CHARACTERIZATION AND FORMULATION OF WAX COMPOSITION FOR INVESTMENT CASTING PATTERN

ABSTRACT

In this study selected natural waxes were blended with other selected synthetic waxes like paraffin wax and microcrystalline wax for the purpose of investment casting. Beeswax, carnauba wax and microcrystalline wax were selected to be blended with paraffin wax at the proportion of; 100%-0%, 80%-20%, 60%-40%, 50%-50%, 40%-60%, 20%-80%, and 0%-100%  for  each  blend.  Wax  properties  like  ash  content,  density,  specific  gravity, melting point, surface finishing, and shrinkage were determined. The results showed that blend F2 (20% beeswax with 80% paraffin wax); blend F3 (20% carnauba wax with 80% paraffin wax); and blend B1 (80% microcrystalline wax with 20% paraffin wax) gave better improvement over the natural waxes. Casting of a gear using wax pattern made from the blend F2  produced  cast  with  smooth  surface finishing  and  dimensional  accuracy. Blended wax of F2, F3, and B1 were recommended for the production of wax pattern in investment casting.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background of the Study

Casting is one of the most versatile forms of manufacturing process for producing components, because there is no limit to the size, shape, and intricacy of the parts that can be produced by casting and it offers one of the cheapest methods with high strength and rigidity of the intricate parts.

In casting operation, there are many processes that can be carried out for the production of metallic parts. These processes include; sand casting, No-bake casting, Resin shell mound casting, permanent mould casting, die casting, expendable pattern casting (Lost foam), vacuum process moulding, centrifugal casting, investment casting (Lost wax), etc. These processes are used to produce metallic materials (both ferrous and non-ferrous metals) into a desired shape and dimension (Horton, 2008). However, amidst them all, investment casting can produce metallic parts with excellent accuracy, flexibility of the design, it is best use for casting alloys that are difficult to machine because there is no flash to be removed or parting line tolerances.

Investment casting is an accurate casting process that is used to create metallic components from different metals and alloys. Any metallic components that require complex and thin wall castings can be produced by investment casting. Also, it may also be used for components whose intricate, detail, accuracy and shapes cannot be achieved using another method (Bonilla, Masood, and Iovenitti, 2001).

To produce wax pattern with the same shape as the finished cast part in investment casting process is the first step. This type of pattern is called expendable pattern because it is made from wax, and it will melt away in the process. Specifically, the pattern is made by pouring in wax into the metallic die (mould). The wax pattern is coated with slurry that is made of sand stucco; the process is repeated several times until the thickness needed is achieved. The raw materials and the detailed description of the design of the finished product will determine the thickness mould. According to Rezavand and Behravesh (2007), the applications of investment castings include: machine tool accessories, pumps, air compressors, aircraft, computers, automotives, weapons, medical equipment, textile machinery, food equipment, and other electronics.

Wax play important role in the production of an investment casting part. The main part of the investment casting is the wax pattern. Because the quality of the final cast product is depend upon wax pattern. It is produced by pouring the molten wax into the permanent mould of desire shape and size. A wax pattern should have some properties for making the pattern like lowest thermal expansion, low melting point, low shrinkage, good surface finish, environmental friendly etc.

In another way, Gebelin and Jolly (2003) explained that, the quality of the wax used for patterns has a significant effect on the dimensional accuracy and surface finished of the cast part. They also concluded that, it is common for the investment casting operators to employ precision-machined full–metal dies for the production of the wax patterns when large quantities are needed with highly dimensional accuracy of the component.

Waxes are of different forms/types; there are natural waxes, synthetic waxes (blended wax). The natural waxes include; beeswax, carnauba wax, candelila wax, Montana wax, ceresine wax, ozokerite wax, petroleum wax, etc. In this work, combination of the natural waxes (different proportion)  that will give all the properties which are desire to make the pattern wax  for investment casting will be investigated.

An important requirement for a wax composition to be used as a pattern material for high precision application is that it must exhibit low shrinkage as the pattern cools from the injection temperature, at which the wax is liquid or semi-solid, to the ambient temperature at which patterns are usually employed to make the refractory moulds. However, convectional moulding waxes exhibit marked changes in volume as they pass through their respective  melting  points,  making  it  difficult  to  maintain  the  wax  patterns  to  the dimensional accuracy required.

Another characteristics  that is desirable for investment casting waxes is that the wax composition have low ash content. If the wax composition, when melted out of the ceramic investment, leaves behind ash, the ash deposits cause imperfections on the metal reproduction.

1.2       Statement of the Problem

The purpose that wax served in production of parts in investment casting has demanded much concern for modification of the nature and composition of wax that should be used for the investment casting. There are various types of waxes which possess divers’ forms

of properties like ability to resist shrinkage, melting point, percentage of ash content, hardness and strength.

Many at times, the wax patterns used for investment casting do shrink even when filler materials were used and this can affect both the surface finishing and the strength of the cast part. Ash content in the wax also affect the products of investment casting, if the quantity of ash is high in the wax use for pattern, there will be much inclusion in the cast and the ash in the mould can block easy flow of the molten metal. These show that where good surface finishing casting is required, waxes of high ash content and high shrinkage will not be suitable.

To overcome the problems associated with wax pattern production in investment casing, properties of wax have to be investigated and the method of moulding both the pattern and the ceramic mould can be examined.

1.3       Significance of the Research Work

Production of wax pattern for investment casting has been facing challenges which reduce the perfection of the product. The challenges include shrinkage, high percentage of ash content, hardness, and strength. Even though, these factors affect the performance of wax pattern, wax is still the best material for making pattern in investment casting because, it is cheap to get, easy to shape and can gives high degree of accuracy.

In order for wax to be effectively use for pattern, its ability to shrink and its ash content must be highly reduce. Due to this, formulation of wax composition is important in order to increase the efficiency of the wax pattern which is used in investment casting. Therefore formulation can use different waxes that are easy to acquire and give better properties.

1.4       Aim and Objectives of the Project

The aim of this work is to formulate suitable wax composition for wax pattern use in investment casting, and to cast a simple casting in order to examine the functionality of the formulation.

The specific objectives of this work are;

1.   to analyze the properties of some natural wax;

2.   to blend different waxes together at different proportions;

3.   to examine the properties of the blended waxes;

4.   to reduce shrinkage and percentage of ash content in the wax used for pattern; and

5.   to improve the surface finishing, dimensional accuracy with no flash or parting line of cast part.

1.5        Scope of the Project

The study will cover the properties test of the different natural waxes. The waxes examined include beeswax, carnauba wax, candelila wax, Montana wax, investment casting wax, paraffin wax. In addition, the study covers blending those waxes together at different proportion and the properties examined are melting point, ash content, shrinkage, density, surface  hardness,  and  specific  gravity.  The  formulation  of  the  wax  composition  for

investment casting wax pattern is expected to reduce the percentage of ash content in the wax, to reduce wax shrinkage, to give excellent surface finishing of the cast material and better dimensional accuracy

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