| HotFlo! | Non-inclined Die Casting Machines | |
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Most Die Casting machines are designed with the platens at an inclined angle of between 5 and 10 degrees, as depicted at the right. This is to allow the metal in the nozzle to run back down into the gooseneck when the plunger is retracted. This process of 'draining back' is very important, and even more so for a hot sprue system. All the molten metal in the sprue must run back into the machine nozzle and from there into the gooseneck. |
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However there are some Die Casting machines which have no inclined angle; the platen faces are vertical and the nozzle bore is horizontal. With a non-inclined machine there are some important differences in the way that this 'drain back' is made to occur, which is explained below. |
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After the plunger has retracted, liquid metal remains in the horizontal nozzle. Some of it drains out the back end, around the bend in the gooseneck and into the shot system. When the die opens, there is still some metal lying in the bottom of the nozzle, and, with the sprue now removed, it can flow forward, spilling out the sprue hole and onto the die face. This is an unacceptable situation because it can prevent the die from closing, damage the die face and disrupt production.
To avoid this problem, machines with a non-inclined platen, have a special plunger return sequence known as 'plunger overlap'.
An extra limit switch is mounted on the plunger to detect an intermediate return postion which is just before the plunger uncovers the filling hole in the liner. After taking a shot, the plunger retracts to this intermediate position. This creates a vacuum inside the gooseneck.
When the die opens, air rushes into the sprue hole to fill this vacuum. As it does so, it pushes any molten metal along with it, back down the nozzle and into the gooseneck. After the dies are fully open, the plunger retracts to the top of its stroke, allowing the shot sleeve to refill with metal for the next shot.
When using a hot sprue system on a non-inclined machine, this 'plunger overlap' feature must be used.
Additionally, when using a hot sprue system on machines with a small inclined angle (5 degrees or less) or on machines which drain back slowly due to their gooseneck design, the use of plunger overlap will be a definite advantage.
It should be noted, that there can be circumstances when a plunger overlap system will not work satisfactorily. This can happen when the casting shot size is very small relative to the machine shot end. There may be insufficient volume of vacuum to push all the metal back into the gooseneck, resulting in metal spill from the sprue hole.
One possible solution is to reduce the diameter of the hole inside the nozzle, reducing the amount of metal which has to drain back. If this is not possible, a smaller machine or gooseneck must be used.
| Document Control | Issue 1 | 1st July 2000 | www.hotflo.com |
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