HOLES IN THE CERAMIC BODY
These are cavities within the ceramic mass, close to the surface, of irregular dimensions but never larger than 1-5 mm. The volume of the hole may be entirely empty or may contain some body in a 'doughnut' configuration; its perimeter may he regular or jagged.
Wrhere they are on the surface such defects can be repaired or refined without compromising the value of the ceramic; yet if they are embedded deeply or ver large, they may result in rejection of the piece at a later stage in the production cycle.
Holes have a multiplicity of causes and are, in any case, attributable to the presence of bodies and fluids that are foreign to the slip. Water inside the body causes the doughnut-like blemishes on its stu fare. Water contamination may be the result of liquid residue inside the service tubing.
If the defect has a rather regular perimeter and the inside is empty then the defect is caused by air bubbles in the slip. Air bubbles can be created by defective operation of the slip feed pump or by imperfectly sealed pipes that draw in air as the slip passes through them. Whenev er the feet! system has been poorly designed
(e.g. bottlenecks, numerous branches and a particularly high feed rate), air bubble may form. The presence of air can be avoided by holding the body for a sulficien time inside a storage tank and stirring at '>-h2 rpm: this is fast enough to maintain rheological characteristics but slow enough to prevent turbulence.
W hen using a high pressure machine the defect in question is more serious casting conditions ai t- generally more intense and filling pressures higher. Moreov during casting moulds are treated w ith w ater; the latter can remain on mould surfa or inside the delivery tubing.
On a pressure bench, the instruments used to introduce the .slip into the mod cavity are of particular relevance. They represent the last obstacle encountered by the slip and can have very small cross-sections with problematic right-angled delivery piping.
CRACKS ALONG THE HOLLOW-CAST / SOLID-CAST INTERFACE
These trace the geometry of the interlace between hollow-cast and solid-cast parts. They have a jagged surface, which v aries depending on the moment at which they are formed. In traditional casting such cracks are symptomatic of an excessive hardening time, but may also indicate excessive softening of the piece, w hich, as it deforms, breaks at the weaker interface. The problem is solved by optimising the thickness forming time/hardening time ratio.
In high pressure casting the causes of such cracks t an be traced not only to the excessive hardening time but also an accentuated deformation of the piece or an insufficient hygroscopic content of the resin mould. In this case too the solution lies in optimisation of the thickness forming/hardening ratio and optimisation of the hygroscopic content of the mould at the start of the casting cycle.
Date: 2015-04-20; view: 928