Shakeout: separating the casting from its mold.
Separating the casting from its mold is a necessary step in the metalcasting process. Usually referred to as shakeout, a well-planned and designed system can improve overall productivity and reduce cleaning and finishing operations.
After a metal casting has been poured off and allowed to cool sufficiently, it must be removed from the mold for further processing. In the sand foundry, this step in the metal-casting process is generally referred to as shakeout.
With an efficient shakeout, the mold is broken up, the castings and sand are separated, and mold lumps are reduced in size. To accomplish this, most modern foundries use a vibratory or rotary shakeout system.
Ideally, a shakeout operation should accomplish the following:
* separate the sand,
casting and flask (if used);
* clean adhering sand from flasks (if used);
* remove as much adhering sand from the casting surface
* decore the casting;
* break up any large mold lumps;
* remove sprue, risers and fins where possible.
Again, the ideal shakeout would accomplish each of these. But given the nature of the various metals cast, the molding system, itself, and other variables, it is not always possible to achieve all of the above. With steel castings, for example, gates and risers often have to be burned off with torches; with aluminum castings, they may have to be sawed off.
In addition, the shakeout operation has to meet production requirements without damaging the casting, and generate a minimum amount of dust and noise. Return sand quality requirements also must be considered.
A variety of equipment, which may be used singly or in combination to perform shakeout and lump reduction operations, is available. Your choice will vary depending upon production rate, casting size, molding methods used and degree of automation within the foundry.
Mold punchout is the method most commonly used to separate sand and castings from the flask on automatic lines with molds produced by high-pressure squeeze or impact. The punchout ram can be fitted with brushes or scrapers to clean adhering sand from the flask.
After punchout, there remains the separation of sand from castings and lump reduction, which is completed in a second operation using either a vibrating deck or rotary drum. Punchout is inherently quiet and, because it is automatic, almost total enclosure is possible. This allows for reduced air requirements to achieve effective dust and fume control.
Vibratory decks are most commonly used to perform the shakeout operation. The vibrating deck in its simplest form consists of a heavy-duty frame constructed from steel and a perforated grid on the frame's top face. The frame is carried by springs that isolate induced vibrations from the supporting structure.
This type of deck is capable of fulfilling most shakeout operation requirements. The action of the vibrating deck is usually to impart about 900-1800 impacts a minute to the flask and mold while breaking down the sand compact. Continuing vibration usually is sufficient to clear the flask of adhering sand. The same applies to the casting, which as the sand lumps are broken down, comes into contact with the deck and is cleared of cores and adhering sand.
The size of holes in the shakeout deck has to provide the optimum compromise between too large, which gives rapid sand throughput but allows large sand lumps and tramp iron to pass, and, at the other extreme, too small, which slows passage of sand and leads to excessively long shakeout times. Holes are often elliptical with back draft to prevent sprue hangup.
High-frequency shakeout decks are design critical and, while very efficient for sand removal, are susceptible to self-destruction due to metal fatigue.
Rotary drum shakeouts are becoming increasingly popular, particularly with high-pressure flaskless molding systems. Because they do not conveniently handle flasks, an additional piece of equipment often is needed to break flasked molds before sand and castings enter the drum.
The rotary shakeout consists of two concentric drums. The outer unit is supported on rollers and may be gear or chain driven at typically 3-8 revolutions a minute. The inner unit has perforated areas that allow sand to flow into a ring between the two drums. The inner drum may have flights fixed to its inside diameter to assist the movement of castings and flights on its outside diameter to direct (into the ring) the sand flow, which can be in the opposite direction to the castings.
In addition, rotary drums are capable of high cooling rates due to water evaporation if there is sufficient water in the sand. Because of intimate mixing of sand and castings, the castings themselves are rapidly cooled. Some systems add water to the sand for evaporation requirements. While fast cooling is desirable, it also can lead to problems with castings cracking or having high hardness.
The perforated region of the inner drum, which influences the casting cooling rate, may be located for sand separation at an intermediate point or at the end point of the drum length. The earlier that the sand is removed from the drum, the slower the casting cooling rate will be.
However, there are penalties for early sand removal. First, sand lumps are not given as much time to break down. Secondly, there is a risk of cracking fragile castings because the cushioning effect of the sand is not there. More noise will be generated for the same reason.
On the other hand, late sand removal results in processing castings through the drum while helping scrub sand from the castings, homogenizing the sand and breaking lumps. The feed also can be dual with green sand being transferred on the reverse portion and core sand on the forward section.
Because rotary drums tend to be physically large, they require substantial floor area. However, pits are not required with these systems and the drums are mechanically simple. This results in lower maintenance costs. Also, sand and castings are delivered to fixed points, which simplifies subsequent mechanical handling equipment.
Vibrating and media drums also are used for shakeout operations. The nonrotating drum, which applies a vibrating action to its contents, is a recent innovation recognized for gentle handling of castings and the ability to remove sand from external surfaces and internal passageways.
When shakeout sand and castings enter the drum, the vibratory action does not tumble the contents as in the rotary drum, but provides a rotary motion by friction that: rubs sand and castings together, removes sand from the castings, and breaks sand lumps into granular size while achieving uniform temperature of the drum contents.
As in the previous systems, moisture may be added to assist in cooling both castings and the sand. Compared to the conventional vibratory deck and rotary drum systems, the vibrating drum, when used as a shakeout, should yield castings with less sand on external surfaces and internal passageways. At the same time, the scrubbing action of the material contents results in removing runners, risers and fins in addition to reducing core chunks to granular size before being discharged.
When sand is processed through the drum without the castings, temperature sensing devices may be used to add water to assist in cooling the sand. The action within the drum provides an excellent method of blending water into the sand without use of a cooler, such as a fluidized bed.
For large volumes and extreme temperatures, the unit, placed in the system ahead of the cooler, provides a system that eliminates the surge loads. The sand lump reduction by the vibrating drum allows the cooler to perform as designed. The use of vibrating drums for casting finishing operations also has been successful when used with media other than sand (such as stars and garnet.)
A fairly recent innovation in shakeouts is the Rotary Media Drum. This patented process combines functions into equipment providing sand and casting separation, sand screening, casting cooling and casting cleaning. The system also can process very delicate castings and reduce shotblast time, thus speeding production.
Upon entrance into the machine, the media starts to break down the sand lumps while protecting the castings from the tumbling action. The media engulfs the castings thoroughly by scrubbing off the sand--even in deep pockets. After being homogenized and screened twice before discharging, the sand is free of clay balls and core butts.
Due to the shape and surface area, the media also acts as a heat sink that cools the castings while conveying them through the machine. Just before discharge, the media is separated from the castings and recirculated back toward the intake. It then is reintroduced into the drum's main body.
This is accomplished in an outer chamber with a reverse helix where the media cools back down before re-entry. The dual chamber design also allows the green sand to be separated from core sand. The machine's interior or lining is designed for various applications.
Finally, shakeouts should be capable of handling unpoured molds. Aeration and lump breaking can be handled separately from the shakeout system but this presents numerous compromises.
PHOTO : Vibratory decks are commonly used to perform the shakeout operation in the foundry. As the
PHOTO : castings work their way through shakeout, the sand mold is broken up and falls through the
PHOTO : holes in the deck and is transported back to a preparation area for reuse.
PHOTO : While much work has been done to automate the shakeout operation in foundries, in some
PHOTO : cases it may still require employees for casting handling.
PHOTO : Typical construction for a vibrating deck shakeout consists of a heavy-duty steel frame
PHOTO : that carriers on its top face a perforated grid. The frame is carried by springs that
PHOTO : serve to isolate induced vibrations from the supporting structure.
PHOTO : The rotary media drum type of shakeout can separate the sand from castings, screen the
PHOTO : sand, while at the same time cool and clean the casting.
PHOTO : The rotary drum shakeout provides a straight-through operation where mold and casting
PHOTO : enter one end of the drum (above) and are separated, the casting decored, large sand lumps
PHOTO : broken up and sprues and risers removed (left).
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|Date:||Oct 1, 1989|
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