Experimental researches regarding cutting width in laser machining of glass reinforced composite polymeric materials.

Visan, Aurelian ; Ionescu, Nicolae ; Doicin, Cristian 等

1. INTRODUCTION

Composite materials laser beam cutting processing has become nowadays one of the main industrial and research application (Iliescu et al., 2007). That is because of the characteristics of this processing such as: wide availability, a proper stability, the lack of processing forces. The applicability of the processing is though limited due to the insufficient knowledge of the main characteristic that causes both the machining accuracy (the width of the cut respectively) and its dependence of the process factors.

In order to solve the problem, one should establish the cutting width and its dependence on electrotechnological parameters.

The authors' new proposed and designed solution consists on the experimental modelling of the process, on finding out of "the out width cutting" process function determination and on its dependence of the main electrotechnological parameters such as: the laser beam power, the material pattern/nature--by experimenting three materials and their thickness.

The authors' results lead to the future development of the research by elaborating a mathematical model to optimise the process parameters such as defining the goal function and the restrictive functions, among which the "width of the cut" to represent one of the restrictions of the model.

2. METHODOLOGY AND MEANS OF RESEARCH

Process functions and the variables. In the instance of laser cutting we appreciate that the independent global variables are, basically speaking the processed material and the work regime. For the specific case of the laser cutting of the polymeric composite materials, a series of parameters connected to the material are particularly important, like the thickness of the material, the nature of the material of the basic matrix, the type of fibbers, the arrangement of fibbers, the fibber glass concentration, etc.

The material pattern was taken into consideration by designing three representative composite polymeric materials based on the base material polyesteric resin and armed with fibber glass, for which names have been chosen depending on the shape, the dimensions and the fibber arrangement, as follows: STRATIMAT, symbolised in the paper as ST, for which the fibbers are long, and arranged in layers (Figure 1a), FIBRA TOCATA, symbolised FT, for which the glass fibbers are shorter than 10 mm and randomly arranged (Figure 1b) and TESATURA, symbolized TS, for which the glass fibbers are long and resemble a woven cloth, arranged in several layers (Figure 1c).

[FIGURE 1 OMITTED]

The main characteristics of the glass fibbers are: Fibber diameter--13[micro]m, Density--2,54 g/cm, Hardness--6,5 MOHS and Tensile breaking strength--1,47 GPa. The fibber glass contents are 25-30% for FT, 30-35% for ST and 35-40% for TS (Amza, 2007). The base matrix is an orthoftaltic polyesteric resin averagely reactive (Hadar, 2002).

Work regime, based on the preliminary research we established the following parameters: the power of the laser [P.sub.L] (W), the thickness of the material g (mm) and the cutting speed v (m/min). The pressure of the processing gas--nitrogen, was mentained at the constant value of 6 bar.

This paper presents the research on "The out width cutting" process function for those three materials under research (namely Yi = Lei = Lei (PL, v, g) [jim]) under a general form (Gheorghe et all. 1985):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

We established, for those three materials, i = FT, ST and TS, the absolute process indexes [I.sub.aFT], considered of reference, [I.sub.aST] and [I.sub.aTS], that will be calculated for the central values of the independent variables, respectively PL = 1330 W, v = 2,6 m/min and g = 3,5 mm. Also, we established the relative process indexes [I.sub.rFT] = [I.sub.aFT]/[I.sub.aFT] = 1 [I.sub.rST] = [I.sub.aST]/[I.sub.aFT] and [I.sub.rTS] = [I.sub.aTS]/[I.sub.aFT].

Means of inspection and the measuring process. The out

width cutting have been measured in the Laboratory for Quantity Analysis at the POLITEHNICA University of Bucharest, using a computerised image analysis line. For each cut, the out width cutting were measured in 5 points, the first one 20 mm from the edge of the material, the distance between each two points also being 20 mm. Each number obtained as a result of the measuring is certified through an analysis report (Figure 2).

[FIGURE 2 OMITTED]

3. EXPERIMENTAL RESULTS

The following process functions was determinated after conducting the experiments (Popescu, 2004) and mathematically processing the data:

[L.sub.eST] = [e.sup.5,519] x [P.sup.0,151.sub.L] x [v.sup.-0,324] x [g.sup.-0,345] (2)

[L.sub.eFT] = [e.sup.5,225] x [P.sup.0,122.sub.L] x [v.sup.-0.129] x [g.sup.-0,051] (3)

[L.sub.eTS] = [e.sup.4,298] x [P.sup.0,218.sub.L] x [v.sup.-0.090] x [g.sup.-0,058] (4)

Absolute dimensional precision indicators have been determined based on (2), (3), (4) functions [I.sub.aSTmed] = 351,898; [I.sub.aFTmed] = 370,697 and [I.sub.aTSmed] = 301,088 and relative dimensional precision indicators: [I.sub.rSTmed] = 0,949; [I.sub.rFTmed] = 1,000 and [I.sub.rTSmed] = 0,812.

4. THE INFLUENCE OF PARAMETERS OF WORK REGIME ON OUT WIDTH CUTTING

The influence of power on the out width cutting. For all three materials being studied, we find a considerable increase of the out width cutting when the power increases, as we can see in figure 3 realised for ST material.

[FIGURE 3 OMITTED]

We notice that the average out width cutting has the lowest values for the TS material, followed in order by the ST and FT. We consider that the lower values of the out width cutting for the TS and ST can be accounted for by the denser structure of these materials that causes, on one hand, the laser beam to be reflected, leading to a further increase of the in width cutting, and on the other hand, because of the "voids" generated by the deficitary penetration of the matrix material through the fibber, the laser beam to be dissipated and thus to penetrate with more difficulty through the thickness of the material.

The influence of the speed on the out width cutting. For all three materials, ST, FT and TS, we find a considerable decrease of the out width cutting when the cutting speed increases within the limit 0,5, ..., 5 m/min, as we can see in figure 4 realised for ST material.

[FIGURE 4 OMITTED]

This situation must have undergone the following explanation: when the speed increases, the time of contact between the laser beam and a certain area of the material decreases and thus the amount of heat transferred to the material decreases and implicitly, the mass of vaporised material decreases.

We notice that the average out width cutting has the highest values for the materials type FT and ST.

The influence of the thickness on the out width cutting. For all three materials being studied, we find a considerable decrease of the out width cutting with the increase of the thickness of the piece as you can see in number 5 figure, a figure designed for the ST material.

[FIGURE 5 OMITTED]

This can be accounted for by the influence of the thickness and structure of the material on the degree of dissipation of the laser beam on the thickness of the material. We find that the average out width cutting is at its highest for the material ST, for most of the work regime.

5. CONCLUSIONS

Based on the facts introduced so far, we can draw some very important conclusions, as follows:

1. The out width cutting Le is directly dependant on the power of the laser PL and it is indirectly dependant on the cutting speed v and on the thickness of the processed material g;

2. For the material ST, the strongest influence on the out width cutting is exercised by the thickness of the material g and for the other two materials, by the power [P.sub.L].

3. For most of the used work regimes the out width cutting Le has its lowest values for the material TS, followed by the materials type ST and FT, so we have the hierarchy [Le.sub.TS] < [Le.sub.ST] < [Le.sub.FT];

4. With very few exceptions, for regimes characterised by speeds close to the lower limit, (0,5 m/min), for all three materials being studied, the value of the in width cutting is higher than that of the out width cutting.

6. REFERENCES

Amza, G. et al., (2008). Monitoring the Processing Temperature of Polymeric Matrix Composite Materials, Plastic Materials, No. 1, MPLAAM 45(1)2007, pp. 61-66, ISSN 0025/5289

Gheorghe, M. et al. (1985). Algorithm for regression functions, Scientific Bulletin of POLITEHNICA University of Bucharest, Series D, ISSN, 1220-3041, tom XLVI-XLVII, pp. 176-189

Hadar, A. (2002). Stratified Composite structures, Romanian Academy Publ., ISBN 973-27-0961-8, Bucharest

Iliescu, M.; Spanu P. & Costoiu, S. (2007). Glass Fibres Reinforced Polimeric Composites--Statistic Models of Surface Roughness, Plastic Materials, No. 4, MPLAAM 44(4)2007, pp. 365 - 369, ISSN 0025/5289

Popescu, D. (2004). Contributions in determination of technological characteristics of laser beam machining, PhD Thesis, POLITEHNICA University of Bucharest