Agriculture Reference
In-Depth Information
that only a certain amount of hot material is injected. After pushing the material
through the nozzle into the mould, the screw withdraws to wait for the next cycle.
The flowing material inside the mould forms a frame-like structure, with the 'sprue' as
the main supply line, and the multi-cavities are fed with hot plastic material through
'runners' and 'gates', which are part of the frame. This feed system can be cut-off and
recycled. Some moulds are designed in such a way that it is stripped automatically
from the parts on de-moulding. The mould undergoes a cooling cycle, after which
the moulded parts are ejected from the mould. This same cycle is then repeated.
The quality of the moulded parts will depend to a great extent on: the quality of the
mould; correct processing parameters such as temperature, pressure and cooling cycle;
the mould design. The closed mould halves must fit exactly with no gaps otherwise
small leakages of the hot material can cause 'flash'. If filling a new or unfamiliar
mould for the first time, in which the required shot-size is not known, a technician
should reduce the nozzle pressure so that the mould fills but does not produce a flash
with excess material. Sometimes this may have to be done gradually by trial and
error, especially if dealing with very expensive or intricate moulds. An operator will
have an electronic console in which readings can be taken periodically of the pre-
set parameters. If a statistical process control (SPC) system is used, recording of the
'cushion' and related calculations will be of great importance to ensure the process
is in 'control' and within the tolerances to ensure good parts.
Probably the most common injection-moulding machines are hydraulic oil machines
that use the force of hydraulic fluids to move the mould through the mould. Also
popular are the toggle injection-moulding machines that use mechanical links to
open and close the mould. In addition, hybrid hydraulic/toggle machines have come
on the market and become popular. These hybrid machines can eliminate some of
the problems of the standard machines and also increase moulding rates as well as
the rapid wear and tear of machines. With ongoing research, newer models with
improved concepts are emerging for the benefit of moulders. Whatever the design,
the basic principle of injection moulding has not changed.
11.5 Guidelines for Injection Moulding with Composite Resins
When polymer composite resins with wood fibre appeared on the market, injection
moulders were cautious in their approach to these new composite materials. However,
as they became more acquainted with them and considered the advantages of using
these environmentally friendly materials and with constantly improving material
quality and processing techniques, they are now using these composite resins with
confidence. The availability of PCRH is opening up a whole new frontier with exciting
possibilities.
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