Given that some high-end multi-cavity automotive and medical part molds can exceed £500,000 (US $554,000), mold tooling doesn’t come cheap. In fact, some cost more than the injection molding machine itself! A trusted safety system which can prevent irreparable damage to these sensitive tools is on every molders' wish list.
Safety of the mold can be compromised if components do not eject properly, if a foreign object enters the mold in production during the closing and opening sequence, and when too much clamp force is applied.
Very often, mold protection doesn’t receive the attention warranted. In part, this can be due to the complexity of setting up the system, which requires a certain skill level, particularly with regard to understanding clamp forces. Additionally, few injection molding suppliers offer active mold safety solutions that can sense and react instantaneously to an obstruction.
Sumitomo (SHI) Demag provides customers with a range of options, catering for the very latest hi-tech, all-electric machines, right through to hydraulic machines, and everything in between.
Typically, machinery suppliers use motor torque or tie bar loading measurements to gauge the mold status on electric machines. Sumitomo (SHI) Demag remains the only known company to offer a mold safety system featuring high-resolution sensors built into the toggle system itself, claims UK managing director Nigel Flowers. These sensors are positioned close to the point where force is applied and links to the NC5 machine control panel, detecting and transmitting changes in the force signature during the mold closing sequence.
Forming part of the company’s active range, the sensor safety system — known as activeProtect — is available as standard and can be retrofitted to machines featuring dynamic servo drives. This includes the company’s all-electric IntElect models and fast-cycling El-Exis SP range.
On top of safety
On these machines, the high-resolution sensor is located on the first link directly behind the moving clamping platen on the offside. The benefit of positioning the sensor close to the point of force application is even the smallest tensile or compression strains are measured. It means that any condition that may result in the set mold safety force being exceeded is instantly addressed.
Nigel 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 mold faces touch. It’s like a chain reaction, with the entire mold safety decision and response taking place in less than 10 milliseconds.”
For added mold protection, activeProtect has now been extended to monitor the opening strokes. This helps to ensure mechanical mold elements, for example unscrewing devices, are protected from damage.
Other technology available
Additionally, users of the IntElect machines can benefit from enhanced ejector control technology. Sumitomo (SHI) Demag’s latest advancement — Synchronized Ejector Operation — ensures that components are clear of the mold before it closes again. As well as preventing falling parts being crushed, it helps to mitigate damage to expensive mold tools.
"In an ideal world the parts are ejected with zero inertia and drop in a central line out of the mold space. It isn’t so much of a challenge for users of hydraulic machines, as plastic parts flow out of the mold 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 molded components will fall,” explains Nigel.
Through the application of this technology, which enables synchronous operation of mold and ejector, the ejector speed is adapted to the master mold speed. If the mold speed is slowed down, the ejector is prohibited from moving closer than 20mm towards the mold.
Thermal imaging is another technique that can be used to control the optimum point to eject parts from the mold. It can also help to ensure that the mold is clear of components prior to mold closing. Attached to the fixed platen on the machine, this infrared technology uses heat to determine if parts have been ejected from the cavities.
The benefit for many is thermal imaging provides users of hybrid and hydraulic machines with a safety solution. “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,” notes Nigel.
Help for hydraulics
For the large volume of molders still using hydraulic-only machinery, safety systems are conventional and more limited. “Should an event happen within a mold 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 mold,” emphasizes Nigel.
“Setting these safety parameters is reliant on machine operators having the skills and finesse to identify the region in the closing stroke where the mold is most at risk,” adds Nigel.
Typically, this is just before the two mold faces touch. Operators then set a slower speed and force based on the hydraulic pressure level. Getting it wrong can lead to increased cycle times and consequently lower output.
The passive nature of this safety system is its biggest drawback. It relies on an operative incrementally converting the toggle system characteristics and cylinder diameter to the hydraulic pressure 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 mold stops.
Additionally, the safety system has to account for and overcome the mechanical resistance of the mold (cheeks and slides etc.). This often requires increased closing force, which again can damage the mold.
A slightly more sophisticated safety option was introduced with second generation machines. Featuring toggle clamping technology and more powerful control systems, machines were able to calculate the mold 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.
“Given that a well maintained precision mold 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 mold safety system the attention it warrants,” ends Nigel.