Sep 01 2008


Published by at 2:21 pm under Uncategorized

When you’re so bored you’re ready to scream, try this little financial exercise:  Look up the cost of any mold in your shop that has a ‘high’ volume.  Because material prices are so flakey simply look at the running time and pump in a press rate per hour.  Now with a little spreadsheet magic you can calculate the TOTAL cost of the part by factoring in the lifetime volume of the part.  OK, So What?  First conclusion – the mold cost isn’t a large percentage of the overall project cost.

Most molders take a modest markup on material but they are really selling machine time translated into dollars per thousand parts. Now that you’ve got your spreadsheet warmed up, do the copy/paste.trick, but this time see what impact it would have if you could save (for instance) 5% of the cycle time. That’s reducing a 20 second cycle to only 19 seconds.    Repeat yourself again but this time look at the impact of a 15% reduction.  AMAZING!  An absolute bucket of money that is all profit because you were the low bidder and have no screaming guilt to share it with your customer.  You produced you parts faster thereby effectively raising the dollar per hour machine rate!

Nice, you managed to fill up a few hours when you could have been staring out the window.  Now what?

Let’s re-visit molding like you would if you were to give a tour to a bunch of high school kids.  At its very basics injection molding is an exercise in heat exchange:  We melt the material, squeeze and pack it into the mold, then cool it down until it’s solid.  Heat in and heat out.

First let’s look at heating – Go back and do a calculation of ‘machine load’ on your original cycle time.  This is the percentage of the production machine-year devoted to this mold:  Direct production, scrap allowance and mold changeovers.  If you want to fudge the calculation to make the numbers come out better find another mold that runs in the same machine you can add into the mold load that uses the exact same material.  The magic number is 60% or more of a machine’s capacity.

Now write a very nice letter with your calculations, shot capacities and machine information to the folks who make screws inquiring about the feasibility of a custom screw that that particular material.  The people from Westland or Spirex and their peers will quickly respond back to you with the cost of a new screw and nozzle along with their anticipated cycle time savings because a custom screw built for a specific material can always produce a better melt faster than a generic General Purpose screw. As a freebie (for the tree huggers in the office) custom built screws also require less energy per pound of melted material than their general purpose cousins.

What?  No money?  Horse Hockey!  With the pirate’s fortune you’ve saved in cycle time, the cost of a new screw has an ROI calculated in weeks.  So much for the heating side.

Now let’s look at cooling:  The mold is a heat exchanger.

Question: Why do we normally use ¼ inch diameter fittings, hoses, connectors and such?
Answer:  Because we always have and we have bunches of spare connectors, fittings and hoses in that size

Question:  Why do we have internal loops in a mold either cavity to cavity or a circuit that looks like gridlock on the LA freeway?
Answer: Because we always have and if our machines have a manifold with eight circuits, its poor form to ask for twenty four.

BIG QUESTION:  Are any of the two questions and answers justifiable?
Answer:  Nope.

Manifolds are big pipes with valves on them.  Explain why you can’t drill, tap, splice, adapt and install a set with bigger size valves? NO, Don’t try to explain.  It will give you a headache.

If you don’t like that, why not put a big honker manifold on the mold and have one ‘fire hose’ connection the same size of the machine manifold connect to the mold’s manifold?  In a previous article I sited a molder who intentionally used small connectors and hoses on relatively large molds because he initially started out in business with small molds and machines that necessitated small connectors and hoses.  As he grew he ‘standardized’ with the excuse of how much it would cost him to change over or have two sizes of hoses and connectors.  He completely forgot at a mold running in a 300 Ton machine with 1/8 inch connectors was probably running 30-50% slower because of this lame justification.

Now let’s look at the mold:  Keep in mind manifolds are cheap so having one with a “U” adaptor then two trees of a dozen or more connectors each is no sin.  I’ve even seen them looking like a candelabra.  Two dozen valves for one size connectors on two trees, two dozen of a different size on the others. So what’s the problem with your mold specs requiring twice as many circuits as you are currently use to and instead of having 7/16 inch diameter lines put in bigger ones?

But “Ah HA!!’ you say  “We can’t get enough water delivery”.

Well you’re right and wrong.  Right in the sense that you don’t think you can deliver enough water right now.

Let’s do a mind experiment: We all know we need a Reynolds Number of 4,000 or higher to get Turbulent Flow (the mystical concept combining flow and the diameter of the flow channel to get the water to tumble thereby maximizing heat exchange).  If we’re to get the maximum heat exchange capacity from our mold; another variable in the calculations is the surface area of the flow channel exposed to our turbulent flow media (water).  We now get into the area where people with many letters after their names begin to drool – Thermal Tranmissivity and several other esoteric ‘constants’, ‘variables’ and ‘effects’ named after lettered individuals enter into the calculations.

To make the complex simple – Flow is Flow.  Ignoring the pressure it takes to make it happen, one gallon per minute is a gallon per minute regardless of the ID of the hole it flows through.  But a hole that is very small has an equally small surface area to pick up heat.  Larger holes have more surface area and therefore can pick up more heat.  Surface area is length times Pi times the radius squared relationship.  Do a little algebra and you’ll find being only slightly larger gives massively more surface area.

Conclusion? So long as we can get our minimum Reynolds Number, larger waterline diameters will pull heat out faster than smaller ones.

When we get down to the short strokes, the real question is: Does your heat exchange unit have the pumping capacity to keep up?  If it does, you’re home free.  If it doesn’t higher capacity pumps aren’t particularly expensive.  Use this free Reynolds Number calculator to play with the numbers. (It’s also in the ‘free stuff’ section of the website.)  In practical terms a Reynolds Number of 8,000 is no better than one of 4,000 when it comes to creating turbulent flow.  When worrying about pumping capacity in multiple lines remember that it is flow and NOT pressure (or pressure drop) that determines the Reynolds Number.  This is why you use larger fittings and hoses.

Specifying a mold with a maximum of water circuits that are oversized along with oversized hoses and fittings is actually free because it can be part of the mold build RFQ specifications. The reason you don’t want one inch lines and quarter inch fittings is the Reynolds Number that makes sense is not the one you measure off the hoses, but the one you calculate from the cooling circuit in the mold.  The larger the lines, the easier the flow. But using small diameter connectors it is like trying to get adequate flow through a sewer pipe using a garden hose.

Larger manifolds (and perhaps connectors and hoses) are your only one time expense.  Since the essence of cooling a part is massive calorie exchange with turbulent flow; larger circuits and more of them will reduce the temperature differential you normally have in a mold – let’s say you are going to eject the part at 110 Degrees F.  Most people put water in at 60-70 degrees F if not cooler.  With enhanced heat exchange your water will be up around 90-100 degrees.  Result?  Less molded in stress and a cycle determined to be as fast as the plastic can give up enough heat so it can be ejected.  This one Tricki Tip alone can find you several seconds in a cycle.

Let’s do the math.  A custom screw can find you between 5% and 25% depending on the machine load.  This estimate is best left to the pros giving you advice.  Putting in gobs of extra less restrictive water lines with larger diameters will have the potential of giving you between 5% and 30% improvement.  What’s nifty about the waterline thing is you can model this on one of the many mold simulator programs in your CAD system.  Kinda fun to play with, but the results will leave you kicking your self for not doing it earlier.  While the two numbers don’t exactly add together their combined effect very profitable.

Using both of these Tricki Tips, you’ll be amazed as how fast the mold runs and how productive it is.  Of course you’ll keep the excess money generated by it because you were righteously the low bidder in the first place.  But the real laugh-riot comes when your customer pulls the tool and sends it somewhere else because he thinks just because this job is highly profitable, you shouldn’t have it.  The folks who said they could run it faster and cheaper (1) don’t have a custom built screw or (2) they’ll have to loop the mold because their machine manifold doesn’t have enough inlets and outlets while you don’t have to.  The new guy with the job will have to slow the press down to even run the job. HA!

There is no sin to an ‘unfair technological advantage’.  If you can process parts better and still are the low bidder, good for you!  You’re just using your head while the other guys aren’t.

* * * * * * * * *

This article is Virtual.  You might read it to relieve the above mentioned boredom then go back to your thousand yard stare. Or, you might do a little keyboard thumping and look at the calculations.  It might even motivate you to see if what you read just might make you a “Maharishi of Molding”, the “Mold Meister”, or the “Pundent of Precision Plastic Parts” if you do a little work by simply providing the leadership and equipment to improve your productivity.  OR you can delete it and hope you’re not the one who gets a mold with tapped holes too large for your connectors and too many circuits for your manifold system that your salesman said you could run like a race horse.

It’s your choice.

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