Scientific Molding:Definition and application

Injection molding came a long way in a relatively short span in the realm of the industrial age. In the beginning, molding was more of an art form than it was a science. John Bozzelli and some of the other pioneers in decoupled and scientific molding changed all of that. They developed new molding techniques based on processing variable replication that revolutionized the methods of an entire industry. Today's article defines the practice of "scientific molding" and will touch on some of the fundamentals that this application supports.

Scientific molding is best defined as an extension of decoupled molding theory. When using decoupled molding procedures, a process is established with the part filling out at 95-98% of being completely filled with pack and hold pressures set at 0. A typical part at this percentage would appear slightly unfilled, or with a sinky appearance. Pack and hold are then added back into the process adding just enough to pack out the part(s).

Where scientific molding takes a step further is the extension of this procedure. The procedure of decoupling is completed, and then all data that can be recorded is taken..in essence taking a "snapshot" of the process while it is in a good running state. Typical molding variables to be accounted for are melt temperature,cushion, fill time, recovery time, peak pressure, cycle time and any other actuals data that can be recorded. The ones listed here are the basics, but I'm a bit more thorough. For instance, I like to get:

- actual mold temps in several locations(running state)
-water pressure(to and from process)
-barrel temperature actuals(each zone, using probe)
-replication of water set up(hard plummed, or water set up sheet, repeating flow directions)
-decomp.aft. rotation actual

The primary point of scientific theory is to repeat the process set-up and actuals to assure that a good process results by repeating the previous values. Because of this, it is only logical that the more data you are able to document when a job is running well, the more likely you will be able to repeat the molding condition the next time around.

That is a basic description...now for a little more advanced version, let's touch on some additional testing procedures...

-In addition to melt temp, a hold pressure study should be completed to determine the gate seal time
-With the shot size set at 85%, back pressure should be raised in increments...stopping the screw after allowing soak time during cycle to perform a melt temperature reading. This data should be recorded on a graph. When the melt temperature stops rising, this is the max setpoint for back pressure. Back pressure should be set at the point in the graph that major temperature increases falter, and melt temperature rises are more subtle in their rise.

-With the shot size set at 85%, velocities should be adjusted from slow to fast, graphing as described above. This time, fill time is the factor to watch. When fill stabilizes and the time stays equal, you have maxxed out your velocities. Again, optimally the velocities should be set at the setpoints recorded that show fill time going from rapid increases to a gradual rise.

It is important to realize of course that sometimes your process will require adjustments to be made in order to get a process running good parts again. Whenever possible, the technique above should be followed as closely as possible to assist you in running your processes at optimum efficiciencies.

When a part requires velocity profiling, each stage of the profile should be treated as a single velocity. Adjust the speed for each position from slow to fast, again monitoring the fill time as described above.

Coupled with set up standardization, adding scientific method to your process will greatly improve your control of each of your processes. It is important to note, that when a process has been established as consistently performing well it is a solid framework and should be followed closely with only minor changes to process. If you see a drastic change in process variables, it is important to realize that something changed. All of your recorded variables should be examined for change and when a change is discovered it should be analyzed for potential causes in the state of that condition(in ex: melt temp change due to a heater band going out).

Look for more on this subject in later articles....