Injection molding compounds exhibit a certain amount of shrinkage，i.e., in a cooled condition, the volume is somewhat smaller than in a heated condition. If parts with accurate dimensions are needed, allowances must be made for this shrinkage when establishing the dimensions of the cavities.
Shrinkage also causes the molded parts to sit tight on the cores since usually parts are ejected after they have cooled down. As a result, special measures are required to be able to eject finished parts.
These measures include first of all careful polishing of all mold surfaces that come in contact with the plastics melt as well as a certain draft in the direction of draw.
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 parts
Only 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.
However, in special cases, it may be advisable to choose a parting line that will cause the molded parts to be retained more strongly by the stationary half than by the movable half. In such cases, special measures must be taken to eject the molded part from the stationary half during the opening.
Generally, a molded part will sit more tightly on the half on which it can shrink onto the cores. However， this is only the case if this core is of sufficient length relative to the outer surface. In the case of a container, the core for the interior will generally be located on the movable half, because the molded container shrinks tightly on this core, whereas the exterior will be locat¬ed on the stationary half. In special cases, the part can be made to adhere more strongly on the movable half through appropriate variation in draft and polishing.
In cases of doubt in which it is difficult to decide whether the molded part is more likely to be retained by the stationary half or the movable half, it is possible to apply a spring force to suitable ejectors to assist in loosening the part from the stationary half to make sure that the part is carried by the movable half and is not retained by the stationary half when the mold opens. The springs need only travel within a narrow range of approximately 2 to 3 mm since the part，once it is detached, is certain to be carried by the movable half.