3D WERK
PRINTED WAX · INVESTING AND BURNOUT
INVESTING AND BURNOUT GUIDE
From a wax tree
to a clean, hot mould.
Your tree is built and weighed. Next it is set in a flask of investment, and the kiln melts and burns out the wax to leave a mould ready for metal. This guide explains how each step works and what to check in your investment maker's instructions. Their numbers always come first, because every powder is a little different.
Before you start
This guide is general information to help you get the most from your WaxJet. It does not replace the Flashforge manuals, the safety data sheets for your materials or your own safety checks. Where they differ, follow the manufacturer's instructions and safety data sheets. Isopropyl alcohol (IPA) is highly flammable, and warm IPA gives off more vapour. Flashforge's manual uses a mix of PPG400, ethanol and IPA at 40 °C, never above 43 °C. Ethanol is also highly flammable. Keep it away from heat, sparks and open flames, heat it only with equipment suited to flammable liquids, use it in a well-ventilated area, and wear suitable PPE such as gloves and eye protection. Investment powder contains crystalline silica, and wax burnout and metal melting give off fumes, so use ventilation and the PPE your safety data sheets list. As a business, you are responsible for work health and safety in your workshop, including risk assessment and how you store, use and dispose of chemicals. See our Terms and Conditions.
THE NUMBERS AT A GLANCE
38 to 42
g of water per 100 g of powder, as your maker specifies
8 to 11 min
typical working time, from powder into water to the vacuumed flask
1.5 to 2 h
typical bench set before burnout, up to 6 h with R&R powders
About 730°C
top of a standard gypsum investment cycle. Never above your maker's maximum
1 h min
at casting temperature before you cast, longer for big flasks
WHAT YOU NEED
Investment powder
fresh, sealed, stored dry
Mixing bowl and mixer
or a spatula
Burnout kiln
programmable, vented
Digital scale
for powder and water
Vacuum investing unit
bell jar and pump
Flask tongs
plus heat resistant gloves
Distilled water
and a thermometer
Flasks and tape
collar and perforated tape
P2 respirator
fit tested, for the powder
Before you start
Read your powder's instructions. Ratio, mixing time, bench set and cycle all differ between powders.
Get everything ready first. Once powder meets water, the clock is running.
Fresh, dry powder. Investment absorbs moisture, so keep it sealed and off the floor.
1
Prepare the flask
Check the fit. At least 6 mm from any pattern to the flask wall, and at least 19 mm of investment above the highest pattern, as in our casting tree guide.
Solid or perforated. Perforated flasks are used for vacuum casting, where the vacuum pulls through the flask wall. Cover the holes with perforated flask tape before investing. The tape burns off in the kiln or peels off once the investment has set.
Add a tape collar. A band standing about 25 mm above the rim holds the mix as it rises when you vacuum the flask. Seat the flask firmly on the rubber base so nothing leaks at the join.
Debubblizer. As in the tree guide: it helps stop air bubbles clinging to the wax. Recommended with a bell jar setup like this one, optional only if an investing machine mixes and pours under vacuum. Follow the debubblizer maker's instructions for drying time.
2
Weigh and mix
Weigh the water as well as the powder. 1 ml of water weighs 1 g, so the scale beats a jug. Take the amounts for your flask size from the maker's chart.
Common ratios are 38 to 42 g of water per 100 g of powder: less water for heavy pieces, more for fine filigree, as your maker advises. Be exact. A 5% error in the water can shift the setting time by 30 to 45 seconds.
Distilled or deionised water at about 21 to 24°C. Warmer water shortens the working time, colder lengthens it.
Powder into water, never the other way round. Start the timer, wet the powder out within about 30 seconds, then mix for your maker's time, typically 3 to 5 minutes.
3
Vacuum the mix
Vacuum the bowl until the mix rises, breaks and starts to boil, then only as long as your maker says past the boil (Kerr about 20 seconds). Keep it short: R&R's Ultra-Vest sheet says no more than 2 minutes in all.
4
Pour and vacuum the flask
Pour down the side of the flask, never over the patterns. The slurry fills from the bottom up and pushes the air out ahead of it.
Vacuum the filled flask for about 1 to 2 minutes, as your maker specifies. The collar catches the rise.
Finish inside the working time, usually about 8 to 11 minutes from powder into water. Slurry handled after it starts to thicken gives fins, water marks and loose investment in the casting.
Example: Goldstar's hand mix sequence for Omega+ and XL, 8 minutes in total. Gloss-off time from Stuller. Other powders and machine mixing differ.
5
Let it set
Watch for gloss-off. Around 10 minutes after the powder went in, sometimes a little later, the shine turns matt. Do not move or knock the flask while it sets.
Bench set for your maker's time, commonly 1.5 to 2 hours: Goldstar Omega+ and XL 90 minutes, Kerr 2 hours, R&R 2 to 6 hours. Too soon in the kiln and the mould can crack or shed investment into the casting.
Keep it moist. Flasks go into the kiln moist, not dried out: the steam helps push the wax out. If they must wait, for example overnight, follow your maker. R&R's Bandust sheet says past 6 hours, rewet, cover with a wet cloth and seal in a bag.
Before loading, remove the base and collar, and clear loose investment from the sprue opening.
Printed wax: what to expect in the kiln
It melts early. The WaxJet's red model wax melts at 68°C, so it softens and drains in the dewax stage, before the flask sees casting heat.
It pushes on the mould first. Wax expands as it warms. In a 2024 lab test, a solid sample of one FDM printing wax pushed about twice as hard as an injection wax, though its porous printed parts did better. The WaxJet's wax was not tested, so use a good investment, the full bench set and your maker's ramps.
It still has a carbon stage. Flashforge publishes no ash figure for this wax, so keep the full high soak. Use your powder's cycle for wax patterns, not a resin cycle.
6
Load the kiln and burn out
Button down, raised. Load the flasks sprue opening down, on a mesh or stand at least 25 mm above the floor, so air circulates and the wax drains. Preheat the kiln only if your maker says so.
Leave space around each flask. The carbon left by the wax needs air to burn off.
Bigger flasks need longer cycles. R&R has revised its Ultra-Vest schedules by flask size, with cycles from about 7 hours for small flasks to 15 hours or more for large ones, so use the sheet that came with your powder. A full kiln needs more time too.
HOW A BURNOUT CYCLE WORKS
The shape of a typical cycle, not a schedule to copy. Use your investment maker's schedule. Drawn from the Satin Cast 20 (now R&R, formerly Kerr) eight hour cycle for a 3.5 x 4 inch flask (holds of 1, 2, 1, 3 and 1 hours), with ramps of about 5°C a minute. Cycles for standard gypsum powders such as R&R Ultra-Vest, Kerr Satin Cast 20 and Goldstar XL peak at about 730°C (1350°F). A few powders are rated higher, such as Goldstar Omega+ at up to 850°C.
1
Dewax, about 150°C. The wax melts and drains. Steam from the moist investment helps carry it out.
2
Slow ramps and holds. The last water leaves and the silica goes through its steepest expansion. Rushing cracks moulds. Stuller suggests about 2°C (4°F) a minute.
3
High soak, about 730°C. Burns the wax's carbon out of the mould and cures the investment. Above your powder's maximum, the gypsum breaks down and gives off sulphur gases: stained metal and gas porosity.
4
Down to casting temperature, then hold. The core of the flask lags behind the kiln display, so hold at least 1 hour. Stuller waits 2 hours for a tall flask 100 mm (4 inches) across.
SAFETY
Silica dust. Many investments are more than half crystalline silica (cristobalite and quartz). Breathing the dust can cause silicosis and lung cancer. The Safe Work Australia limit is 0.05 mg/m3 over 8 hours. Wear at least a P2 respirator, fit tested and clean shaven where it seals, when you handle powder and again at breakout. Use local extraction where you weigh and mix.
Clean up wet. Never dry sweep or use compressed air. Wet wipe, mop, or use an M or H class HEPA vacuum.
Kiln fumes and hot flasks. Burning wax gives off carbon monoxide and irritating vapours, including acrolein and formaldehyde, so vent the kiln to outside and keep the room ventilated while it runs. Use non-slip flask tongs, heat resistant gloves, protective clothing and eye protection. To look into a glowing kiln, wear infrared rated glasses (shade 3 or darker is common; check your kiln maker).
FLASK TEMPERATURE FOR CASTING
Heavy, plain pieces such as men's rings
KERR SATIN CAST 20
STULLER, KARAT GOLD
370 to 480°C (700 to 900°F)
425 to 540°C (800 to 1000°F)
Fine, detailed pieces such as filigree and findings
KERR SATIN CAST 20
STULLER, KARAT GOLD
480 to 540°C (900 to 1000°F)
590 to 700°C (1100 to 1300°F)
Your alloy maker's data sheet wins, and it can run hotter: Cooksongold lists 550 to 650°C for some yellow golds. These are typical starting ranges from two sources, and they differ. White gold usually runs at the high end, and a mixed tree is set closer to what its lightest pieces need.
Platinum is different. It needs a platinum investment, such as a phosphate-bonded one, with its own cycle, not a gypsum powder.
TROUBLESHOOTING
Fins or flash on the casting
LIKELY CAUSE
Cracked mould: wrong water ratio, slurry worked past its working time, flask moved too soon, or heated too fast
WHAT TO DO
Weigh water and powder, finish in time, full bench set, follow the maker's ramps
Small metal balls (nodules)
LIKELY CAUSE
Air bubbles on the wax: no debubblizer, mix not vacuumed enough, or a weak vacuum pump
WHAT TO DO
Debubblizer, vacuum the bowl and the flask fully, service the pump
Water marks or a rough surface
LIKELY CAUSE
Investing too fast, wrong water ratio, slurry handled past its working time, or a rough pattern
WHAT TO DO
Pour steadily down the side, weigh accurately, stay in time, smooth patterns
Investment inside the casting
LIKELY CAUSE
Sharp corners on the sprues, flask into the kiln too soon, or heated too fast
WHAT TO DO
Fillet every join, full bench set, slow ramps
Incomplete fill
LIKELY CAUSE
Sprues too thin or too few, burnout not finished, mould or metal too cool, or not enough metal
WHAT TO DO
Check the sprues, full cycle, correct flask temperature, weigh the metal
Gas porosity or a dark, stained surface
LIKELY CAUSE
Carbon left in the mould, flask or metal too hot, or investment taken above its maximum
WHAT TO DO
Full high soak with air around the flasks, never above the maximum, correct temperatures
BEFORE YOU CAST: CHECKLIST
Flask taped, collared and sealed on its base
Water and powder weighed, water 21 to 24°C
Mix and flask vacuumed inside the working time
Flask left alone through gloss-off and bench set
Button down on a raised stand, kiln loaded as your maker says
Maker's cycle programmed, never above its maximum
Keep a burnout log
Write down the powder, ratio, water temperature, bench set, cycle and flask temperature for every flask. When a casting goes wrong, the log shows what changed. Change one thing at a time, within your makers' ranges.
WHERE TO NEXT
GUIDE 9 OF 13 · CASTING
THE BIG PICTURE
1 · DESIGN
2 · PATTERNS
3 · CASTING
4 · AFTER CASTING