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

On Fri, 15 Aug 2008 18:16:12 -0700, mpate wrote:

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 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 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 don't want the hot
plastic leaking around a loose gap.

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

James Waldby 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.


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

On Sat, 16 Aug 2008 03:01:46 -0400, Wes wrote:
James Waldby 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.

What temperature does this run at?

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

On Fri, 15 Aug 2008 18:16:12 -0700 (PDT), the renowned
wrote:

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.

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.

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?

School supply company or eBay.

Thanks,

Michael


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

James Waldby wrote:

What temperature does this run at?

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

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

4079 +32° to +600°F 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

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.

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).

I think I am in this range from the CTE calcs of CRS and alum.


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.

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.

On Sat, 16 Aug 2008 13:50:59 -0400, Wes wrote:
James Waldby wrote:
What temperature does this run at?

The block has 2 ea. 175W heaters [...] I set the controller for 450F

I was wondering how a kalrez o-ring would help your situation?
[...] Material Kalrez® Perfluoroelastomer Durometer Hard
[...] Highest Temperature Range +501°F to +600°F
[...] I cut and pasted that from the mcmaster carr website. Pretty spendy
though
AS568A Dash Number 109
[...]4079 +32° to +600°F 9568K421 $36.83

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

On Sat, 16 Aug 2008 13:09:40 -0700 (PDT), the renowned
wrote:

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.

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).

I think I am in this range from the CTE calcs of CRS and alum.


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.

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.

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.

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.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com