A mold assembly for injection molding of a plastic pipe fitting (2, 3), the pipe fitting comprising an elbow-shaped or tee-shaped internal flow channel comprising at least two channel parts (5, 6, 7) arranged at a first angle (α) in relation to each other, the channel parts (5, 6, 7) each having a circular cross-section and a smoothly radiused inner corner face between each two channel parts being at said first angle in relation to each other, the mold assembly comprising - a pair of mold members (9, 10) adapted for movement between an open position and a closed position to define a mold cavity having a shape conforming to the exterior geometry of the pipe fitting (2, 3) to be molded, - a pair of retractable core packages (12, 13) each comprising a first core member and a second core member, each core package (12, 13) being movable between an advanced position protruding into said mold cavity and cooperating with each other to define the internal flow channel in the pipe fitting (2, 3) to be molded, and a retracted position substantially withdrawn from said mold cavity, the first core member and the second core member having distal ends with end faces (16, 17) for abutting engagement with each other when said first core member and second core member are in advanced positions to define the internal flow channel with said channel parts (5, 6, 7) at the first angle (α) in relation to each other, the first core member defining an outer portion of the internal flow channel, and the second core member defining an inner portion of the internal flow channel, and including a curved inner edge disposed generally at a distal end thereof, wherein said curved inner edges of the second core member cooperatively define the smoothly radiused inner corner face of the internal flow channel;, characterized in that the first core member includes an inclined guide member extending longitudinally and at a second angle (β) in relation to a longitudinal axis, the second core member being movably slidable along said inclined guide member, that the second core member is movable along the inclined guide member on the first core member upon movement of the core package (12, 13) toward said retracted position subsequent to molding of the pipe fitting (2, 3) within the mold cavity to shift the second core member towards a center axis (x) of the internal flow channel for withdrawal of the core package (12, 13) from the mold cavity and the pipe fitting (2, 3) molded therein, and that at least one of the first core member and the second core member of the core package (12, 13) comprises a built-in cooling arrangement for cooling of the core package (12, 13), the cooling arrangement extending longitudinally inside said core member over a substantial length of said core member. - a pair of mold members (9, 10) adapted for movement between an open position and a closed position to define a mold cavity having a shape conforming to the exterior geometry of the pipe fitting (2, 3) to be molded, - a pair of retractable core packages (12, 13) each comprising a first core member and a second core member, each core package (12, 13) being movable between an advanced position protruding into said mold cavity and cooperating with each other to define the internal flow channel in the pipe fitting (2, 3) to be molded, and a retracted position substantially withdrawn from said mold cavity, the first core member and the second core member having distal ends with end faces (16, 17) for abutting engagement with each other when said first core member and second core member are in advanced positions to define the internal flow channel with said channel parts (5, 6, 7) at the first angle (α) in relation to each other, the first core member defining an outer portion of the internal flow channel, and the second core member defining an inner portion of the internal flow channel, and including a curved inner edge disposed generally at a distal end thereof, wherein said curved inner edges of the second core member cooperatively define the smoothly radiused inner corner face of the internal flow channel;, characterized in that the first core member includes an inclined guide member extending longitudinally and at a second angle (β) in relation to a longitudinal axis,