posted April 01, 2003 01:23 PM               

Some of the "newer" guys are saying that to properly measure a feedscrew or barrel, that they need to be up to temperature, like 500 degrees F.

To which I say, when the manufacturer ships them, they document them at "ambient" temperature so the readings will be different.

Which do you think is best?

posted April 02, 2003 10:05 PM               

I guess we all thought you were baiting us on April fools day.

I assume your only refering to barrels and screws.

As far as I know, all parts are drawn and made to dimension at room temperature conditions.

The times you get into real trouble are:

1) When the potential for big differences in temperature are not accounted for in design. Example; A full diameter hot screw in a cold feed throat.

2) When there are big differences in thermal expansion rates between the screw and barrel metal of construction.

3) When there is much higher or lower thermal conductivity in the screw than the barrel.

There have been times when combinations of the above have caused real problems.

Likely the issue being refered to is measuring the barrel alignment. Using laser measurment, all kinds of effects can be seen as the barrel heats up. There is an interaction between the barrel, gear box mounting, barrels supports, and heater efficiency. Barrels can take on a curve like a bannana which will not be seen at room temperature. I have seen some serious wear problems until this effect was found. The real problem was a dead heater half near a vent port.

Hope this helps.

Tom Cunningham
PlastxAlliance

posted April 03, 2003 12:00 PM               

Some quick background here. I took over the molding operations just about a year ago and have been trying to get the shop on track. (I hesitate to use the over used term of World Class)

So the issue at hand is at what temperature do we check them? I implemented a program whereby we will pull the feed screw and check it and the barrel for size, to establish wear rates.

Some of my newer engineers are telling me that we need to do it at 500 deg. F, to which I say Poppycock!

They brought up the argument about thermal expansion, and I replied that the manufacturer would take that into consideration when building them.

So what is the answer - ambient or elevated temperatures.

I know what side of the fence I'm on, but as an old dog, I'm open for suggestions to newer ideas.

posted April 03, 2003 01:16 PM               

If the screw and barrels were 2 1/2" or less I would measure them cold and never look back. The issue is the gap between the screw and the barrel. That can change with a number of factors as described. The screw diameter increase and barrel diameter increase at temperature can be calculated to fair reliability. Any interference or large gap problems can be calculated. What else is can anyone be concerned about?

What I don't know about is the end of screw valves and such on IM screws.

If the barrel is larger than 2 1/2" having a laser alignment at temperature might be worthwile, but it is quite expensive.

posted April 04, 2003 08:48 AM               

IM screws are a bit different. I've been amazed at the lack of concern about screw design for specific materials in the injection molding industry. In extrusion, as we all know, the feed screw is the heart of the system. Tooling and downstream equipment can be "spot on" but if the right screw isn't in the machine, quality suffers.

In fact, Spirex has a full page ad running on just that subject. To paraphrase the ad, they say " a new approach to Plasticating process" and say things like "a melt processing system when properly designed will increase revenues, quality and profitability"

What a novel concept!

posted April 04, 2003 09:45 AM               

Interestingly enough SPR's extrusion simulation program that I use and represent "Extrud 2000" has a version that is able to simulate the plasticating side of an injection molding machine. Some of the big IM machine makers use it to design screws. It is not popular with end users though.

It is able to help with cycle and shot size matching, adjusting melt temperature, solving feeding problems, predicting plasticating capability as the screw moves and other things.

posted April 04, 2003 11:58 AM               

How does the program deal with the fact that the molding process is intermittent?

The holy Grail of extrusion is to reach a "steady state" of operation, screw speed, temperatures, residence time etc with plastic moving at a constant speed and never stopping?

Just curious . . .

posted April 04, 2003 01:00 PM               

There is some compensation for hold time and such. Its been a liitle while since I've run the IM version. I'll report on the details.

posted April 10, 2003 06:15 AM               

So which way did you decide to go with the barrel & screw measurement? We are pulling the screw from our 1650 Husky at Easter and we will be measuring at ambient.
Perhaps if these 'new' blokes are so insistent that the screw & barrel be measured at 500degF, you could offer them the opportunity to get in there and measure it themselves. I'm sure it wouldn't take long for them to agree that measurement at ambient would be satisfactory (and a lot more comfortable!)

posted April 10, 2003 07:34 AM               

Measuring a hot screw? Well, I did not bother, as it turned out ot be a worn and severly galled barrel. However, I used the guidleines once published by SPE and ordered a new screw as well.

posted April 10, 2003 08:32 AM               

Guess you could say that I have not been "sold" on the benefits. The argument that a given temerature is the right temperature doesnt hold up well in my mind as the screw will lose heat fast once pulled.

Further, I'd be willing to bet that at 500 degrees with the screw out, compared to 500 degrees with the screw in, the barrel temperatures would be different.

posted April 10, 2003 10:32 PM               

I have also used a light source at the opposite end of the barrel to highlight any damage on the barrel bore but without the slug-on-a-string. What was the purpose of drawing the metal slug down the bore of the barrel? Was it to cut down excess reflected light? Or were you looking at the "corona" around the slug to get an idea of clearance between the slug & the barrel?

Regards,
Rod.

posted May 14, 2003 04:34 PM               

quote:

---

Originally posted by Xavier:
Gentlemen,
If I may be so bold as to apply the principles applied in metrology labs, measure at room temperature. This is how all the measuring equipment is calibrated and components are measured regardless of the application temperature.
Does an auto mechanic or a Jet engine mechanic measure the components of an engine at operating temp? NO. Why should the screw and barrel be any different?
How about the mold? I sincerely hope your tool room isn't running a thermolator in the mold prior to measuring the cavities and cores to determine wear in the steel.

---

GOOD ANSWER!

I will use that logic when I "convince" them, that ambient is the way to go.

Come to thnk of it, aren't some micrometers equipped with insulators so that the temperature of the operator's hand does not cause expansion and thus a faulty reading?

:0