Injection Molding Machine - Gingery - Machining Tolerances

I am building the Gingery plastic injection molding machine right now and have a few questions for people who have already built them.
The heater block assy is CRS and you are supposed to drill and ream a 0.501 inch bore. The plunger is CRS and is 0.5 inch rod, which we have measured at 0.5009 inch diameter. What is a reasonable tolerance for the 0.501 inch bore, so that it will inject the plastic (pmma and polycarbonate mainly) into the mold and not leak around the plunger and squirt or shoot out the top?
I am concerned about similar metals and very close diameter to bore clearance and having things gall up when I try to use the parts together( it was usually aluminum not CRS), but I dont want the hot plastic leaking around a loose gap.
Some other questions about the process. With aluminum molds, what kind of mold release spray or material should I use? With an aluminum mold if I can get it nickle plated will this give me a better surface finish, like on a plastic lens? If so what mold release spray or material should I use with this material?
I read some great safety tips in some older posts on this group - thanks for the heads up.
Anybody have a good source for 1-5 pounds of polycarb or pmma pelets?
Thanks,
Michael
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On Fri, 15 Aug 2008 18:16:12 -0700, mpate wrote:

I used aluminum rather than CRS for the heater block, and 3/4" stainless for the plunger. Drilled a hole through the block, then bored it out to size on a lathe. Ended up with a sloppy fit (the bore might be as much as .001" oversize) but the viscosity of the various common thermoplastics I've molded with is high enough that leakage around the plunger isn't much of a problem. Given the small amount of plastic molding I do (a few dozen parts per year) it's not enough of a problem at present to be worth fixing.
I have the plunger attached to an arbor press rather than to the lever shown by Gingery. Driven by the same force, your .5" plunger would develop more than twice as much pressure as my .75" plunger does. Anyhow, at higher pressure you will see more leakage. When I see leakage, it means the mold is full or it wasn't hot enough and the plastic congealed before filling it.

I don't have much info for you re your other questions -- mold release spray / nickle plate on aluminum mold / sources of polycarb or pmma pellets -- since I haven't used any release spray, and have reused old plastic, cut up on a bandsaw, rather than buying pellets. The parts I mold are small and relatively easy to remove from the mold. Some ebay sellers have yellow Acetal/Delrin at about $1/#, and Dupont Hytrel and Surlyn at $.25-$.40/#.
-jiw
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What temperature does this run at?
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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On Sat, 16 Aug 2008 03:01:46 -0400, Wes wrote:

The block has 2 ea. 175W heaters, one on a 500-degree-F controller, the other manually switched and only turned on to bring the temperature up faster when starting from cold. Molding small parts from cut-up plastic coathangers, I set the controller for 450F, which IRRC may be 50F higher than Gingery recommends. My molds heat up by conduction, which works ok for limited output of parts. If I were producing larger parts or more parts, I'd attach a heater/thermocouple/ controller to the mold and set it for about 200F. -jiw
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I was wondering how a kalrez o-ring would help your situation?
Type     O-Ring Cross Section Shape     Round System of Measurement     Inch Material     Kalrez Perfluoroelastomer Durometer     Hard Durometer Shore     Shore A: 75 Lowest Temperature Range     +1 to +32F Highest Temperature Range     +501F to +600F Color     Black
I cut and pasted that from the mcmaster carr website. Pretty spendy though
AS568A Dash Number     109 Type     O-Ring O-Ring Type     Standard Cross Section Shape     Round System of Measurement     Inch Width     3/32" Inside Diameter     5/16" Outside Diameter     1/2" Material     Kalrez Perfluoroelastomer
4079     +32 to +600F     9568K421     $36.83
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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On Sat, 16 Aug 2008 13:50:59 -0400, Wes wrote:

I wasn't aware of hi-temp o-rings like that, so hadn't considered using them. Even so, for the next version I don't plan to use o-rings but instead will attach a short close-fitting end-piece to the plunger. My main concern with current version isn't the sloppy fit (.001"), but instead that the plunger cools the block as it enters. Like the mold, the plunger heats by conduction, but since I don't cycle it very fast, the effect isn't significant at the moment. Anyhow, with an endpiece a little wider than the plunger this effect would be reduced and possibly I could lower the temperature setpoint somewhat.
-jiw
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On Fri, 15 Aug 2008 18:16:12 -0700 (PDT), the renowned snipped-for-privacy@oscintl.com wrote:

You could look at the recommended tolerances on ejector pins, typically in the range 0.3 to 0.9 thou (pins -0.3 to -0.6 and hole +0 to 0.3 thou).
Closer to the other limit, the vent depth (optimal depth varies by plastic) is 1.5 to 2.5 thou for PC, and about a third of that for runnier polypropylene.

School supply company or eBay.

Best regards, Spehro Pefhany
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snipped-for-privacy@interlog.com Info for manufacturers: http://www.trexon.com
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I think I am in this range from the CTE calcs of CRS and alum.

What is the vent depth? I dont think Gingery talks about molds with vents, he talks about the molds similar to the ones in Industrial Supply Company catalog - like guitar picks, golf tees, etc.
For Acrylic(PMMA) and Polycarbonate do you think it will be possible to get optical quality surfaces with just bare aluminum molds that are polished well. I only care about small quantities maybe 10's to 100 parts. I ask about nickle plating as most optical lens molds seem to be steel pins with nickle plated surfaces.
Thanks.
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On Sat, 16 Aug 2008 13:09:40 -0700 (PDT), the renowned snipped-for-privacy@oscintl.com wrote:

I'm talking about parting line vents-- usually a surface-ground strip or something like that which allows the air to escape from the mold but is thin enough that flash does not form under full injection pressure. Except for the fact that your plunger probably is a bit hotter (and thus less viscous) than the melt when it hits the parting line, and the pressure will be a bit lower at the parting line, it's pretty much the same scenario.
All molds have vents, even if it's just rough grinding (leaving lateral scratches) for the air to escape. Otherwise the mold would never fill. Sometimes the vents get plugged on a real injection molding machine, which causes partial fills and burning where the air is compressed at thousands of PSI very quickly and heats to a very high temperature.

Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
snipped-for-privacy@interlog.com Info for manufacturers: http://www.trexon.com
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