Lettering or numbers on injection molding

Lettering or numbers on injection molding

Lettering or numbers on a molded piece may be either raised or depressed. As a general rule, the raised letters are less expensive due chiefly to the fact that they can be stamped in the cavity. The exception to this rule is found when a hob is used to produce each impression. Furthermore, any depressed lines are usually filled with some contrasting color and this finishing operation is quite expensive. The height of raised letters does not have to be more than 1/32″ to give the necessary effect of relief and depressed letters also can be shallow as long as they are not too wide. The method of wiping in the color pigment necessitates a very narrow width letter of sufficient depth to hold the pigment when the operator removes the excess paint from the surface.

It is of utmost importance that no thin edges or sharp points appear anywhere in either the plastic injection mold or the piece itself, for constant wear soon dulls any sharpness and hence the dimensions are not held within the required tolerances. Tolerances vary on molded parts in accordance with the design. The vertical tolerances should be about ±.008″ but can be held to ±.005〃 if necessary. On the horizontal dimensions, that is, the areas perpendicular to the line of ram action, tolerances of ~.002″ can be maintained, but it is customary to have ±.005″. The reason for the difference in the vertical and horizontal limits is that the closing of the press may vary a few thousandths while the horizontal areas remain constant and are affected solely by the shrinkage of the material itself. Molds are built to allow for shrinkage and hence very close tolerances can he maintained on the horizontal dimensions.

Where inserts are placed in corners the wall section around them should blend in with the interior of the piece itself and particular attention must be given to having sufficient thickness so that the exterior will not crack.

Sometimes a piece must have its parting line on a curved surface, but wherever possible this should be avoided. It is a difficult task to remove the fin under such conditions. Very often a buffing operation has to be resorted to in order to completely eradicate the flash line. Wherever possible the parting line should appear on the sharp section of the piece so that filing or grinding will tend to improve rather than destroy the desired appearance.

There are innumerable rules of a definite nature which govern correct die design for plastic molding, and a thorough understanding of all these fundamentals is highly essential to the competent manager, sales engineer, draftsman, and estimator. Every item is different and every new proposition presents a problem in itself which requires utilization of a general knowledge of die design. The sales engineer should know at a glance whether or not a proposition presented to him is practical from a molding standpoint and should also be prepared to suggest any changes in design which might facilitate the molding operation. The estimator should carefully check the prints for possible changes while he is preparing his quotations, and the tool room should also be on the alert for any errors which might have inadvertently passed the attention of the engineering department. With a thorough checking of the work it becomes quite improbable that anything but the best design will result, and therefore the most efficient mold will undoubtedly be constructed.