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Plastic injection processing has experienced massive change in the last 20 years. The days of molding by time and pressure have given way to molding by position, peak pressure and process repeatability. Don Paulson, RJG and John Bozzelli were major pioneers in the development of decoupled and scientific molding, as the procedures were developed and given structure. Plastic injection as an art gave way to plastics processing as a science of repeatability and standardization.

In recent years, the term "scientific molding" has become a buzzword of sorts. There are many organizations claiming to be scientific molders and trainers. It is important to note here there are many variances from one training program to another. Great care should be taken when choosing a training source for scientific molding theory and applications. 

First, we need to understand what scientific molding is as an application. It is the science of process development, recording, standardization and repeatability. These variables are strictly dependent upon machine and mold validation and design. Poorly functioning machines, or molds that fail from a design perspective, may inhibit or remove a manufacturer’s ability to develop a process that is repeatable. For instance, the chances of controlling a process are poor if the cavitation has not gone through the process of balancing the runner to ensure equal flow into each cavity. Equally, control is hard to achieve if a barrel is worn to the point that it is impossible to maintain a consistent cushion. Moldflow is an excellent source of testing mold design prior to its inception and construction. From a machine validation standpoint, John Bozzelli and RJG have tools and training available for machine and process testing.

Once the machine and tooling have passed validation, a process is developed using the decoupled molding technique, which then is further developed into a science-based process and application through testing and recordable data. One analogy that describes the development of scientific structure is this:

We have all seen a pit full of balls that children love to jump into. Imagine that one of these pits is filled with nearly all red balls (the red balls representing processes that do not work and produce scrap or cannot be controlled) and a few blue balls (representing processes that are repeatable and profitable, efficient with little to no scrap). Imagine reaching into the pit with your eyes closed trying to grab a blue ball. With so many red balls, it takes time to finally grasp a blue ball in your hand. When you finally achieve this, you take great pains to record everything you had to do to grab the blue ball so that the next time you try to get it, it becomes easier and faster to obtain.

In a nutshell, a science-based process is a process that has been validated as profitable and repeatable; following validation, a "snapshot" of sorts is taken (through process recording) of all settings and monitoring actual. This helps to ensure that the next time a press is set up, the process can be repeated. It also stores a standard of recordable data that can be compared historically, which helps to identify changes within the core molding system. In essence, the more care we put into documenting every recordable condition from a previously successful run, the easier it becomes to repeat that process.

The following variables should be recorded and monitored as scientific molding data pertaining to the plastic injection industry standards. When changes in molding conditions are noted, these variables can help determine the cause of change, and repeating previous molding variables often will return the process to a good running state.

This article outlines the basic scientific molding approach. We will provide you with multiple reference sources to further examine the applications and theories of scientific and decoupled molding. Some of the primary variables that affect process consistency are listed below.

The key to any successful molding operation is to record all data that is available when the process is producing minimal scrap and is at optimum efficiency. By replicating these variables at machine start-up, you ensure that you are repeating your previous run. Here are some other factors that can help to determine the success of your operation:

Fill time is the amount of time taken from the beginning of shooting material to the point of reaching cut-off.

Peak pressure is the maximum pressure achieved at the point of velocity cut-off prior to dropping off into hold pressure.

Mold temperature should be measured at various points in the mold in a running state. Measure individual cavities, runner system, bushing area etc.

Screw rotate time is the amount of time it takes for the screw to recover.

Melt temperature is the actual temperature of material as it exits the nozzle tip. This measurement should be taken while barrel is in running state.

The amount of time taken for each shot to be produced.

Cushion should hold steady between 0.15 and 0.35, depending on part size.

Gallons per minute measurement taken prior to mold entry.

Compared with "to process" pressure for calculation of pressure drop.

Actual running temperatures of barrel zones. Comparisons should be made between barrel temp actual and setpoints to ensure barrel temperature is in control.

Actual time mold is open between shots.

Actual pressure held during recovery stage (PSI).

Material moisture is a critical control that should be measured regularly to ensure that material has been properly dried.

Regrind usage should be controlled to ensure that process variance is minimal and consistent. It is important to maintain consistent regrind usage through proper blending or reextrusion.

It is important to note that there are several steps that are part of establishing a robust process. These studies include:

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] .

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