Method of injection molding a plastics component includes providing a first mold member with a mold cavity, defined by an internal surface, and a second mold member with an external surface complementary to the mold cavity shape. Either the internal or external surface carry an elongate formation with an elongate apex. Then, the second mold member is advanced into the mold cavity such that the apex is spaced from the opposed one of the internal surface and the external surface by a predetermined distance and the internal surface of the first mold member and the external surface of the second mold member together define a mold space. Molten polyolefin material is then injected into the mold space to substantially fill it. The predetermined distance is so dimensioned that the polyolefin material does not completely fill the space between the apex and the opposed surface and the slit is thus formed.