Heating element suggestions for line cutter - Page 1
Forum section: electronics / projects

author: Winston
author_rank: Regular Contributor
timestamp: July 22, 2015, 07:02:50 pm
content: I'd appreciate any suggestions for an inexpensive off-the-shelf heating element for a hobby project thermal line cutter. I need very fast (ideally instantaneous, a few seconds at most) heat-up to at least 297 °F (147 °C) using a 3.7 380-500mAh 20C lipo cell.The line to be cut will be 30lb rated Spectra/Dyneema fishing line (it has a considerably lower melting point than the more common nylon monofilament line) which will be under tension and fixed in place (i.e. the line will not be moving over or through the heating zone). Right now, a homemade coil of 24 ga nichrome embedded within high temperature stove/furnace cement is what I plan to try with the line to be cut passing through the coil.Any other ideas like passing the line between opposing through-hole or SMD power resistors or anything else that's off-the-shelf? I've tried passing the line through opposing PWM driven glow plugs which worked great, but the glow plugs had a very limited lifespan.Also, are there any ceramic-like epoxies that don't cost a fortune that could be used in place of the stove/furnace cement?

author: Ian.M
author_rank: Super Contributor
timestamp: July 22, 2015, 07:27:41 pm
content: Commercially available thermal rope cutters use a thin V-notched blade clamped beween two very low resistance posts. The V notch concentrates the heat and thus the resistance in the center of the blade. I would suggest stainless shim stock for the blade to improve its corrosion resistance as it will be exposed to melted plastic.You wont find much in the way of epoxies that can even stand 200 deg C for long. There are a few that can withstand higher temperatures for a short time but the are rather expensive and difficult to obtain.

author: ajb
author_rank: Super Contributor
timestamp: July 22, 2015, 09:21:19 pm
content: 30lb fishing line isn't terribly thick, as cordage goes, so I'd expect that you could get away with nichrome wire like you've got, unless you need something more mechanically robust. Keeping the element small will help keep your heat-up time and power requirements low due to lower mass and surface area, but you'll need to do some testing to ensure that the element once heated has enough thermal mass to cut the line cleanly. This will also be affected by your required cutting frequency.

author: pelud
author_rank: Regular Contributor
timestamp: July 22, 2015, 10:21:01 pm
content: This blade designed for a thermal wire stripper might be of interest to you:http://www.digikey.com/product-detail/en/0010-001/PTS-101-ND/10156My understanding is that the actual stripper has a low voltage transformer in it, and uses only about 7W.http://www.digikey.com/product-detail/en/PTS-10/PTS-10-ND/20493Edit: Too bad digikey doesn't have any of these V shaped ones, they would be better for the line if it's under tension:http://www.digikey.com/product-detail/en/0050-001/PTS-501-ND/102968John

author: penfold
author_rank: Frequent Contributor
timestamp: July 23, 2015, 08:20:15 am
content: Piano wire is great for this application, with even only a little bit of tension its very stiff and its available in quite narrow gaugues, you just have to watch out for the temperature coeficient of resistance with it being steel

author: Winston
author_rank: Regular Contributor
timestamp: July 23, 2015, 04:32:54 pm
content: Many thanks for the great suggestions, but I think I've found exactly what I need thanks to e-cigarettes. I read in various eBay ads that these are around 1.8 to 2 ohms, 2A or less thus required from the 3.7 lipo which could easily handle far more. The coil is wrapped around a wick which I'll remove if it can't handle heating while not wet. Note the grooves in the protective ceramic case which will allow the fishing line to pass in close proximity to the coil. There are also no-wick types which have the heating element behind a metal mesh screen. I'll buy some those, too, just in case the type with a wick doesn't work out for some reason.It'sweirdthat I've been trying to think of something to use for many weeks, have already ordered and had shipped the 24ga nichrome wire for my less than ideal coil idea, finally decide to ask for help here, then immediately find something to use that almost looks like it was made for my application! The Great Electron must have had a hand in this.:http://www.ebay.com/itm/10-Glass-Globe-replacement-COIL-Nail-ATOMIZER-vaporizer-VAPE-PEN-Free-Gift-/181809507301