1. Field of the Invention
The present invention relates to an abnormality detector for an injection molding machine.
2. Description of the Related Art
In the mold opening/closing operation and molded product ejecting operation in an injection molding cycle for manufacturing a molded product using an injection molding machine, a reference load on the motor driving a movable unit is stored in association with the time or the position of the movable unit and an actual motor load is sequentially compared with the stored reference load in association with the time or the position of the movable unit, so that, when the actual load deviates from the reference load by more than a predetermined threshold value, an abnormality in the mold opening/closing operation or ejecting operation is detected and the injection molding machine is stopped to avoid damage to the mechanism section and/or mold.
Japanese Patent Application Laid-Open Nos. 2001-30326 and 2001-38775, for example, disclose techniques for avoiding damage to the mechanism section and/or mold in an injection molding machine by setting as a reference load a load under which a normal mold opening/closing operation and/or ejecting operation was performed at least once in the past or a moving average value calculated from the loads under which a normal mold opening/closing operation and/or ejecting operation was performed several times in the past. The techniques described in these patent documents, however, the threshold value for abnormality detection must be set by the operator and the threshold value setting operation could be a burden on the operator.
Japanese Patent Application Laid-Open Nos. 2004-330529 and 2005-280015 disclose techniques for controlling an injection molding machine by determining a threshold value from an average value or variance of the motor currents detected in the past. In the techniques described in these patent documents, a monitoring range is set on the basis of the average value and/or variance of the motor currents detected in the past. The distribution of motor current variations might differ, however, from a normal distribution when the mold guide pins fit into the guide bushes during a mold closing operation or when the mold intermediate plate touches the movable plate. Some shapes of distribution could cause the threshold value calculated on the basis of the average value and/or variance of the motor currents detected in the past to become lower than an appropriate value and cause erroneous detection of abnormalities.
Although it does not relate to a technique for detecting abnormalities in an injection molding machine, Japanese Patent Application Laid-Open No. 2006-228181 discloses a technique and device for displaying variations of predetermined qualities of a plurality of products manufactured in a manufacturing facility, wherein quality data having a frequency distribution not approximating a normal distribution is transformed so as to approximate the normal distribution, an average value and various statistics are then calculated from the transformed quality data, and inverse transformation is carried out with respect to the calculated average value and various statistics to produce an average value and various statistics for the quality data.