Hello friends,

I participated in a seminar on manufacturing processes and in the process of wet layup with vacuum bag the instructor told us that in the laminate we have the right time to apply the vacuum and the last moment to apply the vacuum.

He stated the following information:

• EARLIEST TIME TO APPLY VACCUM - 100,000 CPS
• LATEST TIME TO APPLY VACUUM - 500,000 CPS
This is based on curve the viscosity increase x time.

All this so as not to have excessive removal of resin generating dry areas.

Does anyone use this method, does that statement apply?

The problem with that is how do you measure viscosity of the resin in a laminate?

it is quite plausible that those figures are accurate from mechanical testing and enough research but they are not really that helpful in that format.

It makes sense that as a resin system begins to cure, the viscosity will rise, and in practice a point will be reached that vacuum bagging would have minimal or no impact on consolidation because the resin has thickened up too much.  So the statement and theory is fine. 

In practice, with no viscosity measurement, most people are likely to ignore the first part.  In practice, by the time you have laid up the part and then got the vacuum bag on it, it is likely to be fine for pulling vac on.  I have not heard of people having issues from vac bagging too soon (not to say it cant happen.).

In terms of the second statement, in practice most people tend to aim to work quick enough so the resin has no where near gelled by the time the bag pulls down so again it is unlikely to be an issue for most users.

In my experience, the most common factor for dry spots is either poor application and consolidation of the first layers of resin and fabric and/or  excessive vacuum used during the bagging stage.  Generally these are problems found with inexperienced users and with experience, people tend to improve their  laminating technique and find a vacuum level sweet spot that works for them.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

+x

 

Warren (Staff) - 7/19/2018 9:17:50 AM

The problem with that is how do you measure viscosity of the resin in a laminate?

it is quite plausible that those figures are accurate from mechanical testing and enough research but they are not really that helpful in that format.

It makes sense that as a resin system begins to cure, the viscosity will rise, and in practice a point will be reached that vacuum bagging would have minimal or no impact on consolidation because the resin has thickened up too much.  So the statement and theory is fine. 

In practice, with no viscosity measurement, most people are likely to ignore the first part.  In practice, by the time you have laid up the part and then got the vacuum bag on it, it is likely to be fine for pulling vac on.  I have not heard of people having issues from vac bagging too soon (not to say it cant happen.).

In terms of the second statement, in practice most people tend to aim to work quick enough so the resin has no where near gelled by the time the bag pulls down so again it is unlikely to be an issue for most users.

In my experience, the most common factor for dry spots is either poor application and consolidation of the first layers of resin and fabric and/or  excessive vacuum used during the bagging stage.  Generally these are problems found with inexperienced users and with experience, people tend to improve their  laminating technique and find a vacuum level sweet spot that works for them.

Measuring the viscosity in the laminate is practically impossible. This information it relies on is technical information that resin manufacturers provide (but I know that not everyone has this information).

Here is an example of a technical detail of a resin:



As for the lamination, I will try to carry out the good practices that you have passed me.