Product category:
Diecasting machines and equipment
News Release from: Frech | Subject: Diecasting magnesium successfully
Edited by the Manufacturingtalk Editorial
Team on 08 August 2002
Magnesium diecasting success is in the
chamber
For the diecaster producing components in magnesium alloy, making the right choice between hot and cold chamber manufacturing routes is fundamental.
For the diecaster producing components in magnesium alloy, making the right choice between hot and cold chamber manufacturing routes is fundamental Keith Higginbottom of machine builder Frech weighs up some of the pros and cons
This article was originally published on Manufacturingtalk on 26 Mar 2002 at 8.00am (UK)
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In an ideal world, my advice (and therefore this article) would be commendably brief - hot chamber for smaller, thinner-walled parts; cold for everything else.
But, as we all know, the world isn't perfect, so ten words just isn't enough.
The reality is that the diecaster working with magnesium has to contend with a long list of "ifs" and "buts" that clamour for his attention.
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Many considerations, often seemingly contradictory, have to be taken into account and brought into the right balance to achieve a successful result.
Without a specific case on the table for discussion there is bound to be a fair degree of fence-sitting in the outline set out below, because both methods have advantages suited to different circumstances.
At the risk of some over-simplification, it might help to clarify the issues involved if we approach the subject under three broad headings - costs, product and process.
Costs - here, we have both capital and running costs to consider, and it is the former that predominate in the case of the cold chamber option.
In fact, setting up a cold operation could well require double the investment needed for the hot chamber counterpart making the same part.
Much of this is down to the fact that the cold machine must have a locking force greater by 50-100% to counter the higher injection forces needed to move the rapidly solidifying magnesium into the die.
Equally important in determining capital outlay is the means by which liquid metal is supplied.
Dosing is the preferred method in the overwhelming majority of cases, because it delivers a double benefit - the pressurised inert gas that fires the dose into the shot sleeve also provides anti-ignition cover over the surface of the metal in the holding bath.
Whereas the cold chamber process incurs most of its costs 'up-front' in the form of capital investment, it is the hot chamber machine - which needs much more maintenance attention, as explained later - that makes the greater demands in the long term; and these, of course are on an open-ended basis.
Both options need to be costed carefully over the lifetime of the project to determine which is likely to give the best return.
Product - the component that is the end result of the whole process naturally has a major influence on the choice of method.
Apart from the general principle already stated that the hot route is likely to be better for small parts, especially where thin walls are specified, a component needing a locking force above 300 tonnes would normally go onto a cold chamber machine.
In fact, there are very few hot chamber machines above this size being used for magnesium.
Size, weight and configuration of the part will all influence the decision; but equally important is the quality of finish that the customer requires.
If it is particularly demanding, it could pay dividends to invest in the greater force of cold chamber, even though in other respects a hot machine would be adequate for the job.
Process - although magnesium's position in the diecasting mainstream now seems assured, the fact remains that in the day-to-day production context it is a relative newcomer and most machine operators are still learning how best to work with it - so it is important that concerns from this quarter are given due weight in the decision-making process.
It is its casting temperature above all that makes magnesium on a hot chamber machine problematic for many; and certainly no-one relishes the prospect of changing a gooseneck with molten metal at almost 700degC in the furnace.
Erosion of wear parts is another issue that has to be addressed.
Since it is caused by the repeated heating and cooling of the machine, hot chamber magnesium diecasting gives of its best on projects where demand is sufficient to keep machines running round the clock - an ideal situation, even given continuing growth in demand, that may not always be attainable.
If the perspective of the people making the castings was the only one that counted and price was no object, there's no doubt that the cold chamber method would carry the day - although a hard-pressed production manager aware that the hot route will probably take a good 15 seconds off his cycle times may take a different view!.
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