Abstract: A plurality of invert forming mold members are bolted together and have their ends received in internal frames provided at adjacent pipe openings in a manhole. The mold members are formed of relatively thick and heavy polyurethane and are of U-shaped cross-section. A mold supporting plate is provided in each of the internal mounting frames and has an opening configured to matingly receive the endmost mold component. The mold components are provided in a variety of linear lengths, angular curved configurations and elevation changing members so that they can be associated to form inverts required for any particular manhole configuration. The assembly of mold members is extremely strong and rigid and does not require any additional internal bracing.