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Please help: cannot get enough stretch from my plastic

 
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mojomandan
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Joined: 10 Aug 2015
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Location: South Jersey

PostPosted: Tue Aug 11, 2015 7:12 pm    Post subject: Please help: cannot get enough stretch from my plastic Reply with quote

Hello guys,

I have just made a vacuum forming box/table so I may make plastic RC car bodies. The mold itself is made mostly from plaster of paris, with some Apoxie (sculptable epoxy with a very long working time) features. The mold is 22” long, 12” wide, and 6.5” high. My platen is 26” long and 16” wide (as is the plastic sheet I use).

I have watched numerous Youtube videos of failures and successes from people who are doing what I am attempting with a mold just as big, and platens that are smaller, the same size, or larger. But my failure is different than any other I have seen or read about…

I have tried using .020 PetG and have heated a sheet to allow sag of 3.5”. The result: I could not get enough of a stretch to fully plant my plastic holding frame down onto my platen to create a vacuum seal.

I have also tried using .060 polycarbonate and heated to the point where I achieved a sag of 3” on one attempt, and then a sag of 7.5” on another attempt. The results are the same: cannot plant my frame down so the vac cannot do its job. I am 200 lbs and my full weight will not push the frame down. I saw a video where a person was successful with a sheet .080” thick and a mold the same size as mine. He did not need to kneel on his frame like I do.

What am I doing wrong? I am very frustrated with my lack of progress. Every amateur thermoforming video I have watched, with plastic thicknesses ranging from .020 to .080”, have at the very least created a vacuum seal. Why can’t I?
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IMMark
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Joined: 18 Nov 2010
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Location: Columbus, Ohio

PostPosted: Tue Aug 11, 2015 7:32 pm    Post subject: Reply with quote

You state you are using a platen that is 26 x 16 and your plastic size is 26x16?
At least in my design, the platen is smaller than the frame holding the plastic, so that the seal comes from the platen making contact with the soft plastic.

If your plastic is 26x16, assuming a 1" grip area; the platen would be closer to 24x14.
That's just a guess what is going on? You can also post pictures of what you have built, that may be of some help. Others might have more ideas. Also, just fyi, HIPS is very inexpensive and easy to form. Might be good for practicing.
Best of luck
Mark
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crashmann
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Joined: 27 Sep 2005
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PostPosted: Tue Aug 11, 2015 11:13 pm    Post subject: Reply with quote

I had a great deal of trouble trying to vacuum form polycarbonate. It is easy to overheat creating in bubbles in the plastic, and the forming time is very small - only a few seconds!

When preparing to make a mold for vacuum forming, one of the recommendations is to take the height of the mold (6.5" in your case) and have that much space extending out from the base of your mold. This provides enough plastic to stretch down the vertical sides of the mold without becoming too thin. Of course, ultimately it depends on the shape of your mold. If it has a natural flowing slope gently rising up from the platen, then this recommendation can be relaxed. However, more vertical sides require more space.

What type of vacuum forming rig did you build? Over under, or flip flop? Or something else? If you could post a picture using Photobucket with the [img] and [/img] BBCode, then we can provide more advice.

Charlie
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mojomandan
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Joined: 10 Aug 2015
Posts: 7
Location: South Jersey

PostPosted: Wed Aug 12, 2015 12:01 pm    Post subject: Reply with quote

IMMark wrote:
You state you are using a platen that is 26 x 16 and your plastic size is 26x16?
At least in my design, the platen is smaller than the frame holding the plastic, so that the seal comes from the platen making contact with the soft plastic.

If your plastic is 26x16, assuming a 1" grip area; the platen would be closer to 24x14.
That's just a guess what is going on? You can also post pictures of what you have built, that may be of some help. Others might have more ideas. Also, just fyi, HIPS is very inexpensive and easy to form. Might be good for practicing.
Best of luck
Mark


Hi Mark,

Thank you for your response! I am sorry, but I was too general when I listed the sizes of my plastic frame and plastic sheet size. The plastic sheet and platen are 16” x 26” and my plastic holding frame is 16 Ύ” x 27”. The frame is large enough to firmly hold the plastic when I use 1 Ό” screws spaced every 2”.

I will post pics soon.


Last edited by mojomandan on Wed Aug 12, 2015 12:38 pm; edited 1 time in total
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mojomandan
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Joined: 10 Aug 2015
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PostPosted: Wed Aug 12, 2015 12:03 pm    Post subject: Reply with quote

crashmann wrote:
I had a great deal of trouble trying to vacuum form polycarbonate. It is easy to overheat creating in bubbles in the plastic, and the forming time is very small - only a few seconds!

When preparing to make a mold for vacuum forming, one of the recommendations is to take the height of the mold (6.5" in your case) and have that much space extending out from the base of your mold. This provides enough plastic to stretch down the vertical sides of the mold without becoming too thin. Of course, ultimately it depends on the shape of your mold. If it has a natural flowing slope gently rising up from the platen, then this recommendation can be relaxed. However, more vertical sides require more space.

What type of vacuum forming rig did you build? Over under, or flip flop? Or something else? If you could post a picture using Photobucket with the [img] and [/img] BBCode, then we can provide more advice.

Charlie


Hi Charlie,

Thank you for your response!

Regarding the size of my platen, I did think to make it considerably larger like you were suggesting (to minimize the amount the plastic had to stretch thin) until I saw the vids showing someone with a small platen and successful pulls with thick polycarbonate. His platen is barely longer than the mold! Here is a link to that Youtube video:

https://www.youtube.com/watch?v=MVISj9DT1sQ

I will have to create a Photobucket account to post my pics, which I will do shortly.

Dan
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mojomandan
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PostPosted: Wed Aug 12, 2015 12:15 pm    Post subject: Reply with quote

This shows the mold sitting on the platen with the plastic from my last failure. You might be able to see the bottom edge of the plastic does not reach the rubber seal (I could not get it to stretch any more).


[/URL]

Below shows my vac rig: the mold on the platen, and the plastic holding frame down on the seal (no plastic mounted on the frame).

[/URL]
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crashmann
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PostPosted: Thu Aug 13, 2015 10:16 pm    Post subject: Reply with quote

Wow, I don't know how the guy in the YouTube video is getting such good pulls using a kitchen oven, a shop vac with .080 polycarbonate, and so little space around the perimeter!

Using the kitchen oven, you lose precious seconds moving from the oven to your forming platen. The window in which you can shape the plastic is shrinking as you walk across the floor and position the frame.

A shop vac will pull 5 to 6 inches of mercury. It moves a great volume of air, but the strength of the vacuum is what pulls the plastic down on the mold and makes the details come through. To get a better seal, place rubber gasket around the perimeter of the top of your platen.

If you could make your forming surface bigger, so there's more space around the mold, and therefore more plastic to work with, you will get much better results.

Good luck!

Charlie
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mojomandan
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PostPosted: Fri Aug 14, 2015 9:23 am    Post subject: Reply with quote

crashmann wrote:


When preparing to make a mold for vacuum forming, one of the recommendations is to take the height of the mold (6.5" in your case) and have that much space extending out from the base of your mold. This provides enough plastic to stretch down the vertical sides of the mold without becoming too thin.


So, if my mold is 12" wide and 22" long, should my platen (and plastic) be 18.5" wide and 28.5" long (add 6.5" total to length and to width), or 25" wide and 35" long? (add 6.5" to the front, back, left side and right side).
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spektr
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Posts: 425

PostPosted: Fri Aug 14, 2015 12:51 pm    Post subject: MOLD DESIGN. Reply with quote

Nice Mold.....
I do see a few issues with it. Expecting the plastic to pull all the way to the bottom of the mold and turn a 90 degree corner is a bit optimistic. If it was mine, I'd make it at least 1 inch taller by adding a spacer under it that is cut at a 45 degree angle. It is so much easier for the plastic to turn that corner. The plastic sheet itself is too small. You need enough material to get the job done and I'd bet that 4 inches around the base of the mold, after adding the 45 degree spacer would work. Trying to draw that much air under the tool using a single hole platen is in my opinion foolish. Far better to drill a large number of #21 holes all around the perimeter of the tool bade and swiss cheese the base with #21 holes on about 3/8 ctc spacing. the object of the game is to move a shitload of air really fact and pin the plastic to the tool while it is still in the plastic deformable heat range, not the cooler stretchable phase. Also, you might want to drill 3 or 4 #60 holes in every surface depression in the tool and vent them to the underside of the tool. Once the sheet seals to the tool, any undercuts cannot pull down unless there is a vacuum port path available. The guy pulling the 080 for a car body is using that because he is suffering from such severe thinning on the sides due to overstretching. The thickness og the sheet is giving him a little bit od "grace" in the process because it cools slower than .032 would. The sheet shutoff point also can be moved up off the plenum plane bu as much as 4 inches adding a 60 degree ramp on the inside of the mold. This makes the part stretch less and gives you a more uniform part thickness. I know this is a lot to get in 1 note, so if you need to expand on anything here, just ask away.....

Scott.
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mojomandan
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PostPosted: Fri Aug 14, 2015 12:59 pm    Post subject: Reply with quote

Hi Scott,

The advice you are giving may be more appropriate for any problems AFTER I have created a vacuum seal.

Please read the original post...I am not able to get my plastic down over the mold to create a vacuum seal and form the plastic. Once I solve this problem, then I will consider your other recommendations.

If its a matter of using a larger platen and plastic sheet, then I suppose I will have to do that as step 1. Though, to reiterate what I stated before, I have seen numerous videos where people are using a plastic sheet barely larger than their mold to create RC bodies and are successful. Their molds are VERY similar in dimension to mine.

I was hoping to have the same success as those people.
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spektr
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PostPosted: Fri Aug 14, 2015 1:11 pm    Post subject: Reply with quote

I don't think you understood what I suggested.
The reason his part "works" is because he is accepting a huge variation in sheet thickness inside the part.
Ditch the platen and make a dedicated mold base.
You have to move a ton of air quickly to do thin sheet.
Move the seal plane up to minimize stretching.
Vent void areas to the mold base to improve surface details.

Making parts is my day job and I hope you will reconsider.
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mojomandan
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PostPosted: Fri Aug 14, 2015 5:08 pm    Post subject: Reply with quote

Hi Scott,

Ok, I guess I have no choice but to try your suggestions. By "tool", do you mean mold? If so, as I understand it, I need to:

1. create a mold base with a 45 degree cut all around. Should it be bigger than the bottom dimensions of the mold, or the same size? Do I drill holes in this base, or just my platen? Your use of terms like "tool" and "base" are a bit confusing to me.

2. increase my platen and plastic size. You mention 4"....does that mean add 4" to each side of my mold (that is, 4" left, 4" right, 4" to the rear, and 4" to the front?)

3. drill about 4 #60 holes in depressions of my tool (does that mean mold?). I will need to buy a 6" #60 drill bit.

By the way, the guy pulling .080" polycarb stood on one of the bodies he made in a separate video. Would that suggest his finished product had fairly uniform thickness? Supporting the weight of an adult can't be easy for one of those RC bodies.
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spektr
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PostPosted: Fri Aug 14, 2015 10:21 pm    Post subject: Reply with quote

My wifes off to the airport tomorrow, over the weekend. Ill shoot a few pictures for you, it gets real clear that way. As far as a 6 inch #60, 1 inch will be long enough. I'll show you how.
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