Injection Molding Machine - Gingery - Machining Tolerances

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

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

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

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

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 +32°F Highest Temperature Range +501°F to +600°F 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

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

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.

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

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