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Tools/castingcrucible/induction: Difference between revisions
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{{inductioninfo}}
The [[Tools/castingcrucible|casting crucible]] is one of the most useful but also most hazardous items of equipment rLab has. Because of this people seeking induction will have to provide more safety equipment than is usual, inductees must have
* Cotton or other non-synthetic overalls required
* Heavy trousers, Jeans are good
* Leather shoes - NO SYNTHETICS, Steelies preferred. - Trainers are NOT acceptable
* If wanting to cast lead or other hazardous metals then a mask rated for FFP3 and metal fume. For Aluminium and copper castings then we have masked rated FFP3
Casting can only be done outdoors and absolutely must be kept dry. This limits the times when inductions can occur to Sundays when the forecast indicated no possibility of rain. Sundays because that's the only time the carpark is likely to be empty and when dry because of the risk of explosion if the crucible is rained on.
Equipment capabilities
* Types of casting
** Sand casting
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* How the mold is mounted to the boards, creates the cavity which can then be filled with metal, what is cope, drag and board.
* You have to be able to align the 2 parts of the mold using pins
* You have to be able to
* Using cores if you need complex shapes but we won't cover that
* Consider how the metal will flow, small cavities are hard to fill, there needs to be a path for metal to get in and gas to get out
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* What are Sprus, Gates and Risers and why you need each of them
* Design them into the mold, or cut them afterwards once you've made the sand form
* The possibility of designing, 3D printing, Casting and then finishing on the [[Tools/boxford|Boxford CNC Mill]].
* Printing out a mold, carving a mold
Getting set up
* Casting must only be done outdoors, and only in good weather
* Getting out the casting
* Using the dry sand for safety
* Getting out the crucible and setting it up in the casting tray
* Selecting which crucible to use - Aluminium reside in the default crucible
* Choosing your metal and checking it over carefully
* Keeping the gas bottle well away from the burners
* Mounting the regulator
** Left-hand thread on the regulator
** Do not use PTFE tape on the thread
* Burners are self-lighting and have auto-shutoff
** Turn the blower on first and off last
* Selecting suitable metal
** Reused old castings are cheapest, but have increased risk of voids, porosity and poor strength
** Virgin metal of an approved casting alloy will give the best results but at the highest cost
** Choose depending on budget and mechanical loads the part must withstand
Making the mold
* Mounting the board to the cope and drag
* The sand is builders sharp-sand and bentonite clay with a small amount of water to make it stick together, too little water and it falls apart, too much and steam will prevent filling
* Screening the sand to remove lumps
*
* Packing the sand down hard and completely filling the drag
* First sand layer must be very fine and carefully forced into the smallest of gaps, later layers can be larger and less careful
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* Tapping and releasing the mold
* Touching up any defects, large defects mean starting again
*
* Piercing gas holes to release steam and other gasses
* Reassembling the mold
* Using the
Safety equipment and hazards
* Hazards
** Any water getting into molten metal WILL EXPLODE, so we can't cast on days with even the slightest chance of rain. Drinks must be kept well away from the crucible. Check your metal stock carefully for liquids or voids as voids will also explode on heating. Check the sand in the casting tray is dry
** Sand tends to make melt stop and set, it'll run long distances on other surfaces and start fires at considerable distances
** Burns are an obvious hazard, basic information about burns treatment and when to call an ambulance
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*** Check all gas equipment for any leaks
** Sudden temperature changes causing cracking/shattering, Cold metal should never directly touch hot, pre-warm stuff
** Risks of bystanders - Let people at rLab know,
* Safety equipment
** Overalls required - NON-SYNTHETICS ONLY!!
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* This means that before metal goes into the crucible it must be pre-warmed around the outside of the forge, and then only added in small amounts so it doesn't radically cool the crucible. Little and often is the way to add metal.
* Additions must be made carefully to avoid splashing.
* The maximum capacity of the crucible is about 3Kg of aluminium, or 10Kg of copper. That equates to the crucible being about 75% full. This is approx 1.5l of liquid metal.
Making the Pour
* Positioning the casting box under the spout and fitting the pouring cup
** Getting the box level
** Making sure the cup is accurately over the spru
** Consider where splashing and run-off will go
** Weighting the box down if pouring lead, brass, or other dense metal. Not always needed for Aluminium
* Possibility of using dross filters
* Checking temperature of the melt using the thermocouple
** 750ºC-800ºC for Aluminium
** 950ºC-1050ºC for Brass
** For other metals consult your supplier
** High end of the range for small castings or fine detail, lower end for bulk castings with rough surfaces
* Degassing
** Using the degassing flux to get trapped gasses out of aluminium otherwise your cast will have voids and porosity
** It's very concentrated, you don't need much
** Pre-warm the plunger
** Use it to press the flux down to the bottom of the crucible
** Keep it there till the bubbling stops
* Skimming
** Using a tool to skim off the oxides that will have formed
** Must be done immediately before pouring
* Pouring
** Full safety gear required!
** Final temperature check to make sure we're still in the right range
** The burner will cut off as the crucible begins to tip and re-light when it's replaced
** Warn everyone not directly involved to clear the area
** Pour in one smooth motion, no stopping and starting, try to maintain the pour rate to keep the pouring cup half full
** Gasses and possibly flames will come from the risers and vent holes, this is expected behavior
** "Bumping" or "Burping" that's forceful enough to overflow the pouring cup is NOT expected and indicates a problem, STOP
** The pour is finished when either
*** There's metal visible in all risers
*** Metal stops draining from the casting cup into the mold
** Metal still in the pouring cup is likely to overflow out of the risers, let this happen, fight fires with sand
** Sprinkle dry sand onto the top of the pouring cup for insulation
** Be aware that the burner will re-light once the crucible is returned to the upright position, do not allow the metal in it to become overheated.
After the Pour
* The cast needs a little time to cool before it can be moved or it'll distort
** How long depends on the size of the thickest part of the cast, from a few seconds up to several minutes
* Once the cast has cooled a little then move the casting box away from the crucible
* Pouring off excess
** Position a mold under the spout, either a trough or the baking tray and pour out the remaining metal
** Do not let metal cool in the crucible, it'll be very hard to remove later
Unmolding
* The cast needs to cool down a lot before it can be unmolded
* Times are likely to vary from as little as 5 minutes for a small cast up to an hour for a very large one.
* Once cool take the casting box over to the table and break out the sand to reveal the casting
** Wear welding gloves! Parts of the casting and the sand may still be several hundred degrees
* Collect the sand and riddle it, returning all unburnt sand to the store
* Burnt sand goes in the bin
Casting Flaws
* Examine your finished casting for flaws
** Poor Surface finish - Mold not packed well, mold of poor surface finish, failure to talc the mold, metal too hot, lack of gas vent holes
** Small bubbles in the casting - Poor degassing, sand too wet, metal too hot
** Sunken areas - Spru too small, bad part geometry
** Sand inclusions - Mold not packed well
** Slag inclusions - Poor skimming, bad pour control
** Unfilled areas - Insufficient gating
** Cold joints - Bad gating design, metal had to flow too far through small passages
** Excess flashing - Mold not clamped tightly
Cleaning up
* Letting everything cool properly, this may take several hours, allow time for this.
* Examining crucible, discard if visibly cracked and arrange order of new one
* Returning casting equipment to proper location
* Sand into the suitable boxes
* Sweeping up everything
* Putting the casting tray back
* Returning unused metal to store
* Dust masks to bin, they cannot be re-used
* Re-order any PPE that was damaged
[[Category:Inductions]]
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