Nov 07 2005

A Universal Method of Troubleshooting

Published by at 5:28 pm under Uncategorized

There actually is a common method to troubleshoot ALL molding defects.  It isn’t theoretical, it doesn’t require a degree in plastics.  You don’t need a computer.  This only requires some simple steps and knowing how to ask the questions.  Before you begin knob twisting and the usual fiddling with the controls, you go through your mind of all the places to look and given yourself the reason (or lack of one) that would cause this defect.  As you look at the causes you check to see if the process/machine/tool has somehow wandered into the place where your reason fits.  Now you can begin your adjustments.

Here is an example or this thinking.  It’s universal.  Try it, youll like it.
I was recently discussing flash with a guy.  He whipped out his calculator did some serious finger thumping then announced flash was not possible with the conditions at hand. I hasten to say he had a degree in PLASTICS from a snooty East Coast university. Then I showed him the part complete with flash. Kinda funny. All defects stem from a cause.  The Fickle Faeries of Fate don’t just come down a curse the mold. Flash is probably the simplest and most common defect to understand. Molded parts are actually ‘created’ by the concept of flash:  Molten material got into an area, cooled and became solid before ejection. More simply put  if there’s a hole and the liquid plastic can get in it, it will.  Let’s go through the various areas of mold to see if we can highlight some of the causes: ENVIRONMENT- this is the plant’s cleanliness, temperature, operating procedures or something else.  The only cause of flash that can be considered in this section is some kind of mechanical blockage.  I’ve actually seen screwdrivers etc. left in the mold when it closed.  The next shot had flash. The next day the employee head count had one less person.

Operating procedures are always a cause of every defect.  You MUST show the plastic an acceptable set of conditions, the same way, for every shot if want consistent good parts.  These must be universal conditions. Understood and agreed to by every technician.  Look at a molding machine:  Megawatt heaters, a couple inches of steel from the outside of the barrel to the screw, an excessive amount of pressure to drive the screw forward, material shooting into a mold at such enormous speeds and pressures that unless we wrap the cavities in at least a couple inches of steel the cavities will crack.  Now, with that said . . . . Why should every shift have to change operating conditions because the plant cooled down and night and warmed up during the day?

Catch a clue – The plastic is too dumb and it’s too dark in the machine and mold for it to know day from night.  However setting process conditions so close to the ‘edge’ that a few degrees difference in the plant temperature causes changes OR a burned out heater is compensated for by over heating adjacent heaters OR every tech hooks up water lines differently OR the plant cooling water (the infamous and ever-variable ‘tower water’ that is sensitive to how machine machines are hooked into the system as well as the differential from the plant temperature to the evaporative ability of the tower) changes by a few degrees from day to night  YOU’LL NEVER GET A CONSISTENT PROCESS.

Another classic cause of techs changing process conditions is the plant temperature. During the day it’s hot.  They move in these 36″ fans to keep the operator cool. Unfortunately they also cool down the barrel and overheat the material.  At night the fans are shut off, the high settings must be lowered.  And so the battle goes.

If you have this degree of variability in your plant, don’t read any further.  You’ll cry and get a headache  Fix this most basic problem first.

THE EQUIPMENT – This has two sub categories: the machinery and the mold.
1.    The MACHINE –
a.    Pressure is resistance to force If the force on the plastic is greater than the force holding the platens together the mold is ‘blown’ open. This can occur through leaky valves, no ability to maintain pressure heads etc.  This requires a maintenance solution.
b.    Using the principle in #1 if the platens aren’t square in relationship to each other, one side of the mold will close up very firmly while the other will be lose. This too can cause a drop of clamp pressure. This requires a maintenance solution.
c.    Over tightening bolts in the mold clamps will begin to pull the threads out of the platen. This will leave high spots on the platen.  While the actual platens might be square to each other the high spots will not allow the mold to close properly. This requires a maintenance solution: check each time a mold is hung by scraping the platen with a straight-edge.  Any high spots encountered should be stoned flat.
d.    While this can be either a machine or mold cause look at the two temperatures of each half.  If they are more than 10 degrees F from each other consider the hot half is usually the cavity holding the guide pins and the cold half is usually the ejector side holding the ejector bushings. Cool down the bushings and heat up the pins at just before full line-up it is possible to keep the mold from closing because the pins jam into the holes without full engagement.
e.    Can the machine tonnage generate 3.5 tons per square in of projected area for the mold face?  If not, use a larger machine the machine can’t generate enough clamp pressure.
f.    The wrong side of the 80/20 rule. The machine shot capacity should be between 80-20% of the required shot.  If it is lower than 20% it is like shooting rabbits with an elephant gun – too much power and no control.  Use a machine with a smaller barrel. If it is over 80% you won’t get a consistent melt.  Shooting a Kodiak bear with a .22 caliber.  It only makes him mad.

2. The Mold
A.    Components
1.    part?
2.    runner?
3.    An artificial vent i.e. a paperclip, duct tape, metal shim etc. etc.?
B.    Look at the front and back sides of the mold before clamping it in. Are there any loose bolt heads stopping the mold from hanging square?
C.    Dumb checkup: Use a torque wrench and tighten all the bolts on both halves of the mold to the same amount.  Different torque can cause the mold to bend.
D.    If this has been a continuing problem, look at the ejector system.  Are there enough pillar supports so that the ejector plates aren’t bending under the pressure of injection.
E.    Are the ejector half holding plates thick enough to not bend? Do they have enough bolts holding them together?
F.    Have you used Prussian Blue  (if you don’t know what this is ask a mold maker), put it on one half of the mold then under full clamp pressure checked to see if every cavity is shutting off?  If not, machine away material beyond the shutoff to increase the effective tonnage as well as reworking it.
G.    Is the mold shutting off square?  If not, why not?  Fix it.
H.    With a flashlight look into the leader pin holes.  Remove all the crushed, impacted pellets/parts/crud that lives in the bottom of the hole.  If one or more holes are filled with something, the mold can’t close regardless of pressure.
I.    The most obvious cause of flash – Plastic always fills a cavity.  Did somebody close up on something? OR did they simply smoosh the mold on a part and bent or damaged something.  Most operators/techs know when this is done intentionally it’s an easy ticket to the parking lot and a career move to the fast food industry.  They’d rather blame it on a Freak Occurrence. Regardless of who the Freak is, fix the mold.

PROCESS VARIABLES
1.    HEAT
A.    Check the mold heat as described above.
B.    Material heat has little to do with flash.
C.    Mold heat has little to do with flash, EXCEPT in the extreme case of heating the side with the guide pins and cooling the side with the bushings.  Since these have a precision fit when almost closed if the pin is heated (expanded) and the hold is cooled (contracted) if it hasn’t seized up already the mold will probably not fully close.
PRESSURE
1.    Pressure only blows the mold open after it is filled. If several cavities are filling early, they have the possibility of blowing the mold open. If you try to put too much material under high pressure you can also blow the mold open. Fill pressure and Fill time are related.
2.    Fill pressure is really only used to get the material moving.  Once moving its viscosity drops to where the machine usually can’t keep up with it.  However you can overdo it.
3.    If packing pressure is overly large you can blow the mold open and create flash.
TIME
1.    Fill time should be the least amount until 95-98% of the mold is filled.  The last 2-5% of the time will be consumed from going from fill to pack.  If you slam fast moving, low viscosity melt into a full mold it is relative easy to flash it.
2.    Packing time should only be maintained until the gate freezes.t should have little to do with flashing.  Cooling times never flash anything.
SPEED
1.    The speed of fill is what allows the viscosity to drop.  Fast fills coupled with adequate venting and appropriate melt temperatures fill molds and do not flash them. Switching to packing mode slows the speed, thereby increasing the viscosity thereby reducing the possibility of flash.  Like having a pickup truck full of bricks going 60 miles per hour coming up to a stop sign. The whole trick is to know when to put on the brakes.
2.    Heat is a minor component of inherent viscosity.  Therefore the speed of screw recovery (also back pressure) should be considered.  In an ideal process, the screw stops turning and is fully decompressed almost exactly when the mold opens.
POSITION
1.    This is a combination of both machine and process maintenance.  The screw must return to the same spot every time. (see above) Thinking differently, the cushion must be maintained.  Cushions are designed to pressurize the plastic in the cavities to offset the shrinkage of cooling material.  However trying to put 10 quarts of anything into a gallon bucket is overdoing it.  Cushions too large don’t transmit pressure to the cavity, too small and there’s not enough material to transmit pressure.  Find the balance.  The amount of cushion will be determined by the return position of the screw.  Since most processes are ‘switch on position’ if the screw picked up too much material the mold would have already been filled before it switched to packing mode.  Maintain the machine, monitor the process.
2.    The Full Open position should be the minimum space allowable to eject the shot.  However when a fat setup tech gets in between the platens to dig out a stuck part he usually doesn’t reset the platens.  Position is proportional to speed and time. More distance changes the cycle time, that changes the temperature, that changes the viscosity that could be a cause of flash.

Well be visiting several of the common defects in the coming months.This all is described in TROUBLESHOOTING INJECTION MOLDED PARTS sold in the bookstore.  If you simply run through this technique:
1.    Variables
a.    Environment
b.    Machine
c.    Mold
d.    Heat
e.    Pressure
f.    Time
g.    Speed
h.    Position
2.    Find the Cause
3.    Remove the cause either by changing the machine, mold, or process:
You’ll usually solve the problem.  Remember causes A, B, and C require ‘hard ‘or permanent solutions.  This means you permanently change something (a tooling or machine correction):’ A one time expense.  Causes D, E, F, G, and H usually use ‘soft’ or process solutions.  This means every time you run the mold you have to remember and implement this little tweak.  Soft solutions are the most expensive and scrap prone. It is really your choice – do two setups and call me in the morning.

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