Injection molding info...

The tonnage of the machine is in reference to the force required to stretch
the tiebars on the clamp assembly. It is not the weight of the machine.
The machines I am building will weigh in at under 750 lbs. Yes they will fit
in the garage. Typical dimensions will be around 12" deep, 24" wide, and 42" tall.
They are designed to fit on a good sturdy workbench.
They will require 220v single phase, and 120 psi shop air.
Plans and semi kits in works.

Also concerning tonnage...the melt viscosity of plastics resin is very heavy.
Much like semi frozen honey, and even thicker. I don't have the CP data handy.
This translates in to substantial required pressure.
When you couple that with the fact that the plastic is typically 350f or hotter,
and molds are usually room temp or cooler, the plastic wants to immediatly "freeze off".
Therefore you must have adequate pressure and velocity to fill the cavity.

The shot weight of a plunger type lab/hobby machine is based upon polystyrene,
which has a density of 1gm per cm3. The actual part weight can vary
according to the actual density of the material you are using.
Most small machines are woefully inadequate for real world applications...
unless you deal in miniature or micro components exclusively.
When you find a machine of adequate capacity, say at least 3/4 oz to 1 1/2 oz
(styrene std.) you pay out the kazoo. I am hoping to fill a need here.

The industry standard for tonnage is basically this:
1-3 tons of clamp force per square inch of part area for parts .062" thick and thicker.
3-5 tons of clamp force for parts thinnner. This can be fudged with changing
the melt flow, and to some degree, the melt and mold temperature.

The formula to calculate the actual available injection pressure in a given
machine is: Surface area of piston in air cyl X air pressure in PSI, divided
by the surface area of the actual ram pressing against the plastic.

Example: 6" air cylinder, 120 PSI shop air, 1/2" diameter plunger-
Area of cylinder=Pie x R2= 3.1417 x 9 = 28.275 sq. inches
Area of 28.275 x 120 psi = 3,393 pounds of force
Area of plunger= Pie x R2= 3.1417 x .0625 = .1964 sq. inches
3394 lbs of pressure divided by .1964 in. = 17,281 PSI

That is a serious amount of available hydraulic pressure.
Hence the need for clamp tonnage, and a good air regulator.
Hope this helps...

Joseph Delgado

P.S. PID temperature controllers are nice,
but standard on/off relays work just fine
for small machines.