Zig Zag Vents = ZZVs & Zed Vents = ZVs
ZZVs are deep Zig Zag Vent shaped flat channels, which are cut close together in groups for connecting a die mould 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.
ZZVs on Heatsink Dies
Generally cut 0.6 mm deep, ZZVs can be from 0.3 up to 1.6 mm deep, according to size of die and other factors.
The ZZVs connect to a peripheral vacuum gallery, which is adjacent to a peripheral seal. The gallery also draws air and lubricants away from the the product cavity via the shut-out imperfections between the parting faces.
Very large vent flow areas are possible with this method to give rapid vacuum pull-down, or massive natural venting.
The total ZZV channel flow area in use on the largest ZVAC die to date, exceeded 1,350 mm2 (>2.1 in2 ).
ZZVs have financial, practical and reliability advantages over the traditional corrugated 'chill-vent' and 'on_die' vacuum valve methods.
ZZVs have small mass compared to overflows, so transfer less heat into the die, but provide very high air extraction flow rates, which have significant beneficial effects on reducing porosity, greatly exceeding those of overflows alone or with old style vents, and ZZVs also reduce metal mass melting energy needs.
ZZVs enable overflows to be reduced in size or sometimes eliminated, thus enabling reduction of shot weights, throughput of molten metal and energy saving.
Overflows perform three main functions:
- add heat to a die,
- flush out metal that might have over-chilled or turbulently entrained gas bubbles,
- reduce the gas compression ratio of the cavity, so that less mass of gas gets trapped as porosity.
History of ZZVs:
The Zig Zag Vent (ZZV) method was conceived, designed and developed between March and May 1991, while I was Technical Manager of Dyson Diecastings Ltd.
Initially, vacuum was applied in January 1991 to three conventional dies, two of which were in continuous use for making the same product on two quite old 400 ton hot chamber machines being run day and night.
In the first dies, simply by adding peripheral sealing and an adjacent vacuum gallery, air was sucked out via many existing conventional straight flat vents.
The benefits were instant and astonishingly good, but the time to pull down the vacuum in the cavity slowed the cycle time by about two seconds.
By measuring the actual vacuum pressure in the die each cycle, it was realised that there were still even higher quality levels and increased productivity possible, if the ultimate vacuum in the die could be reduced and rate of pressure drop increase; hence there was a motivation to improve vent gas flow rates out of the die , and if possible, ideally without changing the die construction or spending more money.
Examples of metal flow in ZZVs .
The search for more efficient vents and shorter cycle times led to the concept of the Zig-Zag-Vent.
Some inspiration came from a device known as a 'Fluidic-Diode' that I learned of at a symposium on Fluidics at Birmingham University (UK), around 1970/71. It was a device that exploited the Coanda effect of thin boundary layer flows across surfaces. Fluid flow in one direction through the device was smooth and 'free', as vortices in corners rotated like rollers on the sides of the laminar jet stream, but in reverse flow condition, the vortices still rotated in the same direction as for 'free' flow, but now opposed and chopped the incoming jet stream.
By simplifying the flow profile of the fluidic diode, I realised that the flow-back characteristic could be replicated in diecasting vents if they had acute corners.
I was also familiar with the Company's caster's 'trick-of-the trade' of having massive venting by putting 0.025 inch thich steel bailing strip (from alloy ingot bundles) between the parting faces at the tops of dies. So, my first ZZVs were cut 0.5mm deep. and then 0.6 and then experimentally deeper and with larger plan view geometry according to die size.
The result was that air flowed through the vents with vortices that were sympathetic to the direction of flow, but when molten metal flowed into the channels, the 'standing' vortices became viscous blobs of rapidly solidifying metal, which at each corner progressively restrict the flow.
About two years later, a fourth die was made which incorporated all the technique's advantages developed on many other dies that were converted to vacuum, to enable reduction of wall thickness and product weight, and elimination of overflows.
To recap, the objectives were:
Several geometries of vent tried, but the simple zig zag patern with acute corners between 50 to 55 degrees, gave the best results. The vents became known as ZZVs, and the vacuum method was named both 'ZVAC' and 'ZZVAC' in 2004.
- To maximise vacuum in the die cavity.
- To get maximum vacuum in the die cavity as rapidly as possible, in order to enable the pause during first (slow) injection phase of hot chamber plunger to be set less than 1.5 seconds for die mould cavity evacuation.
- To optimise machine cycle time and maximise vacuum pressure and quality improvements, without using expensive poppet vacuum valves or troublesome chill vents on the die.
Corner angles of 53° derive by default from having the pitch and the height of ZZV centre line equal, which also happened to enable close nesting with suitably acute corners, to cause the metal to flow back on itself at each corner.
LATEST ZZV GEOMETRY
Based on trials of many ZZV configurations over many years, it is the writer's current opinion that the following general relationship provides close to optimum ZZV geometry and performance.
Angle between ZZV channel legs = 50°
ZZV channel pitch centres = 3.5 x channel_width (at parting face).
ZZV Channel depth = channel_width x 0.05 (can be deeper, depending on the application)
ZZV Channel Edges given 2mm radii (preferred), or 45° chamfer.
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 the acute corners, 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 obstructs the following flow once the void in the channel has filled. That flow pattern progressively obstructs and halts further flow.
The progressive filling of ZZVs is shown in photos
The inertial impact of the injection piston is gradually dissipated 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 mould 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 corrugated chill vents, so enabling better casting quality.
ZZVs do not overcool the holder block or die inserts, which can happen with massive water cooled chill vent inserts whether of steel, beryllium-copper or bronze.
ZZVs can be mapped onto curved parting faces, and cut with 4mm ball nose cutters, or flat faced end-mills with 2mm corner radii using 5axis CNC technology.
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