Since self-acting slight lubrication of the mold surfaces that come in contact with the molded parts is possible only in rare cases?there is a danger that scratches and cracks will develop when the mold is opened or the parts ejected, particularly if the molding compound is very hard. A mirror finish, which is easily obtained by buffing a hard steel surface?is in no way adequate; it is far more important to grind and polish the areas in the direction of ejection with an oilstone in order to eliminate any scratches and indentations. Wherever possible the surface should be lapped, since even microscopic scratches and indentations are filled with plastics melt under the high injection pressure, thus preventing smooth ejection. Proper surface finishing takes up a considerable portion of the total mold production time.Ejection of flat partsOnly completely flat parts, which in addition must have a pronounced draft toward the direction of ejection, can be ejected without special ejector pins on the parts themselves. However, this requires the runner system, that is?the channels between the sprue and the molded part as well as the gate, that is, the small opening leading from the runner to the cavity?to be of sufficient strength in order to be able to eject the actual molded parts from the center by means of the ejector pin.The sprue ejector pin is always required?since the sprue adheres relatively firmly in the sprue bushing, even though the bore has a draft of approximately 1.5 to 2°. To make sure that the sprue is pulled out of the sprue bushing on the stationary half of the mold?a well with a depth of about 4 to 5 mm should be provided on the movable half of the mold in line with the sprue. This well?with the help of grooves or reverse draft retains the sprue on the movable half of the mold when the mold opens. A sprue ejector pin is located in the extension of the well to eject the runner system with the molded parts connected to it as the ejector rod actuates the ejection mechanism in the machine.To make sure only a slight draft on the outer surface or have holes? ejector pins must be placed on the parts themselves. Their diameter and number depend on the depth of the parts in the direction of ejection. As a rule?the parting line between the movable and station¬ary halves of the mold should run in such a way that the molded part is retained more strongly by the half from which it is to be ejected than by the other half. Generally, this will be the movable half of the mold.