Why manufacturers need effective mould safety systems

Nigel Flowers, UK managing director of moulding machine specialist company Sumitomo (SHI) Demag, explains the importance of an effective mould safety system.

In some cases, mould tooling can cost more than the injection moulding machine itself. Therefore, a trusted safety system to prevent irreparable damage being caused to these sensitive tools is essential.

The safety of the mould can be compromised under certain circumstances, such as when components do not eject properly, if a foreign object enters the mould in production during the closing and opening sequence, or when too much clamp force is applied.

It is thought that often mould protection doesn’t receive enough attention. This could be due to the complexity of setting up the system, or due to a lack of knowledge in understanding how clamp forces operate. Additionally, few injection moulding suppliers offer active mould safety solutions that are able to sense and react instantaneously to an obstruction.

Sumitomo (SHI) Demag aims to provide their customers with a range of options which cater for the very latest hi-tech all-electric machines right through to hydraulic machines.

Typically, machinery suppliers use motor torque or tie bar loading measurements to gauge the mould status on electric machines. However, Flowers comments that Sumitomo (SHI) Demag offer a mould safety system which features high resolution sensors built into the toggle system. These sensors are positioned close to the point where force is applied and link to the NC5 machine control panel. Their role is to detect and transmit changes in the force signature during the mould closing sequence.

The high-resolution sensor is located on the first link directly behind the moving clamping platen on the offside of these machines. The benefit of positioning the sensor close to the point of force application is even the smallest tensile or compression strains are measured. This means that any condition that may result in the set mould safety force being exceeded is instantly addressed.

Flowers explains: “The signal produced by activeProtect is so accurate it can detect even the slightest change in force patterns. Force values are converted in the transducer into a 0 to 10-volt signal, which is immediately transmitted to the machine’s NC5 operating control system.

"At this point, the real values of closing force, piston speed and stroke are compared with the master calculations, enabling the machine to react and stop before the mould faces touch. It’s like a chain reaction, with the entire mould safety decision and response taking place in less than 10 milliseconds.”

Users of the IntElect machines can benefit from enhanced ejector control technology, and Sumitomo (SHI) Demag’s latest advancement - Synchronised Ejector Operation - ensures that components are clear of the mould before it closes again. Therefore, as well as preventing falling parts being crushed, it helps to mitigate damage to expensive mould tools.

“In an ideal world the parts are ejected with zero inertia and drop in a central line out of the mould space. It isn’t so much of a challenge for users of hydraulic machines, as plastic parts flow out of the mould in a cascade-like waterfall. In all-electric machines, this constant stream can be difficult to replicate. That’s because when the toggle clamp operates faster than the ejector mechanism, there’s less control over how and where the moulded components will fall,” explains Flowers.

Synchronous operation of mould and ejector is enabled through the application of this technology. The ejector speed is able to be adapted to the master mould speed, and if the mould speed is slowed down, the ejector is prohibited from moving closer than 20mm towards the mould.

Another technique that can be used to control the optimum point to eject parts from the mould is thermal imaging. This can also help to ensure that the mould is clear of components prior to mould closing. This infrared technology is attached to the fixed platen on the machine and uses heat to determine if parts have been ejected from the cavities.

Flowers comments: “Compared to vision systems, thermal imaging is approximately a third of the investment cost. Additionally, thermal imaging can also be used to monitor the temperature of parts, and document important production parameters." 

For the large volume of moulders still using hydraulic-only machinery, safety systems are typically conventional and tend to be more limited. Flowers emphasises: “Should an event happen within a mould in a hydraulic machine, the closing phase will continue, albeit at a slower speed, pushing gently in the hope it will come to a standstill before any permanent damage from the obstruction is done to the mould.

“Setting these safety parameters is reliant on machine operators having the skills and finesse to identify the region in the closing stroke where the mould is most at risk.” 

The passive nature of this safety system is thought to be its biggest drawback. The system relies on an operative incrementally converting the toggle system characteristics and cylinder diameter to the hydraulic pressure in order to determine the most likely collision point. If, as the clamping proceeds, this pressure level is exceeded, the system pressure relief valve opens and the mould stops. Additionally, the safety system has to account for and overcome the mechanical resistance of the mould. This often requires increased closing force, which again can damage the mould.

A slightly more sophisticated safety option was introduced with second generation machines. This system features toggle clamping technology and powerful control systems. Here, the machines are able to calculate the mould position from the crosshead position by storing a model of the toggle system in the control. With this system, a consistent force is accomplished by adjusting the pressure according to how close together the platens are.

Flowers concludes: “Given that a well maintained precision mould running 10-second cycles should have a lifespan exceeding 10 million cycles over five years, let alone the price tag these tools command, it’s worth giving your mould safety system the attention it warrants.”