Injection-moulded plastic throttle body adaptors have two main advantages over their older aluminium sand-cast counterparts. Not only are they much lighter, they are also cheaper, since no secondary machining operations need to be performed on them after they have been made.
Nevertheless, before they can be attached to an engine manifold, the throttle bodies must undergo a process in which threaded brass inserts and steel-compression limiters are inserted into them to ensure they can withstand the assembly-process conditions.
The steel-compression limiters are designed to protect the plastic assembly from the compressive loads generated by the tightening of bolts that are used to mount them; the knurled bodies of the brass inserts lock tightly into the plastic, providing permanent threads into which screws can be inserted.
To fully automate the insertion process, Nottingham-based automation specialist TQC has developed a machine for automotive components manufacturer Mahle Filter Systems (UK). The machine can insert three cold-steel compression limiters and six hot-threaded brass inserts in each of two throttle body mouldings, enabling one moulding to be produced every 20 seconds.
The machine takes a two-stage approach to the insertion process. First, a twin-headed fixed pneumatic press at the front of the machine inserts the compression limiters, after which they are transferred along a linear guide to the back of the machine, where a four-axis servo-controlled, pick-and-place gantry manipulator performs the hot-insertion operations.
The approach provided the company with two main advantages, according to TQC sales engineer Mike Gordon. It limited the amount of work-piece movement that was needed, helping to minimise machine process time. Second, it enabled the company to create a compact machine that takes up little space on the factory floor while still maintaining the flexibility to accommodate future changes in the throttle-body design.
By using a servo-position system, changes in the positions of the inserts can easily be programmed into the machine without expensive re-tooling of the fixtures. In addition, by using Z-axes servo drives capable of both torque and position control, the hot-insertion process can be closely controlled to achieve the required balance of insert temperature and insertion force. In this process, it is important that the inserts ‘melt’ into the high glass-content body material without ‘burning’, which would cause localised vaporisation of the material and create a weak insert.
Before any component insertion takes place, proximity sensors on the machine check to ensure that an operator has correctly placed the pair of plastic throttle-body adaptors onto location fixtures at the front of the machine and that the compression limiters have been located over the mandrels, which will then be used by the machine to maintain their position while the pneumatic press inserts them. At the same time, moulded location pin features on the throttle body are checked for correct length using a relative-displacement sensing mechanism.
If all parts are present and correct, the cold insertion of the steel-compression limiters is then automatically performed at the front station by the twin-headed pneumatic press, which pushes home three of them per part, after which it verifies their correct position relative to the surface of the throttle body.
The fixture holding the two body-adaptor mouldings is then retracted into the hot insertion station of the machine along the pair of linear guides. There, threaded-brass inserts are fed from a vibratory bowl feeder via twin vibratory linear tracks to two temperature-controlled stainless-steel heater blocks, where they are heated to approximately 350ºC and queued along a track ready for use. Thermocouples placed around the exit of the heater blocks ensure that the correct temperature of the block is maintained.
The hot inserts are then transferred from the track into the throttle mouldings by a Festo four-axis servo-controlled pick-and-place gantry manipulator with two pneumatically powered grippers. Each throttle moulding in the machine has three inserts successively pressed into its top face. Then, the support fixture holding the mouldings is rotated by a pneumatically powered rotary actuator, so that a further three inserts can be pressed into the moulding’s opposite face.
Once the brass inserts have been pressed into the throttle mouldings, small air jets cool the inserts and the surrounding area to stabilise the insert, after which the mouldings are checked to ensure that the inserts have been pressed into them at the correct position of flush to 0.2mm proud, again using a relative displacement sensing mechanism and a position sensor.
If the machine is paused during the insertion process, the insert will cool below its optimum temperature, rendering it unsuitable for use. To ensure that such a cold insert is not used after the machine is restarted, the machine will reject the pair of inserts and the system will select another pair of hot inserts from the heater blocks.
Ensuring the high-speed cycle time of the hot-insertion process was one of the key challenges for the software team at TQC, which had to create a set of software algorithms that would precisely define the velocity, acceleration and deceleration of the two vertical-axis insertion actuators to ensure the correct force was applied to the inserts during the insertion process as they are pressed into the throttle moulding.
This is necessary to ensure the component will withstand the pull-out forces necessary to meet the throttle-body adaptor’s specification. Using servo systems for this process also enables easy adjustment of the insertion process to maintain the quality of the product.
It is at this hot insertion stage where TQC claims the new machine has a significant advantage over its competitors.
‘Whereas our competitors proposed using individual, physically separate pneumatic-insertion mechanisms for each phase of the hot insertion process to improve machine cycle times, we found that we could achieve the same results using a four-axis servo-controlled manipulator to perform all 12 hot-insertion operations,’ claimed Gordon.
After the final hot-insertion operations have been successfully performed, the fixture is returned to the front of the machine, where the operator manually fits a seal to each moulding. The machine then verifies the position of the seals and providing that everything is correct, places a ‘pass’ stamp on each completed throttle-body assembly.
The machine uses computer, electrical and pneumatic technology to perform the automated insertion process. The overall control of the machine is handled by a Mitsubishi Programmable Logic Controller (PLC), which is connected via a Profibus-DP interface to the Festo intelligent servo-drive system. This enables the PLC to control the movement and configuration of the axes, in real time, to successfully complete all of the insert operations.
In addition, the PLC is programmed to display feedback to an operator via a colour touch-screen Mitsubishi HMI (Human Machine Interface). During automatic operation this not only provides details of the status of the machine and its constituent subsystems, it also graphically highlights whether each set of parts has passed or failed the insertion process. The HMI also provides a convenient interface for manual operation of the machine for diagnostic and maintenance purposes and X-Y-Z position setting operations for each of the servo system’s insert positions.
The TQC automatic insertion machine was successfully installed earlier this year at Mahle Filter Systems, where it has produced a batch of throttle-body adaptors for engineers at the Ford Motor Company, who are now using them to develop a prototype engine.
Gordon predicts full-capacity production of the throttle-body adaptors will start later this year.