TIPS FOR POLYURETHANE CASTINGS
Take Measurements To Determine Your Needs
Caculate the quantity of casting materials required to complete the job, then verify your supply of materials. It is costly and time consuming to abandon incomplete jobs.
Pre-Test Your Castings
When you have adequate materials on hand, pre-test them before starting the tool-making project. This precaution allows you to forecast the casting quality. It's especially important to check materials from previous projects. Simply mix a small cup size sample batch. After curing inspect and test it. Then date, identify and save the sample for future reference.
Prepare The Enviroment
The workplace enviroment contributes to high quality work and safety. A clean draft-free and well-lit mixing and casting area is fundamental. So are even temperatures, adequate ventilation and sufficient work space. Conveniently located support systems, such as electrical outlets, air supply lines, vacuum equipment, etc., also enhance casting success.
Use The Right Tools And Supplies
Have all equipment ready and positioned for use. Quick-setting urethanes require easily accessed tools and equipment. Suggested items to use: Flexible polyethylene buckets are great mixing and pouring containers, since they clean easily and can be reused ( hardened materials release by flexing the bucket.) When using the mixing buckets, short-fill them since this permits faster mixing without spilling. Have clean, throw-away rags and paper towels nearby.
Oil-dry ( a cat-litter type of absorbent clay is usefull for accidental spills. Laquer thinner is a compatable clean-up solvent. Note: any type of alcohol thinner must be avoided because it will chemically contaminate the casting system. Other basic tools include paint can opener, mixing spatulas, hammer, rubber mallet, drill motor, jiffy mixer, waxed cold drink cups ( for test samples), putty knife, wide blade scraper, duct tape, demolding wedges, modelling clay, level and casting box form.
Avoid All Forms Of Moisture
The liquid stages of the polyurethanes components both react differently to moisture. The surface of the "a" material hardens and becomes crusty: the "b" material absorbs moisture and creates foam and swells when mixed with the "a". So work in a de-humidified, air-conditioned enviroment on warm, humid days. Also make sure that the lids are kept on containers when not in use.
Equalize 3 Temperatures
Seventy degrees ( 70 f ) is the ideal work enviroment temperature for the "a" and "b" polyurethane components and parts to be cast. If the temperatures vary considerably one from another ( such as reaching the dew point), moisture or potlife problems are imminent. Warm materials that are stored on a cold floor. Like-wise, cool materials stored over-head near a warm ceiling. Dascar plastics low shrinkage features is possible because of it's low exotherm temperature. Temperatures over 70 f negate this feature and shortens working time.
Prepare The Casting Table
Unlike tables that are solidly constructed and secured to the floor, the best casting table is intentionally shakeable. This way it's capable of being vibrated to release air bubbles. The best table top material is one-inch polypropylene. It is ideal because of it's non-stick, easy to clean surface. Otherwise, use a 3/4 inch marine grade plywood sheet (that's lacquer sealed, waxed, release coated, and polished). Structurally, the top must be strong enough to remain level under full load conditions. If casting with-out an assistant's help, mount a motor-driven vibrator to the table. Remember, an unlevel table creates non-uniform wall heights and will produce wedge-shaped castings. Check the surface level in all directions before pouring material.
Plan Your Pattern Placement & Removal Technique
When the pattern, model, form or part is in the casting position and about to be poured, verify it's secured so it wont move or float during the pour. If you are making a single section , visually inspect for any backdrafts that would prevent pattern removal. Also double-check the primary "demolding" technique to be used and plan an alternate method just in case the first removal technique fails.
Seal Moisture Out
The pattern or model material, whether it's wood, plaster or of another porous composition, must be moisture proof. If not, the porosity will harbor moisture that can combine with the casting and flaw the mold surface. Pores can also bond the pattern and plastic together which makes demolding nearly impossible. Prevent these problems bt applying lacquer-sealers (such as deft) by brush or spray can. It dries in minutes, seals porosity and builds quickly to a thick surface. As each coat dries, lightly sand with very fine sandpaper. The final coat becomes extremely slick with extra fine steel wool. As mentioned before, avoid alcohol-reduced sealers, namely shellac.
Wax & Polish Pattern
Guarantee a good release by applying a hard wax and a high temperature parting agent on the sealed pattern. First use a carnuba wax, buff, then apply a coat of parting agent (release # 1628). ( Using two dissimiliar materials is good release insurance.)