September 2011: What are your recommendations to reduce machine tonnage for plastic injection molding?

Homogeneous melt at optimum injection speed with meticulious profiling, optimal mold and melt temperature, when injected through larger nozzle orifice diameter in balance runner design lower locking force.with smooth curve with coldslug always attribute lower locking force. Sequential gating, pulse cooling , injection before tonage build up and stack as well tandem molds are also supportive features in machine as well as mold..

Pressure Drop Formula to minimise drop and hence to run at lower tonnage.

Pressure = Viscosity x Die Resistance x Flow Rate
P = VxRxQ
Die Resistance = (12 x L) / (W x T3)
L= Length of flow; W=Width of flow; T=Wall Thickness.

Ashish Rajput's (FERROMATIK Milacron) comments on less pressure drop, less clamp tonnage are right on point and verey astutely summarized.

Injection molding machines are defined by the tonnage of the clamp force they provide. The required clamp force is determined by the projected area of the parts in the mold and the pressure with which the material is injected. Thus a larger part will require a larger clamping force. Equally important, certain materials that require high injection pressures may require higher tonnage machines. The size of the part must also comply with other machine specifications, such as shot capacity, clamp stroke, minimum mold thickness, and platen size. Injection molded parts can vary greatly in size and therefore require these machine specifications to cover a very large range. As a result, injection molding machines are designed to each accommodate a small range of the larger spectrum of machine specification values.

Donald Rosato has comprehensively brifed tonnage need , based on part area, average thickness & polymer density as well as flow ratio.(L/T).
However, this complexity can be summerised by simple thumb rule as follow.
Do ensure & verify the locking force as given below (This is for Polyolefins)
Above 1.5mm thickness/150mm depth = 1 Tons/inch2
Above 1mm thickness/150mm depth = 1.5 to 2.0 Tons/inch2
Above 0.5mm thickness/150mm depth = 2 to 3 Tons/inch2
For 0.5mm thickness/150mm depth = 3 to 4 Tons/inch2
Any further increment in flow length for every inch shall increase the tonnage by 10%.
Filled material , alloy plastics, plastic blend etc. shall add tonnage based on melt viscosity.
In short , plastic being non-Newtonian fluid varies viscosity with Shear Rate and hence one should not surprise if the same article with different weight run at different tonnage, only because of differential rate of filling and cooling. Both should be as short as possible.
For engineering polymers , you can jump to next level and for high end polymer further upper level.

First minimise the projected area by smart design

Limiting the discussion to the regulation of the press the complexity of the options can be synthesized by a very simple rule:
"Optimize by increasing the shear rate in the various phases of the filling"
This means:
1)Exploit the greater clamping force by unity of surface filled in the initial phase
2)Minimize the formation of the cold skin
3)Decrease, very often a lot, the polymer viscosity (shear thinning)