A mold assembly (1) for injection molding of a plastic pipe fitting (2, 3). The pipe fitting comprises an elbow-shaped or a tee-shaped internal flow channel (4). At least one of the first core member (14) and the second core member (15) of the core package (12, 13) comprises a built-in cooling arrangement (20) for cooling of the core package (12, 13), the cooling arrangement (20) extending longitudinally inside said core member over a substantial length of said core member. The pipe fitting (2, 3) comprises an elbow-shaped or tee-shaped internal flow channel (4) 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 (8) between each two channel parts being at said first angle in relation to each other, the at least one of the channel parts having an inner diameter D, a length L from central corner point to the end of the channel part, the inner corner face having a rounding radius R. The ratio (D/R) of the inner diameter D and the rounding radius R is in the range 2 to 5, and the ratio (L/D) of the length L and inner diameter D is in the range 8 to 3.