A large percentage of molded work includes the use of metal insert moulding. Usually these are specified when threaded parts are required. By using inserts, the need of any unwinding at the press is obviated and the straight-draw method of ejection can be successfully utilized. During the plastic molding process the inserts are imbedded in the material and held in place as a result of the specially cut cross-knurl found on the exterior of the insert. In order to place these inserts in the required position small pins are provided in the plastic injection mold over which the threaded piece is placed. It is much more advantageous to have the mold constructed so that the inserts can be placed in the cavity for in that position they are held on the pins by gravity. Where they have to be placed in the top half of the die, however, a slight taper is given to the insert pins and they are then held in place by a wedge action when tapped lightly. It is, of course, a much more arduous task to place the inserts in the force, but very often it becomes necessary due to the design of the insert moulding piece. So far as the choice of using inserts or including threaded sections in the molded part itself is concerned, it sometimes becomes optional which method is selected. By using a threaded insert the material cost is increased, but to offset this there is a definite labor cost saving, for the use of inserts obviates a lengthy unwinding process. So far as strength is concerned, the metal insert is generally recognized to surpass the average molded threaded section. A molded thread rarely strips, but the possibility of this happening with a brass or other metal insert is almost impossible. Finally, a metal insert permits unlimited screwing and unscrewing while such practice is often questionable with a plastic material. The diameter of the threaded section actually determines to a certain extent which method should be used. A threaded cosmetic jar, for instance, could not utilize a metal insert, while a deep threaded hole about %〃 in diameter would seldom be made without the use of metal. Hence where small diameters or minute threads are specified the metal inserts should always be used.
Before leaving the subject of inserts, it should be pointed out that sufficient material must always be provided to surround the insert in order to avoid any cracking out of the material. Plastics and metals have different coefficients of expansion and hence a wall thickness of no less than should be specified. In other words, 1/4″ should be added to the insert diameter to obtain the required wall protection.
Lettering or numbers on a molded insert moulding 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 mold or the insert moulding piece itself, for constant wears soon dulls any sharpness and hence the dimensions are not held within the required tolerances. Tolerances vary on molded insert moulding 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.