Here is a method I use to make low-relief patterns for both sandcasting and lostwax work using a dot-matrix printer... *Get some heavy weight (oven-type) aluminum foil at the supermarket. *Generate your graphic, lettering, etc on the computer. *Carefully unroll a piece of the foil, avoiding any wrinkles. *Feed it into the dot-matrix printer, using a piece of soft blotter (construction) paper as a backup for the foil. *Printout your graphic onto the foil. *(You'll have to experiment with printhead gap, paper backing, fill patterns on the graphics, etc, etc). *Depending on the effect you want, positive or negative relief, you may have to flip the graphic in your draw program before printing. *Take this printed (embossed) foil, apply a spray-on mold release (commercial or Pam cooking oil spray), remove excess with cotton ball. *Pour catalyzed Alumilite or thinned Bondo on the foil and let it setup. *Use the hardened plastic directly as a pattern or carve to get further relief. *(All this takes some playing to develop the technique but I find it very useful. Had this as a dream a couple years ago ... might be in common use but I haven't heard of anyone doing it.) Alumilite is a two-part, fast setting (2 to 5 minutes) casting urethane. It can be easily carved but the electrostatic attraction of the dust makes it a PITA. It is available from Micro-Mark or factory direct ... Alumilite Corp. 225 Parsons Kalamazoo, MI 49007 1-616-342-1259 I use it for lots of permanent sandcasting patterns. You can add fillers to it. After it is solid, you can heat it with a hairdryer and bend to new shape.
I have used a no-bake method of core preparation for the last 12 years or so; before that, I used traditional baked cores. The no-bake binder I use is Chem-Bond #12 by Thiem Corp. (now Acme/Borden) which is a sodium silicate based solution with "secret ingredients" (sugar + ?). Sodium silicate when contacted with carbon dioxide reacts to form a silica-gel and sodium carbonate. Dry sand wetted with this Chem-Bond and then gassed with CO2 under pressure becomes a bonded solid mass due to the binding action of the formed silica-gel and carbonate, sort of like natural sandstone formations. This method of making cores is very fast and the prepared cores have a lengthy shelf life. You can add other things like bentonite clay, iron oxide, sugar, wood flour, etc during the initial prep to change the hot collapsibility of the cores. You can also use this sand preparation to make complete molds for patterns with undercuts/locks. While Chem-Bond wetted sand will harden when exposed to the open air, this is usually not good enough for core or mold preparation (slow for bulky cores and the bonding is weak). Pressurized CO2 in the usual method of gassing the packed sand. (Chem-Bond #12 is available in one quart bottles from POWER MODEL SUPPLY, see the "Getting started" section). Chem-Bond coremaking.... *Use 250 cc per one gallon white sand as the "normal mix". (You can use less but I don't recommend more ... cores too hard). *Hand-mull the Chem-Bond and sand until there are no "wet spots" evident. (Wet spots are concentrated sodium silicate and will cause core-blows). *Store in Zip-Lok bags or plastic snap-top containers ... seal tightly. (Exposure to open air will cause the CO2-sodium silicate to react). *I use pure CO2 at 30 psi for injection. *Small cores, 10-15 seconds of injection ... large cores, 30 seconds at multiple injection vent holes. *Always thoroughly wax the core box before starting ... wax good for about 6 cores, then rewax. (The sodium silicate is alkaline and attacks mold releases ... wax core boxes heavy and often). *Prepared cores have a shelf-life of a couple months. *Small cores (<3/4" diameter) can be used immediately. I usually let larger cores stand overnight to lose excess moisture and avoid core-blows. *Always use LOTS of scratch and through vents with Chem-Bond cores ... they give off lots of gases during metal pour. *You can use this prepared sand to sandcast patterns with undercuts, locks, etc, by hand-building the mold in jagsaw puzzle sections. *Super Glue works great on broken cores or multi-part cores ... holds up even with bronze pours. *You can change the hot-strength (collapsibility) of the cores by adding bentonite, wheat/corn flour, etc to the sand during initial prep (doesn't take much). *I use washed white silica sand for cores, 100 to 120 grain size.
Expendable Pattern Casting (EPC), often called Lost Foam Casting, is widely used in industry. I have used the technique in the past and will use it in the future. Basically, the process involves imbedding a foam (polystyrene) pattern in a sand mold and pouring the molten metal on the plastic. When making these foam patterns, there is no need to worry about draft angles, undercuts, etc since the pattern will be left in the sand and vaporized. I do NOT recommend that the home-hobby caster attempt this casting process without extensive reading on the subject due to the many hazards associated with the method. Here are some of the points to consider about the EPC process: *Strong possibility of mold explosions if not properly vented. *Toxic fumes generated during the foam plastic vaporization. *Proper foam selection very important ... NEVER use urethane foams. *Strong possibility of gas defects in the castings. *As-cast surface finish of castings usually poor. Again, I strongly suggest that you read some professional references before attempting this casting method.
"Do not try to make sheetmetal via sandcasting." Sounds simple but it is a common problem most beginning casters have. I cast a lot of reproduction furniture hardware, some of which was made originally as sheetmetal stampings ... pretty thin stuff to use as a casting pattern. Here are some of the things I do to get complete mold cavity fills during a pour: *If it is a flat-back piece, built up the backside with 5 to 10 layers of masking tape. *If the piece is back-carved, build up the backside with Bondo, following the original contour. (Remove the Bondo by soaking in acetone for a couple hours). *Use multiple step-gates. *Use a tall cope, giving added hydraulic pressure. *Use LOTS of scratch vents on the pattern. *Use extra superheat on the melt. *Use fresh metal in the melt ... too much recycled metal will give sluggish flows, especially with yellow brass. On thin patterns, there is a tendency to have sand inclusions at the edges fartherest from the in-gate; to avoid this being in the casting, I cut some "sand trap" channels in the cope face sand exterior to the mold cavity. These don't have to be very big ... just use a spoon tool to make your scratch vents at these points, rather than using a vent wire.
Commercial parting compounds are usually powdered waxes and are sometimes expensive and hard to find in small amounts. I have used ordinary CORN STARCH for several years; I like it better than the waxes.
I often have to make short runs of items that have irregular parting lines. When only a few items need to be cast, the preparation of a permanent follower is not justified. I have several 3" deep frames made which are slightly larger than my flasks; I keep these frames rammed with sand and use them as temporary sand followers. I cope down into these rammed sand frames and imbed the pattern to its parting line, dust and ram the drag on these followers.
Silicon bronzes do not really need a flux during the melt but I use one made from 50/50 broken clear glass (food grade) and borax. This gives a good gummy, floating cover whereby you can remove any trash just before pouring. Use about 2 tablespoons per #10 crucible.
I don't use any added flux with yellow brass, letting the zinc flare do the work of flux.
For tin-bronzes and aluminum alloys, I use commercial flux and degassing compounds. (Power Model Supply has several commercial compounds packaged in small amounts).
Silver solder different size ball bearings onto 1/8" wire handles for slicks.
Stainless steel kitchen spoons, from baby size to soup size ... use for cutters and slicks. Cut the bowl off half way back and use for gate cutters.
Nickel welding rod, 1/8" diameter, can be hammer formed into special shaped spoons and tools.
You don't want the flask pins to bind during opening and closing. If the fit is too loose, you may have registration problems of split patterns. After ramming the drag half of each flask, I form sand registers in the drag sand face using an old dome-ended screwdriver handle ... tap 3 to 6 impressions about 1/4" deep. This will give you corresponding dimples in the cope ram. Even if your flask pins are really loose, these sand registers will realign your flask sections on final close.
Professional foundry fonts are expensive and require special pattern boards ... here is an inexpensive alternative. Most large office supply stores offer cast acrylic fonts for use on bulletin boards; these are usually flatbacked letters, still on the sprue and sizes range from 1/2 to 2 inches.
Prepare your own pattern boards from lacquered wood or plastic. Stick the plastic lettering to the pattern boards with "school glue" (washable white glue). Allow the the glued lettering to dry overnight. To remove the lettering from the board, spray with water and the glue will unbond in minutes.
The technique I use for clean molds .... after the pattern is pulled, I go around all mold edges with a dry, fine brush to loosen any weak sand, then I blow out the mold with 20 psi compressed air. I have found that what won't blow loose with 20 psi air most likely isn't going to wash loose during the pour. If you have a good solid ram, 20 psi air won't damage the mold faces. I also usually print-rap (impression-rap) most patterns before pulling (automatic habit).
The primary use of a riser on a casting mold is to supply molten metal
as the casting proper solidifies. I see a lot of beginning casters fill the
whole mold through the sprue and then can't understand why they still
have casting defects (shrinks, voids, cracks, etc). The riser MUST have
the hottest metal during the pour and be the last metal to solidify in the
mold. My basic technique of pouring a mold with a riser is to fill the
primary pattern mold through the inlet sprue until the riser is about 1/3
full, then back-fill ("hot-top") the rest of pour through the riser basin; this
insures the hottest metal is in the riser and will be last to solidify.