Nov 05 2006

Changing to New Materials and/or New Colors

Published by at 5:57 pm under Uncategorized

Why use that very expensive purging compound?  Isn’t cheaper and easier to run the screw dry then plop in the new material?  Sure you have mixed material parts but this was has got to be cheaper.  WRONG!  There’s a lot more to it than that.

I was at a client’s shop with dozens of large molding machines.  One product came in several colors.  It was common practice to simply let the machine ‘run dry’ then load the new color and mold parts until everything was flushed clean and the new color came through.  They had a few things going in their favor: (1) color match wasn’t particularly an issue; (2) they were using pre-colored material, avoiding the messy-ness of purging out liquid color or color concentrates; (3) this was a color and not a material change; (4) the dimensions from color to color were different but the tolerances were large enough that it didn’t require any process adjustments.  What was fascinating was they were going from a Forest Green to a Fire Engine Red on a large flat part.  They’d run the machine for more than eight hours and you could still see green stripes in the red.  Yes, we all will probably admit that if a material changeover that takes this long is a definite waste of money.

Many company owners get sticker shock when they look at the price of Purging Compound.  Yes, the price per pound is spendy, right up there with the engineering compounds known only by letters. However if you read the instructions it generally takes less than three times the machine’s shot capacity to make a successful changeover.  So it’s not like you have to use an entire 50 lb bag for each 10 ounce machine. Balance this against wasted time and it’s a bargain.

Purging compounds generally come in grades according to temperatures:  A general purpose grade, a high temperature grade and a separate grade designed to purge hot runners.

While it might seem obvious to some, it is amazing to note the ignorance of others on two significant points:  (1) Hot runner mold should NEVER be put into storage when using a material that cannot withstand a high level of heat-soaking.  PVC turns into nasty acid and ugly fumes in a very short period of time.  The nylons/acetals turn into formic acid.  However HDPE or even Styrene can live happily in a runner system and be easily purged out during a start up.  (2) Filled materials such as anything with long fiber fiberglass and such will not only burn in the runner system but also leave clumps of the filler that will clog up the system.  Filling a hot runner system with purging compound before pulling the mold neatly eliminates this problem.  Clumps of filler mixed with burned material have a difficult time going through the machine.  As they do, they abraid everything they touch.

Most molders don’t realize the kind of wear molding highly filled or filled high temperature materials does to their equipment.  They end up paying for a new screw and barrel from premature wear out which is really caused by the customer’s specifying this particular material.  The use of purging compounds reduces wear and extend the life of the equipment.

When I first got my fingernails greasy in molding there was a very risky practice in startups:  The techs would look at the temperature controllers and the minute they were even ‘close’ to the set point temperatures they would begin purging.  With a material like LDPE (which can actually be run with no temperature assistance) nothing happened other than the startups came on line a little faster.  However  if there was glass filled nylon in the barrel and you ignored the fact the temperature readouts were really the temperature of the thermocouples and not the barrel; starting the screw up in the best of all worlds would either stall the motor or break an internal sheer pin.  Usually either the screw broke or the drive motor splines broke causing a lot of down time at a huge cost.

The machine companies quickly saw this as a problem and required not only (1) that the heaters were at a proper temperature, but also (2) they’d been there for a period of time; thereby allowing the barrel to come to heat.  If these two conditions weren’t met, the screw wouldn’t turn.  This ‘fail safe’ is excellent for startups.  Keeping the ‘fail safe’ in mind, consider this scenario: You are molding a high temperature material like LCP or PEEK. And, the next material to run is ABS.   Interestingly enough all the requirements for the Fail Safe have been met because you are going from something that melts in the 600-700oF to a material that melts several hundred degrees lower and therefore it is at or above the set point for the sufficient time.  If the tech doesn’t empty the molten high temperature material and put in a heat stable intermediary material before putting in the ABS, the LCP or PEEK will be solid when he hits the Screw Rotate button.  If you’re lucky the barrel will see an incredible amount of abrasion and the Non Return Valve system will see some significant abuse.  Normally you break the screw tip. Soon you can no longer hold an appropriate shot because very quickly the tolerance between the screw and barrel opens up because you’re grinding cold unmelted abrasive material against the steel.

Purging compounds not only have the capacity to clean the screw, barrel, and tip; but also have the ability to withstand heat soaking at different temperatures for long periods of time.  This eliminates burned material, clumps of filler, excessive abrasion, broken tips and extends the life of the equipment.

Be a banker:  Do a study; not on SMED but how long it takes you to change materials.  If you’re changing colors – not using liquid color concentrate – going from light or less intense colors to dark or more intense colors is relatively easy.  Doing anything different is time/material consumptive.  Because you rarely have the luxury of scheduling colors in the proper order, purging compound saves a considerable amount of time and material.  Liquid color concentrate has this nasty habit of hiding in the root of the screw threads.  Without purging compound you spend a lot of material and frustration trying to get the screw completely clean.

Now go look at the money spent on replacement screws, screw tips, re-plating or replacing barrels as well as the associated down time.  Factor in the time it takes to change out this equipment.  Factor in the pain, agony, and frustration of pulling a screw then wire brushing the screw and barrel as well as cleaning out the Non Return system.

Calculate the cost of Mixed Material/Mixed Color parts.  This is scrap.  Multiply this cost by 3X:  The time and material lost initially, the time and material you must take to mold those parts that were scrap and not saleable product, and the time and material your folks must take to 100% inspect before they release the new color to production. Add up the costs.  They are an eye popper.  An intense program of training techs on the use of purging compound will usually show results buy cutting the sum of these expenses between 50-75%.  In a moderately sized shop this adds up to be a little less than $100K annually.

The savings are a free fall to net profit.  It also has the added bonus of making you far more competitive because you’ve lowered the costs of manufacture and made more machine time available that the guys down the street don’t have..

Something to think about.

As always you can print this article, possibly implement some of these suggestions and look like a hero or show it to your boss and he’ll take all the credit.  Equally, you can use it a bird cage liner for the Company Vulture or use it to scare away the critters in the warehouse.

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