Simple gravity resin insertion lends simplicity, but voids can easily form since there is no pressure. Cheap and quick. 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.
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. 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. The Thin Wall Rim method can produce copies of RP patterns at about three or four per hour.