| PROBLEM | CAUSE | REMEDY |
|---|---|---|
| Bubbles - Complete Spheres. | Investment to 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. |
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+44 (0) 1782 622 285 
+44 (0) 1782 636 982
hoben@teg.co.uk