13312 Injection MoldingSlip Casting

High-volume production methods such as injection-molding or slip casting [23] are well established, and several casting parameters effect component uniformity. In the injection-molding process, the distribution of organics (binder, plasticizers, and mold-release agents) used as the carrier for the ceramic powder is important. The distribution effects homogeneity, green density, local densification rates, and mechanical properties. Yeh et al. [24] have listed typical defects and causes in injection-molded parts; see Table 13.3.

Two NDE methods have shown promise in determining the distribution of organics. These are X-ray computed tomographic imaging [16-18] and nuclear magnetic resonance (NMR) imaging [25, 26]. X-ray computed tomographic imaging with its inherent high sensitivity to small variations in density has been used to determine variations in organics. In a set of experiments, multicomponent polymer binder content was varied by using inserts in a cylinder. By examining the gray-scale values of the X-ray images and relating this to the organic content, the relation shown in Fig. 13.2 was established. Note that the linear relation holds regardless of whether the organic content was changed for either cold-pressed or injection-molding composition. Imaging solids by NMR presents problems. The main issue is the NMR signal to be detected decays rapidly (i.e., short T2) and requires special imaging methods. Special NMR instrumentation has been developed that combines high gradient field strengths (ca. 10 G/cm gradients) and short gradient switching times. Further details of this instrumentation are given by Ellingson et al. [25] and Carduner et al. [26].

TABLE 13.3 Injection-Molding Defects and Causes

Type of Defect

Cause

Incomplete part

Improper feed material

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