occasional shrinkage | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Rodney Schares Posted: Sep 05, 2010 04:39 PM	occasional shrinkage Mason, Why does 2# ccSPF have a tendacy to recess/shrink after it has had the skin removed?
mason Posted: Sep 05, 2010 04:56 PM	All foam plastics will shrink or grow under certain conditions. a 2 lb density foam typically has a humid aging percentage volume change of between 8-15 % when exposed to hot and humid conditions (ASTM D 2126, 158 degrees F at 97 to 99% humidity.) At low temperatures, it tend to shrink but not as much. The exterior skin of the foam is a higher density and acts as a coating that minimizes the dimensional instability to some degree. So when it is trimmed off, the foam is more likely to shrink or to grow depending on the circumstances, Also, when you cut the foam, you open the cells and they can collapse on themselves easier causing shrinkage. Other factors that affect the dimensional stabilty of the foam are; 1. how thick the foam is installed with one pass. the thicker the foam lifts, the lower the density of the foam and the weaker the cell structure. 2. If the foam is still on the substrate. Foam that has been cut of the substrate that it was sprayed to is much more likely to grow and shrink than when it is still on the substrate. For example a foam that has 15% volume change at 12 x 12 inches by 2 inches thick, might only have 8% volume change when adhered to a substrate. 3. Irregular shaped foam also tends to have greater dimensional instability. So it helps to spray more unformily. 4. Temperature of the substrate and of the foam chemicals. The hotter the substrate and the liqued components, the greater the rise of the foam, which makes the foam a lower density and causes weaker cell structure and dimensional stability. 5. Off ratio foam. B rich foam will be more dimensionally unstable than on ratio foam.
Rodney Schares Posted: Sep 06, 2010 10:31 AM	Thanks for the quick response and insight.The funny thing is that I have been trimming the skin off foam and grinding it down to a smooth plane for 15 years now and since the whole blowing agent change we never had a problem with shrinkage. I have thought about all of the 5 things you have listed as shrinkage causes.The one I have concern with is B rich foam.How far off are the chemical guages to be concidered b rich foam? 100,200 or more pounds? With our gusmer H20/35 The closest I can bring the gauges to balance is 100lbs.
mason Posted: Sep 06, 2010 03:35 PM	I would not worry about balancing gauges that are 100 lbs apart. That can be caused by the guages just being off. Also, the viscosity of the material can cause pressures to be off up to 200 lbs. So, the best way to tell is to check your foam. See if the density is right, are the compressive strengths where they should be or is the foam soft. How is the cell structure, is there color variations, weird shaped cells, hairy like filaments at the substrate interface. Is the foam soft and gummy or hard and brittle? You can also check ratio by just pumping out a and b side from the gun block into separate cups and measuring the volume. It should be close to identical.
steven argus Posted: Oct 13, 2010 10:30 AM	Hi Mason, Would you mind explaning this process in detail? Some guys on this forum might see this as a stupid question, but we have some nagging issues w/ our gear and I just want to get the process right. "You can also check ratio by just pumping out a and b side from the gun block into separate cups and measuring the volume. It should be close to identical." Thank you.
mason Posted: Oct 14, 2010 08:56 AM	Take the gun off the coupling block. Open the valves on the coupling block so that a stream of material goes into separate containers. Turn on the proportioner and pour out material into separate containers for 20-30 seconds. (make sure the heat is off so the B side doesn't froth) Measure the volume of material on each container. It should be equal, if not then there is an equipment problem that you need to trouble shoot. This technique would not tell you if there were a problem in the spraygun.

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Comments

Rodney Schares

Posted: Sep 05, 2010 04:39 PM

occasional shrinkage

Mason,

Why does 2# ccSPF have a tendacy to recess/shrink after it has had the skin removed?

mason

Posted: Sep 05, 2010 04:56 PM

All foam plastics will shrink or grow under certain conditions. a 2 lb density foam typically has a humid aging percentage volume change of between 8-15 % when exposed to hot and humid conditions (ASTM D 2126, 158 degrees F at 97 to 99% humidity.) At low temperatures, it tend to shrink but not as much.

The exterior skin of the foam is a higher density and acts as a coating that minimizes the dimensional instability to some degree. So when it is trimmed off, the foam is more likely to shrink or to grow depending on the circumstances,
Also, when you cut the foam, you open the cells and they can collapse on themselves easier causing shrinkage.

Other factors that affect the dimensional stabilty of the foam are;
1. how thick the foam is installed with one pass. the thicker the foam lifts, the lower the density of the foam and the weaker the cell structure.
2. If the foam is still on the substrate. Foam that has been cut of the substrate that it was sprayed to is much more likely to grow and shrink than when it is still on the substrate. For example a foam that has 15% volume change at 12 x 12 inches by 2 inches thick, might only have 8% volume change when adhered to a substrate.
3. Irregular shaped foam also tends to have greater dimensional instability. So it helps to spray more unformily.
4. Temperature of the substrate and of the foam chemicals. The hotter the substrate and the liqued components, the greater the rise of the foam, which makes the foam a lower density and causes weaker cell structure and dimensional stability.
5. Off ratio foam. B rich foam will be more dimensionally unstable than on ratio foam.

Rodney Schares

Posted: Sep 06, 2010 10:31 AM

Thanks for the quick response and insight.The funny thing is that I have been trimming the skin off foam and grinding it down to a smooth plane for 15 years now and since the whole blowing agent change we never had a problem with shrinkage.

I have thought about all of the 5 things you have listed as shrinkage causes.The one I have concern with is B rich foam.How far off are the chemical guages to be concidered b rich foam? 100,200 or more pounds? With our gusmer H20/35 The closest I can bring the gauges to balance is 100lbs.

mason

Posted: Sep 06, 2010 03:35 PM

I would not worry about balancing gauges that are 100 lbs apart. That can be caused by the guages just being off. Also, the viscosity of the material can cause pressures to be off up to 200 lbs. So, the best way to tell is to check your foam. See if the density is right, are the compressive strengths where they should be or is the foam soft. How is the cell structure, is there color variations, weird shaped cells, hairy like filaments at the substrate interface. Is the foam soft and gummy or hard and brittle?

You can also check ratio by just pumping out a and b side from the gun block into separate cups and measuring the volume. It should be close to identical.

steven argus

Posted: Oct 13, 2010 10:30 AM

Hi Mason,
Would you mind explaning this process in detail? Some guys on this forum might see this as a stupid question, but we have some nagging issues w/ our gear and I just want to get the process right.

"You can also check ratio by just pumping out a and b side from the gun block into separate cups and measuring the volume. It should be close to identical."

Thank you.

mason

Posted: Oct 14, 2010 08:56 AM

Take the gun off the coupling block. Open the valves on the coupling block so that a stream of material goes into separate containers. Turn on the proportioner and pour out material into separate containers for 20-30 seconds. (make sure the heat is off so the B side doesn't froth)

Measure the volume of material on each container. It should be equal, if not then there is an equipment problem that you need to trouble shoot. This technique would not tell you if there were a problem in the spraygun.

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