For high volume products like a DVB-RCS terminal, manufacturing by plastic injection molding and metallization techniques are considered to be key technologies to make low cost ground terminals feasible. In this paper we report on the electrical design of a smooth-walled feed horn, where the horn profile is optimized to obtain low cross polarization both at 20 and 30 GHz. The optimized feed horn is manufactured by plastic injection molding and metallization techniques. The optimized horn has an aperture diameter of 38 mm, length 42.9 mm and is fed from an 11 mm diameter waveguide. The feed horn has XPD > 25 dB within the Rx-band and XPD > 32 dB within the Tx-band, and return loss is better than 25 dB in both bands.