SEMI-POSITIVE TYPE MOLD
Because of the difficulty of fitting positive molds, and because scoring is detrimental to the finish of the casting, a semi-positive mold is a good compromise design. Fig. 28 is a diagram of a semipositive type mold. The telescoping portion is short, and its clear proper, and cavity depth can be less than in a full positive mold, and still provide enough volume to take care of the bulk factor.
A more shallow cavity is preferable, too, when inserts must be placed in the bottom of the mold. The decrease in height results in better heat Semi-Positive Type Mold
An auxiliary telescoping portion provides a guide to prevent scoring of the pallet mold proper, a larger space for material, and a land area to absorb excess pressure, and to limit the thickness of the casting. The positive portion of a semi-positive mold provides full pressure just as the pallet mold closes.
The material well is greater in area than the molding when the mold is heated between hot plates. Scoring on the positive portion is limited to a very short section, and it is largely prevented by the auxiliary telescoping of the force in the material well. It is possible to have a generous radius on the well portion to facilitate guiding, and to have a sharp edge at the top of the cavity.
A semi-positive mold provides the same control of flash thickness as does the positive type mold. In a semi-positive mold, the material charge is not as completely trapped as in a full positive mold. A slight excess may be used, which is allowed to escape. Weighing need not be as accurate as in the full positive mold. This is an advantage because powder charges can be weighed more quickly, and preforming machines do not have to be watched as closely.
Other Arrangements of Semi-Positive Molds
This mold is especially adapted to operation with the force on the bottom, which allows the casting to be retained on the force, until the mold is open, and then to be pushed off retainers on the force by a bottom knockout arrangement. Its principal advantage is to avoid knockout pin marks on the cavity portion. With this design the force portion may contain undercut retaining pins to assure that the casting is pulled out of the cavity.
The land surfaces on both the force and cavity are easily accessible for surface grinding. Note how much simpler it is to grind the land area of this mold as compared to the mold which is a simpler type of semipositive mold than the one shown , since there is no full positive portion. This type of mold is in very common use, and is preferred if the full positive is not required or specified.