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45.1 SAND CASTING
Sand casting is one of the older techniques. In this form a mold is made from sand, and the part is cast into it. When the metal has hardened and cooled the part is removed, and the sand removed.
Typical stages of operation include,
2. Cores may also be made at this time. These cores are made of bonded sand that will be broken out of the cast part after it is complete.
3. Sand is mulled (mixed) thoroughly with additives such as bentonite (clay) to increase bonding and overall strength.
4. Sand is formed about the patterns, and gates, runners, risers, vents and pouring cups are added as needed. A compaction stage is typically used to ensure good coverage and solid molds. Cores may also be added to make concave, or internal features for the cast part. Alignment pins may also be used for mating the molds later. Chills may be added to cools large masses faster.
7. Metal is preheated in a furnace or crucible until is above the liquidus temperature in a suitable range (we don't want the metal solidifying before the pour is complete). The exact temperature may be closely controlled depending upon the application. Degassing, and other treatment processes may be done at this time, such as removal of impurities (i.e. slag). Some portion of this metal may be remelted scrap from previously cast parts - 10% is reasonable.
9. As the molten metal cools (minutes to days) the metal will shrink. As the molten metal cools the volume will decrease. During this time molten metal may backflow from the molten risers to feed the part, and maintain the same shape.
10. Once the part starts to solidify small dendrites of solid material form in the part. During this time metal properties are being determined, and internal stresses are being generated. If a part is allowed to cool slowly enough at a constant rate then the final part will be relatively homogenous and stress free.
11. Once the part has completely solidified below the eutectic point it may be removed with no concern for final metal properties. At this point the sand is simply broken up, and the part removed. At this point the surface will have a quantity of sand adhering to the surface, and solid cores inside.
12. A bulk of the remaining sand and cores can be removed by mechanically by striking the part. Other options are to use a vibrating table, sand/shot blaster, hand labor, etc.
13. The final part is cut off the runner gate system, and is near final shape using cutters, torches, etc.. Grinding operations are used to remove any remaining bulk.
14. The part is taken down to final shape using machining operations. And cleaning operations may be used to remove oxides, etc.
45.1.1 Molds
The basic components found in many molds are shown below,
The terms for the parts of a mold are,
pouring cup - the molten metal is poured in here. It has a funnel shape to ease pouring accuracy problems.
runner/sprue - a sprue carries metal from the pouring cup to the runners. The runners distribute metal to the part.
riser - a thermal mass where excess metal will remain in a liquid state while the part cools. As the cooling part shrinks, the molten metal in the riser will feed or fill in the shrinkage. Risers can also be used to collect impurities that rise in molten metal.
parting line - a line of separation that allows the mold (made in two pieces) to be put together to make a full cavity. Note that this line does not have to be a straight line, and is often staggered to make the mold making easier.
There are a number of interesting points about patterns,
- patterns are made of wood, metal and plastics - the material must be stronger if a large number of molds are to be made.
match plate - the split patterns are mounted in a single plate. This allows gating on the drag side to match up with the runners on the cope.
- a slight taper should be added to the sides all patterns this will make them easy to remove from the completed mold. i.e. a cone is easier to remove than a cylinder.
Cores are typically used for more complex shapes. Some point of interest,
- The cores are added when the cavity are made, and they act as part of the mold during casting, but they are rigid enough to allow internal features on parts.
A mold might undergo a hardening process,
skin-dried - the sand is hardened by drying in an oven or air. Higher strength, but distortion and lower collapsibility.
When the pattern and cores have been inserted into the sand it is compacted. There are a number of techniques for doing this,
Squeeze Molding Machines - automatically insert and compact sand. The processes used are designed to produce a uniform compaction. Jolting is sometimes used to help settle the sand. These molds are made in flasks.
Vertical Flaskless Molding - the molds halves are made by blowing sand against a vertical mold. High production rates are possible.
Impact molding - an explosive impulse is used to compact the sand. The mold quality with this technique is quite good.
Vacuum molding - an envelope of plastic is created about the sand using plastic sheeting. Air is drawn from the sand, and the vacuum leads to compaction.
45.1.2 Sands
The sands used tend to fall into the following categories,
The sand effects the following aspects of the casting,
granule size - a coarse grained sand will be porous and allow gases to escape during casting. a fine grained sand leads to a stronger mold.
collapsibility - if the sand can shift during cooling of the part it will reduce stress tears and cracks
Green sand molding refers to a slightly wet condition of the sand (much like `green wood'). At the right level of humidity the moisture will increase sand binding. But in excess this moisture expand when heated during pouring and blow metal back out of the mold (i.e. explosion). This is one of the least expensive molding techniques).
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