Temporary Void and Reinjection
When the removal constraint is too high for a standard complete impression, you can leave a temporary void inside the impression so it compresses past the undercut. Once it is out, you reinject product into the void to give the impression its rigidity back before measurement.
When to Use It
Use this technique when a complete impression is mandatory because you must measure a diameter, and the removal constraint is beyond what the products accept. F30 Max, the highest of the range, accepts 40%. If a profile is enough, take a partial impression instead, as it is simpler.
The Principle
You leave a void on purpose inside the impression. The hollow impression is less rigid, so it compresses and passes the undercut without tearing. Once it is out, you inject product into the void to give the impression its rigidity back. This keeps it from deforming during measurement.
Step by Step
You build the complete impression in two injections so it can pass the undercut without damage.
- Prepare the part: degrease it, let it dry, mount the nozzle, and check the test drop.
- Inject partially to fill the detail areas and leave a central void. You can use a straw or a pen to hold the void and remove it afterwards.
- Wait for complete polymerization and check it with a fingernail on the test drop.
- Pull straight out. The hollow areas let the impression compress past the undercut.
- Reinject F20 into the void to rebuild the missing geometry.
- Wait for the second polymerization.
- Check before measuring that the useful zone has no bubble and that the geometry is continuous.
A Worked Example
Take an internal threaded bore with Dmax = 10 mm and Dmin = 6 mm, where a diameter must be measured. The removal constraint is (1 − 6 / 10) × 100 = 40%. F30 Max accepts 40% at most, so the impression would be at its limit and could tear at the ends. Leaving a central void is the safer route.
Practice First
This is an advanced technique. Both the position of the void and the quantity injected change the quality of the impression. Practice on a scrap part before a production part, because the joint between the first and the second injection can leave small offsets that are measurable on tight tolerances.
