How to make a temporary crown? What we need to do before the milling is what we call the silicone key. What we can see on the next picture is the negative of our preparation. There is two molars that are already milled for the last practice. This time we are going to do two premolars.
After this key we are ready to do the milling. The mechanical requirements are: retention (vertical forces) and stability with horizontal or oblique forces. Retention involves parallel walls with at least 3 grades.
- Parallelism
- Conicity
- Narrow shaft of input/output
- Height
- Surface
- Auxiliary retention
This picture shows the best form. If we chose too conicity, like the B, the crown will easily peel off. So what we want to do is to reduce the shaft of the input/output.
Next picture shows how parallel has to be our preparation. We prefer 3ยบ at least. If it's more there is going to increase the shaft of the input/output. Otherwise if we make inverse, the crown will impossible not go inside. 
The problem comes when is a molar very small. If we don't put a retention the crown will turn around like in the case A. So if we mark some retentions we are going to reduce the shaft of the input/output, there is going to be an unique form to put and output the crown.
We have to keep the anatomy of the teeth but reducing the preparation. We also have to think about the marginal adaptation and the biologic space. 
The chamfer is very important. Is the end of the preparation at the marginal level.
The first day we put our teeth in the plaster and we did the milling of two molars.
The second day we did the key and the milling of two premolars.
Finally what we have to do is to polish the crowns and cementing in our preparation.
