In the area of the increasingly popular two-component injection-moulding technology, a frequent question is how the process for production of two-component parts should be organised for the best (most economical) results. Suitable injection moulding machines are available from many manufacturers and the automation can be handled by competent jig builders or by the general contractor responsible for the overall system. Toolmakers will gladly supply the appropriate moulds and dies. But this is where the main problem lies, since the combination of the mould and the parts handling system determines the output of the machine and its efficiency and productivity.

Depending on the complexity of the two-component part, it is first necessary to decide whether a complicated rotary two-component mould is to be used, which means that parts handling is relatively simple, or whether it is better to use a considerably simple standard two-component mould together with a more costly, intelligent parts handling system.
For reasons of cost, technical availability and process reliability, the combination "simpler two-component mould plus intelligent gripper system" is usually better.

Another advantage of this solution is that the decoupling of the system components "mould" and "gripper system" permits their manufacture and testing to be carried out in parallel, saving a lot of time for the user. Proof of this was recently supplied by Messrs. Geiger-Handling GmbH & Co. KG in D-72175 Dornhan, who have gained a good reputation over the past years in the area of automation and system solutions for the handling of plastic parts. In close cooperation between Geiger-Handling and the vacuum specialist J. Schmalz GmbH in D-72293 Glatten, two high-performance two-component injection-moulding systems with integrated robot handling were constructed for Messrs. ROS in Coburg, an established manufacturer of high-quality injection-moulded plastic parts for the automobile industry and for medical equipment.

 
Multifunctional vacuum (mechanical) gripper
improves productivity and saves costs

The ROS Group currently uses about 135 injection-moulding machines, and most of these are automated with the aid of parts-removal devices or linear robots from Geiger-Handling. Schmalz, in turn, have for many years supplied Geiger-Handling with the parts for the vacuum gripper systems, i.e. all relevant vacuum components, and their product range includes suction pads specially developed for the handling of high-quality injection-moulded parts.

However, the two-component systems presented all participants with a grater challenge, since ROS favoured the use of standard two-component moulds and intelligent parts handling with the aid of a 6-axis industrial robot and a combined vacuum/mechanical gripper system. The robot was selected because access to two-component injection-moulding machines from the top is difficult, which means that standard linear robots cannot be used to lift the parts out upwards. In addition, the 6-axis robot can be used for several different functions.

The handling tasks for the two two-component injection-moulding systems were as follows: removal from the sides of the left and right injection-moulded parts from the mould, transfer of the workpieces between the mould cavities, placing the left and right workpieces on a conveyor belt such that packing in cardboard boxes is simplified.
The two systems are used to produce very flexible plug-connector housing of different types for use in the electrical systems of automobiles. The one system uses three different moulds, while the other uses two different moulds.

The required flexibility and productivity could be achieved only by the use of robots and intelligent gripper systems. The actual grippers require particular attention, since these must be matched to the requirements imposed by the moulds, by the surfaces of the workpieces and by the handling tasks.

The high-quality grippers made by Messrs. Geiger consist of aluminium plates which form the body and ensure a long lifetime and the necessary stability and precision of the grippers. Since each gripper is designed specifically for handling one workpiece, conversion of the machine to produce a different part always involves changing the gripper. Geiger pays particular attention to a very simple connection between the gripper and the robot. The use of a single hose for compressed air, a single cable for the electrical connections and a quick-change coupling ensure that the grippers can be changed quickly and easily.

The only correct (vacuum) gripper
for each injection-moulded part

The gripper systems, which are constructed specifically for each mould, each have two suction pads per mould cavity. Each gripper has four stations, which means that there is a total of eight suction pads. Each of these is a suction pad of the type FSGA SI from Schmalz, a bellows suction pad made of silicone, with 1.5 folds and a diameter of 11 mm. The suction-pad materials silicone resists high temperatures and is very hard-wearing. The operating lifetime of the suction pads, and thus the technical availability of the system, was further increased by selecting the largest possible suction-pad diameter. Since the surface finish of the connector housing is not critical, suction pads made of silicone can be used. In cases where the surfaces have to be painted or treated in other ways, special HT1 suction pads from Schmalz would have to be used instead of silicone. These are made of a special material which leaves no marks on the workpiece surface (they are free of substances which could interfere with the wetting of the surface by the paint) and can withstand temperatures of up to 160° Celsius.

The cycle time for the moulding process and the safe and dynamic removal of the parts from two cavities in the mould and transfer to two other cavities is 40 seconds. The time available for parts handling is six seconds, including transfer, on system I and three seconds, without transfer, on system II. The total cycle time can be reduced by quicker handling of the parts, which means that the fast and reliable operation of the gripper system is a decisive factor. Fast operation, in turn, requires a powerful vacuum and this is generated, in this case, by compact ejectors of the series SCPM MiniPump. These are mounted in blocks of four, with one ejector providing the vacuum for a pair of suction pads, i.e. for one cavity.

Locally installed compact ejectors ensure fast
evacuation and consume little compressed air

The SCPM MiniPump compact ejectors require very little space and are extremely, which means that they can be installed close to the suction pads with no effect on the dynamic operation of the robot. This local installation – without long hoses – also means that the vacuum is available immediately when it is needed. The integrated vacuum monitoring system permits the system to check that the parts have actually been gripped, whereby a switching output of the miniature vacuum switch which is integrated into the SCPM generates the necessary signal.
An automatic air-saving function permits a further reduction in the amount of compressed air used – something which cannot be ignored on systems which run in three shifts.
A further integrated function of the compact ejector which deserves mention is the blow-off pulse, with which the workpieces can be placed more quickly and precisely on the conveyor belt.

The combination of a two-component injection–moulding machine, a 6-axis industrial robot and precise vacuum/mechanical gripper systems is an intelligent, modular solution. Only the gripper systems need to be designed and constructed specifically for each mould, but these almost certainly contain most of the process expertise which, in turn, is based on the functionality and performance of the vacuum components – the suction pads and the compact ejectors.