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1 posts from April 2006

April 11, 2006

Silver Threads Among The Gold - OR, How Did You Get That Thing to Unscrew, Anyway?

New project came in over the transom the other day, and started the latest round in the ancient debate all moldmakers get to participate in: If I have a threaded product to injection mold what am I going to do about getting the parts off the mold? Want to hazard a guess how many ways to do a threaded tool are going to have to be looked at before the first pack of pixels ever get to the CAD screen? ( Pixels: Tiny points of light made by even tinier Pixies with flashlights who live inside your monitor, and keep flipping the lights on and off in response to mice bothering them.)

Having been at this a while, I've had time to work out an equation that predicts the magic number. Take the number of designers on the staff of the design house ( At our place that would be 4) multiply by the number of mold shops who will be quoting the job (usually 3), multiply THAT number by the total number of designers at each of the shops quoting the job(usually at least 2 per shop), and then multiply THAT number by the total number of customer personnel involved in any design meetings that may take place(usually at least 6 by the time a decision approaches), and you get pretty close. Total number of designs that are each the only way to do the job properly will be roughly 144, assuming you don't ask the injection molding machine manufacturer the same question. As a contract manufacturer, this makes your likelihood of everyone being happy with your design some fractional number smaller than the reciprocal of 144, before the project has seen a single chip fly off a mill in anyone's mold shop.

The most fun you can have in the relatively dry world of injection mold design, if you have a lot of experience and a sufficiently twisted sense of humor, is that first part of the mold design process, when you assign ANYBODY ELSE the task of deciding or recommending the ideal design approach for an uncrewing tool for your client's injection molded part.

To make it simple, let's specify that this is a threaded cap for a wide mouth juice jug, say 38mm threads for a self seal application, made of polyethylene, with no particular fancy design quirks to add problems. Top center outside pinpoint gate. 50,000 pieces per day production rate required. Here are some of the general design choices we get to have our guy sort through before committing time (and money) to the actual design:

1. Forget unscrewing, lets make collapsible cores. Sure they're expensive, but it's so EASY.

2. Uncrewing with retracting cores, Hyd. Cylinder driven, no other plate actions required. (Robot or sweep must be used to get the caps off the stripper plate)

3. Uncrewing cores with spiral drive and planetary gears, no hydraulics needed, retracting cores, clean operation driven by press action on opening. Let's blow the caps off the stripper rings with air.

4. Retracting cores are a pain to design, go with Rack unscrewing cores with cams to lift and pop the stripper plate, HYD. Cyl. driven, using press hydraulics to pull the racks. Old Newark design.

5. It's polyethylene, just pop the caps with fast acting stripper plate action, no uncrewing used. With any luck, the threads will not deform too much to still work on the bottle.

6. Use mold mounted hydraulic motors and gear sets or chain drives to uncrew cores and drive stripper plate lifters in sequence, timed to press opening and using press corepull sequence for power. Stripperplate advance keeps unscrewing lugs engaged, cores only rotate, no retract action.

7. Spiral drive all cores individually to uncrew and drop the parts by extended press open travel, no drive mechanisms required. Separate Spiral drive also elevates the stripper plate to keep uncrew lugs engaged, sweep or robot removes the caps off the stipper plate face.

8. Rotating cores are a nightmare for wear, damage, cooling problems, etc.  Leave the cores fixed, spin the stripper rings with rack and gears, HYD Cyl. Drive and cams on the drive gear to advance the stripper plate. Add a fast ramp "pop" at the end of the stroke to shock eject the caps off the stripper plate.

9. Rotate the strippers, but use spiral drive and press action, not extra hydraulics that can fail, or leak oil and contaminate the parts. Robot or sweeps to get the parts out.

10. Eliminate all the press action sequencing issues by using rotating cavities to spin the caps off the cores during press slow-open/break sequence, then air eject the caps from the cavity pockets.

Of course , with all of the above, you also get to do the whole range of mechanism dances; Do you mount your unscrew cylinder (if in use) above the mold on a platform so it can pull the rack from above, mount it underneath, so any leaks go on the floor instead of onto the parts (look out for all that hardware traversing your parts bin at every cycle), or do you set the whole mold out from the moving platen on rails, so the cylinder can be built in behind the mold, and drive the racks via a bridge coupling to cut down on the height above platen that a top mounted drive ram would otherwise require? Also, since press corepull circuits are NEVER strong or fast enough to operate any serious sized unscrewing molds, do you make the required hydraulic power unit a part of the mold build, or stick the molder with the cost of that "Auxiliary Equipment" that your mold won't work without?  How much of the wiring for actions and safety do you need to provide, and what part of responsibility for proper setup is with the moldmaker?

As to the other details, central spiral drive/gear systems require presses to have very significant die opening power in order to supply the extra force needed to turn the drivers and the gears, etc., and may not work at all with toggle clamp machines that are nooriously weak on initial opening force. Hydraulic motor drives, gear and chain assemblies require precise sequencing, retract and extend positioning and safety interlocks to prevent damage to molds or personnel. Who has to take responsibility and absorb costs for all that? Collapsible cores are a great solution for many applications, as long as the parts are not too small, or too big, or out of the specific sizes of stock cores, or require more than mediocre cooling, or are going to have to run for extended time without being pulled for complete cleaning. Also a poor setup, double shot into a filled mold, or a single instance of flashing into the core sectors can demolish this type tool.

Sooooooo, those of you still awake are likely asking, "OK, hotshot, so what do you do to get past all this mishmash and start cutting some steel?" The answer is that there is no correct answer, only the one that makes the majority of the players step back and let you do your job.

Moldmaking secret of the century is: The mold that is going to be best for any application is the one that makes life easiest for the largest number of people in the food chain the job brings to you.

The tool we're working on for the application that started all this rambling is going to be as close as possible to an exact duplicate of the last mold the client bought from us. They have a building FULL of these molds, and everyone down to the last materials guy on the third shift has been looking at the same design, in the same applications, in pretty much the same injection machines, for at least the last 15 years. The new mold will make a slightly different unscrewing cap for a slightly different big mouth juice jug, but it will be a dead bang replica of a Newark Die style rack and gear, rotating core, stripper plate cam advance, eject popper at end of stroke, HYD Cylinder driven tool. It will have the cylinder behind the mold so that it neither leaks onto the parts from above nor hangs down into the parts bin in the way of conveyors, or makes mold setting more difficult by having fragile parts outside the shield of the mold plate stack. It will also be a 3 plate TC gate cold runner mold, even though everyone knows hot runners are more efficient, because all the molds in this house are cold runner.

Would we be making this mold this way for this exact same part, for another molder, in another location?  Never happen. Mold design, ultimately, is intensely client-specific. Most moldmakers start the design process with a couple of critical, success-or-failure-determining, and entirely NOT design oriented questions: "Which molds are you running now that you like the best?" and "If you had to pick the kind of molds that you would expect to fail the fastest in your shop, what would they be like?" From that point on, anyone wanting to get repeat orders on injection molds from a given client had better remember the answers to those questions, because the designs that are "popular" in a given plant are going to survive, and the others, even if they may in fact be better designs and built better, will ultimately fail if they are not accepted by the folks running the show.

One aside about unscrewing molds working with outboard cams and lift bars that drive the stripper plate to hold the stripper rings up against the plastic part as the cores uncrew:  In some cases these molds will be stored upright, which causes the cams to extend the stripper forward, typically held that way by cam lift bars on the 4 corners of the mold. IF THE MOLDSETTERS MOUNT SUCH MOLDS TO THE MACHINE WITHOUT MANUALLY PUSHING THE CAM BARS DOWN TO RETRACT THE STRIPPER PLATE, THEN CLOSING THE PRESS CAN CAUSE THE 4 CAM ARMS TO EXPLODE OFF THE MOLD JUST LIKE BULLETS, GENERALLY RESULTING IN SEVERE INJURIES TO ANYONE IN THE PATH OF THESE FLYING BARS. One of my favorite mechanics made this mistake at a facility I was running at the time, and caught a face full of flying mold parts and door plexiglass as a result. Oddly enough, after a 3 months recovery period, the fellow made the same mistake on the same mold his first day back from medical leave. Happily the vacation time at least improved his reaction time, as on the second round he remembered to duck and had all the mold parts and door glass fly over, rather than into, him. No hospital bills this time, just $3500.00 worth of broken mold parts and another two sheets of plexiglass. You can teach an old dog new tricks..........just not more complex ones.


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