Avaliação da retificabilidade de polímeros aplicados à indústria automobilística.

Abstract

Polymeric materials have occupied a prominent position in recent years, because with the development of new materials of this class and with properties that are compatible with metals, the number of applications in the industry has increased, especially in the automotive industry. Some examples of application range from reservoir caps (Nylon), bearing liners (Polyurethane) and even gears (Polyacetal). Depending on the design requirements, e.g. low roughness values and dimensional deviations represented by international tolerance grade, varying from IT06-IT03 grade, many of these workpieces will have the grinding as an alternative manufacturing process. Grinding is an abrasion machining process in which the workpiece material is removed by the action of grinding wheel grits. However, in grinding a greater number of variables need to be controlled in relation to milling, for example, and having many peculiarities. Among of them, is the high specific energy that causes large amount of heat generated in the process to be directed to the workpiece when using conventional grinding wheel. Depending on the thermal gradient, defects on the workpiece such as distortions can be generated to the workpiece, thereby leading to its uselessness. Although there are many studies that analyze the influence of cutting parameters on the integrity of metal parts, for polymers the literature is still scarce, so understanding the grindability of polymers becomes important. Grindability is defined as the ability of a material to be ground in relation to another, for example, a material with good grindability may present less occurrence of cracks, better finish and less variation in hardness after grinding in relation to another with lower grindability. Thus, this work aimed to analyze the grinding of 3 important polymers used in the automotive industry (Nylon, Polyurethane and Polyacetal) in terms of roughness (Ra and parameters) and difference between initial and final workpiece diameter. The input variables were workspeed (Vw = 5.64 m/min and 7.848 m/min) and radial depth of cut (ae = 25 μm and 50 μm). The output variables were the final diameter (dimensional variation of the workpiece) and. As a result, it was observed that in general the increase in radial depth of cut (from 25 μm to 50 μm) combined with the higher workspeed (7.848 m/min) of the workpiece resulted in lower roughness values when machining Nylon and Polyurethane, unlike what occurred with when machining the Polyacetal. The increase in the workspeed (from 5.64 m/min to 7.848 m/min) resulted in a reduction of the Ra roughness for Nylon and Polyurethane (decreasing of about 62 % and 26 %, respectively, while the Ra roughness was lower when using Vw = 5.64 m/min combined with ae = 25 μm for Polyacetal (Ra = 0.24 μm, a reduction of 37%) in relation to the workpiece prior to grinding. Considering the greatest reduction in Ra (-61,79%) and machining condition with Vw = 7.848 m/min and ae = 50 μm, the order of grindability (from the best to the worst) was: Nylon, Polyurethane and Polyacetal. Regarding difference between diameters, grindability order was as follows (from the smallest difference to the highest difference): Polyacetal, Polyurethane and Nylon.