Bridge expansion

[number1]

Good morning.

How do I adjust my investment ratio for better fitting long span bridges? I follow the wax/sprue techniques in some of the forums and use Fast Fire investment. 30 minute bench set with a 60 minute burnout at 1450. After casting, bench cool to touch. I find that the individual units fit ok, but the bridge as a whole does not. It seems to only happen on big bridges with multiple pontics. Simple 3 or 4 unit bridges are no big deal. Is there any merit to adjusting the ratio? And if so, which way?

Thanks

 

[cadfan]

Good morning.

How do I adjust my investment ratio for better fitting long span bridges? I follow the wax/sprue techniques in some of the forums and use Fast Fire investment. 30 minute bench set with a 60 minute burnout at 1450. After casting, bench cool to touch. I find that the individual units fit ok, but the bridge as a whole does not. It seems to only happen on big bridges with multiple pontics. Simple 3 or 4 unit bridges are no big deal. Is there any merit to adjusting the ratio? And if so, which way?

Thanks

make wet in wet investment stump with fit liquid powder ratio and bridge span with another liquid powder ratio. You need more material but save a lot of time and time is ..... But wet in wet dont wait to long that the stump investment binds form and gets hot you get deformations or loose parts if its only wax.

 

[2thm8kr]

If your not casting non precious. Cast your bridge in sections and solder.

 

[DevonR]

Good morning.

How do I adjust my investment ratio for better fitting long span bridges? I follow the wax/sprue techniques in some of the forums and use Fast Fire investment. 30 minute bench set with a 60 minute burnout at 1450. After casting, bench cool to touch. I find that the individual units fit ok, but the bridge as a whole does not. It seems to only happen on big bridges with multiple pontics. Simple 3 or 4 unit bridges are no big deal. Is there any merit to adjusting the ratio? And if so, which way?

Thanks

We use the whipmix system. To get more expansion, you alter the special liquid to distilled water ratio. More special liquid = more expansion

 

[rich green]

Good morning.

How do I adjust my investment ratio for better fitting long span bridges? I follow the wax/sprue techniques in some of the forums and use Fast Fire investment. 30 minute bench set with a 60 minute burnout at 1450. After casting, bench cool to touch. I find that the individual units fit ok, but the bridge as a whole does not. It seems to only happen on big bridges with multiple pontics. Simple 3 or 4 unit bridges are no big deal. Is there any merit to adjusting the ratio? And if so, which way?

Thanks

When you say does not fit do you mean rocking? or differences in length? or just what kind of problem? Just thinking do you use same material for pontics as copings, meaning do you use plastic forms for pontics and wax for copings? Possible differences in expansion there. Maybe even if you use different waxes. just a thought

 

[charles007]

Good morning.

How do I adjust my investment ratio for better fitting long span bridges? I follow the wax/sprue techniques in some of the forums and use Fast Fire investment. 30 minute bench set with a 60 minute burnout at 1450. After casting, bench cool to touch. I find that the individual units fit ok, but the bridge as a whole does not. It seems to only happen on big bridges with multiple pontics. Simple 3 or 4 unit bridges are no big deal. Is there any merit to adjusting the ratio? And if so, which way?

Thanks

" IF " the individual abutment crowns fit the same as single unit crowns, there is never a reason to change the expansion ratio ! Three units bridges should fit exactly the same as 6 unit bridges.
The problem could be the preps are so parallel the bridge fits tight because of the draw.. path of insertion.
The other problem could be your techniques in spruing and your seeing a slightly warped bridge.
Are you using hard plastic sprues as your runner bar.. these must be dipped/coated in with wax..
How do you join each connector ... technique must be precision..

 

[charles007]

Cut the next bridge to solder and join together with wax, zapit, etc to check the fit.
1. If the bridge now has a perfect fit,your bridge warped with your spruing technique.
2. If the bridge now fits but scrapes the walls of the dies when you take off the bridge, and seat back on the model, you need to start using a surveyor on your larger bridges before waxing..

I have heard of labs using different ratios when using multi packs of investment, but never needed to do that..

 

[charles007]

Ok, last idea
If your not sure if the bridge will draw or not before waxing, and you don't own a surveyor ! make a quick coping on each abutment and place a steel mandrel, etc over the abutments and lute together. Now lift off to see if any of the wax
coping crack or break. You may learn you need to make a reduction coping so the bridge will draw...
If you want to learn more about spruing techniques, start a new thread.. or buy a scanner for printing or milling your larger bridges.... lol

 

[ter01475]

When I wax large bridges...I cut all my connections and relute them with dead soft wax...then do the same when spruing. My burn out is a multi stage burn out that is much slower than yours....and even then when i check the fit under a microscope...I need to section and laser weld the bridge. Plan on cutting and welding!

 

[number1]

" IF " the individual abutment crowns fit the same as single unit crowns, there is never a reason to change the expansion ratio ! Three units bridges should fit exactly the same as 6 unit bridges.
The problem could be the preps are so parallel the bridge fits tight because of the draw.. path of insertion.
The other problem could be your techniques in spruing and your seeing a slightly warped bridge.
Are you using hard plastic sprues as your runner bar.. these must be dipped/coated in with wax..
How do you join each connector ... technique must be precision..

I actually worry about the draw when it is in metal. I find that I can relieve most undercuts then. I dip the copings in Rapid Wax, and build the pontics and collars with Maves inlay wax. I sprue each unit with a 10 guage sprue, and use an 8 guage plastic runner bar. ( I have not tried coating in wax). Then 2 8 guage sprues to the base. I lute everything with Slaycris wax, including the connectors, adding little amounts and cooling in between. It takes me about 30 minutes to sprue. Lots of cooling time. Then I will pop the pins from the base if it is a 3 or 4 unit, or draw with the runner bar if there are many abutments. I don't like to take the wax of the dies multiple times because of the fragile margins.

 

[user name]

Most likely you are having distortions...either in your wax/sprue technique, or as a result of how you cast. You need to calculate how much alloy you will need to just cast the bridge and most of the runner bar. If you have excess alloy, ie button, then that mass of metal is pulling away from your bridge and warping it as it cools. That will also leave you with micro porosity in your framework that will cause distortion during your bake cycles.

 

[charles007]

I actually worry about the draw when it is in metal. I find that I can relieve most undercuts then. I dip the copings in Rapid Wax, and build the pontics and collars with Maves inlay wax. I sprue each unit with a 10 guage sprue, and use an 8 guage plastic runner bar. ( I have not tried coating in wax). Then 2 8 guage sprues to the base. I lute everything with Slaycris wax, including the connectors, adding little amounts and cooling in between. It takes me about 30 minutes to sprue. Lots of cooling time. Then I will pop the pins from the base if it is a 3 or 4 unit, or draw with the runner bar if there are many abutments. I don't like to take the wax of the dies multiple times because of the fragile margins.

WOW.. you need to get back to a basic spruing technique.. deal soft wax on connectors,, this technique is shown in the old Belle De Claire and now Kerr video. I think you can view online ?
I now use direct spruing and lute my reservoirs together with wax, not sticky wax, and use glue on connectors.. The direct spruing technique is shown using Dentifax sprues/any brand of sprue with work. Maybe you can see this technique online, Dentifax International ?
On hard plastic sprue bars and coating with wax. I'm talking about the solid clear plastic sprues that can warp. Newer softer plastic sprues and hollow sprues don't need to be coated..

 

[Mohammad Khair]

Try to do follow this

1-When you fit the wax bridge to the funnel make sure to make the bridge plane perpendicular to the longitudinal axis of the ring (for example if its full arch the acclusal plane must be parallel to the base of the casting ring as much as possible)

2- make all units to have equal distance from the center of the ring as much equal as possible, and nearly equal distance from the top of the ring.

3- even if the investment material is fast, try to slow down the heating rate between room temp to 600* and hold for 20 minutes at 600*.

4- try to use a bigger size ring if the investment material around the bridges is very little.

5- place the ring in the center of the preheating furnace and turn it 90* each 10 minutes.

6- remove the investment material from the plastic/silicon mold as soon as it is start to produce heat caused by setting reaction ( to allow for even setting expansion),and if you use metal ring try to make thicker thermo-liner.

7- reduce the liquid/water ratio a little than usually used for singles. (reduce a little the liquid, add a little more water)

best wishes

 

[number1]

WOW.. you need to get back to a basic spruing technique.. deal soft wax on connectors,, this technique is shown in the old Belle De Claire and now Kerr video. I think you can view online ?
I now use direct spruing and lute my reservoirs together with wax, not sticky wax, and use glue on connectors.. The direct spruing technique is shown using Dentifax sprues/any brand of sprue with work. Maybe you can see this technique online, Dentifax International ?
On hard plastic sprue bars and coating with wax. I'm talking about the solid clear plastic sprues that can warp. Newer softer plastic sprues and hollow sprues don't need to be coated..

What is considered "soft" wax? As for the plastic runner bar, they are solid

 

[charles007]

What is considered "soft" wax? As for the plastic runner bar, they are solid

Solid sprues have great memory, and tend to break down investment. 2 stage burnout helps, but you need to coat with wax
Kerr makes a dead soft wax sold in a small can, but I can't remember the name. Its white, almost clear. Red rope wax is really the same wax and soft margin wax is basically the same..
You don't want to use a wax that has a lot of shrinkage that you normally use to wax with like Slaycris for your connectors.
To be honest, if I were waxing large bridges a lot, I would design to presolder to get the best fits since soldering is so simple to do.(Talking about large difficult round house cases.)
Or send out to be designed on a scanner and milled in wax or printed. You can send models and preop models to copy and also view over teamviewer for your approval before milling/printing.
PM your tel # to me... I'm sure I can help you with a few simple changes in techniques and different materials...
Check your Inbox

Charles

 

[rkm rdt]

Just dip your plastic runner bars in your dip pot.

Then switch to zirconia

 

[number1]

Most likely you are having distortions...either in your wax/sprue technique, or as a result of how you cast. You need to calculate how much alloy you will need to just cast the bridge and most of the runner bar. If you have excess alloy, ie button, then that mass of metal is pulling away from your bridge and warping it as it cools. That will also leave you with micro porosity in your framework that will cause distortion during your bake cycles.

Hi. Just rereading this thread. What are your thoughts are on how much button to have when casting full gold crowns. I use a reservoir sprue and usually end up with a button twice the size of the crown. On very heavy crowns I will occasionally have porosity. I know there are many possibilities for this, like overheating the metal, burnout oven to hot, or improper spruing. I'm just wondering if button size plays any role. Thanks

 

[CatamountRob]

Hi. Just rereading this thread. What are your thoughts are on how much button to have when casting full gold crowns. I use a reservoir sprue and usually end up with a button twice the size of the crown. On very heavy crowns I will occasionally have porosity. I know there are many possibilities for this, like overheating the metal, burnout oven to hot, or improper spruing. I'm just wondering if button size plays any role. Thanks

The reservoir should be filled, but little more. I use 8 gauge (I think) direct sprues. Make sure the reservoir is in the center of the ring, the crown close to the top and little or no button at the bottom. What temp do you burnout at?
I use a rapid burnout, I put the ring into the furnace preheated to 1250 F for 30 minutes, then I turn the oven down to 950 F for 30 minutes. I find it casts nicer at the cooler temp but if you don't heat it above 1200ish, you won't get rid of the carbon.

 

[rich green]

Hi. Just rereading this thread. What are your thoughts are on how much button to have when casting full gold crowns. I use a reservoir sprue and usually end up with a button twice the size of the crown. On very heavy crowns I will occasionally have porosity. I know there are many possibilities for this, like overheating the metal, burnout oven to hot, or improper spruing. I'm just wondering if button size plays any role. Thanks

Could be a couple of reasons. 1.If the sprue is not large enough it will cool before the crown. This will prevent the reservoir from doing its job.2 If you are using ringless casting be sure the crown is positioned correctly in the ring, because ringless systems have a different heat zone than rings and the button has to remain in the heat zone during cooling to allow the crown to draw on the reservoir during that time..

 

[technician]

Good morning.

How do I adjust my investment ratio for better fitting long span bridges? I follow the wax/sprue techniques in some of the forums and use Fast Fire investment. 30 minute bench set with a 60 minute burnout at 1450. After casting, bench cool to touch. I find that the individual units fit ok, but the bridge as a whole does not. It seems to only happen on big bridges with multiple pontics. Simple 3 or 4 unit bridges are no big deal. Is there any merit to adjusting the ratio? And if so, which way?

Thanks

Hello, just to make you understand whats happening when you invest a large bridge with a radius of in this example 7 cm.
Because of the thermal expansion of about 2.5% your bridge will now be 7.5 cm in radius so NO way way of good fit. So to make it fit you will have to adjust inside of all the copings.

I think that the best way is you use different expansion ratios (liquid/Water) so you fill the inside of copings with normal expansion and the rest of the flask with no expansion.
A nice trick is to use less expansion on big molars and more on lower incisors.

Off course you can also cut it in smaller sections and later solder it together.