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View Full Version : Reparing A Injection Runner Nozzle *WARNING ACTUAL MACHINING QUESTION WARNING*



Liger Zero
12-01-2010, 01:07 PM
Ok, the material flows through the injection system, into the cold runner manifold and the flow splits eight times and fills six cavities.

Due to a combination of factors, some material bypasses the nozzle and ends up "stuck" where it should not be! This causes the sprue to stick, and the part to tear.

I think at this point it's a question of wear on the original set of nozzles, so I am going to have a toolmaker make a new set... eventually.

I got a couple of spares to play with... I have two ideas.

1: Build up a bead of weld around the outside and grind it into final shape. Only thing I'm not sure of... thermal expansion of dissimilar materials... I'd have to find a wire/rod that matches my tool-steel fairly close.

2: Machine a groove into the nozzle and fit some kind of ring there... I see various fiber, non-fiber, and metallic options for doing this.

Of the two options I think #2 would be best right now because I can do that on my lathe in about an hour. Question is... I'm thinking a brass ring. However, again I'm not sure how much the material is going to expand when heated.

Can anyone think of a third option? I'm open to suggestions.


http://i1198.photobucket.com/albums/aa443/Moar_Work/NozzStickingIssue002.jpg

http://i1198.photobucket.com/albums/aa443/Moar_Work/NozzStickingIssue001.jpg

http://i1198.photobucket.com/albums/aa443/Moar_Work/NozzStickingIssue003.jpg

JoeCB
12-01-2010, 01:19 PM
I have no expierence in this area but I agree a metalic ring seal would be the fix... I vote for copper, how about a ring cut from a piece of copper tubing of the correct diameter.

Joe B

BillDaCatt
12-01-2010, 01:25 PM
I'm not very familiar with injection molding but I would think that if the plastic is seeping out around the injector that the nozzles are not seating tightly to the mold. So it would seem that you need to look at your mold set up or the mold itself and verify that all of the nozzles seat into/onto the mold equally.

The nozzles are threaded, are some shorter than the others or are they all the same size?

Toolguy
12-01-2010, 01:28 PM
An Aluminum Bronze ring would probably wear a long time. Also, you can get A2 or D2 tig welding rod.

Mike of the North
12-01-2010, 01:56 PM
From what I know about injection molding, rubber is very watery, making a tight seal very important. With the drop not being on the part means it does not have to look pretty, it needs to just function. A soft metal ring that can be pressed in should work.

JeffKranz
12-01-2010, 01:58 PM
Well, I not sure what you are asking but if this runner system and the puck under the drop is all scrap you have an option of peening (I think that is the correct word) of the mold steel around this nozzle tip to reduce the gap between the nozzle and the mold steel. This distance should be only about .001 gap or smaller. There must be some gap since when this tip heats up it needs some room to expand. This gap is really dependent on what resin you are using since some resin flashes at a very small gap.

The correct fix would be to either replace the nozzle tip or weld and re-cut the mold steel. Normally you check them both and see what is incorrect and fix that.

gary350
12-01-2010, 02:00 PM
Call DME in Michigan they sell dies, unit holders, sprue male and female and everything you need. Water cooling in the sprue makes a world of difference. Sprues last a long time and if one goes bad you can usually polish it with a die grinder and get a few more months of service. Then when it is time to change it, it is only a 1 hour job. I have been getting several years out of each sprue on the 11 machines that we have. Usually only the female half goes bad. The water lines sometimes plug up after 5 years on the male half run some acid through it and its as good as new.

Liger Zero
12-01-2010, 03:05 PM
This is thermoset rubber, the stock temp doesn't exceed 200 degrees F, it only flows under pressure.

The cold runner is maintained at 190F and the mold sections are heated to 375 to cure the rubber.

There is a thread on here where I was asking about mold insulation not that long ago.

Anyway, these screw up into the mold into the cold runner. Heat is being conducted through them from the mold up into the body of the cold runner.

I talked with a rubber molding expert and he agrees these nozzles need to seal tighter. He suggests that I replace them (naturally) using his company. :D

They ain't no good until they are reduced to swarf on the lathe or melted into slag by my welder. Replace them.. HA!

Liger Zero
12-01-2010, 06:49 PM
I'm not very familiar with injection molding but I would think that if the plastic is seeping out around the injector that the nozzles are not seating tightly to the mold. So it would seem that you need to look at your mold set up or the mold itself and verify that all of the nozzles seat into/onto the mold equally.

The nozzles are threaded, are some shorter than the others or are they all the same size?

All the same length.

There is an orifice inside. I actually have 16 of them per mold (plus a few "extra"), two different orifices. Depending on how the rubber flows through the manifold I change the nozzles to balance the flow.

It looks like the previous owner or one of his "talented" assistants took a wire-wheel to them.

Tomorrow I am going to chuck one in Mr. Lathe and see what I can come up with. Going to try the copper-pipe idea first since I can get that first thing in the morning.

macona
12-01-2010, 08:00 PM
I think you could just weld it up (Tig) and reshape. Even if you have to redo it every once in a while as it wears it will work better than anything non-ferrous. Just about any standard filler rod will work.

-Jerry

Waterlogged
12-01-2010, 08:04 PM
If you had asked about your cat, dog, computer, favorite lathe color, how to reside your house, or any of the normal conspiracy theories, I might have been able to offer advise. But a machining related question........what's that?

kf2qd
12-01-2010, 08:18 PM
How much pressure are these pieces under from the injection unit pushing against them? 2 reasons I ask, 1 is because you may need to use some hard filler because these parts will tend to deform under the mechanical forces. 2 if they aren't under a great deal of pressure might it be easier to just remake them? By the time you weld and remachine you might be able to produce a new one and have the old one for a pattern/sample that is still functional.

Liger Zero
12-01-2010, 08:30 PM
Kf2QD.... it's a 600 ton press, and I wish it was 800 tons like the other one. :D

They screw into the cold runner body, full engagement. Its not the material coming down I'm worried about it's the material seeping up between the mold and the nozzle.

I suspect a picture of one of these in it's native habitat is in order.

gda
12-01-2010, 08:42 PM
We micro-weld mold cavities up all the time in pilot molds - and have seen it done in molds that run hundreds of millions of cycles and hold 0.002" tolerance. This is you might not have the equipment and may want to send it out.

boslab
12-02-2010, 08:12 PM
long time ago i used to patch blow moulding dies by silver soldering the nicks and dents and polishing, it worked ok
mark

Liger Zero
12-02-2010, 09:50 PM
Bobslab: Mold is heated to 375 degrees, will there be any issues with that?

boslab
12-03-2010, 06:58 AM
it works well, oxy/acetylene and a bit of flux, bingo, die saved
http://www.jm-metaljoining.com/products-pages.asp?pageid=70&sectionid=3
you can choose a suitable solder for most bits, the dies i used to do were 250 dia ring dies for blown film extruders so finish was vital
regards
mark

Liger Zero
12-03-2010, 07:08 AM
Thanks Bobslab. I'll give the silver solder a try.

Not too worried about finish other than "smooth." Trying to make a runner stop sticking by sealing a gap.

Liger Zero
12-03-2010, 11:47 AM
This is how the noz looks mounted in the machine. I am only showing the mold plate. THAT mounts onto the manifold, and THAT mounts onto an insulator and the whole thing gets held in place with T-bolts and hold-downs.

The rubber comes out of the hex, fills that volume (there is a matching puck space on the bottom and flows out the runner.

The red ring you see denotes where my gap is.

This plate is heated to 375 degrees. The noz itself contacts this plate, and is also screwed into a cold manifold... so there is some serious heat movement here.

http://i1189.photobucket.com/albums/z432/Arrghh/NozInNativeEnviroment.jpg

mc_n_g
12-03-2010, 03:31 PM
We still only do compression rubber molding, no nozzle or runner system like you. The pressure you have on the rubber is very high and I do not know how you could actually 'seal' the plunger tip. The rubber would want to flow to any gap no matter how small as it usually does under the high pressure.
My only suggestions would maybe be copper rings. Unless your plunger is prefectly centered and doesnt flex the rings will wear quickly on one side.

Liger Zero
12-03-2010, 03:40 PM
We still only do compression rubber molding, no nozzle or runner system like you. The pressure you have on the rubber is very high and I do not know how you could actually 'seal' the plunger tip. The rubber would want to flow to any gap no matter how small as it usually does under the high pressure.
My only suggestions would maybe be copper rings. Unless your plunger is prefectly centered and doesnt flex the rings will wear quickly on one side.

Injection, not compression/transfer. :)

This part doesn't move at all. The wear seems to have been induced wire-brushing or other abrasive method.


I have a few transfer molds running on a 300 ton French Oil press, there's no getting around flash with those. Fascinating process... very SLOW though.

boslab
12-03-2010, 06:39 PM
from memory, and it was a long time ago, ive seen similar wear in the sprue and runner on a unhardened die useing regranulated PE and adding schulmans masterbatch too, the white was the worst prob due to TiO2 once we got them nitrided they [the dies] were running 1 x 10>4 parts before they were needing repairs/polish/tlc
all the best
mark

Liger Zero
12-03-2010, 06:49 PM
What I don't get is why they wirebrushed that surface to begin with. That needs to make a tight seal, as the rubber comes out of the hex and fills that puck then shoots down the runner.

The rubber flows up between the nozzles into the space between plates, the first dozen or so shots are underfills until enough builds up to seal everything off.

moldmonkey
12-03-2010, 11:18 PM
I only have experience with hot runner molds running stuff like polypropylene and polyethylene and am still a little unclear on your application without the part cleaned up. I am assuming there is a step in there under all the flash fits into a cbore and that your part screws into one plate without a shoulder to seat against and the other end seals on the next plate. If so, thats a crappy design as it would be real hard to get/maintain the proper stack-up.

Anyway the same principles should apply. In my experience any place where plastic was transferred between two parts (sprue bar/bushing to hot runner; machine barrel to nozzle tip) the standard was .000-.001" stack-up between the parts. In other words the boss was from the same height to .001" taller than the corresponding counter bore was deep. In the case of the hot runner where plastic was transfered between two flat surfaces the stack-up standard between all the heights of all the components and the depths of the pockets in the mold plates was also .000"-001". Heat expansion and being torqued the hell down is also a big factor. In the case of nozzle tips, the final torque was done with everything up to heat.

The seal was all on the faces. Any nicks or worn/hobbed surfaces and it was impossible to stop it from leaking. It was standard practice to clean up the faces and reestablish the stack-up when that happened. I have seen copper rings/washers used in some applications on the presses and it might be the short-term answer to your problem. I would true up the surfaces and start with .003-.005" thicker on the copper washer than the gap. Total WAG on the thickness, some things you just have to experiment with until you get a feel.

As far as the wire brushing that shouldn't be a problem. Virtually every night for the 8 years I worked there, i and everyone else used a wire brush to clean plastic off of runner parts. Metal surfaces wear under the pressures and countless cycles of molding.

Hopefully my jargon is close to yours. I'm my experience, there isn't standard jargon in the plastics industry.

Liger Zero
12-03-2010, 11:44 PM
I understand what you are saying :)



The cold runner manifold splits the flow into eight drops, and the eight drops feed six cavities



* *
--- ---
* *
--- ---
* *
--- ---
* *



Stars being the nozzles I am having trouble with, lines being the cavities.

There are runners from the nozzles to the individual cavities, due to the nature of the part the runners are quite simple, but fairly large in diameter. Each cavity has four gates fed by two runners supplied by two nozzles, each nozzle being fed by a manifold drop.

Got it? :D Edit: (again) I'm going on beer-enhanced memory here, so it's not exactly clear even to me ROTFL a picture sure would help!

There is no seal behind the nozzle. The nozzle rests against a ledge, but there is space between the noz body and the mold plate itself... Wiggle-room and lots of it.

The noz is secured by screwing it into the directly into the runner drop. It's designed this way so I can pull the cavity plate off and change product sizes with minimum downtime.

It looks like someone may have enlarged those holes in the cavity plate. Previous owner admonished me to have another locating-ring made because "you'll have trouble aligning everything and end up crushing it." Well in all the years I've molded I've never crushed a locating ring. Six hours on the lathe now I have three of them just in case he jinxed me. :D

Anyway. He tended to use non-sentient labor supplied by "vocational rehab centers" rather than experienced techs. I could see him enlarging these holes to "make easy" the alignment and prevent damage to the runner drops at the cost of sealing.

I also suspect those nozzles got those grind-marks and flat-spots from ham-handed "rehab workers" trying to clean the caked on release agent with a wire-wheel or worse.

What this is going to come down to, I have a feeling... is the mold has to go out to a rubber-mold specialist to be cleaned up, re-ground/re-trued and new nozzles fabricated.

In the meantime I'm going to try silver-solder on one and make a ring seal on another. I'll keep everyone posted as to my progress.