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IMMark · Posted: Sun Jul 03, 2011 3:40 pm Post subject: Lot's of webbing

My 1st attemp with petg. The mold is 7.5" tall, 12" x 8"
Is there a better clear plastic to use?
Improvements to mold design?
Changes in heat and or vacuum?
I used a 24 x 32 size plastic.
The mold is a temp, just to see if I can get things to work.
I did also get horizontal lines in the plastic (but I made the mold from plywood) so if that is the cause, no worries as I will make a proper mold.
Pictures of mold and the draw can be seen here;
http://s255.photobucket.com/albums/hh123/immark/Vacuum%20form/

Any suggestions???
Thanks
Mark

jegner · Posted: Sun Jul 03, 2011 4:28 pm Post subject:

Not sure if it's my connection or not, but I can't get the images to load.

IMMark · Posted: Sun Jul 03, 2011 5:36 pm Post subject:

I just checked again, link seems to work for me?
Anyone else able to see the pictures?
Also, how do you post pictures to this discussion board...I am sure there is something simple I am missing?
Mark

jegner · Posted: Sun Jul 03, 2011 5:46 pm Post subject:

I'm seeing your pics now. The box or cube you are forming has walls that are too straight up and down. Thats what is causing the webbing.

To post a pic, you have to have the image URL and use the button on this page called URL both before and after the URL address of the image.

IMMark · Posted: Sun Jul 03, 2011 6:04 pm Post subject:

Glad you can see them.
Unfortunately I am replicating an original (that was also vacuum formed), so I have to stay as close to original as possible. I just rechecked my original, and I can increase the draft angle (which I will do). Is there any other suggestions, should the plastic on the softer end (with a deep draw)?
Also, thanks for the picture instructions. I will post one, just to make sure what I am doing.
Thanks
Mark

jegner · Posted: Mon Jul 04, 2011 7:43 am Post subject:

You could try a larger base with a deeper taper on it, reduce the temp, and try pulling with less hot plastic, try different thicknesses of plastic.

I have had poor luck forming boxes that deep. Another option would be to pull into a female mold.

grhd76 · Newbie Joined: 28 May 2011 Posts: 5

Go to Google Books and search for "Thermoforming: a plastics processing guide" and then go to page 42 in the preview. There is some good mold design theory there on how to deal with webbing by adding "web catchers".

I've never used this design before, but if you think about it, it makes sense. You're trying to form a cube. If you look at your sheet and try to imagine the cube in its 2D form (pre-draw) you'll have the top side of the cube with the sides of the cube coming off perpendicular from the top-basically a plus sign. Now, if you created this cube by just cutting out the pattern you could simply fold down the sides and have your cube. The webs in your draw are coming from the excess material in between those sides. By adding the "web catchers" you'll be giving that excess material a place to go other than forming a web.

Hope this helps!

Fredo · Posted: Tue Jul 05, 2011 8:46 pm Post subject:

Is this what you're referring to? http://ebookee.org/Thermoforming-A-Plastics-Processing-Guide_884663.html It most be, but I worry about free downloads.........Fredo
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it would be reasonable to shoot back with your own gun...The Dalai Lama..Seattle 2001

IMMark · Posted: Wed Jul 06, 2011 7:58 am Post subject:

Thanks to all that have responded.
I will look into alternatives and keep trying.
I will let you know how things turn out.
Over all, I must say I am very impressed with PETG. Very clear, durable and vacuums nicely (other than my application, which I think just needs tweaked).
Mark

grhd76 · Newbie Joined: 28 May 2011 Posts: 5

IMMark · Posted: Wed Jul 06, 2011 9:57 am Post subject:

I found the book on Amazon, but when I preview...it goes from page 6 and skips to page 200 ?
Mark

grhd76 · Newbie Joined: 28 May 2011 Posts: 5

IMMark · Posted: Wed Jul 06, 2011 12:02 pm Post subject:

That stops at page 23 for me???
That's ok, I appreciate your help....I am sure it is user error Smile

I think that my first attempt to try and remedy this situation will be with a reducing window. Does anyone have any experience with this?
If so, any input on the gap between the mold (ie: 1/2" all the way around, or perhaps a ratio of the draw), just hoping for a starting point.
I want to try this first, as I already have a male mold made (say that 3 times fast).
If I have no luck, I will most likely go for a female mold and see how that does.
Thanks
Mark

jdougn · Posted: Wed Jul 06, 2011 12:44 pm Post subject:

Hey Mark, is the piece of partical board underneath part of the mold? If not, cutting it to the approximate shape of the mold may help reduce webbing. However, with the steep sides on the mold it'll probably still web some and require additonal measures. The web is always created between two points. If you can reduce the size or shape one point you'll often reduce webbing.

That all said, seems like getting rid of webbing is way more art than science. Jegner has some good suggestions to pursue.
dn
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DougN - Just in case you're wondering, I got my "rating" legitimately... by posting aimless drivel, useless advice, and pointless questions.

spektr · Master Joined: 07 Jan 2008 Posts: 425

Webbing.
Fixing webbing is science, not art. All you have to do is to remember what webbing is. OVERSTRETCHED MATERIAL you can't make shrink.
Look at whats going on here. You are attempting to pull the plastic over a box on the platen, make it turn 90 degrees and expect it Not to be overstretched in the process. Why not terminate the part in the air above the elements that cause the issue????? Thats the simple way to fix this. make a spacer about 4 inches tall sloped at 70 degrees and stick it on the bottom of the tool....... This works because the entire part gets cut out of the area of the plastic being stretched.

If you are going to use a marginal vac method, at least give your tool a chance. More design time needs to happen on the tool bases to make them work well. It's far from art, there are solid rules that you shouldnt stray from.

The REAL way to produce a square part like this usiing a thermoplastic sheet is to go to VACUUM SNAPBACK forming. It requires topside tooling and the knowlege to do it. It also gives you consistant wall thicknesses. Something you will NOT get pulling this OML over a male tool.

Scott.