Large-part ceramics are within reach

The challenges associated with thick-part ceramic molding have been so significant that for years applications have been restricted to relatively small, thin-walled components. Voids, shrinkage deformation, and residual stress have doomed many thick-walled ceramic parts. New research, however, has shed light on the cause of such anomalies and offers a solution that might help open new doors for ceramics. 

The defects that appear in thick ceramic parts are sometimes apparent right out of the mold, but many don't present themselves until after debinding or sintering is complete. The cause is usually attributed to the difficult-to-control and unpredictable solidification of the ceramic-polymer material inside the mold cavity. 

Increased hold pressure can help eliminate voids and take the variation out of shrinkage, but this often generates new stress distribution patterns, giving rise to stress cracks. The high hold pressure is applied to compensate for shrinkage of the solidifying part, but when the gate solidifies, the pressure in the isolated liquid core of the mold decays over time. 

If you extend the solidification time of the gate, previous research has shown, you can prevent void formation and stress cracking during packing at lower hold pressures. Modulated pressure, heated sprue, and insulated sprue molding have been used to prolong gate solidification time. 

Research Says 
In an effort to find that long-life sprue, Steffen Krug, an engineer at Ingenieurbüro-AME (Kaiserslautern, Germany), conducted a series of tests molding relatively thick 25-by-45-by-60-mm parts of BASF's Catamold AO-F (POM-alumina) feedstock. The hold pressure was applied for 400 seconds and varied from .1 to 120 MPa. 

The cavity was direct-gated through a sprue made out of a polyetheretherketone, 12 mm in diameter and 10 mm long. This reduced radial heat loss and allowed a significant contribution from axial heat flow from the nozzle. This prolonged sprue solidification time to 300 seconds, up from 26 seconds using a conventional steel sprue. 

To prove that this method produced the desired effect, Krug measured cavity pressure decay using an insulated and conventional sprue (Figure 1). Through a conventional sprue, pronounced pressure decay occurs after about 26 seconds, indicating relatively early solidification. Cavity pressure with the insulated sprue, however, remained steady for about 210 seconds before beginning a gradual decline. 

Before debinding, X-ray radiographs were taken of these parts, which were molded with a constant hold pressure of 60 MPa. The images showed pronounced shrinkage voids and porosity in the center of the conventionally sprued part, caused by thermal contraction of the molten core during solidification. 

No macroscopic voids or porosity were detected in the X ray of the insulated-sprue part. However, after binder removal, a systematic pattern of cracks appeared in all parts produced with the insulated sprue. 

Ideal Pressures 
Although insulated sprue parts were healthier out of the mold, the postdebinding cracks suggested to Krug that pressure settings had to be tested and modified to find an ideal setting. 

Using a pressure-reducing valve, Krug conducted another series of tests with the insulated sprue and hold pressures of 5 to 120 MPa. Figure 2, opposite, shows part weight and density for this course. Krug proved that at a hold pressure of less than 10 MPa, part weight dropped dramatically, but a corresponding decrease in density was not observed. Below 10 MPa, says Krug, the pressure in the molten core is low enough to allow the part to contract and shrink during solidification. 

To determine the influence on residual stress, bars were cut from the part surface and subjected to layer removal from one side. During layer removal, any unbalanced residual stress caused the bars to bend. The direction of bending of the samples subjected to high hold pressures (10 to 120 MPa) indicated tension on the part surface. Samples from parts subjected to hold pressures of about 8 MPa showed no visible deformation, and bending was reversed, indicating compression on the part surface. 

Krug also conducted a series of tests using gas assist to modulate hold pressure, this time varying it from .1 to 1 MPa. Parts molded with this method, after debinding, developed no externally visible cracks. Sectioning and fractography of parts after debinding and sintering revealed no macroscopic voids. 

What It All Means for Molders 
Krug's recommendations, in the end, are predictable. Hold pressure has a significant influence on the development of voids and residual stress in thick-wall ceramic parts. Therefore, prolonged and low hold pressures ensure that such parts cool and shrink more steadily and predictably, producing stress- and crack-free parts. Hold pressures should be less than 10 MPa, and ideally close to 5 Mpa--possibly even lower if modulating with gas assist. Further, large-part defects in the form of cracks that are detected after debinding or sintering generally have their origin in the injection molding stage. 

The complete version of this paper is available from the EPMA; Shrewsbury, U.K.; +44 (1743) 248 899; fax +44 (1743) 362 968; e-mail [email protected]; www.epma.com.