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What causes webbing?

 
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captmike
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Joined: 01 May 2012
Posts: 27
Location: Elberton, Georgia

PostPosted: Tue Jun 05, 2012 7:41 pm    Post subject: What causes webbing? Reply with quote

Too much heat and plastic stretch? I just completed my oven upgrade and I think it may work too well.
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spektr
Master


Joined: 07 Jan 2008
Posts: 425

PostPosted: Wed Jun 06, 2012 7:09 am    Post subject: Reply with quote

Webbing isnt a heat or plastic stretch ONLY problem. It is a bit more complicated than that...... It IS a sign of overstretching in the PRE STRETCH phase of the operation. The vac form cycle is bacically composed of HEAT, PRE-STRETCH/SEAL, VAC APPLICATION/FINAL STRETCH, COOLING.

You see STRETCHING LISTED TWICE. This is because Vac Forming is a stretching operation only, we dont make the part smaller, we dont compress the plastic. The part is never thicker than the sheet goods we start with (in single sheet forming).

Webbing is really a tool design failure where the pre stretch operation pulls the sheet taught and the material cannot be pulled back down into the tool contact face area beneath the sheet. It might be an incorrect shutoff , it might be insufficient vac venting of spaces under the sheet. It is ALWAYS a tool design issue. Overheated plastic with excessive droop causes a failure because it FAILS TO PRESTRETCH at seal cutoff. This means you are trying to shove more material into the space than you can evacuate with the vacuum. This is ALSO a tool design failure because there are ways to make sure that overheated sheets will still function properly if you design the tool properly.

Single hole plenum style platens limit your design options. Platens also limit your options but to a lesser degree. Independent tool bases are the optimal design solution to producing parts simply and accurately..

Scott.
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ANH trooper
Master


Joined: 20 Oct 2005
Posts: 305
Location: UK

PostPosted: Fri Jun 08, 2012 3:15 pm    Post subject: Reply with quote

Tool placement and applying the vacuum too quickly are also factors to consider. If you are forming multiple tools that are too close to each other can cause webbing, especially if you over heat your plastic. Slow application of vacuum is better than just opening the valve fully and hoping for the best.
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jdougn
Guru


Joined: 18 Mar 2009
Posts: 714
Location: Louisville KY area

PostPosted: Mon Jun 11, 2012 6:09 am    Post subject: Reply with quote

Post up some picts of the webbing issue if you can. It'll help with suggestions.
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