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Newsletter >> April 2014 >> Quality Control through Central Monitoring System

Central Monitoring adds value to Injection Molding industries in Germany

By Thomas Mann, Managing Director, Plexpert GmbH

The high quality demands from the customers, especially the automotive industry drives the injection molding companies to look for solutions to control quality parameters. Customers also demand that data of all production parameters be made available to them if and when required. Some suppliers of injection molding machines offer quality control within their own setup unit. But the recording of the data over a longer period of time as well as the storage outside the injection molding setup unit is not given or at least hard to achieve. Moreover most of the injection molding companies are working with machines from more than one supplier. This makes supervision and control of quality parameters even more tricky.

Another requirement that needs to be considered while talking about quality control in injection molding companies is the fact that most of these companies already use a specialized production planning system.
According to these facts the industry demands a central monitoring system that is flexible enough to work with different type of machines as well as has an open interface that can be connected to any production planning system.

Data Acquisition

Technically there are two possibilities to collect data from injection molding machines.
The first method is pretty simple and delivers valuable information about the cycle time of the machine. A 24V signal is taken directly from the control box in order to determine the cycle time.

Using an appropriate software it is also possible to calculate downtimes for each machine through the record of this signal. However the cycle time is not that important regarding quality but it is a crucial value in order to determine whether a running production is profitable or not. This method is the cheapest and simplest one to get data from an injection molding machine.

The second method to collect data from an injection molding machine requires an additional hardware. Each machine from where information shall be taken needs to be equipped with a special host computer board that can communicate with a computer network. The advantage is that one can directly access a great variety of technical data from the machine. It has to be considered that each machine is running its own protocol, hence using its own language in order to communicate with other systems. There are some protocols available (like Euromap 63) that try to form a standard. Often these protocols are used with additional features and deliver a limited number of values.

Critical Parameters

If we take a close look to the machine parameters it is vital to record the following data.
1. Cycle time - this we already discussed for the simple method.
2. Dosing time - this information can be taken in order to validate the quality of the melt and the way of how the material is drawn in by the screw.
3. Injection pressure - this is a significant value that has a direct influence on the part quality. It can also be used in order to determine the functionality of the non-return valve.
But it has to be noted that these values are given directly from the machine itself. The quality of the part depends on these values, but what happens inside of the mold can only be seen by extending this method of collecting data. This is done by installing sensors directly into the cavity of the mold. There are temperature and pressure sensors available that can be used on different positions within the mold. Recording the pressure curve and its integral is the best way to control a stable process and hence a good part quality. Using a combination of pressure and temperature sensors gives the opportunity to calculate even the viscosity of the material. This is used to see changes in the quality of a new batch of material.
Temperature sensors can be used to determine whether the cavities are completely filled. They can also be used to control coolant devices or hotrunner drops in a closed circuit.
In summary, real time acquisition and analysis of above parameters ensures quality control of injection molding process and output.

Case Study

A company with 70 employees and 40 injection molding machines was looking for a system for controlling quality parameters within their production.

The first step is always to estimate what system or what combination of systems is appropriate to reach the given target. In this case 10 machines (Type Sumitomo/Demag) and 10 machines (Type Arburg) had to be connected to a given ERP system. Since the machines where already equipped with host computer board the decision was made to install PlantConnect.

The base software was installed on a central server. From here all required settings could be done so that the user can determine what kind of parameter he want to take from which machine.
The communication with the machine is done using two other computers. One is responsible for the connection to the Sumitomo/Demag machines and one is responsible for the Arburg machines.
On each computer one DAS (data acquisition server) was installed. The advantage is that the DAS is able to run several protocols in parallel and stores data (if necessary) if the central server is not responding.

The data is stored within a Microsoft SQL database. The format is based on XML and could be used from the existing ERP system. The merge of the data coming from the injection molding machine and the production planning is done in the ERP system that takes out information directly from the Microsoft SQL database.

The installation of the software and the required test where done within three days. After this time the recording of quality relevant parameters did start.

The recorded data helps the company to ensure a continuous good quality of the parts being produced.