August 16th 2010

Shrinkage? It is just a matter of control!!

Let us explain a little bit more about volumetric shrinkage; in other words three dimensional shrinkage of a liquid which hardens out by polymerization.

All denture base materials show a certain degree of shrinkage. The shrinkage of pour type of acrylics is slightly higher than heat cure acrylics, simply because of the fact that there is more monomer in the mix. Pure monomer (Perspex industry) has a shrinkage of 21%. Pour type of acrylics around 6,25% and Heat cure acrylics around 5%.

The more voluminous the job, the higher the degree of shrinkage. You are able to influence the shrinkage by a pour type of acrylic to wait a sufficient time of around 8 minutes by Castapress and around 13 minutes by Castavaria under the condition that the room temperature is 22 degrees. The materials must be polymerized at 2,5 bar pressure for 30 minutes at 55 degrees.

To prevent thermal shrinkage we have to cool down the flask without force at the bench at room temperatures. If we refuse to do this, we have to deal with additional thermal shrinkage on top of the voluminous shrinkage.

If we have a model with abnormal proportions, sometimes it is beneficiary to use the pressing technique with trial pressing and afterwards to execute the polymerization process as slow as possible, for instance heating up starting from cold water, increasing it to boiling in 15 minutes and continue to boil it for another 30 minutes at least. Or even the whole night cooling down without a hurry and or stress.