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Larger and more complex moulded parts are
difficult to fill and are susceptible to either over-packing, warpage
or flashing. Similarly, family moulds of different sized parts,
particularly from different materials, create more problems of flow
and have a relatively poor surface finish. Such parts face limitations
in moulding by the conventional valve gated hot runner system.
A technique known as Sequential Valve Gating (SVG), wherein the
valve gate opens and closes through a programmed sequence is increasingly
used for moulding of such parts. Moulded parts of excellent surface
finish which are free from warpage, flashes, etc. can easily be
produced from SVG technique. No wonder that many international suppliers
of hot runner systems are resorting to this technique. SVG technique
also helps in filling the family moulds more uniformly and avoids
over-packing and flashing. The progressive fill through SVG can
help in reducing the clamp-tonnage by about 25%. SVG also helps
in achieving better flow lengths and is hence more suitable for
thin-wall parts.
Automotive market is the first market that adopted SVG system for
moulding of bumpers, fenders, wheel liners, rocker panels, grilles,
fans radiators, air-intake systems and even instrument panels. One
of the most important part moulded from SVG system is a two-component
weather-strip from PP (harder component) and TPE (softer component).
The non-automotive market has also used SVG very effectively. For
instance, palm (PDA) is moulded using sequential valve gating through
11 gates from Carbon filled conductive Polycarbonate in 1.5 second.
Majority of SVG system uses from 4 to as high as 12 drops, with
essentially two different mould filling requirements. In one, SVG
has to fulfill uniform balanced filling of the multi-cavity family
moulds. The other one is to cascade the moulding for long or large
parts to improve surface finish and prevent subsequent painting.
Cascade moulding is designed to emulate the single melt flow front
and lack of weld lines obtained when one gate is used. A cascade
design positions the gates so that the melt flow length from one
nozzle ends just past the gate of the next nozzle. When the melt
passes the nozzle, the nozzle is opened. The new melt comes in behind
the preceding flow front. Hot melt from the new nozzle rejuvenates
the melt in front without forming a weld line. The cascade can be
designed to flow from the center of part out to both sides or from
one end of a part to the other. A third approach may be to open
the gates on both outer part edges and bring the melt streams together
at the center of the part. In either case the melt flow moves before
it sets up. The distance the melt flows from one gate to another
and the timing of opening and closing of gates are very critical.
Proper venting is also essential for SVG to work effectively. SVG
obviously offers advantages but needs to be properly designed.
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