Page 170 - Module DMV20173

EXERCISE 7.2

What are the functions of gates in injection molds?

Answer: The functions of gates in an injection mold are (1) to increase shear rate to
increase viscosity and temperature of the polymer melt, (2) the thinner cross section
of the gate freezes more rapidly to seal off the cavity; and (3) parts can be more
easily broken off the runner at the gate.

EXERCISE 7.3

A. _A granular form of plastic powder (polystyrene, nylon, _polypropylene/pvc/etc_
and polythene) are poured or fed into a hopper which stores it until it is needed.

B. The heater is turned on. This warms up and melts the granular plastic. A _motor__
turns a screw thread which pushes the granules along the heater section causing it to
change to a plastic liquid.
C. The liquid plastic is forced into a mould by _screw __.
D. _Pressured_ air is ‘blown’ into the mould. This forces the liquid plastic against the

sides of the _mould_. In this example it forms the shape of a plastic _bottle_.
E. The plastic is allowed to _cooled / solidify_ and the mould is ‘opened_’ and the
plastic bottle removed. The entire process is _repeating_ hundreds or thousands of
times.

EXERCISE 8.1

A typical alloy whose melting point = 500 °C is to be machined using EDM process.
3
If the MRR is 574.709 mm /min, what is the approximate value for the discharged
current used in the process?

4 -1.23
MRR = 4 x 10 ITw

574.709 mm /min = 4 x 10 x I x 500 -1.23
4
3

I = 574.709 mm /min / (4 x 10 4 x 500 -1.23 )
3

I = 30 amps