Casting Process

The process cycle for sand casting consists of six main stages, which are explained below.

1)Mold-making - The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mold. Further details on mold-making will be described in the next section. The mold-making time includes positioning the pattern, packing the sand, and removing the pattern. The mold-making time is affected by the size of the part, the number of cores, and the type of sand mold. If the mold type requires heating or baking time, the mold-making time is substantially increased. Also, lubrication is often applied to the surfaces of the mold cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.

2)Clamping - Once the mold has been made, it must be prepared for the molten metal to be poured. The surface of the mold cavity is first lubricated to facilitate the removal of the casting. Then, the cores are positioned and the mold halves are closed and securely clamped together. It is essential that the mold halves remain securely closed to prevent the loss of any material.

3)Pouring - The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. The pouring can be performed manually or by an automated machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short in order to prevent early solidification of any one part of the metal.

4)Cooling - The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold can not be opened until the cooling time has elapsed. The desired cooling time can be estimated based upon the wall thickness of the casting and the temperature of the metal. Most of the possible defects that can occur are a result of the solidification process. If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks, or incomplete sections. Preventative measures can be taken in designing both the part and the mold and will be explored in later sections.

5)Removal - After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Once removed, the casting will likely have some sand and oxide layers adhered to the surface. Shot blasting is sometimes used to remove any remaining sand, especially from internal surfaces, and reduce the surface roughness.

6)Trimming - During cooling, the material from the channels in the mold solidifies attached to the part. This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. A larger casting will require a longer trimming time. The scrap material that results from this trimming is either discarded or reused in the sand casting process. However, the scrap material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused.

Classification of liquid crystals

Classification of liquid crystals

• Liquid crystals may be broadly classified into twoThermotropic liquid crystals: are those that exhibit liquid crystalline state on change of temperature alone. Example: para azoxyanisole.
•  Lyotropic liquid crystals: these exhibit liquid crystalline state in mixtureand when the concentration of one of the constituents is varied. Example: soap.

 Nematic liquid crystals:

• These have thread like texture. ( Greek nematos = thread).
• These are formed from optically inactive compounds.
• The molecules do not have positional order but the molecules are arranged parallel to oneanother and hence have orientational order.
• Less Viscous as they are less closely arranged
• Fluidity is more
• p- azoxy anisole is an example for nematic liquid crystals. 

Chiral nematic or twisted nematic or cholesteric liquid crystals.

• Cholesteric liquid crystals exhibit finger print texture.
• These are formed from optically active compounds.
• A group of molecules is oriented at an angle to the adjacent group of molecules such that the director takes a helical path as it travels through the liquid crystal just as a nut is moved on to a screw.
• The distance through which a director travels as it completes one full rotation is called the pitch of the liquid crystal.
• Less Viscous as they are less closely arranged
• Fluidity is more
• Eg for chiral nematic liquid crystal is cholesteryl benzoate.

Smectic liquid crystals:

• Greek smectos = soap
• Smectic liquid crystals exhibit focal conic (broken fan )texture
• Molecules are arranged in layers. At any instant of time the number of molecules within a layer is much more than the number of molecules between the layers.
• Highly Viscous as they are closely arranged
• Fluidity is less
• Depending of the orientation of the molecules( inclination to the director) smectic liquid crystals are called smectic A,.B,C etc.

Discotic liquid crystals:

•  Here the molecules are disk like.
• These may be columnar liquid crystals or discotic nematic liquid crystals.
• In columnar liquid crystals, the molecules are stacked one above the other forming a column. In discotic nematic liquid crystals the molecules have coin like shape and have no positional order but possess orientational order.The columns form definite shape such as hexagon.
• Highly Viscous as they are closely arranged
• Fluidity is less
• Eg: Benzene-hexa –n- alkanoates

Liquid Crystals

Definition

Liquid Crystal is a substance which flows like a liquid but has some degree of ordering in the arrangement of its molecules.

•  In crystalline state, not only do the molecules occupy specific positions but also tend to orient in a preferred direction. Thus the molecules have both positional and orientational order.

• In liquid state, the molecules neither occupy specific positions nor are oriented in any particular manner. The molecules are free to move at random and collide with one another abruptly changing their positions thus losing both positional and orientational order.

• Intermediate between the solid and the liquid state exists the liquid crystal phase, wherein the molecules are free to move but are oriented in a particular manner. The molecules have no positional order but retain some orientation order.
•  Liquid crystal phase is also called mesophase and the molecules, which can exist in
  mesophase, are called mesogens.
• Director : in liquid crystal state the molecules have no positional order but are oriented
  in a particular direction. The preferred orientation of the molecules of a liquid crystal is
  called the Director.