Introduction to Basic Manufacturing Processes and ... - always yours

16. Swells 1. Too soft ramming of mold. 1. Provide hard ramming.
Casting 255
2. Low strength of mold and core 2. Increase strength of both mold
3. Mold not properly supported. and core.
17. Hard Spot 1. Faulty metal composition. 1. Suitably charge metal composition.
2. Faulty casting design. 2. Modify casting design.
18. Run out, Fins 1. Faulty molding. 1. Improving molding technique.
and Fash 2. Defective molding boxes. 2. Change the defective molding boxes.
3. Keep weights on mold boxes.
19. Spongings 1. Availability of dirt and swarf held 1. Remove dirt swarf held in molten
in molten metal.
metal.
2. Improper skimming. 2. Skimming should be perfect.
3. Because of more impurities in 3. Fewer impurities in molten metal
molten metal
should be there.
20. Warpage 1. Continuous large flat surfaces on 1. Follow principle of sufficient
castings indicating a poor design.
directional solidification
2. No directional solidification of casting. 2. Make good casting design
13.11 PLASTICS MOLDING PROCESSES
There are various methods of producing components from the plastics materials which are
supplied in the granular, powder and other forms. Various plastics molding processes are:
1. Compression Molding.
2. Transfer Molding
3. Injection Molding.
4. Blow Molding.
5. Extrusion Molding
6. Calendaring.
7. Thermoforming.
8. Casting
Two major processes from the above are discussed as under.
13.11.1 Injection die Molding
In this process, thermoplastic materials soften when heated and re-harden when cooled. No
chemical change takes place during heating and cooling. Fig. 13.11 illustrates the injection
molding process. The process involves granular molding material is loaded into a hopper from
where it is metered out in a heating cylinder by a feeding device. The exact amount of
material is delivered to the cylinder which is required to fill the mold completely. The
injection ram pushes the material into a heating cylinder and doing so pushing bushes a small
amount of heated material out of other end of cylinder through the nozzle and screw bushing
and into cavities of the closed mold. The metal cooled in rigid state in the mold. Then mold
is opened and piece is ejected out material heating temperature is usually between 180°-
280°C. Mold is cooled in order to cool the mold articles. Automatic devices are commercially
available to maintain mold temperature at required level. Injection molding is generally
limited to forming thermoplastic materials, but equipment is available for converting the
machines for molding thermosetting plastics and compounds of rubber.