this invention increases the length - to - diameter ratio of plastic articles made on injection molding machines without using long and relatively thin core rods . this added length is obtained by having the first part of the blowing operation stretch the parison lengthwise before there is any substantial displacement of the parison in a radial direction . at the start of the blowing operation , a tube surrounds and confines radial expansion of the parison , but this tube is withdrawn progressively as the blowing operation continues .