Hello again,
The difference between your understanding (18 AWG) and my calculation (About 16.5 AWG) was due to fact that I assumed the conductor to be solid and not stranded, hence more copper. I imagine it is 18 AWG if your 7 wire strands have a diameter of (1.2) /3 = 0.4 mm and your 19 wire strands have a diameter of (1.2) / 5 = 0.24 mm.
Now on to your original questions.
Since you are using pressure extrusion, the inside bore of the die defines the outside diameter of the insulation. Increasing the die diameter is not required because you would be giving away plastic for nothing. The diameter of the die is therefore fixed.
Now as far as the tip is concerned, the clearance between the tip and the conductor or core should be sufficiently close enough to hold center without causing wire breaks or jamming of the core. For small stranded conductor, it should be three or four mils (Thousandths of an inch). If you are experiencing high spots on your strand, the issue is at stranding (Tension control, starting and stopping and so on.) and not at insulating. Check by installing a laser diameter on the bare conductor as it enters the cross head or at the stranding machine. Be sure however to protect the laser from damage or destruction if the wire breaks. If you are flying this bare stranded wire off at the extruder, do you have tight control as to the flying direction? I personally prefer live (driven) pay-off reels with no twisting at extrusion.
If your tips are plugging with copper dust, the problem is with the drawing lubricant and I personally would first suspect the emulsifier in the drawing lubricant. If you are having such a technical issue, contact your drawing lubricant supplier immediately. You should also be using deionized water to dilute the drawing lubricant plus you should use clean fresh water in the drawing machine annealer. (A lubricant is not necessary in my experience.) Please do not assume drawing lubricants are low tech nuisance you must have because this is not the case.
I have actually witnessed annealed, preheated, solid copper wire (24, 26 and 28 gauges) broken by high pressure plastic via diameter reduction of the conductor (Plastic pressure say in the area of 7500 psi.) but breaking a larger stranded conductor like yours by having too much clearance in the tip would be very unlikely. There are usually pressure gauges on primary, high speed extrusion lines after the breaker plate to make sure that the tooling in the cross head has been designed and installed correctly. Do you have an accurate procedure to set the position of the tip relative to the die or is this all fixed?
If you use a larger than recommended tip, always keep in mind that the conductor runs on the bottom inside surface of the tip, thus you will have to manually center the insulation by adjusting the position of the die holder (if possible).
This is really all the advice I can give from this side of the World and without inspecting your tooling, seeing the line set up and in operation.
Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212
