ZZVs are deep Zig Zag Vent shaped flat channels, which are cut close together in groups for connecting a die mold cavity to a vacuum channel, or for natural exhausting of the die.
They have also become known as ZVs from 'Zed Vents' or 'Zee Vents', and ZZags from ZeeZags or ZigZags.
Generally cut 0.6 mm deep, ZZVs can be from 0.3 up to 1.6 mm deep, according to application.
Very large vent flow areas are possible with this method to give rapid vacuum pull-down, or massive natural venting.
The largest ZZV channel flow area in use to date, is over 1,500 mm2.
ZZVs have financial, practical and reliabilty advantages over the traditional corrugated chill vent and vacuum valve methods.
See example of metal flow in ZZVs
To recap on their history:
The Zig Zag Vent (ZZV) method was conceived, designed and developed between March and May 1991, by Chris Hoskyns, while Technical Manager of Dyson Diecastings Ltd.
The objectives were:
- To maximise vacuum in the die cavity.
- To get vacuum as rapidly as possible, to enable the pause during first slow injection phase of hot chamber diecasting machines to be less than 1.5 seconds for complete die mold cavity evacuation.
- To achieve cycle time, vacuum pressure and quality improvements without using expensive vacuum valves or chill vents on the die.
Several geometires of vent tried, but the simple zig zag patern with acute corners at 53 degrees, gave the best result. The vents became known as ZZVs, and the vacuum method has become the ZVAC or ZZVAC technique.
ZZVs are most effective when used in closely nested multiple parallel groups.
Each ZZV is a lateral flat shallow channel, typically 0.6 mm deep, which successively changes direction at acute corners with 53 degree intersection angles, where vortices and solidification occur, which further resist the flow of metal. The geometry of ZZVs is important. If ZZVs do not have correct proportions, then the metal flow will not be adequately arrested.
Gases easily pass through the ZZV, but molten metal impacts on the ZZV channel sides which causes some metal flow along one side of the channel to the next corner where it flows back upon itself, freezes in situ and obstruct the following flow once the void in the channel has filled. That flow pattern progressively obstructs and halts further flow.
The progressive filling of vents is shown in photos
The inertial impact of the injection piston is gradually absorbed as molten metal meets increasing resistance to flow in the ZZVs, and so, as a secondary benefit, across parting face dimensions have minimal variation.
ZZVs are so effective, that they now mostly used for 'massive' venting without vacuum, to minimize gas porosity problems at very low cost.
ZZVs make use of the redundant areas of the holder block or bolster plates, so you can get hyper-massive venting, almost for free, and without cutting pockets and inserting huge expensive chill vents, which weaken and over-cool the mold frame.
ZZVs are usually connected directly to the mould cavity, and can reduce or eliminate need for overflows, so reducing shot weights and melting energy costs.
By connecting ZZVs directly to the feed runner, air can be exhausted directly from shot sleeves and gooseneck nozzles, so enabling very rapid vacuum evacuation.
Within the die spaces available, ZZVs can give greater total venting area than that possible by conventional crenellated or corrugated chill vents (CCCCVs); so enabling better casting quality.
ZZVs do not overcool the holder block or die inserts, which can happen with massive water cooled berylium-copper or cobalt-copper bronze chill vent inserts
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