Words | James Hines and Darren Vater-Hutchison, Process and Training Engineers, Sumitomo (SHI) Demag
Clamp force is the pressure required to counter the material pressure during the injection and holding phase of the plastic injection moulding process. Filling of the mould during this phase takes place at high pressure and this must be countered if the mould is not to be forced open. It is usually quoted measured in metric tonnes, imperial tons or the more technically correct kiloNewtons (kN).
At Sumitomo (SHI) Demag, machines are available with clamp forces ranging from 250 kN and 20,000 kN.
Locking design options
There are only two principal ways to build a clamp unit: a mechanical lever that is actuated either hydraulically or by electric servo motors (such as a toggle system); and the fully hydraulic type with closing cylinders and a large clamping cylinder.
Toggles are the most commonly built and used clamping system in Europe. Almost two-thirds of all injection moulding machines of over 100 tonnes clamp-force employ toggles as the clamping system. The design is constantly being developed and optimised, and it is the most reliable and dependable type of clamping system, with distinct technical and economic advantages.
In the case of a toggle clamp machine, the clamp force applied is generated by the stretching of the steel tie bars - the greater the stretching of the bars, the higher the clamp force applied to the mould tool. The amount of stretch induced is maintained within the limit of proportionality for the chosen tie bar steel and is monitored using linear strain gauges to ensure consistency of force application.
Selection of the appropriate clamp force for a particular combination of machine, mould tool and material is an important factor and it is important to consider early in the manufacturing process and purchase of a moulding machine. This is because, once set, the clamp force cannot conveniently be adjusted.
Very often 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 mould tool requires. As a result, technicians may be led down the wrong path of process correction, potentially leading to other process-related issues. Most common are:
- Premature wear of mould tools. 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.
- Increased power consumption - using high clamp forces results in the machine essentially using more power.
- Faster wear and tear of the machine, leading to increased maintenance periods and reducing OEE.
Although in general the larger the component the greater the force required to counter the material pressure, the clamp force is calculated not, as is often believed, by the physical size of the mould, but where the plastic flows inside it. It is a function of geometry and of the type of material being processed. Important factors to consider include the:
- Viscosity of the plastic and whether it has a filler - clamp force increases with higher viscosity plastics, and conversely decreases with low viscosity
- Number of cavities in the mould
- Flow path length - the required clamp force will increase as the plastics flows out at a 90° angle to the sprue bush.
There are a few ways to calculate clamp force, however, some of the information is not easy to come by. As a general rule, clamp force can be calculated using a simple rule-of-thumb formula.
Firstly, measure the plastic flow at a 90-degree angle to the sprue bush, then multiply the area by the formula 0.3t/cm2 (low viscosity) to 0.6t/cm2 (high viscosity). This is an approximation and consideration must also be given to whether a hot or cold runner system is used and the number of cavities. An experienced engineer may be able to further optimise the clamp force during setup by decreasing the clamp force by small incremental adjustments and noting any changes to weight/dimensions that fall outside the required quality.
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 per cent. As an example, a predicted clamp force of 270 metric tonnes would be rounded up to 300 tonnes.
A benefit of using this formula is that a confident and experienced engineer may be able to use it to determine the dosage stroke for a given component that, due to mould geometry, is unable to be subjected to a short shot technique. In other words, the component sticks in the mould.
For example, if using polypropylene with 0.3t/cm2, this is equal to 300kgf/cm2 and 300bar specific pressure. Assuming a machine with a 10:1 intensification ratio, you can set the injection pressure to 30bar hydraulic (or 300bar specific) and allow a full barrel to inject into the mould, noting the screw fully forward position to determine your final dosage stroke. The objective here is to create equilibrium between injection and clamp pressure so that the clamp force is not overwhelmed.
To conduct an exercise like this requires engineering confidence and competence. Moreover, there needs to be careful consideration with cold runner moulds, with respect to the runner system length and pressure drop, as well as the selection of melt and mould temperature and injection speed.
If in any doubt about clamp force, seek advice before investing in a new machine. Sumitomo (SHI) Demag will shortly publish a comprehensive whitepaper about clamp force and including locking options which will be available online.