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. The steps taken when watering a mold are 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 into 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 gallons per minute (GPM) 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 ensure that circuits are not performing poorly because of 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 to track a tool's water capacity, and also helps 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 with 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 circuitry 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 (for example, 80 psi supply water vs. 30 psi 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 the water circuitry. Water rushing from the supply side of a 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 a molder’s 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.
If a mold is run 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 at 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 ensure that the mold is in a "heat-soaked" state.
Draw a picture of the mold layout, and record temperature data in the areas on the diagram where 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 because of 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 set point. When troubleshooting, look for temperature swings between set points and actual temperature. Controllers that are providing a temperature that is much higher or lower than the set point you have established point to a faulty thermolator or chiller that is in need of service.
Proper set up, followed by thorough recording of cooling/ heating data, will help to ensure that consistent and repeatable processing is achievable. By correctly handling and monitoring the mold, a company improves its capability to successfully achieve process control and increased profits.
Garrett MacKenzie is the owner/editor of plastic411.com, as well as a consultant/trainer to the plastic injection industry. He has spent more than 31 years in plastics processing, engineering and development including experience with U.S./Japanese automotive OEMs and handgun manufacturing. He can be contacted at: [email protected].