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DEVELOPED OPTIONS TO THE BASIC ZVAC METHOD
Levels of vacuum and rate of pressure drop can be improved over the basic process by applying the developments described below, but the extra complications mitigate against the excellent reliability of the original simple ZVAC process, and add extra cost.
(1) Mechanical valve to augment die evacuation via zig zag vents (ZZVs), or other vent.
The ZVAC method can be used with a valve that is inserted into the die to give ultra rapid die evacuation. The valve allows air flow and blocks molten metal flow in a channel that links the main feed runner or other part of the die cavity to the peripheral vacuum channel or separate vacuum port. There can be more than one valve on a die (unlikely need). The channel via the valve would normally be entirely on the parting face, but could include an easily accessible subterranean section in certain exceptional cases.
The valve may function by either axial or rotational movement, or a combination of both movement modes. The valve may be operated by pressurised fluid, vacuum or electro-magnetic-mechanical devices, either directly or via lever, cam or geared systems to control its movements and opening or closing forces. The valve may be mounted in or on either die half (holder or inserts), and its operator may be mounted on the moving ejector plate, holders or machine platens or parts thereof.
(2) Prevention of air leaks via ejector pins
Instead of sealing ejectors with heavy bodied grease (best and simplest method) or elastomeric seals (fiddly), the ejector plate and pins (usually) on the moving half are enclosed in a sealed box which becomes airtight when the ejector plate is at its returned position, by virtue of seals around the holes for the ejector plate push rods that are moved by the diecasting machine hydraulic piston.
The ejector box is evacuated as soon as the dies close, to a pressure that will be similar to that which will be applied to the mould cavity, thus momentarily causing air to start flowing from the mould cavity into the ejector box via the ejector pin clearances or other leak paths that might exist due to die construction.
The air flow past the ejector pins is intended not to create a vacuum in the mould, but to prevent air leaks into the mould when the process vacuum is applied to the mould cavity via ZZVs and/or valve prior to injecting the metal.
Thus, in this method, there are two stages to applying vacuum :
- A vacuum valve opens to exhaust air from the back of the ejector die half as soon as the dies close or alternatively while metal is being poured into the shot tube in the case of cold chamber machines, before the injection plunger starts to move.
- A vacuum valve opens to evacuate the mould cavity and shot sleeve via the static vents (usually ZZVs) and or a mechanically operated valve on the die as described in paragraph (1) above.
The vacuum supply valves are controlled by separate timers and logical control, to close the vacuum supply valves after injection of the metal, or in the case of a machine sequence fault, or cancelling of injection phases or opening of the dies.
In the case of cold chamber machines, if the pouring of metal is excessively delayed, then the vacuum valve to the ejector box would be closed, but reopened when the ladle metal pouring sequence is reinitiated.
(3) Evacuating shot sleeve and die cavity via shot sleeve pouring hole and ZVAC method.
After poring the metal into the shot tube, the shot tube is quickly enclosed by another tube that slides over the injection ram and seals against the fixed bolster or shot tube support bush. Vacuum is then applied to the mould via the enclosing tube and pouring hole, as well as via the static vents on the die parting surfaces. This shot tube enclosing method can also be used with the ejector box evacuation method. Connecting tubes and vacuum valves need to be large bore to give rapid evacuation, eg, typically 40 mm for 500 t size machine, and very large receivers are needed to minimise depletion of vacuum due to the additional volumes being evacuated.
(4) Hot work steels and high thermal conductivity alloys in ZZV areas.
The ZVAC method has since its inception simply laid its ZZVs across the parting faces of the mould inserts and holders. However, inserting H13 plates in ZZV regions can prevent thermal heat checking (crazing) caused by the heat from of aluminium and copper alloys, and plates of beryllium copper or bronzes can enable shorter ZZV paths so enabling smaller dies, or deeper vents, which would give better air flow and so better vacuum and less porosity. These extra plates add extra cost and complication to the ZVAC method and give only marginal benefit
CONTACT: Chris Hoskyns
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tel. UK: +44 1908 374 226
tel. China: +86 13052 855 713
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ZVAC® really can solve your diecasting's porosity problems.
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