the neck of a container has a smooth seal surface engaged by the inner skirt or plug of a cap . such surface is free of trim and parting line flaws which are characteristic of blow - molded jars because the surface is formed in such a way that the seal surface is not in contact with mold parting lines and further the trim of the neck is located away from the sealing surface . in one form of the disclosure , above the sealing surface the neck wall slants outward and then bends upward - inward in a short , tapered stretch about 18 . degree . to the horizontal which engages the underside of the cap disk to compress against the disk or a liner or foil . other forms of seal surfaces are disclosed in another form of the invention , above the seal surface the neck extends outward and then upward and then inward in a flange which is trimmed along a line which has a circumference no less than the circumference of the seal surface . the exterior of the neck is formed with shoulders or screw threads which engage over locking beads or threads on the interior of the outer skirt of the cap . caps suitable for combination with the neck , methods of forming the neck and mold apparatus for forming the neck are also disclosed .