Petri dish production achieves 4.8-second cycle

How do you achieve high speed production on a part that must stay absolutelyclean and cannot be air ejected from the mould or handled with suctioncups? The moulding and assembly of petri dishes provides just such a challenge.A set is made up of a polystyrene base and a cover that must be carefullyhandled. A system has been developed (Figure 1) that has demonstrated thehigh speed production that medical product manufacturers seek through preciseinteraction between machine and robot.

The cell consists of a Netstal Synergy 3000 injection moulding machinemounted with an 8+8-cavity multidaylight, or stack mould (manufacturedby Schöttli AG, Diessenhofen, Switzerland). The automation systemdesigned by Hekuma Herbst Maschinenbau GmbH, Eching, Germany, includespart removal, base and cover assembly and stacking, as well as packagingfor each cavity separately.

Part Removal

The horizontal robot for rapid removal operates from the back sideof the moulding machine, and has a handling head with an 8+8 arrangementto remove parts from the two sides of the stack mould. The machine signalsthe robot to move into place while the mould is still opening. The handlinghead removes eight bases and covers at the same time. Figure 2 shows atop view of the plant.

The moulded parts are gripped with dedicated mechanical parallel grippersthat are equipped with single-direction rollers. This ensures an accurate,gentle, and reliable gripping action. Cleanliness requirements demand thatthe petri dishes may not be picked up with suction cups. The parts alsomay not be demoulded with the aid of air, and hence, demoulding is performedby a two-stage mechanical ejector system. During the first ejection stage,the injection moulded parts are gradually detached from the mould coreduring the final phase of the mould opening movement, as the robot movesinto position. Once the gripper is in the correct position, the injectionmoulded parts are rapidly handed over to the robot in the second stage.The gentle detachment of the moulded parts from the mould surface ensuresthat a high moulded part quality is achieved.

During production startup, the moulded parts from the first two to threecycles are taken up by the handling head, directed into a chute on therear of the injection moulding machine, and conveyed into a container.This same procedure is followed when the injection moulding machine signalsthat reject parts have been produced. Good moulded parts are passed fromthe removal station on to a positioning station.

Transfer and Assembly

The transfer station incorporates two arms in a vertical position thatare ready to take the bases and covers for the petri dishes. The positioningarm for the bases takes eight bases from the left-hand arm of the handlinghead and a second positioning arm takes eight covers from the right-handarm of the handling head. After this, the handling head is released againand returns to its waiting position for the next cycle.

These bases and covers are placed in two rows of four parts each (facingup) on eight holders in the assembly station. The two positioning armsare now released and swing upwards into a vertical position, and the armfor the bases slides back into its starting position. The two arms in thetransfer station are now in the correct position for the next cycle.

The holders with the eight covers and the holders with the eight basesmove downwards and are then fixed in position mechanically, and the spaceabove the transfer arms is clear for the next operational step. The assemblyframe with the eight covers rotates 180 (facing down) and is placed overthe eight bases. The assembly frame is then released and rotates back toits home position again.

Postassembly handling

The holders with the assembled petri dishes are conveyed one furtherstep downwards and pass the dishes to the two conveyor belts underneath.The two-lane conveyor belt can be operated in both directions, which meansthat subquality parts can be segregated and conveyed into a container.At the end of the conveyor belts, the quality parts are positioned withthe correct spacing for the magazines. A pick-and-place station puts eightpetri dishes in the two magazines.

The shuttle loop system operates horizontally with five shuttles for fourstacks each. Two shuttles are located in parallel to the long side of theinjection moulding machine in the loading position. The long sides of theshuttles are parallel to the center line of the injection moulding machine.One track transports full magazines, with four stacks each, to the unloadingstation while the other track brings back the empty magazines to the loadingstation.

The feed unit unwinds polyethylene film, clamps the ends mechanically,and cuts it to size. The unloading unit lifts one stack into the open tube,which is folded and welded on both ends. The gripper arms holding the bagrotate the bag away from the moulding machine onto a bidirectional receivingstation. The stacks are then directed towards the appropriate storage tablein accordance with the "good-quality" and "poor-quality"signal.

Reduction Techniques

Several technical developments help to reduce the cycle time. First,a dynamic release-signal communication system means that the movementsof the robot and the moving mould half can overlap. No modifications arerequired to the mould for this. Result: a .4- to .7-second cycle time savings.Second, the first ejection stage of the moulded parts can take place duringthe mould opening phase. Result: .2- to .4-second savings. Third, the mouldhas a rigid return pin located on its ejector plate, while the grippercontains a reset mechanism. Through the forward movement of the ejectorplate, the demoulding, transfer, and release of the injection mouldingparts are all performed simultaneously. Result: .2- to .4-second savings.

Finally, the speed of the closing movement is greatly reduced for the startupprocess without the robot. This ensures a finely tuned mould safeguardsystem, even on fast-running machines. Once the startup process has stabilizedand the mould safeguarding system has been replaced by the robot, the clampingmovement is switched to its higher production speed.

The robot moves into position slowly during the first cycle and removesthe injection moulded parts. It then stops briefly in the mould and presentsthe parts to the machine setter for assessment before moving out againslowly. Once the second cycle commences, it can already move into positionrapidly. It removes the parts again, waits for them to be assessed andthen rapidly moves out again. This procedure is repeated until the settergives the signal for automatic operation to commence. Result: .2- to .3-secondsavings.

Apart from giving an increased quality, the procedures described can alsoachieve substantial cycle time savings. A conventional automated plantfor such products normally has a cycle time of about 7 seconds; with thesedevelopments, cycle time has now been reduced approximately 30 percentto 4.8 seconds.

Contact Information:
Hekuma Herbst Maschinenbau, GmbH
Mr. Kurt Knöpfler
Freisinger Strasse 3b
D-85386 Eching
Germany
Tel: +49 (8165) 633 18
Fax: +49 (8165) 633 55