular aims (e.g. lapping of soft metals) use of a composition with lower abrasive percentages (e. g. down to 20% and sometimes even less) and less aggressive abrasive grains (e.g. quartz, alumina, zircon, etc.) is convenient. The thermoplastic resin, or technopolymer, can be for instance polyethylene terephtalate (PET), or polybutylene terephtalate (PBT) or polycarbonate, possibly modified with elastomers. The shapes and sizes of the abrasive chips may be very different, and they correspond to the shapes and sizes of the conventional abrasive chips. For instance, the chips may be of spherical, cylindrical or parallelepiped shape, or of any other shape that better suits to the pieces to be worked. The abrasive chips can be obtained by screw extrusion, directly into the desired shape, or by pelletisation and subsequent new extrusion or injection moulding. If the percentage of the abrasive particles in the binding material is very high (indeed, it can be up to 75 - 80%), use of a double-screw extruder is advisable for extrusion. The physical and mechanical properties of the abrasive chips provide essential improvements in the technology of use of the abrasives, more particularly: possibility of using vibration intensities significantly higher than with conventional abrasive chips, so that a great reduction in the working cycle duration is attained. Actually, the abrasive chips including thermoplastic resins do not cause dents on the workpieces, even with high vibration intensities, and they strongly attenuate the effects of shocks between the workpieces; consequently, the ratio "workpiece weight - to - abrasive chip weight" can be increased, with a consequent increase in efficiency; extremely reduced water amount needed during wet cycles in the removal phase (less than 1/1000 the abrasive chip weight per minute); very low abrasive consumption, even for strong removals; possibility of using peculiar and very advantageous technologies, such as: a) feeding, during working, small amounts of abrasive powder. This causes a great increase in the removal capacity even with very low feed of abrasive powder (typically, 1/1000 the abrasive chip weight per hour). Hence, working times correspondingly decrease; b) high water feed (typically, 30% the abrasive chip weight per hour). This causes passing from a strong abrasive action to a buffing action, with quick attainment of lapped surfaces. During such step, abrasive removal and consumption are substantially zero, so that water can be directly recycled; c) very interesting is the possibility of automatically passing from a phase of strong removal to a polishing and finishing phase, by simply stopping abrasive powder feed and increasing water feed, without any change in the working parameters. At the end of the polishing cycle, after loading new workpieces, restoring the starting conditions is sufficient to go back to a strong removal; no introduction of substances requiring chemical purification into waste water, minimum foam production and possibility of total water recycling through a simple decantation or filtering; total absence of any toxic or dangerous product in the mud generated by the abrasive; possibility of producing the abrasive chips by using recycled materials only (typical formulation: PET obtained by grinding used plastic bottles and abrasive obtained from ground and sieved scrap of exhausted grinding wheels). This allows considerable savings in production costs and enables an economically advantageous recovery of largely diffused wastes, such as used PET bottles and exhausted abrasive grinding wheels. Lastly, the particular working technology enabled by such abrasive chips allows a wide tolerance in abrasive load granulometry.

Claims: Abrasive chip suitable for use as abrasive medium in a vibratory or rotary apparatus for finishing parts or pieces being worked, the chip consisting of abrasive grains dispersed in a binding material, characterised in that said binding material is a thermoplastic resin. Abrasive chip according to claim 1, wherein said thermoplastic resin is a thermoplastic polymer. Abrasive chip according to claim 2, wherein said thermoplastic polymer is polyethylene terephtalate. Abrasive chip according to claim 2, wherein said thermoplastic polymer is polybutylene terephtalate. Abrasive chip according to claim 2, wherein said thermoplastic polymer is polycarbonate. Abrasive chip according to any preceding claim, wherein said abrasive grains are corundum grains. Abrasive chip according to any of claims 1 to 5, wherein said abrasive grains are silicon carbide grains. Abrasive chip according to any of claims 1 to 5, wherein said abrasive grains are zircon grains. Abrasive chip according to any of claims 1 to 5, wherein said abrasive grains are quartz grains. Abrasive chip according to any of claims 1 to 5, wherein said abrasive grains are alumina grains. Abrasive chip according to any of claims 1 to 5, wherein said abrasive grains comprise a mixture of two or more materials chosen out of corundum, silicon carbide, zircon, quartz and alumina. Abrasive chip according to any of claims 1 to 5, wherein said thermoplastic resin is obtained through a regeneration process of recycled plastic materials. Abrasive chip according to any of claims 6 to 11, wherein said abrasive grains are obtained through a grinding process of recycled abrasive materials. Abrasive chip according to any preceding claim, containing, in weight, about 25% binding material and about 75% abrasive grains.

Inventors: TESSORE LUCA, TESSORE VITTORIO, TESSORE, LUCA, TESSORE, VITTORIO
Applicants: POLITESS S N C, POLITESS S.N.C.
Application year: 2001
Main classification: B24B31
IPC codes: B24B31