Guide to Good Investment Casting Techniques
|CASTING PROBLEMS||MOLD RUBBER PROBLEMS||WAX PATTERN PROBLEMS|
|Bubbles - Complete Spheres.||Investment too thick.||Use correct powder/water ratio.|
|Vacuum tank/pump may be faulty or too small for the quantity of investment being used.||Ensure all seals are intact.|
Have pump serviced regularly.
Change oil at regular intervals.
|Work cycle too long. Investment is starting to thicken while still being vacuumed.||Ensure slurry temperature is 20-22ºC. Check all mixing equipment is clean. Consult manufacturer's instructions for correct work time.|
|Bubbles - Incomplete Spheres.||Either static electricity or by surface tension on the surface of the wax due to grease, dirt, etc.||Use a wax wash to clean wax before investing.|
Note: Wax wash must be completely dry before investing.
|Incomplete castings and back pressure porosity.||Mould or metal too cold when casting.||Increase casting temperature.|
|Improperly sprued system.||Check sprues are not too long, too thin or too few.|
|Insufficient metal.||Check metal weight.|
|Impermeable mould caused by fine residual carbon as a result of incomplete burnout.||Hold for longer at maximum burnout temperature of 730ºC.|
|Incomplete oxidation of carbon.||Check burnout furnace has adequate ventilation.|
|Inclusions in the castings.||Poor design of the feed system.||Eliminate all sharp bends in sprue system.|
|Mould breakdown during burnout.||Do not heat flasks too rapidly.|
Do not exceed 730ºC maximum burnout temperature.
|Crucible breakdown||Do not use old, disintegrating crucibles.|
|Dirty flasks.||Clean rust and dirt from flasks.|
|Metal contains foreign particles.||Take care when recycling metal and ensure all reused metal is clean.|
|Metal casting porosity.||Metal casting temperature too high.||Use a lower casting temperature.|
|Mould temperature too high when casting.||Use a lower mould temperature.|
|Use of poor quality metal.||Use better quality metal. Do not use more than 50% recycled metal.|
|Metal insufficiently fluxed.||Use more flux.|
|Rough, discoloured or brittle castings.||If burnout of furnace takes place above 800ºC, gypsum binder will start to break down to produce sulphur dioxide or trioxide. The presence of carbon may encourage decomposition at temperatures as low as 750ºC.||Check thermocouple on furnace, maximum temperature of furnace should not exceed 730ºC.|
|Metal may be overheated.||Check metal temperature.|
|Use of poor quality metal. Some reused metal may be high in oxides and sulphides.||Check quality of charge metal.|
Do not use more than 50% recycled metal.
|De-wax temperature too high.||Ideal temperature 200ºC. If possible flasks should be put in a hot furnace.|
|Water marks on surface of finished castings (stringing).||If the slurry is too thin or if the work time is not fully used, then particle segregation will leave water-rich investment which will be attracted to the surface of hydrophilic waxes resulting in erosion.||Ensure that all the recommended work time is used.|
Make sure that investment is not too thin.
|Orange peel surface finish.||Surface finish of waxes may be poor due to a faulty rubber.||Replace rubber if necessary.|
|Static electricity can cause deterioration of surface finish.||Consider using a wash wax.|
Note: Wax wash must be completely dry before investing.
|Flashing or finning.||Mould is too weak||Use correct powder/water ratio.|
Store material in dry atmosphere, use promptly and in rotation. Ensure slurry temperature is 20-22ºC.
|Setting has commenced prior to completion of work cycle.||Check all mixing equipment is clean. Small amounts of set investment will shorten the available work time. Use correct powder/water ratio.|
|If work time is not used up, or too much water is used, settlement will occur. Heating causes greater expansion at the bottom of the can resulting in horizontal flashing, especially at the top of the can.||Check all mixing equipment is clean. Small amounts of set investment will shorten the available work time. Use correct powder/water ratio.|
|Cans may have been disturbed too soon after investing.||Leave flask undisturbed for at least 1 hour if dry dewaxing, 2 hours if steam dewaxing. Do not use mishandled or dropped flasks.|
|Heating cycle during wax burnout may be too severe.||Ensure the slowest heating cycle possible up to 400ºC.|
|Metal may have been cast at too great a pressure.||Use correct spin speed to suit weight of metal and type of work. Reduce metal temperature.|
|Flasks have dried out before burnout.||Keep stored flasks damp.|
|Cut mould does not seal evenly (warping).||Rubber used in packing has begun to vulcanize.||Use new rubber.|
|Mould under packed.||Use sufficient rubber. Close platens down so they are in contact with the mould frame.|
|Mould over packed.||Review mould packing procedure.|
Lay strips in same direction, invisible grain.
|Voids in mould (air entrapment).||Model not filled in with rubber||Pack Rubber pieces into and around model.|
|Platten not at curing temperature.||Preheat vulcanizer to 155ºC.|
|Mould not cured for recommended time.||Tighten vulcanizer ½ turn past first resistance. Repeat every two minutes until platens are in contact with frame and handle cannot be turned withou resistance.|
Allow 15 minutes at 155ºC for every 6mm. For intricate moulds allow 20 minutes at 145ºC for every 6mm.
|Mould delaminates after curing.||Surface of strips not clean.||Use new strips. Do not contaminate surfaces with hands and protect from extraneous dirt or dust.|
|Mould soft and sticky in centre.||Under heated.||Check temperature of platens.|
|Cure cycle too short||Cure for 15 minutes for every 6mm.|
|Mould has pebbly surface.
Mould is sticky and scorched.
Excessive shrinkage. Lack of flexibility.
|Over heated.||Check Calibration of vulcanizer.|
|Mould overfills.||Air pressure too high.||Reduce air pressure.|
|Wax too hot.||Adjust temperature.|
|Plates held with uneven pressure/not covering mould.||Hold plates firmly, distributing pressure evenly over entire mould surface.|
|Mould held too long at nozzle.||Hold for less time.|
|Mould not filling.||Air pressure too low.||Adjust air pressure.|
|Wax not hot enough.||Adjust temperature.|
|Nozzle plugged.||Remove and clean.|
|Mould held too tightly.||Hold without squeezing.|
|Uneven pressure caused by dirty plates.||Clean plates so surfaces are flat.|
|Air trapped in mould.||Make air release cuts (vents) in mould. Be sure wax pot is more than 1/2 full.|
|Mould too cold.||Warm mould.|
|Air bubbles in pattern.||Air pressure too high.||Adjust air pressure.|
|Wax pot low on wax.||Add wax. Be sure that wax pot is more than 1/2 full.|
|Wax too hot or cold.||Adjust temperature and stir wax to release trapped air.|
|Mould held at wrong angle to nozzle.||Hold mould on same plane as nozzle.|
|Wax brittle||Wax too hot.||Adjust temperature.|
|Wax has been reused.||Use new wax. Use no less than 50% new wax with reused. Try avoiding using old wax.|
|Moulds cooling too long before opening.||Inject fewer moulds during production cycle.|
|Using inferior wax.||Use new wax.|
|Wax discoloured (brownish)||Wax too hot.||Clean wax pot (steam or soak in Paracloroethylene); add new wax.|
|Patterns bend easily, wax sticks to mould.||Wax too hot.||Adjust temperature.|
|Opening mould too soon or mould too hot.||Prolong cooling period by working more moulds.|
|Wax Keeps flowing after mould is filled.||Dirt inside the nozzle.||Remove and clean nozzle.|
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Last Updated 6th October 2011