Moulding processes may be classified as hand moulding or machine moulding according to whether the mould is prepared by hand tools or with the aid of some moulding machine. Hand moulding is generally found to be economical when the castings are required in a small number.
· When the number of castings is substantial, the additional cost of metallic patterns and other equipment is compensated by the high rate of production, and the overall cost per piece works out lower than in the case of hand moulding.
· It affords great saving in time, especially when a large number of similar castings in small sizes are required.
· A semi-skilled worker can do the machine job whereas hand moulding requires skilled craftsmanship.
· The castings obtained are more uniform in size and shape and more accurate than those obtained by hand moulding due to steadier lift of the pattern.
Types of Moulding machine:
1. Hand Operated molding machine
a. Pattern draw type
b. Pattern draw and Squeeze type
c. Pin Lift type machine
d. Roll Over type machine
2. Power operated Molding Machine
a. Squeeze machine
b. Jolt machine
c. Jolt squeeze machine
d. Jolt squeeze roll over pattern draw machine
e. Sand Slinger
A squeeze machine is very useful for shallow patterns. A squeezer (squeeze head) plate or presser board slides inside the flask to compress the sand above and around the pattern.
For squeezing action the squeeze piston may by forced upward, pushing the flask up against the squeezer or presser board the presser board being forced into the flask.
The sand is rammed harder at the back of the mould and softer on the pattern face. In other words sand has greatest density at the surface where pressure is applied to sand and sand density decreases progressively towards the pattern.
Moulding force (Mf) = P (π. d2/4)-W
Where, P – Pressure in squeeze cylinder
d – Piston diameter
W – Weight of flask pattern and sand
It combines in single machine the operating principles of the jolt and squeeze machines. Combination of jolting and squeezing produces beneficial compaction effects on sand density and thus a more uniform hardness throughout the mould is attained. A jolt-squeeze machine makes use of match plate moulding.
The sand slinger consists of a base, a sand bin, a bucket elevator, a swinging or movable arm, a belt conveyor and the sand impeller. Prepared sand lying in the sand bin is picked up by the elevator buckets and is dropped on to the belt conveyor which takes the same to the impeller head.
Inside the impeller head, rapidly rotating cup shaped blade picks up the sand and throws it downward into the moulding box as a continuous stream of sand with machine gun rapidity and great force.
The sand is discharged into the moulding box at a rate of 300 to 2000kg/minute. This force is great enough to ram the mould satisfactorily.
In moulding boxes, sand is filled and rammed at the same time. The density of sand which is the result of sand’s inertia is uniform throughout the mould.