When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. Water set-up and design are every bit as important as establishing and recording a repeatable process. How a mold is watered is key to a processor's goal of consistency. If during the design and development stage, watering is put on the back burner as an "unimportant" variable, the potential for lost process control is huge.
Below are some insights into the most important facets of cooling or heating your mold, as well as what recordable data are important in the initial stages of process engineering.
When heating or cooling your tool with water, there are fundamental considerations that factor in to the function of your thermolator or chiller unit's ability to consistently maintain the temperature needed in the molding application you are using. Here are the must-consider functions when establishing mold temperature control:
Pressure is an important factor in process control. Maintaining your equipment to provide an adequate amount of water pressure to the mold watering circuits is a must. The GPM (gallons per minute) should be measured across each individual circuit prior to the first process run. This measurement should be taken from both the supply and return of the circuit to assure that the pressure drop is not substantial. This is especially important on "takeover tools" that may not have been properly serviced to assure that there are not circuits performing poorly due to scaling or obstruction.
Each circuit should be given a unique identifier, and the data for both the supply and return of the circuit should be recorded. This allows a molding company the ability to not only track a tool's water capacity, but can also help to identify circuits that are affecting a mold's process control. When part defects may be attributed to changes in a mold's cooling or heating consistency, the circuitry's GPM can be measured and compared to the original data.
Turbulent flow, best described as the rolling and swirling action of the water as it passes through the mold's water circuitry, is a key consideration in mold temperature consistency. A tool that has poor turbulent flow in its water curcuitry is more likely to have "hot spots" in areas of the mold where the water flow is less turbulent. By improving turbulent flow, the water's heightened agitation not only offers more consistent cooling and heating to your molding application, it helps to make the mold face temperatures more uniform and improves process control.
Many companies today use water return as a means of improving turbulent flow. Water from the supply manifold is resisted by a lower water pressure coming from the return( ex.: 80 psi supply water vs. 30psi return water). The water pressure from the supply manifold overcomes the lower pressure of the return manifold, but the resistance expands the water passing through against the wall of the water circuit, and improves its turbulent flow.
Molders can also improve turbulent flow by using smaller diameter water lines on the return side of your water circuitry. Water rushing from the supply side of your mold meets resistance as it enters into the smaller line on the return side of the water circuit.
There are a number of ways that temperature control factors into your ability to maintain process consistency. A mold's temperature controller can make or break a processor's ability to achieve process repeatability. GPM from both process supply and return should be measured and analyzed for pressure drop and changing conditions.
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If a mold is ran in several presses, a to/from pressure comparison between the temperature controllers of each press can help identify process inconsistencies. In addition, measuring the mold temperature of the mold face is essential. Once a repeatable process has been established, measure the temperature of each cavity, and various points on the mold face of both the ejection and cover sides of your tool. These measurements should be taken after the mold has been running for a significant amount of time to assure that the mold is in a "heat-soaked" state.
Draw a picture of the mold layout, and record the temperature data in the areas on the diagram that the measurements were taken. This is also a great way to identify cooling/heating inconsistencies that are affecting your ability to establish process control. For instance, when a molder is fighting inconsistent cavity filling, this may be happening due to differences in cavity temperature. If the face temperature of one cavity is cooler than that of another, the cooler cavity will fill at a slower rate than the warmer one.
Finally, watch for fluctuation in the temperature displayed by your temperature controller. A controller that is working properly should be supplying water at a temperature that is consistent with the setpoint. When troubleshooting, look for temperature swings, or controllers that are providing a temperature that is much higher or lower than the setpoint you have established. These conditions point to a faulty thermolator or chiller that is in need of service
About the author:
About the author: Garrett MacKenzie is currently working the Molding Manager for B&B Tool and Molding in Muncie, IN, where his responsibilities include technical support staff training, supervision, process engineering, tooling modifications, project/ process development, press/plant maintenance, and scheduling. Since 1996, processor training has also been one of his primary responsibilities, applying training under John Bozzelli and RJG. He is also the founder and editor of http://www.plastic411.com.