Apr 02 2007

Got a Screw Loose?

Published by at 5:37 pm under Uncategorized

It’s an interesting question. I get calls from clients about intermittent short shots, inconsistent processing and even ‘micro voids’ or sporadic ‘void clusters’. They’re looking for the quick fix and instant answer.Somehow they think a call to a consultant will cause a magic wand to be waved, a secret spell to be whispered and everything somehow will be fixed.

While the visual of me standing there in a black robe and a pointy hat with sequined stars on it is interesting, it’s also about as silly:  There just aren’t magic cures.  But there is the Magic Question:  “When did you last pull the screw, check for wear, replace the non-return tip, or for that matter clean the darn thing?”  I usually get the thousand yard stare.  We give it a Sanity Check: “Are you crazy Mr. Bill?!!?”  Well the answer is both yes and no.

[Editorial note:  I absolutely hate using technogarble notably the multi-letter acronym that only the ‘in’ crowds can decode.  But the Non-Return Valve system includes slip rings, ball check valves, wipe valves, and other assorted mechanisms.  Thus I’ll use the generic acronym of “NRV” to refer to all of them.]

Let’s look at the facts:  You’ve been running filled materials for months/years.  The fillers could have been talc, fiberglass, the ubiquitous ‘minerals’, or any other such pixie dust your friendly resin supplier has recommended.  You may also have been running the semi-naughty TPU’s or TPR’s that have this nasty problem of burning if you look at them cross-eyed.  Even worse at some point in time you may have forgotten you had PVC, one of the nylons or acetals and ‘accidentally’ overheated them.  Well not you, but perhaps it was Joey the new setup kid, or you just came in one day and the entire plant had a really funky smell to it.

Every molder complains that his mold washes out when abrasive fillers are used for any great length of time, they complain about the yucky varnish they get deposited from the rubbers and urethanes, and they really throw a snit when the entire plant gets smelly due to burned vinyl or the acids released from the nylons or acetals.  But with all this bitching and moaning they forget (because they can’t see it) that all this chemical attack and mechanical abrasion is also affecting the screw, barrel and the non-return valve system (NRV).

I will freely admit the usual excuse I get comes cloaked in the guise of they don’t have to pull the screw or inspect the NRV because of the extensive use of purging compound and other tricks (i.e. powdered detergent, even the exceeding dangerous putting water into the feed throat).  But when asked the Magic Question I get a look from the setup techs like I took their 19 year old daughter out on a date and brought her back drunk with a tattoo in a place that only her mother would see.  Ugly.

Yes I’ll admit pulling a screw is prone to creating some nasty burn scars, almost tipping over the injection unit, and those who’ve never done it before sometimes forget to pull locking rings/collars and other such devices that cause the screw to NOT come free from the injection unit.  However if you dust off and remove the plastic wrapping from the machinery manual they give you a procedure in plane language and simple words that is easy to follow.

The first time a screw is pulled is kinda funny.  This is usually because over the years, the bolts on the end cap have thoroughly seized and there is a lot of rude language used to unscrew them.  Next there is the series of spacers, cut lumber, shims and other things required to push the screw out.  As the manual clearly states:  REMOVE THE SCREW OUT WHEN IT IS WARM.  Also try to have the screw filled with Styrene or one of the Olefins.  A cold screw simply will not move, and having it filled with most other materials makes life more difficult.

With the screw empty, clean it with a wire brush and a couple of rags.  Shine a light down one end and look for scratches.  Do it from both ends.  There’s a nifty but spendy measuring device what will measure the screw bore diameter.  Most of your wear will occur near the nozzle tip, but if you’ve been mashing highly filled material the entire barrel’s diameter might be a tad large.

Go to a site such as SPIREX.COM (the replacement screw folks) and pull up their specs on the size of your particular screw.  If you’re within spec, you’re OK.  If not, think about getting the barrel re-lined or buy a replacement.  While you’re at the site also look at the screw diameter.  Pay close attention to the fact of whether the measurements are at room temperature or at an elevated temperature.  Measuring in thousandths of an inch the bore of a three inch barrel at room temperature is quite different than it is at four hundred degrees F.  Get the measurements for the screw and measure it also.

With the screw warm, the plastic in the flights will usually peel off fairly easily or use a coarse wire brush on the end of a drill. It is considered good form to wear heavy gloves and safety glasses when doing this unless you want to leave some hide on the wall or have your eye poked a few times, dummy.  Examine the flights and screw.  Is the screw pitted and/or the plating chipped? And, wouldn’t it be nice if the flights weren’t chipped? Sometimes it takes a torch to heat the nozzle tip before you can remove it.  Look at the NRV:  Does it look like a hunter’s Dutch Oven with a small but even coat of charcoal on it?

A NRV with a slip ring is the easiest to clean and inspect.  It comes off in three pieces.  With a coarse wire brush on the end of a power drill you can remove all the plastic and inspect it.  If it is in good shape the seat from the slip ring to the retainer will seem to be a line for line fit.  If it’s galled, chipped, or pitted; there’s your problem.  On sliding ring NRV assemblies, many molders install the seats backwards, easy to do on some models. An easy way to check clearance on these is with feeler gages, and NRV in the barrel bore.  The best ball check valves are nothing more than a housing with a ball bearing in them.  They easily disassemble.  Other versions of this song are a ball with pins in the front of the tip and others are spring loaded.  Some of these valves cannot be disassembled or cleaned.  You send them off to the manufacturer and for a few hundred less than a new valve; they’ll clean it for you.  Go figure.  Go dumpster diving in the screw tip catalogs and get their measurements so you can inspect the tip also.

Since you now have the nozzle cap off the front of the machine wire brush it clean and inspect it also for buildup, chips, dings and other evidence of previous disasters.

Screws, barrels, NRVs, and end caps are the life-blood of a molding machine.  The entire mechanism works on the concept of a self-lubricating bearing.  It’s the molten plastic that lubricates the screw and NRV.  If there is too much clearance, the material back flows and you can’t maintain a consistent fill rate or packing pressure.  Dirty non-return tips are the biggest culprit of “Prairie Dog” defects that pop up in the form of the occasional short shot then instantly disappear because the check valve sticks in one position then works itself loose.

Got black specs?  In this scenario they come from little bits of carbonized resin that got stuck in a pit, ding, or a dead spot somewhere and occasionally have the bad taste to come loose and end up (as clearly stated in Murphy’s Laws) in the most highly cosmetic section of an expensive part.

Mythbuster: For what it’s worth, having a few end caps and mating non-return tips on the shelf are the best insurance you can have for consistent processing regardless of whether you do it for maintenance reasons or your Competitive Edge.  A NRV and end cap are actually both a mated pair AND are material specific.  Some of the snazzy NRV systems actually create so much shear they burn the material.  Other systems are so ‘loose’ they have a difficult time sealing off the very thin melt materials.  Molders “In the know” will swap out end caps and NRVs depending on the material with the intent of faster cycles and lower scrap.

When you put everything back together here are a few tips:
1.  When you re-assemble the screw to the machine, make sure you don’t have any spare keyway clips, split rings or other things that cause the splines to engage.  Don’t put high temp grease on the splines with a putty knife but an injection unit works much better with a small amount of lube.
2.  Put pipe-fitters high temperature grease on the bolts for the end caps unless you’re interested in trying to unscrew them later using a long length of pipe hoping you won’t break the bolt or bend the Allen Wrench.  Put the bolts in by hand first.  Snug them down with a torque wrench second to the manufacturer’s specs.  The same way you tighten bolts on your tires tighten the bolts on the end cap: 12 O’clock first, 6 O’clock second, 3 O’clock, 9 O’clock then in the same manner the others. When the barrel heats up, everything expands. Recheck the torque.
3.  When you re-mount the injection unit, slowly sneak it up to the mold’s locator ring.  It should hit dead center.  If it isn’t use the jack screws on the injection unit until the alignment is perfect.  Repeat – perfect alignment.  Close enough is never good enough.

Checking for lack of wear is usually as simple waiting until you’re running a reasonably rigid material, pulling back the load so that you have a few inches of cushion and turning the packing timer up to 30-45 seconds and watching what the screw does.  Leaking screws will either slowly creep forward or ‘vibrate’ back and forth.  A good seal will show itself by the screw just sitting there under pressure and not moving in any way.  When you see movement 90% of the time your NRV is the leaker.  If you’ve run highly filled materials for months/years on the original screw and barrel, it’s like shooting 100,000 bullets through a rifle.  It has worn out through excessive use.

The next time you quote highly filled materials that have a long life expectancy, or the filled high temperature exotic materials remember you’ll probably wear out a screw/barrel.  You’ll definitely go through a few NRVs.  If this isn’t in your quote, you’ve just thrown a few hundred or thousand dollars of profit into the fireplace.

When in doubt remember the Three Rules of Threes:

The First Rule of Three is Injection Molding requires (1) Technology – the right equipment, (2) Tooling – the proper molds and secondary equipment, and (3) Technique – knowing how to optimize a cycle to get the best profit.

The Second Rule of Three is (1) Training, (2) Training, and (3) Training.  Duh!

The Third Rule of Three is the most forgotten: (1) Maintenance, (2) Maintenance, and (3) Maintenance.  Yes, maintenance costs money.  But the rule of engineering is that if it moves, it wears.  You can have high tech high precision equipment, or high tech pieces of junk.  It’s your money so it’s your choice.

* * * * * * * * *

While I’d like to accept the credit for this article it was done with the keen eye of realistic practicality of  Brent Borgerson of Matrix Tool/Matrix Plastic Products in Wood Dale,  Illinois who read and bled over my drafts like a third grade grammar teacher on a mission.  Brent can be found writing under an assumed name in Injection Mold Magazine’s Forum, and has recently been published twice in the February issue of that magazine.  He’s got a lot on the ball and is probably a definable percentage of Matrix’s profit.

Like everything else I write, this makes good lining for Gerbil nests, or you can print it out and scare middle management with it.

Bill Tobin and Brent Borgerson

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