Characterization of Mold Material for Small Castings

*Dixit V Patel and Piyushkumar B Tailor* 

## **Abstract**

 The casting process has a wide scope for manufacturing small parts with sizes in the millimeter range. The commonly used mold materials are not suitable to prepare the mold, which has been used for small castings due to various reasons. The effort has been made here to identify the appropriate ingredients for mold materials for small castings from literature and investigation has been done on their effects on compressive strength and thermal stability, which is most important for small castings. From the experiments, it has been found that a mixture of three ingredients satisfies requirements better when compared to two ingredients for mold material. The combination of 30% α-hemihydrate, 30% water and 40% colloidal silica gives better thermal stability with an appropriate compressive strength.

**Keywords:** small casting, mold materials, compressive strength, thermal stability

#### **1. Introduction**

The casting process has a wide scope for manufacturing small parts such as; gears for watches, small size turbine or wind blade used to produce 5-10 W electricity, various MEMS components and nozzle plates for precision jetting of various liquids and gases [1–3]. Normally in the casting process, the molds are made from sand, plaster of Paris (POP) or gypsum bond investment (GBI) as these materials have a higher surface roughness and mechanical strength [4]. Hence, the castings of millimeter to submillimeter sizes are difficult to remove after solidification from such molds. The preferable casting process for small castings is a centrifugal casting in which compressive force acts on the mold [5]. It means that the mold required for casting a small part should not break during the process under compressive force and should be easily breakable so the small casting can be removed without damage from the mold. Apart from this, obviously it is expected that mold must withstand an elevated process temperature, which means a higher thermal stability [5, 6]. Hence, it is understood that the required characteristics of a mold for small casting are different to a mold for the normal casting process. To explore this domain with scientific background, the initial literature survey has been conducted as follows.

 Scheu et al. [7] used POP for making the mold. They have found that POP material has low thermal stability and negligible permeability. However, it is not a favorable condition to cast small parts. Luk and Darvell [8] found that the thermal stability of the mold can be increased by mixing α-hemihydrates powder with a different binder material such as cristobalite, beauty cast, nova cast, and deguvest

*Characterization of Mold Material for Small Castings DOI: http://dx.doi.org/10.5772/intechopen.81083* 

 california. Earnshaw [9] shows that the ecompressive strength of heated GBI depends mainly on added ingredients and the use of sodium chloride reduces the compressive strength of investments after heating. Ohno et al. [10] show that compressive strength GBI change at higher temperatures due to phase transformation of calcium sulfate and silica. They have also found that α to β transformation of silica does not have considerable effect on compressive strength of GBI. Brien and Nielseen [11] have observed that calcium sulfate did not decompose until 1200°C. They have suggested that GBI material should not be heated above 700°C because it reacts with carbon sulfate that contaminates or makes the casting brittle. Jones [12] has reported that thermal decomposition of GBI starts from 650°C but it is very fast in a temperature range of 900–1000°C. Matsuya and Yamane [13] reported that decomposition began at about 900°C and forms CaSiO3 and Ca2SiO4. Rath et al. [14] have demonstrated that 0.5 μm surface finish can be achieved using the molding mixture of 20% fine stone, 80% quartz with 19–30 ml of water, which is lower than the surface roughness of 1.1 μm in the commercial investment cast specimen.

It is understood from the above literature that the mold made from α-hemihydrates, silica and water is more appropriate to cast small parts. The work has been done on a temperature of mold but there is scope in a composition of mold for small castings. Hence, attempt has been made in the present work to investigate the role of ingredients on the compressive strength of the mold as it is important for the success of small castings through the centrifugal casting process. For the better filling and castings, it is preferable to use preheated molds and hence the effect of temperature was also studied.

### **2. Experiment**

#### **2.1 Materials used**

The α-hemihydrate fine powder was used as a binder material to achieve a coherent solid mass received from M/S Olympic chemical industry, India. The colloidal silica was received from M/S Lodhika, India and was employed as a filler material as well as to achieve better thermal stability of the mold. Apart from this, the distilled water was used as a chemical modifier. For small castings, it is favorable to use minimal ingredients.

#### **2.2 Measurements and equipment used**

 The universal testing machine (M/S Fine Spacy Associates and Engineering Pvt. Ltd., India) has a loading capacity of up to 600 kN within +1% accuracy with IS:1828/BS:1010 standard. The compressive strength was measured as per ASTM D395 standards and the specimen had a 1:2 diameter to height ratio. For the small casting, the preheated mold was used to minimize the temperature difference between the molten metal and mold to improve the filling rate of the molten metal into the mold cavity or minimize the shrinkage effect of the cast material. For this reason, the specimen was tested at an elevated temperature by preheating in a muffle furnace (M/S B.S. Pyromatic India Pvt. Ltd., India) with a maximum heating capacity of 1200°C with an accuracy of ±3.0°C temperature.

#### **2.3 Experimental procedure**

The best selected binder material, filler material and chemical modifier cannot produce a good casting until they are efficiently and properly mixed and prepared. *Proceedings of the 4th International Conference on Innovations in Automation...* 


#### **Table 1.**

*Used percentage of ingredient to prepare mixture.* 

 The quality of the mold depends upon the manner in which it is prepared. The specimen as a representative of mold material is prepared from α- hemihydrate, water and colloidal silica of different composition as mentioned in **Table 1**. The ingredients are mixed with a mixture machine for 10 minutes and after that, it was casted in a cylindrical specimen as per ASTM D395 for test. The first combination was made of a binary mixture of α-hemihydrate and water, the second binary was made from α-hemihydrate and colloidal silica, and the third mixture was made from three ingredients i.e. α-hemihydrate, water and colloidal silica and was called the specimen of a tertiary mixture.

## **3. Result and discussion**

The effects of individual ingredients on compressive strength are discussed below.

#### **3.1 Effect of water**

 As shown in **Figure 1**, it has been found that a reduction in water content from 50 to 30% increased the compressive strength by 22.4% because low water content slurry produced a homogeneous mixture but further reduction of water content reduces the compressive strength by 9.9% because low water content does not encourage the chemical reaction or embedding of the α-hemihydrate powder.

#### **3.2 Effect of colloidal silica**

 **Figure 2** shows that adding colloidal silica to α-hemihydrate decreases the compressive strength of the mold material. The 30% reduction of colloidal silica in a mixture contributed to a 11.6% increase in compressive strength because of the effective chemical reaction with α-hemihydrate powder by the fine quartz particles of colloidal silica.

#### **3.3 Combined effect of water and colloidal silica**

It was found that when the mixture contained three ingredients (**Figure 3**) i.e. water, colloidal silica and α-hemihydrate, the compressive strength of the mold material decreased. The compressive strength was found to increase with a higher amount of water content than colloidal silica. When a large amount of colloidal silica was added, the compressive strength of the mold decreased. These factors allowed for the removal of the small castings from the mold without any damage.

*Characterization of Mold Material for Small Castings DOI: http://dx.doi.org/10.5772/intechopen.81083* 

**Figure 1.**  *Effect of water on compressive strength in a binary mixture.* 

**Figure 2.**  *Effect of colloidal silica on compressive strength in a binary mixture.* 

**Figure 3.**  *Combined effect on compressive strength in a tertiary mixture.* 

#### **3.4 Effect of temperature**

During the casting process, the mold has to sustain a high temperature and there should be a small temperature difference between the mold and molten metal as discussed in the introduction section. Hence the effect of temperature on mold material as well as on its compressive strength was investigated.

Through the experiment, it was found that cracks were initiated in a specimen having 60% α-hemihydrate and 40% water from 750°C temperature because of thermal expansion as shown in **Figure 4**. Whereas, as shown in **Figure 5**, the added colloidal silica up to 40% in the mixture increased the heat resistance of the

**Figure 4.**  *60% α-hemihydrate and 40% water content up to 750°C temperature. (a) Before preheating. (b) After preheating.* 

#### **Figure 5.**

*30% α-hemihydrate, 30% water and 40% colloidal silica up to 900°C temperature. (a) Before preheating. (b) After preheating.* 

mold material and it could withstand up to 900°C without any cracking. Apart from this, its compressive strength was also comparatively less i.e 7.21 MPa which shows that the percentage ingredients can appropriate for small casting. Hence that has been taken into consideration for further future investigation.
