I was talking with Bubba (25 year veteran of Molding and he Knew — IT —  All, Thank You!):  “It’s a 500 ton machine” I said, “Why is the clamp pressure 450 tons?”  After expertly hucking a wad of chew into a coffee can on the floor he said: “’Cuz the damn thing won’ give me no 500 tons.”  End of a non-conversation.  Not that either of us are right or wrong; but why is set there?

Most designers, engineers, and tool makers use the “3-5 ton per exposed inch, clamp rule.” for a mold/machine fit.  ‘Thick’ materials with low melt indexes need only 3 tons/square inch of the surface area presented at the parting line to keep from flashing.  High melt index materials (‘thin’) are guestimated at 5 tons per square inch.  The beauty of this Rule of Thumb is that it’s polite enough not to describe the difference between low and high melt index, and with the extremely high melt index custom made materials (80+) the rule doesn’t really apply at all.

When you sit down in front a fireplace in the middle of cold winter’s eve drinking something flammable - old enough to be your uncle - and reflect on this rule; you finally get the punch line to this joke:  You can’t apply pressure to a hole.  So there’s either probably more to it, or you just got suckered into a Snipe Hunt and everyone will be laughing at you.  Yes?

Consider the above mentioned 500 ton press.  It should be capable of delivering 500 tons of force.  But when we’re holding a mold closed, we’re not interested in force, we’re interested in pressure.  Pressure is defined as force divided by area.

Thinking on this we take a short sip and visualize the mold. It is a four cavity hot runner eliminating the need to calculate clamp tonnage adding in the sprue and runner system.  Each cavity has worked out to 27 square inches (4” X 6.75”).  27 sq-inches per cavity times 4 tons/sq-inch times 4 cavities works out to 432 tons guestimated.  We consult the (I.A.I.M.) International Association of Injection Molders’ book Rules of Thumb, common sense, and sanity – what they didn’t teach you got your Plastics Degree in that snooty college (unpublished edition) and note for estimating purposes you should only use 85% of the press’s rated capacity. Our figure of 432 tons is a sconch (maybe a couple of sconches) above the 425 ton - 85% figure, but good enough for estimating!

The next step is the MMDTTWTE - Pronounced “Mmm-DEE-TWEET” from the I.A.I.M.’s (mind- mold-design-thoughts-thunk-whilst-thinking experiment).  We mentally lay the cavities out in a 2 X 2 fashion, giving us an 8” by 13.5” group.  We separate the cavity inserts with 2” of steel and get 10” by 15.5”.  We further allow for a 3” perimeter for the outside of the mold to accommodate leader pins and such giving us the minimum length and width of the mold as 16” by 21.5”.  As a last step we make two checks:

Question #1 Is there enough room between the tie rods on our 500 ton machine?
Answer #1 (in Swedish Minnesota-ese): Ah, You betcha’, by golly!

Question #2 What’s the nearest standard mold base that accommodates our 16” by 21.5”?
Answer #2: After a quick dumpster dive into the mold supplier’s internet catalog we come up with a base of 15.875” by 23.5” (close enough).  This works out to about 373 sq-in.  Now the only exercise left is the thickness of the plates, the hot runner system, the ejector box and other silliness.

Just to backstop our MMDTTWTE, we quickly go to the section in the I.A.I.M.’s book under the “Tribal Knowledge of shot sizes” and see if our 500 ton machine’s shot capacity (which has the bad taste to be rated in ounces or grams of Polystyrene) can handle the shot size of our mental mold and land between rule of thumb 20-80% of the press’s shot capacity.  With a little more algebraic gymnastics calculating the ratio of the specific gravity of Polystyrene and our chosen material we happily find out (give or take a few percentage points) the shot size is in the mid 50% range.  The Tribal Knowledge ‘Shalt Nots’ (also know as the “Shants”) chapter proscribe that the shot size ‘shant be below 20% nor above 80% of the machine’s capacity’ – Amen).  All is well and good from a shot size standpoint; so we can continue our cogitation on clamp pressures.

Since we’re on our third glass of 200 octane refreshment, we won’t digress to consider the movement of side actions that will require both their thickness plus an inch of steel added to our perimeter or internal spacing calculations depending on where they reside.

Putting another log on our fire, let’s back-calculate the tons/sq-inch. 425 tons (85% of 500) divided by the sq-inches of the mold plate minus the leader pins’ surface area and the mold cavities.  What do we come up with?   425 tons clamping force divided by 373 sq-in (the mold base) minus 115 sq-in (four 1.5” diameter leader pins = 7 sq-in plus 4 times 27 sq-inches per cavity) works out to 1.65 tons/sq-inch.

Odd, this isn’t even close to our original Rule of Thumb of 4 tons/sq-inch.  “Whazup with that?” you might say. Actually two things come to mind:

Thing one – it was an excellent excuse for the consumption of 20 year old cough medicine and an interesting waste of algebra – the two numbers (calculated Tons/sq-inch v. Rule of thumb) don’t line up.

Thing two – it does say you have room to correct this situation.

If we simply stopped here, there’s a reasonable bet the press at full clamp still wouldn’t be able to keep the mold from flashing.  You can’t change the ‘tons’ side of the tons/square inch calculation unless you go to a bigger, more expensive machine; but you can certainly change the square inch side.

Our ‘Whazup” calculation gave us 425 tons divided by 258 square inches.  So, let’s preload the cavities.

In our mind we clear away steel on the mold base to a depth of about 0.100 inch leaving a perimeter around each insert of about a half inch wide alone.  We also take a portion around the leader pins, let’s say 3 X 3 inches, and cut away to only a depth of .002 inches.  Now as the mold closes the first ‘kiss’ – our preload - on the perimeter around each cavity.  Without getting all fancy-shmancy over the accuracy, it works out to about 43+ sq-inches of steel the machine will clamp on.  425 tons divided by 43 sq-inches is almost 10 tons per sq-inch!  WHOOPIE! More than enough!

As the press continues to close the second kiss will hit the area around the leader pins (we’ll call these standoff pads) and the mold will clamp up completely square.  Now our biggest problem will be to turn down the clamp pressure until we find that happy medium of keeping it from flashing and not crushing the vents.

Before we drift off to sleep after a tiring evening of calculations we can now be assured we need less than the maximum tonnage of the machine to keep the mold from flashing.  Even better, assuming the other pesky factors of residence time, shot capacity, and shot inventory aren’t in violation of the other Rules of Thumb, we can also run the mold in perhaps a smaller machine so long as we can get it between the tie rods.

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As a young pup, I was taken on hunting trips.  My main purpose was to get fire wood, haul water, keep the beer cold, listen and learn from the one-that-got-away stories.  However the Big Rule of hunting learned on these trips was always to “have enough gun.”  Shooting rabbits with a .22 is fine; but use that little plinker rifle on a 2,000 pound Grizzly Bear would probably result making one of you really cranky.  Conversely shooting a rabbit with a bear rifle would leave little for the evening’s stew.

Moral – use the right sized tool properly for the job at hand that will give you the maximum amount of practical flexibility.  With our mold, doing the preload thing gives us more presses to run in than simply trying to stomp the thing closed.

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This article is virtual.  I.A.I.M. is also a virtual International brotherhood composed of those of us who are in the molding industry, with occasional grease under the fingernails.  It has no dues, no membership roster, no formal meetings or conventions, and no secret handshake; although we are quite vocal in the occasional chat room.  It isn’t listed anywhere (the initials are, but citing many different affiliations).  The ‘fore mentioned handbook is virtual and unpublished.  While the rules of thumb are valid and proven; they are generally not consolidated in written format.  But it is fun to dream.

You can read this, call yourself a “Sustaining 4th Degree Mentor” of the Association and both use the information and pass it along to others referring to yourself as the Meister of Molding. As a final note, keeping with the high and lofty rules of the Association, it would be in poor taste to cobble together an invoice for annual membership dues and present it to middle management.  Nor, should you expect the folks with the mahogany desks to pay for the annual convention expenses or other technical meetings in Maui, Monte Carlo, or Melbourne.  It is not polite to fool the front office feeble minded.

You can read this, think it is all a long joke with no punch line and continue to squash your molds trying to get rid a flash.  Or, you can use what you’ve learned. Or, you can patiently try to convince the Bubba’s of the world that there is logic and proportion in the molding process.  OR, having stayed awake long enough to read this article word for word, you can enjoy the giggle and go back to sleep at your desk.

Your choice.

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