PVC tubing bonding

[Jluis]

We have experienced some cracks in the PVC tubing asembly. After investigation, we have the hypothesis that this craks in the joints could come from a non correct extrusión process temperatures, since the tubing appears to have very high shrinkage rates (9-10%). Have anybody experienced the same issues? Thanks for your help.

 

[rickfischer51]

First, if the cracks are very straight and run the length of the tube, they may be poorly formed weld lines cause by spider legs. Second, PVC must have proper fusion to get good mechanical strength The PVC powder is crystalized, and those crystals must complete melt during processing. If they do not, the material will be brittle. This can be determined by analyzing the extrusion with DSC.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Jluis]

Thanks Rick for your help. Tubing cracks appears when we use ciclohexanone in the joints with diferent parts. These craks are in the edge of the parts where cyclohexanone has contacted and are not in the lengh but in the wide. Could these cracks be related with an non correct extruxion process?

José Luis Zafrilla
Plant Technical Manager in Grifols

 

[rickfischer51]

Is this the first time you've used cyclohexanone to bond this material? Have you done this successfully in hte past? If so, has anything changed? New extruder? New resin supplier? Different material for the other part?

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Jluis]

The only change is the plastic screw extruder. There are no changes in the material neither in the parts. He have used always ciclohexanone since we are bonding PVC.

Thanks Rick

 

[rickfischer51]

Is there another solvent you could try, not as a process change, but as a diagnostic exercise.

So you are saying its a new screw in an existing machine? Was this done to replace a worn screw, i.e. new screw has same specs as old screw, or is the new screw of a different design?

Is this rigid PVC or plasticized? My limited experience is with rigid. If this is rigid PVC, you could check your material properties by running a tensile test and comparing elongation to values from the resin supplier. If you have a piece of tube from the previous screw, run tensile on that and compare to the new stuff. If your testing produces a stable neck and draws out, your material is not too bad. If you cant get a good tensile specimen, run an Izod impact test. If you suspect process temperature issues, you should be looking to see if your material has undergone complete gelation. There are many papers dealing with this, one is below.

https://www.perkinelmer.com/lab-sol...C_Samples_for_Gelation_DSC_4000_011615_01.pdf

Rick Fischer
Principal Engineer
Argonne National Laboratory