Tools/castingcrucible/induction

From rLab

Casting Crucible Induction

Some of the tools at the Hackspace are potentially hazardous to use, for these tools members are required to have an induction before they can use them. Inductions provide the most basic information on how to safely and effectively use the simpler functions of the tools, we appreciate that some members may have professional experience on some of these tools and in this case please tell your induction provider and the induction may be very reduced and just cover any risks or procedures specific to rLab. Some tools have multiple levels of induction in order to cover more advanced uses of that tool without making the basic induction take too long, higher induction levels will introduce some of the more advanced features of the tools but as with all inductions are only intended to provide basic information on the capabilities of the tools and how to use them safely. Some members of rLab may be willing to offer more detailed tuition beyond basic induction level or offer guided practice sessions in exchange for beer money or assistance on their own projects.

For all tools you are only required to take level-1 induction before use, after that you may perform any task that you feel confident you can do safely, higher levels of induction may be useful to you in performing more advanced operations but are not required before doing tasks covered in them so long as you're confident of your ability to handle those tasks without risk to yourself, others, or the tool.

PLEASE NOTE : All induction providers are volunteers who are providing inductions to the best of their ability but are NOT qualified instructors. Inductions are provided on a best-effort basis but you and you alone are responsible for your safety while using the tools and for satisfying yourself that you can operate the tools safely. There are professional training courses available from various providers in Reading and the surrounding area if you feel they are appropriate for the level of work you intend to undertake. Reading these notes is NOT a substitute for an in-person induction.

Note for wiki editors : Please do not edit induction pages unless you are one of of the people that gives that induction

The 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
    • Lost-PLA casting
    • Investment (lost-wax) casting
    • Metal mold casting
  • We're only going to be doing sand-casting today, procedure for others is under development
  • Metals we can work with (Tin/Lead/White-metal, Aluminium, Copper/Brass/Bronze, we can't cast iron/steel)

Object designs

  • 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 remove the mold from the sand without breaking it too badly, so allow tapers. You can't have overhangs
  • 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
  • Remember the shrinkage allowance, 2% in aluminium, 1.5% in brass
  • 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 Boxford CNC Mill.
  • Printing out a mold, carving a mold from wood, high surface finish needed

Getting set up

  • Casting must only be done outdoors, and only in good weather
  • Getting out the casting tray
  • 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
  • Powder the mold to help with release
  • 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
  • Leveling off to enable use of a backing board
  • Flipping it over and then repeating on the cope
  • Tapping and releasing the mold
  • Touching up any defects, large defects mean starting again
  • Cutting Sprus and risers if they weren't molded in
  • Piercing gas holes to release steam and other gasses
  • Reassembling the mold
  • Using the pouring cup

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
    • Fumes coming from metal are both acutely and chronically toxic
    • Beware of concrete and stone spalling in case of spill
    • Fire hazard, does everyone know how to use extinguishers?, remember - NO WATER!
      • Get extinguishers out and ready
      • 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, only people directly involved should be within 5m of the casting equipment
  • Safety equipment
    • Overalls required - NON-SYNTHETICS ONLY!!
    • Jeans are good
    • Shoes Leather or natural fabric, NO SYNTHETICS, Steelies preferred. - Trainers NOT acceptable
    • MIG Welding gloves should be used, nothing else, no others have sufficient insulation and are non-melting
    • Googles AND face shield when pouring, remember face-mask will burn if there is an accident
    • Dust masks
    • You can get specialized foundry versions of all of this kit, all of which are very effective and very expensive

Starting to melt

  • How the burner works, it's self-lighting, the crucible tips to pour and the burner goes out.
  • The burner forms a flame vortex around the crucible to heat the metal
  • The crucible is made of clay/graphite composite, it's expensive (£100) and delicate and only lasts a few casts, so we ask for contribution to cover cost of replacement.
  • Inspect the crucible before starting, a cracked crucible will be become very dangerous on heating
  • Remember metal expands as it heats, that will break the crucible if it's packed in tight or gets sideways inside it
  • When cold metal touches hot it can cause local freezing of the hot metal, that can cause large forces that will break the crucible
  • 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
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