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Large update forming ABS - webbing issue, pic heavy
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mrcadman
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PostPosted: Wed Jan 19, 2011 3:17 am    Post subject: Large update forming ABS - webbing issue, pic heavy Reply with quote

Ok, moving to this forum from the mold making forum now.
I NEED GUIDANCE.
Note: The smaller part made of Slo-Kast warped significantly, but it doesn't matter, since their ended up not being a binding issue. Despite the significant slope, the parts pull right off the mold.

Moving on, here's the temp setup.
4 infrared? heaters (2 tubes each), spaced 12 inches away to avoid burning stripes in the plastic, about 5 minutes to heat.
a york commpressor, 4ea 10gal air-brake tanks, 2 stage setup. We can pull 29in, and only drop to 20 or so for a full evacuation. By the time we apply the compressor stage, it's already too late. The platen is 21x44. The part is 6x8x36.
The smaller part of the mold was designed to lift 3 inches to accomodate any locking issue.
The material is .118 ABS haircell, 24x48 sheets.

I am having a serious problem with webbing on two corners. Tried a riser and removed 90% of it, but the last is very stubborn. I have no idea what to try next.

If it matters, we were getting very nicely formed parts (except webbing) when the bottom of our material was 300 degrees, sag about 4 inches.
We tried down to 270, but that would barely pull, with little detail. Worse, it disfigured the texture from stretching, so going to focus on 290-300 for now.
I REALLY NEED HELP solving this webbing issue. I simply must pull at least 4 perfect tomorrow. I just gotta!
Below are some pics to display what we have to work with and our progress.


This is the overall temp/test assembly


This is our mold on our latest riser, in an effort to conquer the webbing issue. Made it worse. Fail.


Our first attempts to solve the webbing, before adding a riser.


Adding the riser still leaves us with this little webbing on both ends. Every time. precise same size too. Tried adding more height, but there was no change. Perhaps we should make another riser, with a wider top?


A close-up of the persistant webbing.


The basement is filling up......


The final product in place


The ugly wiper assembly this cover is intended to hide.


BOY is it difficult to trim the ends.


Mold on riser and platen ready for another fail.


While the large part of the mold didn't warp, the smaller part has bowed on both ends. This just gets squeezed tight when pulling. As there hasn't been a lock issue, probably just going to screw the two parts together.
Dangit, but what an adventure!
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spektr
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PostPosted: Wed Jan 19, 2011 7:22 am    Post subject: Reply with quote

So many things.... Take off the 4 small triangular items if they are not parts. It looks like they bridge to the part at the corners promoting bridging. where are your vacuum port holes in the negative draft areas of the tool?? You cant seriously expect parts to suck down onto the tool surface without them. The bridge wrinkles in the corners are best fixed this way. All in all, pretty good. you might also think of recentering the tool on the platen to put it more diagonally across the sheet, this unloads the corner tension loads, reduces prestretch and reduces webbing.

webbing is caused by having more prestretch in the plastic than you can pull down. reduce pre stretch and increase vacuum applicatiion is the key to doing this...

Hate to mention it, but if it doesnt pull perfectly, you can keep the vacuum on and heat gun stuff to locally warm it up to forming temp and MAYBE save the parts with the small areas that didnt pull because of time or a bad cutoff. think it thru... it isnt rocket science.
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mrcadman
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PostPosted: Wed Jan 19, 2011 9:45 am    Post subject: Reply with quote

indeed, we figured that one out rather quickly. The little filler pieces were 20 pulls back and no longer considered worthy. If you scroll down a bit, there are pics showing that they've been removed and a riser added.

The The riser has a hole in the middle, and the mold itself sits on hardware cloth. There is much air flow between the two mold parts, which pulls our trim-seam great.

Are you saying we should drill a hole(s) along that little valley, in the corner where the webbing is happening?

Sweet to know about the heat gun! I'll try that on the two pulls that failed for lack of seal (we had 2 blowouts) but so far everything has pulled down with perfect definition, except for the 2 webbing spots. Bah, they should make an anti-webbing gun.

So i need to drill holes in the corners??????
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jdougn
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PostPosted: Wed Jan 19, 2011 2:58 pm    Post subject: Reply with quote

Consider having friends help by pushing the hot plastic up to the mold wherever there potential webbing just befor vacuum is applied. Also, increasing the radius of the corners may help a lot.
hth, DougN
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ANH trooper
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PostPosted: Wed Jan 19, 2011 3:06 pm    Post subject: Reply with quote

It looks like one side of the mould is tapering inwards and that is the side of the webbing. If I was forming this shape, I would have it tilted slightly so both sides have a very slight taper rather than one side having a large taper....make sense?

I think if you can tilt the mould slightly it will definately help. Also your mould need to breathe to help it pull the plastic more evenly.

Another thing that might help is if you can make your riser with more of a radius instead of hard corners. The sharp corners are begging to attract webbing, so smoothing them out more will help.

Finally, it sounds like you are heating the ABS too much. I form .080 and it barely has any sag at all. Mind you, that is on a 24x24 machine, but over heating attracts webbing too.
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drcrash
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PostPosted: Thu Jan 20, 2011 2:01 pm    Post subject: a few suggestions on webbing Reply with quote

That's an ambitious shape for vacuum forming in one piece.

I'm not sure I understand the problem it solves... does it have to be that shape, with the undercuts, and be one piece, or could you join two parts without it being aesthetically or structurally objectionable?

Assuming you want do that shape in one piece, it's an interesting problem. I'm not positive you can do it, but if you can, it'll be a great example of how to do it. Be sure to take pictures and put them up.

I agree that you might be overcooking the plastic. You may want it just cooked enough that it's rubbery enough to form---you need it to still be rubbery and elastic at the moment the vacuum sucks it in, so that it can still contract to fit into the undercut. (You don't want it to stretch out and stay stretched, or you'll get extra surface area that ends up folding up into a web rather than contracting back down to fit.)

How fast you do the stretching and how fast you suck it in may matter too. You generally want to suck the plastic in before it's stayed stretched out too long and won't contract back. (Although it's not clear that reasoning is always right. Tricky stuff.)

I think the tall riser is good, even if that's not immediately obvious.

I'll say a couple of more things about that, but I'm thinking the riser shape may not be the main problem---the problem may be the shape of your original plastic sheet, and/or the plastic-holding frame. A simple trick may make things much easier, and make the riser shape less critical.

It'd be good to make a video of a pull and watch it in slow motion and see how webs form.

You generally get webs where the plastic forms a ridge as it's being stretched, and then when you apply vacuum, the plastic sucks inward on either side of the ridge more than it sucks the ridge itself inward. If the plastic sucks in far enough on either side of the ridge, it meets itself before it meets the mold, and leaves a web.

I could be all wrong in my reasoning here, but I suspect the most problematic ridge you've got, at that corner, actually starts when you start to stretch the plastic downward, but before it actually meets the riser at all. You're getting a subtle ridge between the corner of the mold and the corner of the plastic-holding frame, which is itself a sort of peak.

(If you tilt your head and look at the plastic stretched from the peak to the corner of the frame, as though that line was horizontal, then they both stick "upward" and outward in opposite directions.)

That's likely not an obvious ridge, but even a subtle one there may cause problems later.

I think the first step may be to avoid that, by cutting the corners of your frame and making it hexagonal (if the diagonals meet each other) or octagonal (if there's a bit of the original end left).

To do that, you don't actually need funny-shaped plastic or a new frame. Just clamp a couple of triangles of something across the corners, to block the IR from your heater, and the plastic won't get very hot and stretchy there. In effect, you'll have a hexagonal or octagonal formable area of the plastic, and the unsoftened plastic itself will act as part of the frame, more or less.

Now when you stretch the plastic over a mold, you won't get a ridge between the corner of the mold and the frame, but a triangular face between the corner of the mold and an edge of the stretchy plastic. That triangular face won't have as much area as the original shape, and won't have to un-stretch (contract back again) as much to fit into the undercut without webbing.

That said, and I don't know if it's right, back to risers:

I agree that gently rounded riser corners are better than sharp ones---and the worse the webbing problem, the gentler the curve there generally needs to be.

A large-radius rounded corner is a good shape by default, but even that may not be good enough. If you still get a ridge there, between the upper corner (of the mold) and a point on the curve (of the riser), then you want to flatten that curve there, like part of an ellipse with its major axis on the diagonal.

If you just have a diagonal edge to the base of your riser, instead of a corner, (and a basically triangular face to your riser there) that might work. When you stretch the plastic down, you'll get a sort of triangular face rather than a ridge, and it may suck inward okay. If you'll get two gentler ridges that may each have plastic sucked inward on either sides to form a small web. That's why gently rounding between the diagonal and the existing edges of the riser is probably good.

(I don't think that applies so much to the shape of the plastic sheet, because the plastic tends not to stretch as much right at the corners, and is sorta self-rounding. That's why I suggested a simple hexagonal or octagonal shape for the effective frame shape. I'm not sure about that, though.)

If it's not too much trouble, you might want to experiment with a scale or nearly-scale model, reasonably accurate especially near the problematic points, and use smaller, thinner plastic. That could save you a lot of wasted plastic. If you get something that works, you can probably scale it up and fine-tune it.

Whether you're experimenting full scale or at a smaller scale, you may want to videotape your pulls and then watch them in slow motion in a video editing program. Look for where the tent-like ridges form and then get sharped up by having the plastic sucked in on either side of them. (Be on the lookout for gentle ridges and stretched faces that form when you stretch the plastic down, and then see what happens to them when you suck the plastic in.) Then re-shape your riser contour so that the ridges don't form there, and you get gentler ridges on either side, or no noticeable ridges. The longer you can delay the formation of ridges, and the gentler they are, the less likely they are to get exaggerated into webs by the vacuum.

Even if you do all that right, if you have too much of a concavity, it may not be good enough---even the perfect contour might avoid any predictable ridges and webs, but you may get unpredictable ones because the plastic just isn't rubbery enough, and doesn't contract enough into the undercut.

Even in that case, there might be some subtler tricks you could do about it, and looking at a slo-mo and thinking hard about it might give you some insight. (Starting out with a grid of dots marked on your plastic with a felt-tip marker might make it easier to see where and how the plastic is stretching and contracting.)
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mrcadman
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PostPosted: Fri Jan 21, 2011 9:56 am    Post subject: Reply with quote

Wow, lots of great replies from the experts here!
Thanks you ALL very much for the effort. I STILL NEED HELP.
Sorry i'm late getting back with an update. We did pulls and tried most everything suggested (at that time) and since it had been a 30 hour day for me, i kinda fell asleep, and dreamed of hot plastic.

Here's the update and I'll try to address everything/everyone as best i can...................
Note: I approached this as a one-piece unit as i dont have plastic welding experience, and the potential client does not want rivets or seams.
Sometimes we don't get what we want, but i am commited to try.

We did another 16 pulls and STILL little progress. We tried all that we could in the timeframe allowed (remember i had only until noon Thursday to provide a finished part. At the bottom i will aslo post one solution i am going to attempt when we get more material. Again, I would greatly appreciate any and all constructive comments or suggestions.

Comments are underneath each pic of an effort.


We put a frame around the mold and when it doesn't blow out, there is still a little web at the base. Same thing happened on the other end as shown below.



The diagonal approach resulted in a wonderful finish, but still left large webs on both ends (shown below).





Adding a small wedge to the riser did reduce the size of the web, but spread it to the other side now. While on the other side it is located in the waste area, there is still some original webbing in the original location, and right where we do not want it.
This brings to mind perhaps we should simply tilt it even further in order to move all the webbing over to the other side. hmmmmmmmmm


One L=attempt created two webs on this end, but........

Removed the webbing on the small end. Small victory. This one we sacrificed because of a blowout.


When we pull on a riser, we often get a messed up texture, presumably due to the stretching. Also the ABS loses it sheen in some places.


If it snows again soon, maybe i can sell these as baby-sleds or snow shoes?


This is one attempt we will try next time. Gets rid of the problem corners, basically chamfers the risers, but also trims the bottom pointy corner of the mold itself. Note the line showing the cut of the final part needed. Rounded corners not shown, but they'll be large radius.


If the chamfered riser ends up working, then we should be able to just use a smaller frame with shields, smaller plastic, and lose the riser completely. I'll try this smaller frame with a wedge shaped riser too.
If i can move the webbing to the other side and keep it low enough that it doesn't matter, this could work.

As for heating it too much, here is what we know........
We measure using one of those laser doodahs from directly underneath.
We shoot the bottom middle from directly underneath. We don't even bother until all the wrinkles are gone, at about 280.
The bottom middle is always 5-7 degrees hotter than the outsdide edges.
We typically pull when its at 295-300.
We have pulled as low as 270, but it barely can be forced down over the part.
At 310 or above, there is a slight smoke coming off the top of the plastic. (Wasn't gonna have any more kids anyway).

We did try adding holes along the groove in the problem corners. This had no effect. We even sealed across the long sides in order for all the air flow ot travel thru the ends. Still no effect.
We pulled slower at several rates as well.

The mold sits on 1/2" harware cloth, and going slow showed the plastic just pulls so evenly across the whole thing. That was horrific watching the webbing form each time.

So, here are the next efforts i think are worth trying, due to these experiments.........
1. No riser, but double the wedge to move the webbing to where it won't matter.
2. add shields to number 1.
3. try chamferred riser.

If i've left anything out i apologize. PLEASE, if you guys have further observations i am all ears. If i've overlooked anything, scream at me.

Oh, and the kids found a use for the pulls. A fort with windows too!


Last edited by mrcadman on Sat Jan 22, 2011 2:50 am; edited 1 time in total
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Fredo
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PostPosted: Fri Jan 21, 2011 10:00 pm    Post subject: Reply with quote

Have you drilled the mold as Spektr suggested? That could stop the webbing when the vacuum pulls the abs to the mold at those points. Drilling vacuum ports cured the webbing issues I first had with my molds. It's worth a try. Other that, I've got nothing, good luck........Fredo
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mrcadman
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PostPosted: Fri Jan 21, 2011 11:24 pm    Post subject: Reply with quote

indeed, drilled several holes in the corners channeling the air flow, and sealed off the long sides to no avail. What do you think of the 3 strategies?
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jdougn
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PostPosted: Sun Jan 23, 2011 7:43 pm    Post subject: Reply with quote

Without using a sealed base for the mold drilling holes probably isn't going to help. With the mold setting on mesh the vacuum can go anywhere it wants.
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spektr
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PostPosted: Sun Jan 23, 2011 11:18 pm    Post subject: Reply with quote

Gentlemen, you need to drill holes, LOTS of them , not just in the corners. on a mold this size, with #40 holes spaces every half inch in any area with a return or negative draft area. I bet your looking at 200 holes.. Put them in the tool. You cannot expect a tool to work if you are unable to remove air quickly and effectively. this shape is not that formidable. Also, you can rework parts by using infrared heaters on small propane bottles instead of heat guns. i do it when im looking at "issues".
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spektr
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PostPosted: Sun Jan 23, 2011 11:21 pm    Post subject: Reply with quote

Doug. you cant pull this shape undrilled, you cannot pull the air off the tool once the base shuts off.... mesh or not, you gottta drill all negative draft areas on any tool to get it to pull.
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mrcadman
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PostPosted: Mon Jan 24, 2011 2:37 am    Post subject: Reply with quote

Allrighty then, i need to summarize and regroup.

Summary:
In my previous post, i showed evidence of most every effort attempted. The ABS pulls tight into every nook and cranny, and quickly. Air travels underneath all around and also on the entire middle seam, which has a little vertical to it as well. The negative draft pulls in every time, very tight, and within microseconds as the positive draft side. This happens even when we've sealed off the enitre long edges, holes on the corners or not. By the time the bottom is sealed off, our plastic is firmly shaped and looks just as expected, save for the webbing.

ReGroup:
So, here are the next efforts that appear worth trying, due to these experiments.........
FIRST, i'll have my 60 FPS camera record the pulls from now on. Using my editing skills, this will allow frame by frame examination of when the web forms.

SECOND, chamfer the base corners of the mold itself.

THIRD, drill #40 holes, 1/2" spaced all along the groove on the negative draft side. Then seal off that entire long edge.
If that doesn't work, I'll drill the groove of the entire perimeter and seal the base completely. This will direct the airflow solely thru the holes, and of course, the vertical seam between the two mold parts.


After these three things, the following appears to be the next steps to try.
1. Flat on platen, sealed negative draft edge.
2. Flat on platen, all grooves drilled, all edges sealed to platen.
3. Larger wedge base, to move the web to the other side, where it may not matter if it's low enough.
4. Add shielding to the frame so there isn't as much "stretch" in the area where the webbing forms.
5. Add riser, but with chamferred corners.

Does this appear to be the right direction or combination?
Am i understanding the arguments correctly?
Perhaps i should start with a combination of some of the above?
At $8 a pull, this is starting to add up quickly.
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Fredo
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PostPosted: Mon Jan 24, 2011 12:15 pm    Post subject: Reply with quote

Assuming your riser and your mold both sit on 1/2 hardware cloth, it might help if you drill some large holes through the riser for a more direct vacuum access to the bottom of the mold........Fredo
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spektr
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PostPosted: Mon Jan 24, 2011 12:41 pm    Post subject: Reply with quote

PM TIME....
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