A method for molding articles having variable thickness walls from thermoplastic material. An amount of plastic slightly in excess of that required to fill a mold cavity is severed from an extruded quantity and introduced into the cavity. The mold is closed to build up pressure and force the material throughout the cavity. The excess material causes a pressure build up above the necessary molding pressure and causes a plunger in an overflow well to retract under the excess pressure so as to cause the overflow well to receive the excess material. Pressure can be maintained on material in the well during cooling to maintain the cavity filled when shrinkage occurs. Where adjacent thick and thin wall portions are formed in the article, the thick wall forming portion of the cavity may contain another well which receives excess material at a pressure slightly lower than the overflow well and this excess material may then be forced back into the cavity under pressure to insure filling thereof.