PMMA Ti bases do not fit

[helenakr96]

Both labs i have worked for had issues with pmmas. The problem is the channels for the ti bases and the screw channels are too small, but the final in zirconia the fit is perfect. The bite and sizing of the teeth and tissue aren't wrong just the size of the holes for the implants. We have tried changing the settings for the cement gap, margin line, and wall but nothing changes when we put the parts in. We even tried comparing milled and printed pmmas and the printed are only slightly better than the milled but they still need adjusted for the parts to drop in. My boss wants to try and solve this problem but the person who taught me how to do all on 4 pmmas said from the start that this has always been a problem. Has anyone else had this issue if so what was causing it and how can we fix it?

 

[Labslip]

I'm on the edge of my seat waiting for an answer to this one. We've always had the same problem. I had our support people take a look at it. They changed some settings in Millbox although I wasn't watching and don't know what they did. The fits are improved but still far from what they should be.

 

[Andrew Priddy]

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[Toothman19]

I'm on the edge of my seat waiting for an answer to this one. We've always had the same problem. I had our support people take a look at it. They changed some settings in Millbox although I wasn't watching and don't know what they did. The fits are improved but still far from what they should be.

When was the last time the burs were changed?

 

[Contraluz]

Both labs i have worked for had issues with pmmas. The problem is the channels for the ti bases and the screw channels are too small, but the final in zirconia the fit is perfect. The bite and sizing of the teeth and tissue aren't wrong just the size of the holes for the implants. We have tried changing the settings for the cement gap, margin line, and wall but nothing changes when we put the parts in. We even tried comparing milled and printed pmmas and the printed are only slightly better than the milled but they still need adjusted for the parts to drop in. My boss wants to try and solve this problem but the person who taught me how to do all on 4 pmmas said from the start that this has always been a problem. Has anyone else had this issue if so what was causing it and how can we fix it?

This is something I don't understand either I and would like to get an answer to this myself.

To counter act, we increase the cement gap in the designing software to above 0.110mm, if I am not mistaken. Have to check it.

M

 

[zero_zero]

The situation is: for efficiently cutting plastics you need dedicated tools, usually single or o-fluted, high rake angle, polished endmills. The reason is to have efficient chip removal and avoid heating up, thus melting the plastic. Another aspect is how the milling strategies are set up, each material / tool combo has its own optimal feedrate and spindle speed. What you see in tight fits, is a combination of many variables... tool deflection / not enough rigidity / heating up the pmma, which expands then subsequently cools... and the fact that friction heated milled particles can weld back into the puck if not removed quickly enough.
The solution is to revise the milling tools and the strategies or start gradually scaling up the implant base file bit by bit (save it in a separate folder so it wont affect the fit of the zirconia crowns) , until the fit is right.

 

[Contraluz]

The situation is: for efficiently cutting plastics you need dedicated tools, usually single or o-fluted, high rake angle, polished endmills. The reason is to have efficient chip removal and avoid heating up, thus melting the plastic. Another aspect is how the milling strategies are set up, each material / tool combo has its own optimal feedrate and spindle speed. What you see in tight fits, is a combination of many variables... tool deflection / not enough rigidity / heating up the pmma, which expands then subsequently cools... and the fact that friction heated milled particles can weld back into the puck if not removed quickly enough.
The solution is to revise the milling tools and the strategies or start gradually scaling up the implant base file bit by bit (save it in a separate folder so it wont affect the fit of the zirconia crowns) , until the fit is right.

Thanks for the explanation, Zero.

 

[Sda36]

The situation is: for efficiently cutting plastics you need dedicated tools, usually single or o-fluted, high rake angle, polished endmills. The reason is to have efficient chip removal and avoid heating up, thus melting the plastic. Another aspect is how the milling strategies are set up, each material / tool combo has its own optimal feedrate and spindle speed. What you see in tight fits, is a combination of many variables... tool deflection / not enough rigidity / heating up the pmma, which expands then subsequently cools... and the fact that friction heated milled particles can weld back into the puck if not removed quickly enough.
The solution is to revise the milling tools and the strategies or start gradually scaling up the implant base file bit by bit (save it in a separate folder so it wont affect the fit of the zirconia crowns) , until the fit is right.

I've also read somewhere that due to the above by Zero, Milling it a 2nd time gets it done. You're mainly Milling air on 2nd pass but easily gets it done as so little needs to be cut.

 

[tuyere]

I've also read somewhere that due to the above by Zero, Milling it a 2nd time gets it done. You're mainly Milling air on 2nd pass but easily gets it done as so little needs to be cut.

It's called rest milling, and yeah, it's an inefficient but reliable way to deal with a host of issues that all result in too much material being left behind. Kind of necessary with stuff that causes a lot of tool deflection or where tolerances are very tight, not so much with plastics if you've optimized your process for a particular polymer, which rarely seems the case in dental.

 

[helenakr96]

Thank you everyone who gave some solutions. We are going try them out and see which ones solve our issue. I'll report back what ends up working for us.