- Product and Technical Development -

No.25


Small Injection Molding Unit
using thread-like materials


We succeeded in developing a prototype of a samll injection unit which uses thread-like materials. By this invention, an injection molding unit can be smaller, and also can be used as a built-in device in the production line, being held by a robot.

The developed injection unit is 110mm long with a diameter of 50mm. The injection unit does not incorporate a motor for driving the screw. Therefore, in order to rotate the screw, it is necessary to connect the unit with an external motor or obtain rotating force by wire release.

Unlike conventional types, the screw of this injection unit has no grooves. Grooves for passing the material are formed onto the cylinder. Thread-like material is fed from the above of the unit. By rotating the screw, the material is wound and fed to the tip of screw along the grooves. In order for the material to avoid slipping on the screw surface, incontinuous surface is formed along the axial direction. Furthermore, the material is held between the screw and the pinch roller just after the material insertion, so that the material will be moved ahead securely along the grooves.
The material is mixed gradually in the space between the screw and the cylinder, as the material moves ahead. Furthermore, the material is melted by the heater at the tip of the unit, and the melted resin is injected from the nozzle.
Unlike conventional injection molding mahcines, the screw of this invented unit does not move forward and backward in axial direction. And the unit is not equipped with an injection plunger. The rotation of screw brings the thread-like material forcibly into the grooves in a series, and consequently the melted material is pushed out from the nozzle end. The amount of injection will be determined by the rotation speed of the screw, which is a structure with a new concept.
The reason why the grooves are formed onto the cylinder unlike conventional types is that the material is cut off under the conventional structure in which the grooves are formed on the screw. In this invention, the thread-like material is fed from a single inlet port, and the material would be cut off, every time the screw rotates if the grooves are formed onto the screw.
The compression rate is about 10% which is lower than that of conventional screws -- about 50%. But conventional types of material are pellets, while this invention uses thread-like material, and therefore it is not necessary to make the compression rate such high in this invention, because air is hardly included in the thread-like material.
Conventionally, material of pellet type is typically used for injection molding. This is because material is easier to be engaged with the screw, once the material is cut into object of small size. In actual preparation process of material, resin is first to be formed in shape of thread by an extruding machine and then cut off in small pellets. Therefore, if the thread-like resin from the extruding machine can be used as it is, the resin is simply to be wound, thus the conventional cutting operation is no longer needed. Some material suppliers have already started to provide thread-like material.

This injection unit can be held by a robot in production line. Therefore, injection process can be conducted within the production line. Conventionally, resin parts are formed in different places and brought to a production line to incorporate the formed parts in products. But by this invention, resin parts are formed at the position of assembling, contacting dies to the position from both sides.
One major feature of this invention is that the injection is possible in both upward and lateral directions. In conventional methods, pellets are entered into the hopper and are fallen down into the cylinder. But this new invention winds material in thread, so that the material can be securely fed to the tip of the unit, no matter where the robot is directed to.
As a purpose of use, this unit can be used not only as a part forming device, but also as a fastener to fix parts. For instance, if 2 pieces of plates with holes are placed onto each other and resin is injected into the holes, the 2 pieces of plates will be bonded together.
When attaching the unit onto the top of NC milling machne, the unit can be used as a rapid proto typing machine to produce resin models. In the application, the rotating power is obtained from the spindle, and the table is controlled with the NC and forming is conducted one layer after the other to eventually produce the 3-D structure. Unnecessary areas can be ground off on the table. There is no limitation to the materials which can be used, such as optically formed material, and there is also no need of expensive device.
We are now testing on injecting pressure of the new unit. If the pressure is not enough for injection molding, we plan to add a plunger to the inside of the screw.



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