P Lyons wrote:It may be possible to enlarge the internal diameter of the HDPE pipe by heating and manipulation and insertion an object of the same diameter of your sink drain pipe. You can then clamp the HDPE pipe directly to the drain pipe with a ring clamp. Clamping while the HDPE is warm will ensure a good seal. I have used a heat gun in the past for similar applications, try not to overheat and do all heating in a well ventilated area.
John Wolfram wrote:Have you looked into rubber couplings?