The urethane can be gravity-flowed into the mold. This is easy to implement, but not all thin-wall areas are likely to fill. Gravity casting is demonstrated in the last of series of SRM figures.
- Production Rate: Three shots each day.
- Simple gravity resin insertion lends simplicity, but voids can easily form since there is no pressure.
- Cheap and quick.
|Vacuum Casting Molding|
VCM involves using a vacuum jacket on the mold and injection pressure to fill thin-wall areas. The figure below illustrates the vacuum configuration, which is similar to the vacuum jacket used for injection or transfer molds.
- Production Rate: 15-25 parts each week.
- Vacuum costs money and takes tooling time.
- Parts are of higher quality than with Gravity Casting, with thinner walls and fewer voids.
- Materials can simulate ABS, Nylon, Polycarbonate, etc.
|Thin Wall Rim|
The Thin Wall Rim method can produce copies of RP patterns at about three or four per hour. Short cycle time is due to the fast gel time (one minute) of the polyurethanes used. They can be de-molded after about 10 min.
- Production Rate: 3-4 each hour.
- Tooling Cost: About $3000 for a typical consumer product part of 5"x5"x3"(12.7x12.7x7.6 cm).
- Accuracy: As accurate as the SLA prototype: ±0.005" (±0.127 mm).
- Mold Fabrication Time: 10 days from pattern to first part.
Automated mixing heads have helped because of the following:
These automatic injection heads have come down in price, increasing the popularity of auto mixing. Equipment that handles fast-curing resins can now run only $4000. Fast curing resins also lengthen soft mold life, since the curing process is a major reason for mold break down.
Before automated mixing heads, the two parts of the resin needed to be mixed, then de-gassed in a vacuum, and then finally poured into an injector for insertion into the mold. Since these steps were time-consuming, it was impractical to shoot materials that had pot lives of less than a few minutes.
- The two-part resin is automatically mixed in a vacuum.
- The material is shot into the mold.
- Parts coated with hardened polyurethane are disposable.