Here are the responses to why my TPE bubbles.
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Thanks to all.
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James Cobb C.O.
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There is a couple of different factors involved in eliminating the = bubbles.
First of all you should pre heat the TPE in a different oven (if at all
possible) on a temperature of about 100 to 150 degrees for about 1 to 1 =
1/2
hours. Then go straight into your infrared oven at 350 degrees. The = reason for this is TPE gathers moisture very easily and you have to preheat it = to cook it out.
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My experience is mostly with polyethylene, but perhaps it is applicable = to your problem.
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Bubbling is caused by expansion of the material as it is heated. Where = it can slide along the tray, it doesn’t bubble. Where it sticks, a bubble = forms as material that can slide pushes into a “blocked” area – blocked, that = is, by the sticking boundary.
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So the first solution to bubbling is to make the tray slippery. A teflon-coated tray would be nice, and we use one, but even so, you need = to liberally spray with silicone.
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Next, if you do get bubbles, you need to remove the plastic from the = oven about three minutes before it is done, lift each end carefully to break = the adhesion around the bubble and let the plastic back down to the surface = of the tray, then put the tray back in the oven. (We take this opportunity = to reverse the tray, although that is more for even heating than to counter = a bubble problem.)
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The reason your TPE is bubbling is because this material is hygroscopic =
– it
absorbs moisture from the air. You should try to store this material in = a cool, dry place, preferably bagged. However, if it has been subjected = to high humidity (which can happen @ the distributors location and/or in shipment, it can be predried before use by heating it for about 3 hours = in a 150 degree oven. However, one solution if plausible in your lab, is to store it on top of your oven so that the heat from the oven keeps it = dried out and ready for immediate use. =20
If you have other questions, please free to call me.
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According to the plastic extruder we use, TPE is hydroscopic ( it = absorbs moisture). If you can’t store it in a cool dry place then you might have = to dry it before actualy thermoforming with it. We have to dry our TPE for = at least 3 hours at 150 degrees. Thermoforming is done at 370 degrees. Hope this info helps.
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TPE needs to be dessicated like lexan, either keep it sealed in a bag, = or just buy fresh every time you need it. if you have to dessicate it you = can do it by putting it in the oven at 250 and raising the temp 10 or 20 = degrees every fifteen minutes until you get to 350. at that point you should be = able to form it or hold it at 350 just about forever with no bubbles.
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Need to dry out the TPE — it absorbs moisture from the environment = fairly rapidly — this causes bubbles when heating/vac forming. Can dry it out easily by placing in warm oven for a while (call plastic supplier for procedure — I forget details)
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TPE has a tendency to absorb moisture out of the air, and is therefore affected by humidity. We noticed no problem at all when draping in the winter time (dry Canadian winter), but had troubles at other times of = the year when the humidity was greater, and when the TPE had been sitting = out for a long time. We contacted our supplier and they sent some info from = the manufacturer. It recommended ‘drying’ the TPE by placing in the oven = for a number of hours at temperature well below molding temperature. What = happens is that the moisture trapped in the TPE expands and tries to escape when = the plastic is heated, causing the dreaded “orange peel” bubbling. I am = sorry, I am not at my office right now, but if you want to email me on Tuesday = (May
25th) I would be glad to email or fax back the information that we had received.
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TPE absorbs moisture from the air and when you heat it, the moisture = turns to steam and the bubbles result. There are two solutions:
1) Purchase your plastic from LIGON Bros. in = Michigan.
they are an extruder and wrap their TPE in plastic to prevent the = absorption of moisture.
2) You can cut your sheet to size and place it in the oven for about 20 minutes per side at a temp of 200-220 degrees.
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