In the overall AWJM cost, one of the largest components is the abrasive cost, which can reach the 50% of the total cost [1]

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P.Gonfiotti – Investigation on GMA garnet recycling in Abrasive Water Jet Cutting

1.INTRODUCTION

1.1 - Current situation in Abrasive Water Jet Machining

The water jet machining (WJM) is a cutting technique where a pressurized water jet is used as a tool to machine the workpiece by a supersonic erosion process. It’s used to cut a variety of soft materials including paper, food and some plastics.

The abrasive water jet machining (AWJM), where solid particles are added to the water jet in order to improve the cutting ability, is applied mainly in the field of metals and hard materials cutting like stone or composites and is still unmatched by current mechanical or thermal cutting methods. The flexibility and cool cutting characteristics of the AWJ make it an important tool for manufacturers faced with applications for new materials such composites and sandwiched materials that are difficult to machine with traditional machining methods.

AWJM is a continuously growing technology whose industrial application strongly depends even on its cost effectiveness. Actually, the overall cost of AWJM systems is quite high if compared to other machining techniques, despite the effort by the industry to reduce the equipment cost and increase its reliability. System operating costs have been held steady for many years at a high level.

In the overall AWJM cost, one of the largest components is the abrasive cost, which can reach the 50% of the total cost [1]. According to some authors [2], when abrasive disposal is included, this percentage can be even higher.

The high influence of abrasive cost has restricted many opportunities and usage of this technology since abrasives in the water jet process are typically treated as a single-use, cutting medium, which have to be disposed once used and mixed with kerf materials as a slurry. If the abrasive material is recycled and reused, huge operating cost savings can be achieved.

1.2 - Definition of the problem

Until few years ago the recycling of the abrasive was not considered viable for several reasons: first, the slurry of used abrasive and kerf material usually packs into almost a solid

Chapter 1 - Introduction 1

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mass in the tank catcher of a typical water jet machine and this make a challenge the collection of the abrasive. Next, due to the required precision of the nozzles and the cut expectations from these machines, the reuse of the abrasive was usually not considered an option because of quality concerns. Realizing the importance of recycling for the reduction of the water jetting cost, fully automated systems for abrasive recycling have been introduced into the market during the last few years. Despite the introduction of these systems, until now, not many companies seems to make profit by abrasive recycling, especially across Europe.

This can be due to the fact that the most commonly used abrasive across Europe, the GMA garnet [3], has a low price (0.2 ÷ 0.3 €/Kg [4]) compared to other abrasives like Barton garnet [5]. The Barton garnet, that is claimed to perform better, is mainly used across U.S and the recycling systems on the market are designed for its recycling. Moreover, there is still lack of an effective recycling system for all garnet types.

From a literature survey appears that there are not many researches on the cutting with recycled abrasives, especially with GMA garnet recycled abrasive. There are some researches, described in the following chapters, on the cutting with recycled abrasive for Barton and Indian garnet but for the GMA there is still a lack of research. Since the aim of this research is to prove the effectiveness of abrasive recycling with an economical approach, the final result will be the economics of the recycled GMA garnet abrasives applied to Abrasive Water Jet Cutting in industrial application. In order to develop the economics of the recycling of this kind of abrasives, their cutting performance must be investigated too.

1.3 – Organization of the thesis

In the second chapter a general background on the Abrasive Water Jet cutting technique is presented, showing the characteristics of the process and of the system. The Chapter 3 describe the state of the art on abrasive recycling and particle fragmentation, reporting the results of previous studies. The recycling systems and their characteristics taken into account for the cost calculation are shown too. In the Chapter 4 can be found the development of a cost model for the cost calculation of Abrasive Water Jet specific cutting cost with abrasive recycling. The influence of the different cost components on the overall cost are pointed out, in particular the influence of different system set-ups. The Chapter 5 define more clearly the objectives of the project, defined more deeply after the definition of the important parameters for the cost calculation. It follows with the description of the AWJC set up in the abrasive lab at the Technical University of Technology in Delft, where

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the work has been carried out. The Chapter 6 describe the experiment set up for the reusability investigation, the steps of the recycling process and the encountered problems that have been solved. The results are the reusability after I,II and III cut for the recharged abrasive samples and after the I and II cut for the recycled samples. The fragmentation after cutting is investigated through the change of the particle size distribution and some observation to the particle shape with an optical microscope. The Chapter 7 contains the results of the cutting performance tests on recharged and recycled samples with different screening size for comparison and the tests on multiple recharging. The Chapter 8 relate about the experiments for the cutting quality. In the Chapter 9 there is the cost comparison of the abrasive samples and the achievable savings with an industrial application of abrasive recycling. The last Chapter 10 lists the conclusions and the future possible developments of the work.