The 32-cavity mold landed on our bench and looked like any other mold than had been run hard and put away wet. It was covered in grime and resin dust and after a 90-mile truck ride, was still dripping water out of every fitting. The only information we had was a verbal comment that “It ran OK”. It was our challenge now to get the mold cleaned up, checked out and back to the molder as quickly as possible. Looked simple enough—from the outside.
Since the tool came with zero documentation, we had little to go on that would help us create an accurate maintenance plan. The upside was since it was the first time our shop had ever seen or worked on the mold, it would now benefit from a fresh set of eyes and methods of cleaning and repair. Many times, when a mold is run and problems develop, the corrective action plan is focused on doing whatever is necessary to get a mold producing parts again, which usually translates into shims and process tweaks over several production runs. There just doesn’t seem to be enough time or data to analyze a problem—and repair it right, and permanently—the first time.
Types of Obstructions
Usually overlooked until a problem arises, water lines and bubblers can become caked with calcium and rust. Other restrictions—such as small pieces of plastic, rubber, nylon tape (from pipe fittings) and broken off chunks of brass, steel and even thread seal paste—can accumulate in areas hard to see. Sometimes these type of clogs are either caught in filters or are easily seen after the mold is disassembled as they damn up around o-rings or any reduction of diameter like water fountains/bubblers or baffles. Other times, the restriction is inside a plate or cavity block making identification and removal even more difficult.
Production techniques to remove these obstructions are limited to reversing the direction of water or air, hoping to dislodge it—but seldom work—or they land somewhere else in the mold.
On the bench, molds are disassembled to individually check each component, such as a bubbler, to locate the restriction, then blow it out or remove it with an appropriately sized piano wire or pick. A popular bench test involves blowing air through lines by hand with a nozzle while listening to the sound and feeling the amount of air coming out of the outlet line or fountain with the other hand. This subjective method then verifies that everything “feels OK” from line to line. Gauging the amount of air blowing through a water circuit is difficult and can vary from repair tech to repair tech depending upon technique and tools used. A small thing like using an air nozzle with a rubber tip that seals tight against whatever you are blowing through can make a huge difference in the feel and sound of the air coming out of fountains or the outboard fitting. As in most things in mold maintenance—technique is everything.
Cooling Is Critical, Ya Think?
During a production run, the cooling time in the mold often gets extended as a corrective action for product-related issues such as flash, burns, parts sticking, long gates or dimensional defects. Other defects and mold problems can be caused—or aggravated—by incorrect cooling through plating of the waterline from calcium/rust build-ups or reduced GPMs through restricted water lines. Obviously this is not a new revelation—but cooling channels that digress slowly are many times the last thing to check when the effects are finally realized in the part. When they are, process changes are implemented to override the effects of the reduced cooling. If one of these process tweaks works, the toolroom may not be alerted that the mold ever had a problem, so the issue does not get resolved when the mold gets pulled for PM because process was able to work around it.
Two Types of Flow Measurement
There are two types of water circuit checks. The first is a comparative analysis, where the water flow (GPM) of one circuit is compared to a like circuit of the same ID and configuration.For instance, say you wanted to check the water circuits of a 16-cavity mold and the pattern was 4 x 4 or four rows of four cavities. You would then check each of the four rows, one at a time to determine if there was a restriction somewhere in the row of four cavities, demonstrated by a reduced GPM readout in one of the lines. On our bench manifolds, we installed a shut-off valve to reduce the inlet line pressure to 10 psi, which puts the GPMs in the center of the flow meter gauge (1.5 GPMs). This now becomes our test setting for the rest of the water lines. This will allow an internal restriction to be easily recognized by an increase in inlet pressure in combination with a reduction of GPMs .
The second check involves the calculation of a Reynolds number to determine if the circuit has turbulent or laminar flow. To find out if a water line or circuit has turbulent flow capability, the best method is to install a flow meter (set up with quick disconnects) on a mold water line (checking only one line at a time) while in production at the press, so water temps and additives can be factored in. Water temperature, additives ratio, viscosity, pressure and waterline size all affect GPM—thus turbulent flow. A common rule of thumb without all the calculations is anything over 2.0 GPMs usually results in turbulent flow— for a better breakdown and remember that a favorable comparative flow rate does not verify turbulent flow within a circuit.
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.
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.
Watering variables
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:
Water pressure
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
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.
Temperature control
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.
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