This article was written by Andrew Rankin of Sovrin
Plastics and Peter Manser of Battenfeld Uk. This is Page
2 of 3.
Selecting the Right Process (continues
from previous page)
Injection moulding is essentially a high -volume
process. The purchase of a moulding machine requires a high level
of investment; producing moulds with fine detail is time consuming
and expensive, and skilled process technicians are required to optimise
the production process. However, the benefits are well worth the
effort. (Figure 2).
The Micromoulding Process
Because of the particular processing conditions
required to produce small and thin-section mouldings with fine details,
the normal rules of injection moulding do not apply to micromoulding.
Conventional moulding may require several seconds to fill a mould.
In micromoulding the speed of injection into the mould must be fast
to allow the material to pass through the much smaller feed point
of the moulding cavity to fill the thin sections of the moulding.
The cooling time will usually be much shorter than for conventional
thicker section mouldings; typically, up to a tenfold decrease in
time can be achieved, which allows significantly faster overall
cycle times. This approach to the moulding process has required
the creation of a unique micromoulding system, which employs clean,
dust-free and servo-electric drive systems. An extruder plastics
the material; the required amount of material is measured precisely
in a dosing chamber. The material is then injected into the mould
using a small diameter piston, 3-7mm in diameter depending on the
application, which gives controlled and precise filling with minimal
material wastage.
The mouldings are small and they can be difficult
to handle, which is compounded by the creation of static electricity
during the moulding process; the small mass of the micromoulded
parts makes them susceptible to clinging to any nearby surface (Figure
3).
Figure
3: |
Small
gear weighing 1.1 mg. |
|
To overcome this problem, static is eliminated
using integrated deionising units. Also, an integrated robotic handling
device is used to remove mouldings from the mould and enables then
to be presented to an on-board vision system. The vision system
is linked to a separate integrated PC with a generic quality control
software package using pixel counting to check shape, detail of
features and dimensions. Mouldings can be placed into bespoke individual
cavity storage for 100% quality monitoring and tractability. This
concept of total process control can also be extended to the final
packing of moulded parts for sterilisation or subsequent automated
assemble (Figure 4).
Figure
4: |
Blister
Pack strip with gear wheel |
|
By maintaining control of the mouldings they can
be placed in an enclosure, which has a HEPA filter airflow unit
to provide Class 1000 clean-room conditions within the process area.
Lower particle counts can be achieved if the cell is also placed
inside a conventional high-class clean room.
