Method and Apparatus of Judging Quality of Injection Molded Products

This invention relates to a method of judging quality (good or bad) of an injection molded product, and more particularly a method of judging the accuracy of the configuration and dimensions of the injection molded product.

According to a prior art method of judging the configuration of a product of an injection molding machine, a resin pressure or an injection pressure in a mold cavity is measured at different times during a pressure holding time to obtain a time function P=f(t), then this function is compared with a function P.sub.1 =f(t) which holds when a satisfactory product is obtained. Alternatively, a function T=f(t) representing a mold temperature is compared with a function T.sub.1 =f(t) representing the mold temperature at which a satisfactory product is obtained, thereby judging the quality of the molded product.

The state of the resin in the mold cavity changes in accordance with a state equation PV=RT (where P: pressure, V: volume, T: temperature and R: constant). Accordingly, where the quality of the product is judged by measuring only presssure P or temperature T as in the prior art, it has been impossible to grasp volume V.

Accordingly, it is an object of this invention to provide a novel method of judging the quality of a molded product by accurately grasping the pressure and temperature of the injected resin and the configuration of a molded product.

Another object of this invention is to provide a novel method of judging the quality of a molded product by simultaneously detecting the temperature and pressure of a resin injected into a mold cavity as functions of time, thereby accurately judging the volume or configuration of the molded product.

According to this invention, there is provided a method of judging quality of an injection molded product comprising the steps of measuring a temperature T of a metal mold, of a resin in a mold cavity, barrel or nozzle, or of barrel or nozzle of an injection molding machine at a time t elapsed after commencement of a pressure holding step of the injection molding machine; measuring a pressure P of a resin in the mold cavity or barrel or nozzle of the injection molding machine at the time t; calculating a time function A =f(t) using the temperature T and pressure P, where A represents a calculating parameter T/P, P/T, T.times.P, T+P or T.+-.P; and comparing a reference time function Ao=f(t) of a shot at which satisfactory products can be obtained with a time function An=f(t) of another shot.

In the accompanying drawings:

FIG. 1 shows a longitudinal sectional view of an injection molding machine and a block diagram which are utilized to carry out the method of this invention;

FIG. 2 is a graph utilized to judge the quality of a molded product;

FIG. 3 is a graph similar to that shown in FIG. 2;

FIG. 4 is a view similar to that shown in FIG. 1 and showing a modified block diagram; and

FIG. 5 is a graph similar to that shown in FIG. 2.

One embodiment of the method of this invention will be described with reference to FIGS. 1 and 2. The upper portion of FIG. 1 shows a conventional injection molding machine. More particularly, a stationary metal mold 10A and a movable metal mold 10B define a mold cavity 11 therebetween. A raw material resin supplied through a hopper 14 into a barrel 12 is heated by a heating device, not shown, provided for the outer surface of barrel 12. As the resin is moved forwardly (to the left as viewed in FIG. 1) by a screw 13 contained in the barrel 12, the resin is sequentially melted and the molten resin accumulates in a space in front of screw 13. When the quantity of the accumulated molten resin reaches a predetermined metered value, the screw is advanced while rotating from a retracted position by an oil pressure actuator 50 so that the molten resin is injected into mold cavity 11. An ejection pin 15 extending through movable metal mold 10B is normally used to eject a molded product out of mold cavity 11, but in this embodiment, a pressure sensor 16 is attached to the outer end of the ejection pin 15 for detecting the resin pressure in mold cavity 11. In the same manner, a temperature sensor 18 is attached to the inner end of another ejection pin 17 for detecting the temperature of the resin injected into mold cavity 11. A calculator 19 is provided for producing an electric signal representing a ratio T/P (temperature/pressure) using the pressure and temperature detected by pressure sensor 16 and temperature sensor 18 respectively as time functions. The output signal of calculator 19 is applied to a comparator 20 together with an output signal of a T/P setter 21 which is preset with a value necessary for obtaining a satisfactory product. An allowable or permissible value .DELTA.T/P is set in a setter 22. Output signals of comparator 20 and setter 22 are applied to a comparator 27. The output of this comparator is supplied to an alarm device 28.

In FIG. 2, 23 shows a reference line for obtaining a satisfactory product when allowable values .+-..DELTA.T/P are added to reference line 23. When the allowable values are added, the upper limit value and the lower limit value are shown by lines 24 and 25 respectively. When an actual shot line shown by dot and dash lines deviates from the upper and lower limit lines, a comparator 27 detects such deviation to operate the alarm device 28.

In the foregoing embodiment, although the resin temperature in the mold cavity and the resin pressure in the mold cavity were used, the temperature of the metal mold, or barrel or nozzle can be used, and the injection pressure or the resin pressure in the barrel or nozzle can be used.

According to this invention, as it is possible to simultaneously detect the temperature and pressure of the resin injected into the mold cavity as functions of time, it is possible to acculately judge the quality of the molded product, that is the volume or configuration thereof.

Alternatively, setter 21 is preset with the ratio T/P at any one or more times during injection and the setter 22 is preset with .DELTA.T/P at any one or more times.

FIG. 3 is a graph showing this modification. Where allowable values .+-..DELTA.T/P are added to reference values 24a, 24b and 24c at times t.sub.1, t.sub.2 and t.sub.3 which are arbitrarily set at the time of molding satisfactory products, the upper limit values become 25a, 25b and 25c while the lower limit values become 26.sub.a, 26.sub.b and 26.sub.c. Consequently, the actual shot value becomes deviate from the upper and lower limit values 25.sub.c and 26.sub.c. Then, detector 27 detects this fact to operate alarm device 28.

In still another embodiment shown in FIG. 4, instead of measuring mold temperature T and resin pressure P in accordance with time t elapsed after commencement of the pressure hold step, these parameters are measured in an arbitrarily determined time band t.sub.4 (FIG. 5) during time t elapsed after commencement of the pressure hold step. The block diagram shown in FIG. 4 is similar to that shown in FIG. 1 except that a time band setter 51 is added, which is connected to calculator 19. Consequently, the calculator calculates the ratio T/P using the pressure detected by pressure sensor 16 and the temperature detected by temperature sensor 18 in the time band t.sub.4 set in time band setter 51 as time functions. the output signal of calculator 19 is applied to comparator 20 together with the output of .DELTA.T/P setter 22 in the same manner as in the embodiment shown in FIG. 1. The starting point of time band t.sub.4 in time t is set by a timer, not shown. Use of the time band t.sub.4 reduces the number of samplings more than in time t, whereby the construction of the control apparatus can be simplified.