# impulse heat sealer

Discussion in 'The Projects Forum' started by simf14, Nov 18, 2010.

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1. ### simf14 Thread Starter Active Member

Jul 27, 2007
43
0
Hello Kind Sirs:
I am an absolute beginner and need help designing a circuit that will power 50" of 18g nichrome wire to heat seal polyethylene bags. My understanding is that the wire on typical heat sealer is heated by an impulse of electricity, rather than constantly staying hot. This prevents the bags from melting.

Any help would be so appreciated!!
Thank you
Curtis

Last edited: Nov 18, 2010
2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
What is the resistance of the 50" of wire?

The ones I've worked with would start heating once the lever was moved from straight up, heating through the stroke, and when the conveyor belt pulled it free, the lever was let up.

Unsure if there was temperature compensation on the heated wires or not. One was for sealing, other was to cut about 1/4" beyond at a higher temp to cut.

3. ### simf14 Thread Starter Active Member

Jul 27, 2007
43
0
it has to be done by a timed impulse for precise strength of welding of the plastic. To cut then any constant heat is used.

resistance is .400 ohms per ft of 18g NiCr wire
6.5 amps is needed to heat 18g wire to 600 degrees
http://www.wiretron.com/nicrdat.html

4. ### simf14 Thread Starter Active Member

Jul 27, 2007
43
0
here is a circuit I found on the net but I am not sure how to modify it for my larger and longer heating element.

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5. ### JDT Well-Known Member

Feb 12, 2009
658
85
If what you say is true (sounds right) that the wire has a resistance of 0.4Ω and needs 6.5A of heating current then Ohms law tells us that you need:-
0.4 x 6.5 = 2.6V to drive it. It will need 6.5 x 2.6 = 16.9W of power.

There are various ways of electronically getting this voltage and current but the easiest way is to use a transformer from the mains supply as it is perfectly OK to use AC to heat the wire. You then connect a timer switch in the primary (HV side) of the transformer to control the heating time.

You probably will not easily be able to get a transformer with exactly this specification. (It is possibly to get them specially made - even 1off). But 2.6V is only a few turns of wire on the secondary. You can make or modify your own! You need to find a transformer of about the right power rating - 20VA or more. It needs to have a "split bobbin" where the high-voltage primary and the low voltage secondary are on a separate formers (usually side-by side). Something like this:-
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0504593

Very carefully remove the wire on the secondary side without touching the high-voltage primary. Then wind a few turns of fairly thick insulated copper wire on the empty secondary bobbin so as to get the voltage you need. You might need 5 to 8 turns. Best to use a voltmeter to see what you get. Have the transformer as near as possible to the heater wire to reduce losses in the connections. Use wire about 1mm diameter copper for this current.

Because you have been very careful and not touched or damaged the primary, safety is not affected.

On some transformers there may be enough space to wind on your turns without removing the secondary. In this case just leave to original secondary un-connected.

6. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
Those 20VA transformers are usually wound with 6~9 turns per volt.

For 2.6V, that would require some 20 or more turns.

7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,766
928
Remember to allow for the expansion of the wire at high temp. When the wire gets hot; if you have it drawn taught at room temp, it will sag like a wet diaper when hot.

Spring loaded tension supports are one way to cope.

8. ### Wendy Moderator

Mar 24, 2008
20,764
2,534
You can buy replacement parts for existing sealers. They include two nichrome strips with lugs on the ends and Teflon tape. The commercial models use spring loaded electrical posts as has been suggested.

I would have to say the teflon tape, where the wire sits under, is an absolute necessity. It prevents the molten plastic from sticking.

Wires shaped in strips is also probably needed, not a simple wire.

There is a simple pot on the front to set duration. Basically it is a timer (monostable) triggered by the top handle coming all the way down.

Harbor Freight sells the units very economically.

http://www.harborfreight.com/catalogsearch/result?category=&q=impulse+sealer

So do a lot of other people.

9. ### simf14 Thread Starter Active Member

Jul 27, 2007
43
0
Thank you Bill...what do they say, great minds think alike? Just ordered one and will use all the parts to customize what I want. Thank you

10. ### DarenHeras New Member

Aug 4, 2015
1
0
Thanks Bill for suggestion!!

I have same problem and now it solved.

I bought Gramatech impulse sealer and they also helped me a lot to fix my bug.

Thanks to Bill and gramatech.