Sumitomo (SHI) Demag recently hosted its first in a series of free educational online training, kicking off with Clamp Force. Attended by nearly 50 UK professionals, Process & Training Engineer Darren Vater-Hutchison recaps on the theory behind the practice and techniques that can be used to balance the pressures and forces in the moulding cycle.
Clamp force is the force that’s used to counter the material pressure during the injection and holding phase of the injection moulding process. Usually measured in kiloNewtons (kN), selection of the appropriate clamp force for a particular combination of component geometry and polymer material is a critical factor for maintaining component quality and extending the longevity of mould tools and machinery.
Starting with the theory, Darren highlights why balance is so critical and how too little or too much clamp force can result in process-quality issues. When the pressure of the injection unit overwhelms a clamp unit, flash can occur as a result of the mould being slightly open. Oversized components can also be an issue.
On the flipside, if there’s too much clamp force, the machine will consume too much energy. It can also cause premature wear and tear on both the machine and mould tools.
Often, moulding machines are simply set with the maximum available clamp force (e.g. 100t, 300t, 750t). However, this clamp force setting may not necessarily be what the component requires. Additionally, platen deflection can result in dimensional issues. This generally occurs when running a small mould on large platens with maximum clamp force. It can even cause processing issues and damage to a new mould tool, which in extreme cases can be more expensive than the machine itself.
Venting issues
When tools are built they are designed to have venting cut into the tool steel. Prolonged running with too much clamp force will destroy them, leading to gas burns/traps on the components. This can lead to more vents being cut into the mould.
As a transient measure, it’s common practice for tool setters to add their own vents by adding sticky paper onto the tool. Although not a long-term solution, it can be useful as a short-term measure to identify precisely where additional venting is required before making cuts to rectify a burning issue. However, leaving sticky paper on can eventually bruise the tool, leading to metal (welding) being added to the tool to correct a flash condition. From a mould making point of view it’s easier to cut new vents than address a tool to correct flash.
Most injection moulding machines over 100 tonnes employ toggles as the clamping system.
Rule-of-thumb formula
Two steps are advised – calculating the clamp force theoretically to get operatives close to where they should be, and then practically to confirm and verify that the clamp force has been optimised.
The clamp force is equal to the pressure multiplied by the area. Therefore the greater the area, or cavity pressure the higher the force required. Geometries of the components being moulded also need factoring in. Generally speaking a thick component requires less pressure, a thin component greater pressure. Material viscosity, flow path length, filler in the polymer (glass, talc, mineral etc.), the number of mould cavities, and use of a cold or hot runner system, also form part of the equation.
As a general rule, the clamp force calculation comprises measuring the plastic flow at a 90-degree right angle to the sprue bush, then multiplying the area by the formula 0.3t/cm2 (low viscosity) to 0.6t/cm2 (high viscosity).
For a single square impression, multiply the width by height to derive the area. In multi-impression moulds, multiply by the number of impressions. Always measure where the plastic flows only, not the physical size of the mould tool. If using a cold runner, include this in the calculation.
To measure the dimensions of a round component, use Pi (3.142) rather than height multiplied by width. Again, only measure where the material flows and multiply by the number of cavities.
A table is then used to determine the clamping quotient for the material being used. Clamp force increases with higher viscosity plastics, and conversely decreases with low viscosity. Using fillers generally stiffens the flow and requires higher clamp force.
Incrementally decreasing the clamp force and noting any changes to weight/dimensions can further optimise the process.
Once the necessary calculations and/or software derivations have been performed, the estimator will usually apply a safety factor, generally in the region of 10-15%. As an example, a predicted clamp force of 270 metric tonnes would be rounded up to 300 tonnes.
To find out more about future free Sumitomo (SHI) Demag online courses, please contact, [email protected]
