Nov 03 2014

It’s all about the water, Joey

Published by at 8:53 pm under management,processing,tooling

I watched a young tech hook up a fairly complex mold and asked him (my deadly question) “Joey, how do you know which inlet gets what waterline and when to loop? Naturally, he looked at me like I was an idiot.

He was a MASTER because he’d attended several ‘schools’ where he’d learn about calculating Gate Freeze Off and using Apparent Viscosity to set the injection speed but somewhere he’d missed the classes on clamp pressure optimization, how to cool the mold, and hooking up the water.

To him, more clamp pressure was better than less, you used the cooling time to ‘process the dimensions in’, and if the machine had eight ‘ins’ and ‘outs’ on the manifolds; you looped everything until you came out with eight circuits all set at full open AND it didn’t matter what line was looped to any other line.

This method of molding is called the TECHNICIAN’S FULL EMPLOYMENT ACT: He’ll never be ‘downsized’ because he’ll always be struggling with trying to make parts that will be acceptable. Job security at its finest.

HOOKING UP AND BALANCING THE WATERLINES

Do a simple mind experiment: It’s Saturday in the middle of August – your ‘to do’ list is to water the back yard and wash the car. Being a smart guy you have a “Y” connector on the faucet outside your house. You think the lawn can water itself if you set the sprinkler properly with 200 feet of hose in your back yard while with another much shorter hose you wash your car. You position the sprinkler precisely where you did in the past go back to the front yard and turn on the faucet full open. Since you don’t have a spray nozzle, you simply allow the shorter hose and your thumb to wet down the car and rinse it off letting the excess water run down your driveway. After 30 minutes your spouse opens the front door and asks why the sprinkler is only watering about a third of the back yard. You forgot the most basic rule of water:

The greatest amount of flow will always be the path of least resistance.

There are a few simple truths in injection molding:
1. You must take the heat out of the plastic before you can open the mold.
2. The Closed Mold (cooling) portion of the molding cycle is controlled by the time the thickest section of the part takes to sufficiently cool and eject.

It would then follow:
1. Highly pressure resistant circuits have slower (sometimes Laminar) flow and don’t pull heat out very efficiently because water will preferentially flow through the less resistant circuits,
2. If you hook the waterlines up differently each time you set the mold, you’ll get different cooling patterns and therefore a different cycle time and different part dimensions.

While not an earth shattering piece of logical deduction; we’ve now hit on the secret of molding: Efficient cooling controls everything.

Let’s go step by step
STEP 1 – Find out where the waterlines go.
Unless you can shine a light in one end of a waterline and see it come out the other end, all you can tell when looking at the outside of each half of the mold is that (hopefully) “8 IN” and “8 OUT” are the beginning and end of one waterline.
STEP 1A Get the mold designer to make you an isometric sketch of each half of the mold showing where each waterline goes.

STEP 1B have this sketch made into two aluminum engraved paint-filled plaques mounted in a recessed pockets on the appropriate half of the mold.

STEP 2 – Classify the waterlines: Long, Short, Complex
From the mold design calculate the length of each waterline adding in the length of the hoses you’d use as jumpers, Take particular note of any lines containing bubblers or fountains.

STEP 3Draw a waterline map – the soft fix – TRUSTING YOUR PEOPLEwaterline map
A soft fix is a technique that relies completely on your folks ability to follow instructions to the letter. It is not IDIOT PROOF.

This is done in a spreadsheet format. As you can see the machine’s first manifold outlet is connected to ‘3 in’. ‘3 out’ is then looped to 18”out”. ‘18 In’ is looped to ‘1 in’ and so on.

“Circuit 1” is comprised of looping five waterlines together. This was done by measuring all the waterlines together and then after the complex waterlines had their own individual circuits, dividing up the rest of the waterline into approximately equal length groups.

As you can tell “circuit 4” is a single waterline so it is probably complexed by the addition of bubblers etc.

SECRET TRICK – Bubbles rise. This means you always start the water flowing from the bottom to the top. If you start from the top down, you can trap air and unintentionally create a hot spot (water is 20X better at conducting heat than air).
STEP 4 Consider a manifold on the mold – the hard fix
A HARD FIX is something you physically do to the mold that is a permanent addition and is therefore no longer relying on people to implement it (Hard fixes are idiot proof)

Put ball vales on the inlets. If you have to purchase a water manifold. The Inlet side has ball valves on it before the male connections. The exit side doesn’t need ball valves. The manifolds will replace the need for hooking the waterlines to the machine’s manifold. Plug one end and other the other put a fire hose male connector. Take the end cap off the machine manifold and install another male connector. Have a ‘jumper’ fire hose made up with two female connectors to connect the mold and machine’s manifolds together.

All waterlines are permanently attached and stay with the mold. Two fire hose connections for each half make for a very quick water hook up during the set up process.
STEP 5 purchase a flowmeter with quick-disconnect fittings and make compatible flow reversal connectors.

STEP 5A To the fully hooked up manifold, ‘splice’ the flowmeter to each individual circuit and write down the flow in gallons per minute or liters per minute.

STEP 5B You can balance the circuits by slightly closing the ball valves on the circuits with a high GPM rate and forcing more flow into the restrictive circuits. Unless you bought a ‘testing’ manifold with one flowmeter for each circuit (very expensive) this is an exercise in trial and error to get the water flow both turbulent in each circuit as well as ‘even’. “Even” is defined as each circuit’s flow is within 10% of the average flow of all circuits.flow meter

STEP 6 Preserve your hard work.
Remove the handles from the ball valves and bend the stem slightly so that the ball valve cannot be readjusted. Make a waterline map as in step 3 and file it away in case something gets damaged and you have to duplicate this work.

STEP 7 – DON’T BELIEVE ME
Will this take time and probably inadvertently get you soaked trying to balance the hookup? Yup.

BUT do you own experiment – go look at the mold’s cycle time a yield history. With the ‘any hookup is a good hookup” strategy you’ll see the cycle times and yields vary with each run completely unexplainably.

Now, do it the way I suggested and go back and compare the cycle time and yield. You should find:
A. You’ll make less scrap
B. You can cut back on the cooling time because the parts are cooler than they were before.
Albert Einstein once said “Smart people solve problems, geniuses avoid them.” There’s a lesson here. Be a genius

It’s only profits

It’s your choice.

One response so far

One Response to “It’s all about the water, Joey”

  1. Douglas Bryceon 04 Nov 2014 at 6:02 am

    Bill, this is a great article about a subject that is usually downplayed. Your suggestions are spot on and your readers should practice them always. Thanks for posting them. As always your information is accurate, useful, and makes perfect sense.

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