Is making a delrin transfer port a bad idea?

[BTDT]

I tried delrin for o ring piston heads and after time it broke up, cant say if this was due to temperature or the impact.

I once deliberately crashed a lump of acetal into a cylinder end cap at 15.51 M/s, Nick, and it did not show any signs of damage. The fastest crash I've recorded on a second forward piston stroke is nearer 9 M/s, so it should withstand one such crash, and certainly a more 'normal' hard landing ~6 M/s, but multiple collisions may cause cumulative damage.

High temperature would I assume soften the surface and make it less prone to breakage, so impact damage seems the more likely culprit.

[rancidtom]

Another thing that may be worth considering is how will the plastic age harden? As we know plastics can become brittle with age and even if the plastic isn't subjected to impact it may become less able to withstand the sudden pressure fluctuations a transfer port has to bear. One good thing is that there is going to be little if any significant exposure to U.V. in this particular application. Prolonged U.V. exposure is death to a great many plastics!
According to DuPont figures:
http://www.dupont.com/content/dam/du...%20Mod%203.pdf

Delrin will withstand 10⁶ cycles of 31+MPa in flexural endurance tests at 23 deg C, it remains to be seen what sort of endurance it has at elevated temps and due to the low thermal conductivity repeated shots may well significantly raise the temp of the surface in contact with hot transfer gases. Will it melt if you try to shoot repetitive shots as fast as you can? How would it affect the long term life rather than just the immediate life of the port? Many questions that can only be answered by testing in the specific application as there are no tests published by the manufacturer that are remotely similar to the conditions found in a compression chamber/transfer port.

[NickG]

Not necessary, Tony; the o rings in fact bear against a shelf in the steel of the end plug, and lockup is unaffected.

My concern now is not whether the Acetal transfer port can withstand the elevated air temperature of the shot cycle, but the elevated air temperature of 10,000 shot cycles.

Jim , its not the o ring seating in the end of the cap that i am on about,in the standard set up the trans port locks against the end of the barrel and also seats the pellet. If you are relying on just the o rings , there is a danger of the cocking shoe breaking when the gun is shot as the comp tube moves too far forward impacting the shoe . I have had this happen when I put too short a trans port in .

[BTDT]

Jim , its not the o ring seating in the end of the cap that i am on about,in the standard set up the trans port locks against the end of the barrel and also seats the pellet. If you are relying on just the o rings , there is a danger of the cocking shoe breaking when the gun is shot as the comp tube moves too far forward impacting the shoe . I have had this happen when I put too short a trans port in .

Thanks for the clarification, Nick, though to me it’s surprising, as I have in the past also fitted, and fairly thoroughly tested, TP inserts that protruded less, with no lockup nor cocking shoe problems.

Also, as a quick experiment, I have just fully closed the breech of a TX (standard TP) with a pellet skirt proud of the breech, and it was still sticking out afterwards, so the TP did not seat it.

In the light of your experience, I’ll stick with the acetal TP and see how it goes. If the worst comes to the worst, I have a spare cocking shoe in the workshop.

[NickG]

Thanks for the clarification, Nick, though to me it’s surprising, as I have in the past also fitted, and fairly thoroughly tested, TP inserts that protruded less, with no lockup nor cocking shoe problems.

Also, as a quick experiment, I have just fully closed the breech of a TX (standard TP) with a pellet skirt proud of the breech, and it was still sticking out afterwards, so the TP did not seat it.

In the light of your experience, I’ll stick with the acetal TP and see how it goes. If the worst comes to the worst, I have a spare cocking shoe in the workshop.

Hi Jim , If its working ok then fine . Re the lock up while developing the 21mm reduced tune , I found that it has been very susceptible to pellet sealing in the barrel, i test by inserting a pellet and looking down the barell to see if there is any daylight showing round the pellet, if seated properly it can gain .5fpe ( that's in my 21mm ) so I now ensure the trans port does the seating for me . It may not be as important in the 25mm as you have far more volume than I am dealing with and can afford to waste some, though it may be worth trying it (especially as you are short stroking) . As an easy test just seat the pellet flush with something flat .

[BTDT]

Hi Jim , If its working ok then fine . Re the lock up while developing the 21mm reduced tune , I found that it has been very susceptible to pellet sealing in the barrel, i test by inserting a pellet and looking down the barell to see if there is any daylight showing round the pellet, if seated properly it can gain .5fpe ( that's in my 21mm ) so I now ensure the trans port does the seating for me . It may not be as important in the 25mm as you have far more volume than I am dealing with and can afford to waste some, though it may be worth trying it (especially as you are short stroking) . As an easy test just seat the pellet flush with something flat .

I suspect your missing 0.5 ft. lb. is connected to the lower air mass, Nick.

Unless I want to test a particular pellet, I feed my Txs a diet of 7.8gn Express and 8.4gn Field, both of which seat just fine with pressure from the pad of my thumb, and I don't think either allow blow-by, but even if they did, it would amount to little air loss because both have very low start pressures (~150psi and ~200psi respectively).

With FTT (~480psi), Superdome (slightly higher) or especially Hobby (600psi), blow by is a more serious issue, because not only will it waste more of the much denser air, but that air will be at a much higher temperature, so the flow will choke at a much higher velocity (approx 1,800fps for the Hobby).