Product category:
Diecasting machines and equipment
News Release from: Frech | Subject: Diecasting surface quality
Edited by the Manufacturingtalk Editorial
Team on 21 January 2005
Diecasting quality pitfalls - how to
avoid them
Poor surface condition is bad news for any diecaster - especially in zinc - Keith Higginbottom, consultant to Frech UK, identifies some of the pitfalls during the cycle.
Poor surface condition is bad news for any diecaster - especially in zinc, where many parts must take a blemish-free decorative finish Keith Higginbottom, consultant to Frech UK, follows the liquid metal on its short but eventful journey to its cast form, and identifies some of the pitfalls along the way
This article was originally published on Manufacturingtalk on 26 Mar 2002 at 8.00am (UK)
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A good casting starts with good metal in the furnace, which means, amongst other things, that the temperature has to be right.
Too high and the melt will deteriorate or be lost through evaporation; too low and the metal will begin to solidify before it reaches its intended destination, with the consequences described in the course of this article.
For zinc alloys the critical temperature is 460 deg C, at which aluminium will come out of solution and form into globules that will attack the iron furnace lining.
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In addition an overheated melt will begin to oxidise, creating hydrogen oxide that, if not skimmed off, can appear on the surface of the casting as inclusions and hard spots.
Re-melting of returns also needs caution, because a content of more than 40 per cent will progressively degrade the melt through oxidation (the only exception where more may be tolerated being large castings with a small runner system).
For aluminium in particular, the next potential hazard is transfer from the furnace to the die, where the level of metal in the shot sleeve must be as high as possible in order to minimise the ingress of air.
A two-thirds level at the pouring hole is normally sufficient to ensure a satisfactory fill as the metal is lifted into the die cavities.
Although the details of the injection cycle differ considerably between zinc and aluminium, both have the same objective - to inject the metal as fast as possible so that there is a complete fill of metal from the gate right back through the runner to the gooseneck or shot sleeve.
This ensures that there is little or no opportunity for residual gases within the system to be absorbed into the melt during the injection phase.
It is at this point that the opportunities for defects in the finished casting begin to proliferate - and, if the criteria outlined above have been met, the effectiveness (or otherwise) of the die design is put to the test.
The first requirement is to configure the runner system with increasingly narrow channels so that the speed of flow of the metal accelerates continuously up to the gate, where it enters the casting cavity.
Although speeds and volumes will naturally vary depending on the nature of the casting being produced, typical figures are 2 metres per second for plunger speed and volumes of 30 litres a second at the nozzle, rising to 40 litres a second at the gate.
This will achieve the cavity fill time of 20 metres per second that is needed for some higher quality surface finishes.
This acceleration is especially important when producing zinc castings for which the best possible surface condition is required, because the impetus of the metal drives before it the air that causes porosity - on the surface of the casting as well as internally.
As the metal passes through the gate, the air should be vented from the cavity, while any dross at the front of the metal is allowed to overflow and can be trimmed off later.
Ideally, vents and overflows should be positioned where separate streams of metal meet - and predicting such locations is one of the die designers most valued skills.
Equally important is an understanding of the optimum speed for a given volume and surface area, in order to avoid moving the metal so fast that the flow breaks up and admits porosity-causing air.
Making use of gravity by moving metal upwards rather than downwards is another way of avoiding this effect.
The design of the gate is critical, because it is here that the phenomenon known as cold flow (or cold laps) is most often initiated.
Although the surface defects it gives rise to go by a number of different names -lamination, scale formation, flow marks and surface waves, for example - there is general agreement about the cause.
If the temperature of the metal is too low when the first wave passes through the gate, it solidifies as it enters the cavity.
The next wave does the same, but will move slightly further into the cavity because of the warming effect of the first; and so on, creating overlapping waves that are visible on the surface of the casting.
In extreme cases, they can even be felt as undulations.
The challenge for the die designer is to ensure that the different metal streams arrive in the cavity at the same time and at the right temperature, so that it is filled completely and the metal solidifies at the same rate throughout.
No easy task, especially considering that it is also necessary to cool the casting quickly in the interests of the shortest possible cycle; but not so quickly that the part has to be scrapped.
Porosity tends to concentrate in the centre of the casting and is not normally considered a surface condition problem (although it can be an issue with window hardware manufacturers, who often prefer the gate to be positioned where even the slightest evidence of surface pores can be polished away before finishing).
Cracks, of course, are most definitely unacceptable and almost always to be laid at the die designers door, indicating an absence of the radii that prevent the potentially damaging concentration of stresses on edges and corners.
They can also be symptomatic of adjacent hot and cold spots in the die - commonplace when water-cooled dies were the norm, but nowadays entirely avoidable by the use of die temperature control to maintain a consistent temperature profile.
Other causes of surface imperfections include inexpertly applied die lubricant, which if present in excess can create discolouration and even surface porosity.
If there is sufficient to penetrate below the surface in gaseous form, the metal of the casting can solidify above it and then break down as the gas expands if it is subsequently heated - a common cause of failure during powder coating.
Here again, getting the temperature right is the key to success.
If zinc castings can be kept below 200 deg C during surface finishing or other post-casting operations, it is likely that problems of this kind can be avoided.
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