1. The Do’s and Don’ts of Building a Robust Process
Garrett Mac Kenzie
Owner/ Editor (Plastics Technology Contributor - Articles)
Plastic411.com

2. My background: 30 years in plastic injection.
Started as an operator, worked my way up into engineering and management in 2001
Rubbermaid, Stanley Electric, Johnson Controls, Summit Polymers
Process development since 1992— 25 years now
Plastic411.com Owner/ Editor

3. ROBUST PROCESSING: To DO.. Or NOT to Do!

4. First, Let’s define what is meant when we refer to a robust process:
ˈpräˌses,ˈprōˌses/
Noun
A series of actions or steps taken in order to achieve a particular end
ro·bust
rōˈbəst,ˈrōˌbəst/
adjective
strong and healthy; vigorous.

5. So when we refer to a robust process, we are referring to:
A series of actions that lead to a strong and healthy production event.

6. But when is a production event strong and healthy in plastic injection?
First , we need to look at the 4 primary measurables that define the success or failure of our production operation:
Efficiency
The state of achieving maximum productivity with minimum wasted effort or expense.
Down Time
Mold changes, color changes, maintenance events, and poor planning
Yield
Total parts produced after scrap (which also affects efficiency)
Quality
Molded- IN quality, not to be confused with Sorted -in quality

7. These 4 measurables can be monitored and enforced even further, by requiring that:
Process demands meet or exceed quoted cycle
Minimal down time is repeatable between mold changes
Mold changes are standardized and repeatable
Start up is achieved with minimal scrap (ideally, 2 to 3 parts)
Scrap produced during each production run is preferably zero,
but no more than 1.5% of total production
Maintenance events are planned, not unplanned

8. AND Now we must ask, how do we get from: Point A:
The development of a part
To Point B:
A stable and repeatable process that achieves:
Fast and reliable part-to-part change over times
A molding process that quickly starts and offers optimized efficiencies, zero to low scrap and high yields
AND
Production longevity, with little to no production hiccups adding to down time

9. The definition of “Efficient” best describes our approach to developing these type of world class manufacturing systems:
ef·fi·cient
əˈfiSHənt/
Adjective
(especially of a system or machine) achieving maximum productivity with minimum wasted effort or expense.
Preventing the wasteful use of a particular resource.

10. Therefore, process efficiency is accomplished through:
A series of actions or steps taken in order to achieve maximum productivity with minimum wasted effort or expense.

11. So the question arises, “What are the steps?”
This is where we need to not put the cart before the horse. To outline the steps, we must first establish
the engineering phases that occur before a job is turned over to production. Those phases are:
Mold design
Tool building
Press set up
Process Development & Optimization
Process Control

12. These phases are all critical components of a robust process
These phases are all critical components of a robust process! Poor handling of any one of these affects our ability to build strength into our end result.
Now let’s break each phase down, and analyze what each will need to accomplish.
My primary focus is on the processing side, so I will just briefly touch on designing and building the tool:

13. MOLD DESIGN
Mold design is best served as a team participation event. During the design process, include your designer, mold maker and process engineer throughout the development.
This valuable approach will help to prevent major headaches, such as a mold that is difficult to process, a design that is difficult to build, etc.
MoldFlow™ is an excellent software tool that allows for a process viewing based on likely flow fronts and cooling data prior to building the mold. I highly recommend it to help identify problems before the mold is built and the problem becomes a reality that could have been prevented in the design stage.

14. MOLD BUILDING Assure that the mold is cooled/ heated properly
Verify that the movable half has enough support pillars to prevent flexing
Validate that the mold has been sufficiently vented
Assure that draft has been added to areas prone to drags and sticking
Record GPM of individual circuits to achieve base line data for comparison (Historical)
Record Supply/ return PSI of individual circuits for base line data
Record hot runner/ manifold steel temps for base line data

15. SETTING UP THE PRESS
Set up is one of the most important phases of the engineering stage. The goals for developing strong set up procedures are:

16. Standardize the set up to assure that it is installed exactly the same way every time
Develop methods of quickly centering and leveling mold
Hard plumb tooling to assure cooling is repeatable
Dummy proof connectors (water, hydraulic, electrical)
Standardize knockouts when possible
Standardize bushings when possible
Look for ways to eliminate waste of motion

17. Maximize change over efficiencies to make changeovers quick and consistent
Plan and prepare for each change prior to the event (NASCAR)
Don’t be afraid to think and communicate outside of the box
Look for tools or approaches that can make change faster and easier(NASCAR)
Take advantage of the strength and skill sets of your entire production crew (NASCAR)
Provide all the tools needed to complete the job (NASCAR)
Use time studies to identify problems and weaknesses

18. Develop your production operation to be operator friendly
Verify ergonomics
Provide instructions that clearly communicate direction (Lather/ Rinse/ Repeat)
Provide visual aids whenever possible
Let your operators have a voice
Mold in quality
Fast cycle times = less time to inspect
Look for ways to remove human error from the process
Treat your operator like a customer

19. PROCESSING 101: Developing and Optimizing a Process
Earlier we defined what was needed for a stable process as: 
A molding process that quickly starts and offers optimized efficiencies, zero to low scrap and high yields
Now, in the processing stage we add :
Dimensional stability
Repeatability
It all comes down to what steps are taken to identify.. and then clarify, a new process.

20. Let’s envision this ball pit as a multitude of potential processes
Let’s envision this ball pit as a multitude of potential processes. Each of these balls represent a process, both good or bad.
Now, let’s identify that only the yellow balls are good processes.. Because --
YES..
it IS possible to have more than one good process.

21. RECORDING YOUR PROCESS
In a nutshell, a robust process has been validated as profitable and repeatable.
Following validation, a "snap shot" of sorts is taken (through process recording) of all settings and monitoring actuals.
This helps to assure that the next time a press is set up, the process can be repeated.
It also develops a standard of recordable data that can be compared historically, which helps to identify changes within the core molding system.
The more care we put into documenting every recordable condition from a previously successful run.. the easier it becomes to repeat that process.

22. These studies include:
It is absolutely crucial that shot size be established using decoupled molding procedures to assure that parts are filled to 95-98% with no pack or hold pressure.
Once this has been established, hold pressure is added into the process until part aesthetics and dimensions meet established criteria.
It is also important to note that there are several important process studies that are part of establishing a robust process.
We do not have time to cover the procedures individually, but performing this testing is a critical part of every process foundation.
These studies include:

23. Graph: Courtesy -FimmTech
VISCOSITY CURVE STUDIES
Graph: Courtesy -FimmTech

24. CAVITY BALANCE STUDIES

25. GATE SEAL STUDY ( Gate Freeze )

26. FILL TIME STUDIES

27. Design Optimization Experiments

28. Key Injection Molding Fundamentals
The key to any successful molding operation is recording all data that is available when the process is producing minimal to no scrap and is running at optimum efficiencies.
By replicating these variables at machine start-up, you assure that you are repeating your previous run. Here are factors available that can be used to determine the success of your operation:

29. Processing: Data Controls
Process Variable
Description
Fill Time
Fill Time is the amount of time taken from the start of injection 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. In most cases, recovery should be completed within 2-3 seconds prior to the mold opening phase
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 .20 and .35 (6 to 9mm) depending on part size
Water Pressure (To Process)
Pressure measurement taken prior to entering mold (Supply)
Water Pressure (From Process)
Compared to "to process" pressure for calculation of pressure drop (Return)
Barrel Temp (actuals)
Actual running temperatures of barrel zones. Comparisons should be made between barrel temp actual and set points to assure barrel temperature is in control.
Back Pressure (actuals)
actual pressure held during recovery stage (PSI)
Material moisture
Material moisture is a critical control that should be measured regularly to assure that material has been properly dried.
Regrind Percentage
Regrind usage should be controlled to assure that process variance is minimal and consistent. It is important to maintain consistent regrind usage through proper blending or re-extrusion.

30. OTHER KEY POINTS IN ROBUST PROCESSING SYSTEMS
Melt temperature is a key variable that is often overlooked!
Once a process has been validated, melt temp should be recorded
When heat related defects occur, melt temp should be checked prior to making process adjustments
A validated process should not require change.
Process parameters should be able to be 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, pressure limits, etc..
There may be times you need to change your process to correct a molding condition.
-BUT-
Always check for mechanical changes first! Confirm that your machine, material, mold and auxiliary equipment are correct and functioning properly before making process changes.

31. 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 towards bad heater bands, or thermocouple positioning/ failure
Consider recent mechanical changes made while servicing a press, and whether they might be impacting the validity of your process
Verify the set up has been performed according to standardized procedures
Tools should be cleaned no less than once per shift, and materials that are prone to gassing may require twice per shift.
Slides and guide pins should be lubed, but it is important to remember that over greasing can be detrimental to your process efficiency.
Always clean your mold prior to any process change...defects could be directly related to dirty vents or components
A clean and well serviced mold is imperative to any successful molding operation.

32. Determine the optimal virgin to regrind ratio.
The best approach to introducing a virgin/ regrind mix into your molding equation is to treat it as a different material.
Determine the optimal virgin to regrind ratio.
Once you've established an effective blend and process, record the process separately from a virgin run.
You can gain further process control by re-extruding your regrind with virgin base. This will reduce the potential of inconsistent material drop-down rates.
It is important to understand that adding regrind to your process changes material response.
In some cases where aesthetics and dimensional stability are imperative to process requirements, re-extruding your regrind while adding base might be a viable solution. Establish a separate process for the virgin and the blend to assure that repeatability is achievable for both.

33. Process Optimization
Lean manufacturing is a primary driver of profitability. Without lean, organizations find their operations are sluggish and ineffective.
This not only affects a company’s ability to successfully grow and prosper, the capability of taking on new work suffers because current systems have not been effectively streamlined.
One of the first points to address under this topic is that it is possible for machines to run too fast. Every molding job is different, and the following conditions must be satisfied to properly assure that the optimization is successful:
Quality is key to the integrity of every molding operation. Running too fast to provide adequate operator inspection defeats the purpose of effective production. Rework and scrap reduce productivity, so steps must be taken to assure that quality is not affected by optimization.
Keep optimization tactics real, and consider potential failures that could occur due to optimization. For instance, knocking 10 seconds off of a job molding nylon parts sounds fantastic.. but if the lower cooling time increases shrinkage, it could potentially throw parts out of spec dimensionally.
During the optimization process, be vigilant to make adjustments slowly, and when changes are made to the process in clusters: make the adjustments, allow ample time for changes to take affect, take sample parts for part layout and then PUT SETTINGS BACK until the parts and process have been validated. Sample runs should always be run separate from production as a means to prevent suspect parts from getting to the customer.

34. Process Optimization COOLING TIME HOLD TIME MELT & MOLD TEMPERATURE
The following list outlines many of the parameters that can be considered when reducing the cycle time of a molding operation:
COOLING TIME
Cooling time is one of the easiest avenues of optimizing your cycle. In most molding scenarios, cooling time is set 1 1/2 to 2 seconds longer than screw rotate time.
It is important to point out that there are situations that may require a longer cooling cycle (such as dimensional requirements or parts sticking)
HOLD TIME
Hold Time is another major contributor to maximizing cycle time. The best method of accomplishing this is through a gate seal study.
Gate seal is the amount of required hold time needed to cool the runner tip into stasis, preventing material from flowing back into the bushing.
MELT & MOLD TEMPERATURE
Start your process at the lower end of the melt window
Raise temperatures in modest increments until you  achieve process stability
Higher viscosity of a lower melt temperature could lead to defects
Part dimensions or aesthetics may require higher or lower mold temperature
Key is to be a minimalist
Using minimal temperature helps reduce cooling time which helps improve cycle time

35. Process Optimization MOLD OPEN & CLOSE EJECTION
The following list outlines many of the parameters that can be considered when reducing the cycle time of a molding operation:
MOLD OPEN & CLOSE
mold breakaway and mold close- slow speeds are affected by the complexity of slides, horn pins, etc.. . so keep safety
and care for your mold first priority!
Keep Low Pressure Close settings as low as possible for mold protection
Safety and mold protection take first priority over mold speed optimization.
EJECTION
Improper ejection set up adversely affects cycle time
Use only the amount of stroke you need to:
-Remove the part safely
-Prevent sticking
Maximize mold open and close speeds to reduce mold open time

36. Full efficiency while maintaining world-class quality parts.
In closing, Lean Manufacturing requires continuous improvement and maximized efficiencies. When cycle optimization is complete, the resulting process will produce:
the highest yield
minimum to zero scrap
and minimum down time events
It is important to remember that a true processing regimen continuously evolves, improving upon itself.
By applying the steps outlined, we are able to achieve:
Full efficiency while maintaining world-class quality parts.