Vacuum Film Technique - A Quick Explanation

[Brady Watson]

Due to time constraints, I have not had the resources to get a proper ShopBot Web Column article properly done...So until I do, here's a quick & dirty write up of what I call the 'Vacuum Film Technique'

I have been using this technique for the past few years to hold tightly nested, small and/or thin parts securely to the vacuum table without the need for tabs or other labor intensive band aids. I have used it to machine all types of plastics, non-ferrous metals (silver nickel, brass, AL) and even thin wood veneer.

(pics to follow at some point...I'm not at the shop at the moment)

I'll explain the basic concept using a 4 X 8 X .1875" Cast Acrylic sheet as our material to cut. Let's say we have an entire sheet completely nested with parts with little or no scrap, in order to get this job we desperately need to keep afloat.

I'll start by cleaning one side of the acrylic paper with a dust brush. Then I will adhere two 24" wide strips of High Tack transfer tape, taking care to avoid bubbles. Use a J-roller to firmly adhere the tape to the paper on the acrylic sheet. Then I take 6" wide mounting tape, which is very aggressive & double sided, and stick it to the entire back of the transfer tape I just put on. Then I take a $7 4X8 sheet of Coroplast (corrugated plastic) and laminate it to the mounting tape to give me a layup that looks something like this:

Acrylic Layer
High Tack Transfer Tape Layer
DS Mounting Tape Layer
4mm Coroplast Layer

I run the J-roller over the whole deal and make sure everything is bonded really well. Then I place the whole layup right on my universal vacuum bleeder & pull vacuum. I cut my parts, keeping an eye on things to make sure I am not asking too much from this hold down system by cutting too aggressively. If I get part movement, I make adjustments, but it is rare that I have to do this. I cut all the way thru the acrylic, tapes and well into the coroplast. This allows me to cut all the way thru the parts without ever breaking vacuum. The vaccum holds the coroplast...the coroplast holds the tapes to the acrylic.

When cutting is complete, I simply peel off the acrylic along with it's protective paper. The super aggressive adhesive remains attached to the coroplast and the transfer tape easily peels off the paper backer on the acrylic without a hitch. The mounting & transfer tapes are so thin that adhesive build up does not typically present a problem.

That's it! Now you can cut parts you only dreamed of cutting before!

Everything I mention here is from HarborSales.net:

Mounting Tape
6" x 72 yd | White | Double Side Adhesive
216 Double Coated Poly Film Mounting Tape
Price: $64.77 ea

Transfer Tape
#4076 | Conform/RLA | Higher Tack
24" x 100 Yards
Price: $58.00 ea

OK - This needs to be said to many of you. You MUST charge for part fixtures since there is a consumable cost - the coroplast and tapes, plus your labor in laying everything up. If the customer balks at this, then you might want to think about getting some new customers. There is no reason why you should have to pay for this out of your own pocket. I prorate the cost of the tapes and coroplast and charge shop rate for labor in the layup.

You are essentially giving the customer superior edge quality for just a few bucks more than a hack job and tabs. Something for you to consider.

-B

[dana_swift]

Brady- that is an excellent technique. Thank you for sharing. I can see where that could be very useful in my applications.

I can see a rich area to explore there for holding down all manner of parts.

Good thinking!

D

[knight_toolworks]

this may be better then contact cement. think it would stick to wood?

[Brady Watson]

Thanks guys. I'll get some pics together as soon as my life simmers down a little. This method works unbelievably well, compared to every other method of hold down when doing thin or small parts. I have literally cut hundreds of thousands of parts with this technique.

Steve - I have used it for solid & sheet/plywood type material up to 1/4" thick. I'm not saying that it wouldn't work for parts thicker than this, but you have to stop and think about the physics going on in the cutting process. The thicker the material, the greater the force that can potentially be exerted against your hold down method. Even with conservative step downs, it is possible for a spiral bit to lift the material in such a way that it causes the transfer tape to fail. So using thick materials with this method is something you guys should try out - and then report back to the group to share your findings.

Non-porous materials are not a problem. Woods and other porous materials should be cleaned religiously and tested for adhesion. Veneers of any material are a cakewalk with this system. It is VERY important to use a j-roller to really get these sheets to stick together. A professional lamination machine is really the best...but don't bother unless you already have one in your vinyl shop.

-B

[kra]

i used your method today to cut 1/4" pvc and it worked great
a less aggresive application tape seems to works better with pvc
it really sticks to the raw pvc

I ran my cut file again to clean up the welded shavings and nothing moved even though some of the parts were less than 1/2" x 3/4"
almost no sanding

great tip! thanks

[john_l]

Quite a testament Randy.. 1/2 x 3/4" is really small!

Brady, Thanks for posting the tip. I have been using almost the same method except that I usually use our "Premier" Contact Adhesive spray setup instead of your double sided tape layer. This contact cement is pricey and may not be saving me any material costs but I do think it may be faster. But thats only cause I have it here already.

Do you apply the double sided tape (6" wide roll) continuously over the full area you plan to cut? Even if done that way, you could still get about 3 full sheets from a 72 yard roll.

And I fully agree about charging for ANY necessary hold-down, clean-up, or any other reguired procedure regardles of what it might be. I do here, at full shop rate, and I hope all others are as well. Please dont give it away!!

[woodturner]

I'd like to ask for some advice on this technique please. I need to cut a bunch of small (7" +/- long, and tapered from 3/4" wide down to 1/4", think small triangle) parts in 3/16" thick polycarbonate sheet.

My supplier only has the PC sheet with "cling" film to protect it, not the sticky paper like acrylic. And, I am afraid acrylic is too brittle for the application. Has anyone tried sticking the wide transfer tape directly to the polycarb, then gluing that to the corplast? I am set up for spraying contact cement, so I planned to go that route. Will the tape hold the parts and release them OK? Will the contact hold the back side of the tape? I don't have the double sided stuff.

I plan on cutting with an Onsrud single flute at 2" per sec, probably offsetting the first cut and then doing a .025 clean up pass. There are a few tight curved areas, so I might path those separately and go a little slower (rpm and feed). I need a very good finish (flame polish???) since it's for a display. Any thoughts are appreciated!

Kind Regards.

[Brady Watson]

Gene - Looks great. The only thing I would add is a 1/4" or wider 'island' in the center of the vac pocket you have machined there to prevent the material from cupping down when under vacuum.

Ed - You should have no problem cutting those parts with transfer tape right on the acrylic. It is a bit tricky to get a nice smooth lamination because the transfer tape really sticks to the acrylic's ultra smooth surface. It peels off easily though.

Experiment with the transfer tape & your brand of contact adhesive on scrap before you try it on the real deal. I don't see it being a problem as the transfer tape should act as a barrier for the cement bleeding thru. Acrylics are sensitive to many solvents - which *may* be flashing off of the contact adhesive...Test it to be sure.

I would cut these small parts in 2 passes with a single spiral-O flute at 1,0.5 with ramped entry at 18,000 RPM or a double or triple spiral-o flute up to 2 IPS, if the cutter diameter is at least 1/4". Test 1st few to dial it in & adjust accordingly. You may be able to get away with a single pass, but you'll have to test to be sure. No two acrylics cut the same...and color can greatly affect cutting speeds. Check it: http://www.plasticrouting.com

-B