Keywords

1 Introduction

The IC process, also known as precision casting technique is a widely known method around the world to produce dimensionally accurate metal parts bearing very good surface finish. The process utilises a sacrificial wax pattern for getting a hollow mould for casting and its characteristics are ultimately transferred to the cast parts [1,2,3,4]. The major employment of the IC process is in the manufacture of turbine blades and vanes cast in superalloys, which are used in aircraft industries [5,6,7]. The stages involved in the process are as follows: injection moulding of a disposable pattern, ceramic coating, dewaxing, drying and metal casting, followed by minor finishing operations [8]. The wax pattern should possess desirable properties such as low ash content, good surface finish, dimensional stability, environmental friendliness, unreactivity to the refractory mould, sufficient hardness to prevent breakage, etc. [9].

The operational effectiveness of the IC process can be enhanced by improving one or more aforementioned features of the wax patterns. The diverse IC waxes are of vegetable, animal, petroleum, mineral and synthetic types. The exploration by Solomon [10] disclosed that the patterns made from only waxes are deficient in strength and are dimensionally unstable due to solidification shrinkage during and after pattern injection from the mould/die. Horton [11] proposed that the wax pattern characteristics can be increased by the substitution of various substances called additives and fillers, since they have ring-structured carbon atoms, whereas the majority of the IC waxes are of straight-chained carbon atoms, and consequently, a strong link is formed amid them. Rezavand and Behravesh [12] established that the wax pattern characteristics are affected by wax blend composition and different wax injection process parameters such as the injection temperature, the injection pressure, the holding time, the die temperature, etc. However, selecting and varying all the process parameters simultaneously would complicate the entire process. Thus, a few essential process parameters should be chosen based on available literature on wax pattern making process for experimentation. Once process parameters are decided, subsequently, the best possible values of input process parameters should be determined to obtain the best results from it.

The present competitive situation requires a greater consideration of the dimensional accuracy and surface texture of the cast products. Since, the IC process commences with the construction of wax patterns, the feature of the constructed wax patterns directly affects the overall quality of the castings. In the present research work, an attempt has been made to decrease the linear shrinkage of the wax pattern without affecting its surface texture by arresting its shrinkage characteristics owing to the addition of few materials such as filler and a resin compatible to IC waxes.

2 Experimentation

Each chosen constituent influences the properties of the wax patterns in some way or the other. The materials chosen for constructing the wax patterns by injection moulding process were paraffin wax, microcrystalline wax, polyethylene wax, starch powder and teraphenolic resin. The percentage of filler i.e. starch powder in the mixture was fixed to 10%, whereas the percentage of other ingredients was varied as shown in Table 1. The wax blend was forced into a properly lubricated pre-heated metallic die using a wax injection machine. The PLS and SR were measured using a digital vernier callipers and surface profilometer. Initially, the experimentation was done at fixed input injection process parameters viz. DT = 40 °C, WIT = 70 °C and HT = 1 h to determine the optimum wax composition among the chosen blends. It was found that blend 2 demonstrated minimal PLS and SR. Thus, it was considered as the best wax blend amongst all and used for further experimentation in order to determine the optimal input wax injection process parameters.

Table 1 Different wax blend composition
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The wax pattern characteristics are controlled by wax composition as well as wax injection process parameters such as die temperature, wax injection temperature, holding time, etc. In the first part of experimentation, only best wax composition has been determined. Thus, further, experiments are needed to determine the optimum input wax injection process parameters. The process parameters, namely, die temperature, wax injection temperature and holding time of the IC process were varied at three different levels and experiments were conducted as per Taguchi’s L9 orthogonal array to determine the optimum parametric levels. The span of chosen input process parameters is shown in Table 2. The response of the process is pattern linear shrinkage. Figure 1 exhibits the wax patterns made as per L9 experimental runs.

Table 2 Process parameters at diverse levels
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Fig. 1
A photograph of 9 light-shaded wax pattern parts that are in shapes of 3 steps with conical portions at the bottom. The top row has 5, and the bottom row has 4 wax patterns. The wax pattern at the center of the top row has a nail driven through the bottom conical portion.

Wax patterns made by injection process

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3 Results and Discussions

The experimental results are depicted in Table 3. The wax patterns should possess least shrinkage. Thus, Taguchi’s lower-the-better criteria has been chosen for computing the analysis of means (ANOM) to determine the optimal input parameter setting for the chosen response [13]. The ANOM for the response, PLS, is presented in Table 4 and the graph is depicted in Fig. 2. Delta is the difference between the highest and the lowest value of PLS obtained under each process parameter. Ranking is done on the basis of decreasing order of delta. From Table 4, it is obvious that the optimal input process parameters for obtaining least PLS are found to be DT at level 1 i.e. 40 °C, WIT at level 1 i.e. 70 °C and HT at level 3 i.e. 2 h. Wax injection temperature is found to be the most important injection process parameter impacting the pattern shrinkage. The second most influential process parameter is die temperature, as it is ranked no. 2. It is seen that the optimum die temperature and wax injection temperature is at the lowest level of chosen parameters. If these temperatures are higher, then there are chances of wax leakage in-between the joints of the die-halves. Also, more time is available for the pattern for shrinkage, as cooling rate decreases. The dimensional constancy of the disposable wax pattern is affected by wax blend constituents, injection temperature, die temperature, die holding time, surrounding temperature, etc.

Table 3 Experimental results of wax patterns
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Table 4 ANOM table of PLS
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Fig. 2
3 line graphs. 2 graphs plot mean of P L S versus temperature in degrees Celsius. D T increases between (40, 2.75) and (50, 3.1), while W I T increases between (70, 2.72) and (80, 3.18). A graph plots mean of P L S versus time in hours. H T decreases between (1, 2.93) and (2, 2.85) approximately.

ANOM graph of PLS

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The lower the die and wax injection temperatures, the more the wax is in mushy state. Thus, it solidifies soon with very little chance of thermal contraction. However, the wax should not be too much in a semi-solid state that it could not able to flow to the nook and corners of the die smoothly. It would adversely affect the shape as well as the surface texture of the wax pattern. It is beneficial for the wax pattern to be at a higher holding time so that it is cooled properly inside the die and undergoes shrinkage to the fullest. It is clear from Table 4 that WIT is the most significant process parameter affecting the pattern shrinkage properties in the IC process, as it is ranked no. 1. It is followed by DT and the least significant process parameter is HT.

4 Confirmatory Experiments

The optimal input parametric setting i.e. WIT of 70 °C, DT of 40 °C and HT of 2 h is not found in the experimental run done as per L9 array in Table 3. Thus, there was a need to perform additional experiments at this obtained parametric setting. Three supplementary tests were done at the aforementioned parametric setting and the mean of the results are presented in Table 5. The least value of PLS seen in Table 3 is 2.65%. However, the PLS at the optimal condition is found to be 2.53%, which shows that there is a decrease of PLS further by 4.5% from the least value of Table 3, when the experiments are done at the obtained optimal setting. Thus, it can be said that the Taguchi predicted optimal setting is able to reduce the wax pattern shrinkage, thereby reducing the dimensional variations in castings.

Table 5 Predicted and actual value of PLS of the wax patterns
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The waxes chosen for this study are very cheaply and easily available throughout the world. Resins are added to provide strength to the IC waxes and thus, teraphenolic resin has been blended with the chosen waxes in the present study. The wax pattern shrinkage is unavoidable, but can be controlled or reduced to a maximum level. Literature reveals that fillers are added to restrict the wax pattern shrinkage and in this regard, starch powder has been mixed with selected waxes and resin. Ultimately, a combination of the chosen ingredients in the present study proved that it can be used as commercial pattern IC wax material worldwide, as it exhibited good pattern properties and this combination of ingredients has never been used before for investment casting. It is expected that this novel combination of wax mixture may be used by the investment casters for producing the shell moulds.

5 Conclusions

The conclusions drawn from the present study are as follows:

  • Blend 2 containing 40% paraffin wax, 10% microcrystalline wax, 10% polyethylene, 30% teraphenolic and 10% starch powder is the best wax blend for pattern making producing least shrinkage.

  • The Taguchi predicted optimal parametric setting was found to be WIT of 70 °C, DT of 40 °C and HT of 2 h.

  • When the experiment was done at the obtained optimal setting, the wax pattern’s linear shrinkage was further reduced by 4.5%.