vacuum pump size

super 77 doesnt foul epoxy? that is great.

 

I've used it a couple of times, only very sparingly. Doesn't seem to have any ill effects on the layup or the flow.

 

Hi Pat, Iwas in Lowes today looking for shade cloth,, the guy had no I dea and we went to a second guy how could not help either I found some nylon mesh in the garden center dept. It was called tree something can ya give me a few more clues, Thanks

 

Sun Screen Frabric by Easy Garden

Hi,

Here is the link for the product that we used at the Derek Kelsall workshop, it is call Sun Screen Fabric.

http://www.lowes.com/lowes/lkn?acti...94967294&Ntk=i_products&Ntt=sun screen fabric

I see now it is $.175/foot. Whenever I go to Lowe's or HomeDepot I always go by the outside garden centers. What happens is they discontinue colors or a specific manufacturer, they then put what they have on sale. I buy it then since storage is free at my home till I need it. I would expect with the weather growing cooler that it may go on sale now or soon for example. I buy them out when it does.

I hope this helps you out.

Pat

 

Use it very, very sparingly. If you can see it, you have applied too much. Take a distance of 60 cm or 2 ft from the glass to be sprayed.

And when you do not need it, do not use it. Once vacuum is on, things will not fall apart anymore.

Other options are sprayable hotmelt glue (needs shop air and power, but no VOC's) and there are plenty of other glues that can work. I have also seen plastic staples used (Raptor Composite Nails) which work if there is foam or balsa to tack into. Other proven options are Airtac 2 from Airtech, or their Fusiontack, which can be applied using a plant sprayer or paint gun. (I am worried about the amount of glue used, then)

 

Sorry about double post, lets make it a useful one:

About vacuum pumps: As long as they draw enough vacuum (between 0,8 and 0,9 bar vacuum for epoxy, or perhaps slightly less for polyester) you are OK. But keep in mind some small things:

-when the pump reaches maximum vacuum, it also reaches 0 air transport. So a bit more maximum level of vacuum helps retaining air transport.

-when buying a large pump, make sure you have a decent sized vacuum vessel (could be a large, abandoned compressor tank) and a pressure switch (Danfoss RT121). This way you can:
control the level of vacuum
have the pump stop when vacuum is reached, which saves your ears, your money, and our planet
have control over leaks. As long as you do not hear the pump running, everything is OK. When it starts running more often, check things.

Make sure the capacity of the pump and the size of the vessel match roughly. That way the pump will run for a nice number of seconds or even a minute, and then will shut up for a considerable amount of time. Too large a pump and it will switch on and off rapidly, not even generating enough RPM, or the other way round, and the pump will run too long which is not a bad thing, but is annoying.

And the last thing: Make sure your pump will always run within capacity: Most oil lubricated vane pumps have an acceptable pressure range of 0,5-0,9 bar. Running it for prolonged amount of time (10 minutes or more) at 0-0,5 bar will clog the oil separation filters, which will make the exhaust of the pump dump considerable amounts of oil, and which is irreversible. (you need to change the filters to get it back in shape again). So whenever you smell oil when a vacuum pump is running, or when things get "cloudy" inside, you know you need to call the nearest service centre for new filters. (installation is usually no big deal, 5 minute job)

 

Thanks Pat its a great great help

 

What a cycling pump may mean...

I seem to recall a discussion by Andre (forgot the surname, but goes by GRPguru) who said that he would pull a vacuum in the afternoon and go home. If it was still good in the morning, than he had a good bag. Otherwise it was time to identify the fault.
Why this makes sense to me is that besides having a fresh crew for the critical element of the exercise, once the air is pulled and there's no other source for new air, you should be able to leave the stack indefinitely. If there is a leak, it is probably leaving a trail of bubbles reminiscent of the perforations on a magazine subscrition card, and with similar structural characteristics.
Any thoughts on this?

 

Yes, thats a good approach but... most bags and various consumables have micro leaks and over night can still go down on a perfectly good job. Plus we have found that the longer you leave it the better chance you have of birds and small animals in the factory holing the bag. Even the fibreglass strands can hole the bag if left a long time etc. All bags should have a vacuum gauge in it and if you use a digital gauge you can detect a leak in seconds. If good practice is used all "manufactured leaks" are on the vacuum side of the job so these do not present a problem as long as the pump can cope with the built in leaks. It is impractical to track down all micro leaks although it is possible if you have a couple of days on a big job. When I run infusion trainings I get students to build perfect bags. In fact the first two days of training is just on building good bags. They will hold vacuum for about 3 days before going down. These are small say 1x1m. But try to do this on a 50ft hull job and it is not practical. We've found that the bag quality is very important. Some bag film is micro porous and fine for bagging sandwich panels but impossible to get good vacuum for infusion, similiarly with bag tape. Some are not suitable for infusion. I think the biggest problem in the infusion information is the lack of importance put on getting very good vacuum all over the part. We now use double stage pumps on most jobs. These pull 100x the vacuum a single stage pump does and definitley makes a difference. Can't have too much vacuum! Aussie Guru.

 

I see that many people "recommend" the use of a vacuum receiver. This is NOT GOOD. You can't store a vacuum like a compressed air system. All you are doing is putting more load on the pump to keep the extra volume pulled down. You need to minimise the vacuum volume not maximise it! Plus you need a pump that is rated for 100% duty cycle. It needs to run for as long as the resin takes to cure. Plus many people say 80-90% vac is OK that is untrue. This means that you have 10-20% air in the job and air is your enemy with infusion. If you let the job cycle, the job volume is changing. Its getting bigger then smaller, thicker then thinner. The laminate needs to be compressed at a constant volume otherwise that 10% of air thats in the bag will end up in your job when it expands. Most single stage oil vane pumps pull 99.0% vac or better. Cheap 2 stage refrigeration pumps pull 99.99% vacuum which is prefered. A 25m3/hr pump will do a 50ft boat if the bag is built right. I do recommend that if you are building this size boat that you use a bigger pump for various reasons but I have done it and it is technically OK. Theoretically once you pull a vacuum in the job you don't need a pump! But the micro leaks will get you so you need a pump to scavenge these as the job progresses. Plus if you are using styrenated resins you get outgassing which decreases the vacuum, so you need the pump to scavenge styrene as well. Aussie Guru

 

I also agree that on larger jobs it is impractical to eliminate all micro leaks and some time impossible. These days I place a tube on to the outlet of the vacuum pump and then the free end into a container with water and check to see how fast its bubbling.
No need for long pressure drop tests its instant indication.
This volume could also be simply measured and then related back to the job volume to establish if it is below an accepted value.

 

In one of the physics labs where I used to spend a lot of time, it was frequently necessary to pull 1e-6 to 1e-8 torr vacuum. (ie, on the order of 0.000000004 inches of mercury). At this level, you're no longer worried about leaks- more like, are there any fingerprints in the chamber (fingerprint oils will offgas) or are your hoses themselves porous/offgasing (copper and stainless hoses are often necessary).

By comparison, I don't think I've ever pulled a bagged composite part down below ten torr or so (about 0.01 atmosphere). Although, most of my work with bagged composites has been heat-cured prepregs, where minor leaks are not nearly as critical as they are on an infusion job.

The latter of these two cases is by far harder on the pump. I really take issue with any suggestion of HVAC/refrig vacuum pumps being used to bag a composite part- these pumps are meant to run for 20 minutes at a time, pumping nice clean things like air, refrigerants, maybe a bit of compressor oil. A pump sucking down a bagged composite part is running almost continuously for a day or more, often dealing with styrene and other nasties in the flow.

Beefy old cast iron two-stage pumps, vane or piston, are my favourites.... the old '60s and '70s Welch, etc. that run for decades with little more than an occasional oil change.

Andrew- Bubbling the pump's outflow through oil or water is a clever little trick, I'll have to try that sometime....

 

Andrews water trick is really good. We've used 2 stage refrigeration pumps for years no problems. Just change the oil regularly and when you buy check its rated at 100% duty cycle. Its not about the absolute vac pressure its about acheiving a good vacuum over the entire part. If you put an electronic vacuum gauge around a part thats pulled down with a single stage pump you will find areas that are at reasonably high pressure. These agree with dry spots. You can let the job pump down for a while but it won't acheive the ultimate pressure of the pump due to the microleaks. Using the deeper vacuum helps in getting the job at as low a pressure as possible. Something most people don't realise is that at these pressures the water vapour in the glass fabric and other materials is flashing off at room temp. Its best to let the bag sit for a while and let the water vapour leave. If you use an electronic pressure gauge you can see this happen. A Normal dial gauge will say 101kPa so its OK? No, put the electronic one on and you can see the bag is still coming down. After say 30mins the bag comes down proper. The only explanation I have is that water is flashing off. As Marshmat says there can be a lot of outgassing from various materials in the bag. These are the invisible enemies of the infusion. By the way if you read the manuals on pulling down HVAC systems you will see that you are supposed to pull them down for a couple of hours to get rid of water vapour and oil and various things in the system. 20 mins is not long enough to produce a clean AC system so is it long enough to produce a 'clean" infusion? Most AC companies claim that most reliablity problems in AC units are from techies who do not hold the vacuum long enough when setting up the system. Like most people they are in a hurry and if the meter gets to 100micron then thats OK lets fill her up! Peter

 

While you can perform infusions at lower vacuum levels as Peter said there is a lot of moisture in the reinforcements ( especially in humid locations like Brisbane) that can only be removed with a very high vacuum pump.
I think this is one of the pluses with infusion, no other method will allow you to remove this moisture. But having said that even with a two stage pump on a medium sized job ( 12 x 2.5m) it can still take a very long time to dry the reinforcement completely.
The trick I have used is to periodically introduce some air from my scuba diving cylinder ( this is very dry air) into the job to aid with the flushing out of the moisture. An alternative would be to prepare a large bag, blow it up and place some desiccant in it the day before. Then use this dry air to help with flushing the water out of the job.

The only down side with very high vacuum is that even epoxy resin can start to boil, not just the initial de-gassing of dissolved and mixed in air.

 

I had a Welch for some time. A 1394 model I believe. A notice, riveted to the pump said:

"kindly oil every 10 years".

Modern pumps should have the same notice: "throw away every 10 years"

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Dry materials are a good thing. Some builders over here store their materials in an oven at 70 degrees C, before using them (wet layup or infusion).
Both epoxy and polyester resins do not like moisture. I suggest doing a small test with epoxy resin: add 5% water, which you can mix in even without getting a cloudy appearance, and cure it with a slow hardener. Use a batch of 1/2 kg or so. After doing the test, let me know what you think...