Molding center-gated disk

For an L-shaped molded part, its flanks come closer to each other because of the local shrinkage at the comer. This is called the comer effect. The inner warpage of box-shaped parts is also due to this corner effect. For a center-gated disk, if the inner shrinkage is larger than the outer one, the disk will twist, otherwise it will dome. These phenomena are called buckling. 

Figure 13.7. Radial flow in a center-gated disk mold. (Not a possible solution as shown.)...

Differentiating Eq. (8.7) with respect to Q and equating to zero yields the expression for the minimum pressure gradient Po, mm for center-gated disk mold cavity filling. 

Determine the velocity field and the pressure distribution for the flow of an incompressible Newtonian fluid between two disks as shown in Figure 2.13. Flows similar to this occur in several polymer processes including injection molding in a center-gated disk mold or compression molding. 

FIGURE lOR Basic geometric elements to which nmneis and mold cavities can he rednced (a) tuhnlar element, (h) planar element, and (c) center-gated disk. 

Yoon, K., An Experimental Study on Precision Injection Molding of Center-Gated Disks, Korean Journal of Rheology, 7,19-27 (1995). 

A tool was selected for use in completing preliminary molding tests. The tool produces a center, sprue gated disk approximately 4 inches in diameter and 5/16 inches thick. The thickness of the disk can be increased as desired to evaluate large cross section parts. It is fitted with a transducer for the measurement of cavity pressure. 

A mold to form a thin circular disk is shown in Figure 6.1. Molten polymer is fed through a small circular hole at the center of the mold (the gate) and then flows out radially to fill the mold cavity. We assume that the mold is vented, allowing air to escape as the polymer fills the cavity, so the pressure at the polymer/air interface is always close to atmospheric. The disk has a thickness H and a radius Rd. The radius of the circular gate is Rq. The pressure at the gate is (Po, and the polymer enters with a volumetric flow rate Q [Pg.72]

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