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MOLD DESIGN

Posted by admin on July 22nd, 2012
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Key to the injection-molding process,the injection mold forms the molten plastic into the desired shape, provides the surface texture, and determines the dimensions of the finished molded article. In facilitating mold-cavity filling and cooling, the mold also influences the molding cycle and efficiency as well as the internal stress levels and end-use performance of the molded part. The success of any molding job depends heavily on the skills employed in the design and construction of the mold. An injection mold is a precision instrument yet must be rugged enough to withstand
hundreds of thousands of high-pressure molding cycles. The added expense for a well-engineered and constructed mold
can be repaid many times over in molding efficiency, reduced down time and scrap, and improved part quality.

MOLD BASICS

At the most basic level, molds consist of two main parts: the cavity and core. The core forms the main internal surfaces of the part. The cavity forms the major external surfaces. Typically, the core and cavity separate as the mold opens, so that the part can be removed. This mold separation occurs along the interface known as the parting line. The parting line can lie in one plane corresponding to a major geometric feature such as the part top, bottom or centerline, or it can be stepped or angled to accommodate irregular part features.

 

• Choose the parting-line location to minimize undercuts that would hinder or prevent easy part removal.
Undercuts that cannot be avoided via reasonable adjustments in the parting line require mechanisms in the mold to disengage the undercut prior to ejection.

TYPES OF MOLDS

The two-plate mold, the most common mold configuration, consists of two mold halves that open along one parting line (see figure 7-1). Material can enter the mold cavity directly via a sprue gate, or indirectly through a runner system that delivers the material to the desired locations along the parting line. The movable mold half usually contains a part-ejection mechanism linked to a hydraulic cylinder operated from the main press controller.

The three-plate mold configuration opens at two major locations instead of one.

Figures 7-2A through 7-2C show the mold-opening sequence for a typical three-plate mold. Typically, a linkage system between the three major mold plates controls the mold-opening sequence. The mold first opens at the primary parting line breaking the pinpoint gates and separating the parts from the cavity side of the mold. Next, the mold separates at the runner plate to facilitate removal of the runner system. Finally, a plate strips the runner from the retaining pins, and parts and runner eject from the mold.

Unlike conventional two-plate molds, three-plate molds can gate directly into inner surface areas away from the outer
edge of parts: an advantage for centergated parts such as cups or for large parts that require multiple gates across a surface. Disadvantages include added mold complexity and large runners that can generate excessive regrind. Also,
the small pinpoint gates required for clean automatic degating can generate high shear and lead to material degradation, gate blemish, and packing problems. Because of the high shear rates generated in the tapered runner drops and pinpoint gates, three-plate molds are not recommended for shear-sensitive materials such as Cadon SMA and materials with shear-sensitive colorants or flame retardants.

 

Another configuration, the stack mold, reduces the clamp force required by multicavity molds. Typically, multiple cavities are oriented on a single parting line and the required clamp force is the sum of the clamp needed by each cavity
plus the runner system. In stack molds, cavities lie on two or more stacked parting lines. The injection forces exerted on the plate separating parting lines cancel, so the resulting clamp force is the same as for just one parting line. Stack molds produce more parts per cycle than would otherwise be possible in a given size molding press.

Author: ZHILIAN MOULD
Page Link: http://www.zhilianmould.com/blog/mold-design/
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