A closer look at scientific molding theory

A primer on the science of process development, recording, standardization and repeatability
By: 
September 28, 2016

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.

Garrett MacKenzie
Garrett MacKenzie

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:

Process variable

Description

Fill time

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

Peak pressure

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

Running mold temperature

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

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

Melt temperature

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.

Cycle time

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

Cushion

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

Water pressure (to process)

Gallons per minute measurement taken prior to mold entry.

Water pressure (from process)

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

 

Barrel temp actuals

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

Mold open time

Actual time mold is open between shots.

Back pressure (actual)

Actual pressure held during recovery stage (PSI).

Material moisture

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

Regrind percentage

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:

  • Viscosity curve;
  • cavity balance;
  • pressure drop study;
  • gate seal;
  • cool time.

1. Melt temperature is a key variable that is often overlooked! Once processes are validated, it is imperative that melt temp be recorded. When problems occur, melt temp should be one of the primary checks made prior to process adjustments. It is one of the focuses that can identify problems with your machine, or changes within the molding environment.

2. Measure and record the GPM of your tool's circuitry. By measuring this variable on every circuit, you are able to test your mold in the future when molding conditions show signs of overheating. It can help you to determine if the tool requires descaling. Each circuit should have a unique identification number. This is done by repeating your watering procedure. Keep the same supply and return pattern either by hard plumbing your loops, or by establishing a watering diagram that maps out the watering layout. It is also recommended that your circuits be uniquely identified in or out and that color identification (green, yellow, white etc.) be used. This not only improves your watering time, but will reduce the potential of miswatering.

3. A validated process should not require change. Process parameters should be capable of being repeated each time a mold is set and started. Before changing your process, it is important to look at your monitoring variables first! What changed? For instance, if fill time is slower, look at your temperature actuals. Check your thermolator: Is the actual temperature the same as your setpoint? Yes, there will be times when you will need to change your process to correct a molding condition. First, check for mechanical changes. After confirming that your machine, mold and auxilliary equipment are in a correct state, make changes to your process that replicate your process monitoring variables (fill, peak, etc.).

4. Prior to process changes, monitor your molding variables for significant fluctuations. A cushion that varies sporadically can be a sign of a worn check ring or barrel wear. Barrel temperature fluctuations can point toward bad heater bands or thermocouple positioning/ failure. Also consider recent mechanical changes made while servicing a press, and whether they might impact the validity of your process.

5. Always consider whether your operator could be affecting your process. Inconsistent cycles can wreak havoc on your consistencies. Improper part handling can cause defects that might be mistaken for a processing problem.

6. Part weight is a key recordable variable. Once a process has been validated, part weight (full shot, including runner) should be recorded. The data should not only include part weight after cycle, but also part weight with pack and hold removed. This can help you identify where in the process you are experiencing a change. Part weight should be verified at the beginning of each start up.

7. A clean and well-serviced mold is imperative to any successful molding operation. Tools should be cleaned no less than once per shift, and materials that are prone to gassing may need to be cleaned twice per shift. Slides and guide pins should be lubed, but it is important to remember that overgreasing can be detrimental to your process efficiency. Always clean your mold prior to any process change: Defects could be directly related to dirty vents.

8. Material moisture is a key function that is often overlooked when process defects occur. Moisture analysis should be part of your start-up procedure, and is completed prior to start up. Upkeep of your dryers is essential to your success. Dryer filters should be cleaned every shift, and you should routinely feel the supply and return hoses on your dryer. When a dryer is functioning properly, the supply hose will be hot and the return will be warm.

9. It is imperative that you analyze the effectiveness of your fill time. Some materials require a fast fill, but if you max out your velocities, you lose control of consistency. Monitor your fill and verify that your setpoint is being reached consistently. Whenever possible, your fill speed should be determined by performing a fill time study.

10. Hold time is a crucial element of your cycle. Establishing a time that is too low results in part-weight variations and process inconsistency. Too much time adds time to your cycle that isn't needed. Performing a gate seal study not only verifies that you are achieving gate seal consistency, but is a crucial step toward process optimization.

11. It is important to understand that adding regrind to your process changes material response. The best approach to introducing virgin/regrind into your molding equation is to treat it as a different material. Determine the optimal virgin-to-regrind ratio that reutilizes your regrind effectively without increasing your scrap rate. Once you've established an effective blend and process, record the process separately from a virgin run. Also, record the process monitoring data separately, then consistently repeat the blend ratio. You can further your ability to mold consistently by re-extruding your regrind with virgin base. This will reduce the potential of drop-down inefficencies (pellet size/ weight vs. regrind size/weight) and promote consistency.

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 garrett@plastic411.com.

Comments (2)

Please log in or register to post comments.
  • Oldest First
  • Newest First
Loading Comments...