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May 03, 2005

Summertime, and the Living is Easy

Ok, so its not officially Summertime just yet. We are seeing the early symptoms, though, happier, noisier kids around the neighborhood, grass turning up green when you just got used to it being brown forever. And then of course there is everyone in the Injection Molding Fraternity starting to worry about explaining to "the boss" in a few weeks why productivity is off and cycles are getting longer everywhere you look. "Its Summertime!" never seems to be a good opening remark during those late May through September production meetings when the non-technical guys (accountants all, of course) are asking what they think of as the hard questions. Really just one single question, fully dressed up in lots of detailed little self-serving issues and endless repetition and rephrasing....."Where are the parts that are supposed to be in all those boxes we ordered for JIT delivery based on that survey we did on production rates in December???" 

Yep, you guessed it, the parts aren't going to be there next month, at least not if your operation is anywhere on the East Coast, where summer heat combines with high humidity for two to four months of the year to make it impossible to run your molds cold without having the whole press area swimming in condensation. And of course the watermarked parts that the QC people insist are showing flow lines, or stress marks. Hot plastic plus water equals steam, captured in a cavity equals streaks...........yep, its summertime again.

Our company is preparing for the expected panic calls now, expecting to hear from any number of folks out there who are looking for better answers than just "Its the summertime weather dammit!" since everybody knows that drops in productivity mean drops in revenue, and that translates into the production side of the Company assuming the role of target for the folks in management to be scapegoating for the next couple of months. Kind of like being the coach of a team in a slump, isn't it?

So, what to do? With only the one answer, and with some folks working overtime to figure out how to ask the same productivity questions in new and interesting ways every time there's a chance to impress "the boss" at your expense, Summertime looks to be one long, hard path to walk down, right?

Here are a couple "get out from under" moves to make when the heat and humidity get everything twisted out of shape in the processing end. First, go back to the theory for a minute. The cold water running in the molds that helped you be a hero in December does not have to make you look like a nitwit all summer. Remember what it's supposed to be doing.

Cooling water is NOT there to cool the mold. That's incidental to the process. What it's there for is to REMOVE HEAT from the hot plastic you are shooting into the mold. Two entirely different activities.

Probably if you run polyolefin materials what you do is pump the chiller down to around 40 degrees, and then adjust cycles, especially cooling time, so that you run as fast as possible with the cold water and whatever flow rate you need. Probably you restrict the flow too, because you want the mold surfaces to be not freezing cold, which interferes with filling the shot, but using  the cold water temp for maximizing heat removal AFTER the plastic is in there, right? Fast cycles, cold water, decent flow rates, and life is good.

Now comes summertime, and what happens? To keep from drowning in condensation, you either air condition the whole building, or raise the process water temperature a few degrees above the dewpoint and then try to figure out how to keep the production going at all, knowing that a move from 40 degree to 80 degree cooling water means at least double the cure time for most jobs. Doesn't have to be that way. Remember, its not cooling, its heat removal you need.

Go back to the dusty old textbooks and figure out how many BTUs your cold water winter setup was pulling of the mold per second, then see if the same amount of BTUs per second can't be pulled off that mold with warm water if you boost the flow rate. You might find you need 2x or 3x, or maybe 4x the flow rate to get back to the same place with the warmer water, but at least you have a direction, and a solution of sorts, to get the bean counters to quiet down a bit. What they'l really like is knocking off some of the chiller load, since chilled water costs much more to make than warm water in the SUMMERTIME.

Most plants we have worked with also have issues with jumpered waterlines in their molds, less than ideal line sizes and mold water lines, and flow inhibiting factors like scale in the molds, hard water calcium deposits, limited central water system capacities, and other such fun stuff.  Another get out of jail free card for the processors is to play the "mold needs" card.

If you start the summertime setups problem solving with the question "What does this mold need?" in order to run properly in the new environmental conditions, and you can come up with the right answers, you are going to be worlds ahead of the guys who just crank up the cure time until the job levels out again at a 10% to 30% loss in product out the door.

Since we've been answering the same summertime crisis planning questions for a good while, here's a piece of the check list we use to help out our East Coast clients:

1. Calculate the heat removal rate the winter setup typically achieves. Temp incoming/Temp outgoing, Pressures, and GPM will be part of the formula.

2. Calculate what GPM is needed to get to the same result at higher water temps. Then examine the mold water circuits to see if that rate is possible. Base your calculations on the highest dewpoint temperature likely to occur in your area. No point in setting up for a 80 degree dewpoint if the next week you have to change again for a 90 degree dewpoint. Check the local weather bureau. they keep these records and will be happy to give you the bad news.

3. Look at the cooling system risk factors and correct as many as practical to get to the required flow rates for the higher water temperature. A partial list of factors affecting the process: Scale in the mold water lines. Small and too long and too many hoses, quick disconnects with shutoffs that limit flow, waterlines jumpered on the mold, incorrect water circuiting, blockages of any kind, low ratio of inlet/outlet pressures, low general system pressure, high back pressure from a long return line to the central system pumps. (Talk to me about large plants that do NOT employ reversed return plumbing in their central system.)

4. If it is NOT possible to get back to "Normal" by improving flow alone, then and only then does it make sense to start looking at drying the air, insulating the mold and its waterlines, hara-kiri, etc., or trying to convince the management that it is not completely insane to air condition the press room, or at least the molds and their water circuitry.

5. If nothing else can be done and the mold water HAS TO be set below dewpoint no matter what, then don't waste any time or money insulating the outside of the mold or the water hoses that connect it to the chilled water supply. In high humidity, the mold plate faces and the toolsets themselves will pull condensation out of the air, and all the work done elsewhere will be wasted.

6. In condition (5.) above, bite the bullet and tent the molding area, then install a dryer/blower/air conditioner for spot cooling that has enough air flow to maintain positive pressure inside the enclosure, to keep the moist air out while the press runs. Be sure not to include the whole machine, just the area from stationary platen to the farthest travel of the moving platen needs to be inside the positive pressure dry air environment. No advantage to cooling/drying the injection unit. Life is confusing enough without adding more variables.

Needless to say, all this is couched in the simplest terms, and the regular disclaimer is that this is not intended to be like the advice of a doctor, a lawyer, or a paid molding consultant...........hehehe.

Every manufacturing plant is unique, and every mold will have its own capacities and limitations, as will its relationship to the press its being run in, the materials involved, and the people that are doing the processing.

We're just offering  an abbreviated scenario for making summertime easier, while we're all traveling down the old spiral path. 

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