Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Measuring a weight or volume
Patent
1991-11-13
1993-11-09
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Measuring a weight or volume
264 405, 425145, 425148, B29C 4576
Patent
active
052600106
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of detecting the weight of plasticized synthetic resin injected from the cylinder of an injection molder to fill in the cavity of a mold.
BACKGROUND OF THE INVENTION
In continuous production of desired moldings by injecting plasticized synthetic resin from the cylinder of an injection molder through the nozzle to fill in the cavity of a mold connected to the nozzle, it is essential to inject the resin by accurate amount in term of weight corresponding to the capacity of the cavity or the volume of the intended product. If the filling weight is greater than the required value, excess resin is forced out through the parting line of the mold to form flash. Inversely, if the filling weight is smaller than the required value, recesses are formed on the molded surface, resulting in a defective product with sinks.
Conventionally, for continuous production of moldings of a constant quality, it is known that the weight of plasticized synthetic resin injected from an injection molder to fill in a mold cavity is detected on the basis of a position of a screw in the injection molder. The followings are the methods: injection molder before resin injection into the mold cavity. screw positions upon completion of dwelling following the resin injection into the mold cavity: dwelling. the screw position before resin injection into the mold cavity and that after dwelling following the injection.
In any of these conventional methods, however, the filling weight can be detected with a limited accuracy and so forth because of the following reasons:
Method of i)
The pressure of plasticized synthetic resin in the injection molder cylinder is not controlled nor monitored until immediately before resin injection through the nozzle of the molder. Therefore, the pressure of the plasticized synthetic resin does not become constant in the case of the plasticized synthetic resin having compressibility. Particularly where the screw of the injection molder is retreated forcedly for reducing the resin pressure in the cylinder so as to prevent the resin from leaking through the injection molder nozzle, the resin pressure fluctuates due to air drawn through the nozzle end.
The check valve provided at the front end of the screw is open before injection because no pressure is applied to the screw. When the check valve is closed under pressure applied to the screw for injection, small amount of resin flows back in the cylinder. This also results in the poor detection accuracy of this method.
Method of ii)
For the case (1)
The dwelling pressure is varied to meet various molding conditions for different products. The compression amount of compressible plasticized synthetic resin varies depending on the dwelling pressure.
A high detection accuracy could be achieved by comparing the resin amount in the injection molder cylinder under a constant dwelling pressure. However, since the dwelling pressure is one of the essential molding conditions that determine the shape and dimensions of moldings, it is impracticable to maintain a constant dwelling pressure.
For the case (2)
After application of pressure to the screw for dwelling is stopped, it takes certain time for the plasticized synthetic resin in the injection molder cylinder to be relieved from the pressure. Since the time required for this purpose varies depending on the kind and temperature of the plasticized synthetic resin, it is difficult to know whether or not the resin has been relieved completely from pressure.
Although it takes certain time to remove a dwelling pressure from the plasticized synthetic resin, it is general to assign rather short time for this purpose to avoid reduction in the productivity in the actual molding operation.
Method of iii)
The difference in the screw position indicates the volume of injected resin. However, the resin volume changes with the resin temperature and/or the pressure applied to the screw, and is not proportional to the weight of the injected resin. Moreover, the resin temperature
REFERENCES:
patent: 3750134 (1973-07-01), Weisend
patent: 3767339 (1973-10-01), Hunkar
patent: 4816197 (1989-03-01), Nunn
patent: 5057255 (1991-10-01), Sato et al.
Heitbrink Jill L.
Komatsu Ltd.
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