Lasers Protect Moulds from Damage

MMT, a rapidly growing injection moulding business about 50 km northwest of Milan, Italy, had a small but persistent case of a familiar problem: now and then a moulded part would not eject, or only partially eject, and the mould could be damaged on the closing cycle. Apart from reworking the ejector system and/or the mould itself, the simplest solution is to install a mould protection system.

MMT technicians did that, but they found that the optical sensors could become blocked or out of alignment unpredictably, and the original problem returned as if the protection system was not there. The company specializes in complex technical assemblies and wants parts production as automated as possible, which is why mould protection was installed in the first place. Looking for a more reliable solution, they turned to the newest division of their own company, and solved the problem by using lasers in place of photo-optical systems.

MMT has 23 injection machines ranging from 35 to 500 tons, supplied by manufacturers including Negri-Bossi, Sandretto, Engel, and Arburg. Four brothers founded the company in 1983 with the goal of applying their extensive mechanical engineering experience to the production of complete assemblies and subassemblies of technical parts. Naturally, they prefer to concentrate their resources, particularly human resources, where they can add the most value, such as assembling products, and leave the repetitive tasks to robots.

They staff three shifts from a total of 61 people that includes administration, shipping, and everything else. Most of the production specialists are in the assembly area. The front end of production, the moulding of components, has been designed to be as automated as possible. That categorically rules out having someone watch the systems that are supposed to be watching your moulds.

Low Power Lasers and Mirrors

MMT's three-year-old laser division was already furnishing a variety of low-power laser systems for medical and industrial applications when the mould protection problem became intolerable in early 1997. With a belief that laser technology offered a good solution, the company set about designing a practical and affordable system for injection machines.

Gianluca Luoni, the engineer who heads the laser division, says laser mould protection is practical, despite what many of us think when we hear the word laser. MMT's mould protection system, he says, is actually easier to install, modify, and maintain than a comparable photo-optical system. The system MMT designed for itself, and which it began selling to other moulding shops several months ago, is supplied as a modular kit. Installation takes about 15 minutes, and there is no need for a special technician.

The principle of operation is also simple. A low power laser diode is mounted on the side of the mould and the focused beam of light is aimed across the face of the cavity side at a receiver mounted on the other side. During moulding, if the receiver does not see the light from the transmitter, it means a moulding is present, and it will not send the signal that lets the mould close for the next cycle. When the receiver sees the beam it signals that the mould is clear.

Signaling is fast. The examples in the photos show two of MMT's 200+ active moulds in operation. The path of the laser beam has been illustrated since the beam is invisible to the human eye in normal operation. For complex or large parts, or for multiple cavities where one straight line is not enough, the modular system can be elaborated.

Mirrors and prisms are used to redirect and split the beam, allowing various points on the mould surface to be controlled simultaneously, but still using only one transmitter and one receiver. The beam can also be adjusted for greater or lesser distance by turning a nut. All the components are small and lightweight, housed in durable anodized aluminum, and easy to mount and dismount.

Advantages Include Cost, Reliability

Gianluca Luoni lists a number of advantages that the laser system has over photo-optical types, but the main benefit is what MMT has found in its moulding shop. With the laser systems in place for more than a year, there has been no mould damage from nonejected parts and the system has required only normal periodic inspection.

Luoni notes that the transmitter and receiver can be mounted where they cannot be touched by ejected or falling parts, and out of the line of normal mould maintenance activities. The system components can be left on a mould or transferred between active moulds with ease.

The kits include power sources and electronic circuits that are very compact. MMT will advise on positioning of the components, but Luoni notes that moulding shop technicians, being mechanical-minded, quickly grasp the concept. They are drawing a continuous line across the mould, and the line can be angled and split as needed to cover all the critical points.

In terms of cost, Luoni says the laser system for a given mould may cost L 600,000 to L 700,000 (US$ 350 to 420), while an optical system will cost about L 450,000 plus about L 100,000 to install, or about US$ 325. The small difference in price is quickly recovered in the absence of maintenance and downtime. MMT, whose products include shock absorber assemblies for Lancia, kitchen utensils for the French marketer Tefal, bicycle seat frames for Selle Royal, and components for BMW, is using the system throughout its two plants. An additional benefit it has found is the complete absence of the false alarms encountered with photo-optical systems. Those interrupted production almost as long as real problems.