Sintering of alumina ceramics is of the liquid phase type, that is, during sintering, a liquid phase is formed in the body to be sintered. In the case of alumina ceramics, the liquid phase is a highly viscous silicate glass. The presence of such glass in a sintered body gives it a fairly high plasticity at high temperatures, which can lead to various plastic deformations. If there are irregularities in the section in the sintered body or if it is not strictly vertical with respect to the axis of rotation, then such a product may well be bent during the sintering process.

Dear Piyush,

Unfortunately you did not give geometric details on the deformation, this would help a lot in analysing your problem.

Defects can evolve either during shaping due to inhomogeneous densification during CIP process, granulate may be too hard an cannot be completely destroyed during pressing. However due to the large primary grain sizes in such press granulates this is rather unlikely.

The second reason is the sintering as pointed out be Vadim. Classical spark plug recipes are typically low purity alumina materials containing a glassy phase to ensure good densification at moderate sintering temperature. The warping of the insulator may arise from improper placement of the part in the sintering oven (it is probably best if it stands upright) or of inhomogeneous temperature distribution in the kiln. inhomogeneous shielding of the part could also be an issue.

best regards

Frank

Thank you for your prompt responses to my query. I apologize it took me so long to get back to you.

@Vadim Verlotski

I had a follow-up query arising from your answer. The insulators are packed together tightly in sagger boxes before they go in, however due to densification happening during the sintering process they shrink down and consequently are not standing upright anymore. Now I am not able to understand if them not being perfectly upright is the major reason for the defect.

I am uploading a picture of insulators I find on Google, please use it for your reference. Further information: the bending usually happens on the foot of the insulator (the side with the smaller diameter).

@Frank Kern

I am not entirely sure what you might mean by geometric details, however, the internal cylindrical gap/hole isn't uniformly vertical, it bends to a side (usually near the foot, but can happen at any section).

As for your first theory, like you said, it is highly unlikely that is the problem as there are a lot of processes preceding the pressing to make sure the powder is homogeneous.

As for the second theory, i am not very knowledgeable in the field, so I don't know what a high purity alumina to you might be. However, here we use calcined alumina with a few additives, which I'm not sure if I'm allowed to discuss here but they contain calcium silica and magnesium, which probably form the glass you and Mr(Dr?) Vadim are referring to.

With the standing upright part, they are packed into boxes tightly so they're upright as they go in, but they fall when they come out of the kiln because they're not packed tightly anymore. That means they probably fell down admist the sintering process, do you think that might have had an impact?

Lastly I wanted to ask how sintering geometrically occurs, is it from top to bottom or is it radically from outside to inside, as a whole volume. That might be helpful in analysing the defect properly.

Regards,

Piyush Saraf

Dear Piyush,

If your main defect is on the foot of the product, then I have a simple explanation for this phenomenon and even a ready-made technical solution. The fact is that the deformation of the sole of the sintered product is a general sintering problem, which has the following nature:

During sintering, the product shrinks in size, but contact with the substrate prevents free shrinkage due to high frictional forces. It is the friction of the sole of the part with the substrate that causes plastic deformations in the foot.

There are two simple solutions to this problem:

Regards

Vadim

@Vadim Verlotski

There seems to be a misunderstanding, when I called the section most prone to bending the "foot", it was because that's what it's referred to as with respect to the assembly processes and hence referred to as the same. The insulators before sintering are placed on the "head" side, which has a higher diameter. The same orientation as shown in the picture attached to my previous answer.

The bend happens on the side with the smaller diameter, which during sintering is on top.

Regards,

Piyush Saraf

If the deformation is from the top, then the reason may be the temperature difference between the top and bottom of the product (the top is hotter). Are you using a lid or an open-top parts box? If there is no lid, then the top of the parts will definitely be too hot.

@Vadim Verlotski

We use open top boxes, and that is a very good point.

The loading to the kiln is done 64 boxes at a time on a cart. In that case, the boxes on the top should have higher deformities.

Let me test the theory and get back to you.