In my previous micromolding blog, entitled “Micro Molds, One Key to Success as a Micromolder, I mentioned “Scientific Molding,” a methodology that is also sometimes referred to as “systematic molding” or “scientific processing.”  Many molders who still harken back to the early days, when injection molding was more art than science, think it is impossible to scientifically micromold.

The chief reason for this thinking is the overwhelming popularity of Decoupled MoldingSM, a method first popularized by RJG Inc. founder, Rod Groleau.  This technique, which has since evolved into three distinct types, has been a major influence in the application of scientific principles to the process of injection molding.

Decoupled MoldingSM (and other “scientific methods”) breaks the molding process down into specific fill, pack and hold portions.  A key principle is how the injection of the melt is separated into fill (1st stage injection) and pack (2nd stage injection) portions of the stroke.  In later versions of the method, the injection portion of the cycle is divided even further.  But a major tenet of all these scientific methods is transferring the fill at about 95% of a volumetrically filled part. It is this premise which leads many molders to believe they cannot apply these methods to the molding of micro parts, many of which cannot be seen without the aid of a microscope.

Scientific Molding is much more than just separation of the injection phase though; it is all the steps taken when molding a part with best end properties, using a robust and repeatable process. These steps rely on the following prerequisites:

· A properly designed and constructed mold.

Without a robust mold, a robust process is next to impossible.

· A properly selected, sized and maintained molding press.

If the mold is great but the machine poor, you get a poor process.  Barrel size must not be too small or too big for the intended shot size, and the press must possess abundant injection pressure to avoid a possible pressure limited condition.

· Carefully chosen and handled resin.

Handling includes properly drying (neither over- nor under-drying the resin) and researching the resin to be used.

· Molding the resin at the correct parameters.

Do your research, and set the mold as well as the melt temps correctly.  It is usually best to begin mid-range, but much depends on the part geometry and wall thicknesses.  Also use suggested pressures and speeds.  This includes screw rotation speeds and back pressures.

 

Once the above prerequisites have been met, the initial sampling of the micro tool is done.  And if the mold functions properly and produces visually acceptable parts, the optimization and validation of the scientific molding process is performed.  This is usually comprised of of 6 steps:[i]

 

1. Viscosity study (or melt rheology study):

Usually cannot be done with micro parts.

2. Cavity balance:

This can be done on a multi-cavity micro mold.

3. Pressure drop study

This usually cannot be done completely on a micro mold, but watch your injection pressure so that you are not nearing the maximum.

4. Process window:

This study can and should be done for micro parts. Both the aesthetic and dimensional portions should be done.  The result is also known as the MAD or Molding Area Diagram, and it illustrates how robust the process is.

5. Gate Seal (freeze):

On a micro part, this can be a bit hard to do and requires a very accurate gram scale (which is a prerequisite in micromolding.)  Remember that you pay a lot for each place to the right of the decimal.  If you have a lost weight moisture analyzer, remember that it has a super sensitive scale.

6. Cooling Study:

Many times in molding, the sprue/runner set up determines the cycle time.  Nowhere is this more evident than in micromolding. Chances are the parts will have achieved optimum ejection temperature long before the sprue/runner will.  Of course, in the case of a hot sprue/runner, a molder can do the cooling study.

 

At Matrix Tooling, Inc. /Matrix Plastic Products, we don’t accept that scientific molding cannot be applied to micromolded thermoplastic parts.  In fact, we apply these proven techniques to develop robust processes every day, regardless of whether we are running large parts, small parts, or micro molded parts.

I’d like to thank my friend, Suhas Kulkkarni, Molding and DOE expert, author, teacher, consultant and Principal of FIMMTECH Inc.  Suhas offered me advice and let me post questions about this topic on his Injection Molding Online forum, where other molders also offered greatly appreciated input.

Brent Borgerson
Senior Process Engineer (Older Molder)
Matrix Tooling Inc./ Matrix Plastic Products
Wood Dale IL



[i] Robust Process Development and Scientific Molding, by Suhas Kulkarni (published by Hanser) is a good reference for the details in performing the 6 step optimization process.

 

In my previous post on structured problem solving, I discussed the “5 Whys” technique.  Although it is a very useful method, it can potentially lead you astray as a problem becomes increasingly complex and an intuitive answer (often guided by experience) is not apparent.

In these cases, it may be more beneficial to use a Cause and Effect, or Ishikawa Fishbone Diagram.   Karou Ishikawa (1915-1990) was a Japanese industrialist and statistician, whom we will meet again later when discussing other problem solving tools.  He was a contemporary and disciple of Dr. Deming (1900-1993.) He also shared great friendships with other North American quality notables such as Joseph Juran (1904-2008.)

A Fishbone Diagram can help us to identify possible root causes, sort and relate possible root cause interactions, and present them in an organized manner.  It works under the premise that all problems can be attributed to one of the following six causitive factors (or to a combination of these factors):

Manpower

Methods

Materials

Machinery

Measurements

Environment

Originally, only the first five factors were considered and were called the “Five Ms”, but environment was soon added to the list.  Many variations of this “5 Ms and an E” list exist, including: 8 Ps, 8 Ms and 4 Ss.  At the top of article is an example of a Fishbone Diagram using a short shot/small dimensional reject as the problem:

The above is a simplified Fishbone Diagram, but it shows how the main causes and subsequent sub-causes lead to the effect: in this case, a rejected shipment due to short shots and dimensionally small parts. This is why it is also known as a Cause and Effect Diagram.  It is a visual analytical tool that is especially useful to the injection molder in solving complicated problems.

Brainstorming, the technique we used out on the floor to quickly and informally solve molding problems, is also key when constructing a Fishbone Diagram.  Cross-functional problem solvers representing tooling, design, quality, maintenance, and molding gather around the conference table.  Everybody offers their ideas, and if the group agrees that they are valid, the ideas are posted as “bones” or spines on the diagram as possible causes or factors.  Later, the group decides which causes are critical factors and which are minor.  In the above diagram, they may decide that a cold molding room is a minor factor not worthy of further investigation.

There are 3 main rules for Brainstorming:

Everybody contributes ideas.

There are no “crazy” ideas; even those that are seemingly “off-the-wall” can lead to other relevant concepts.

Do not criticize others’ ideas or get personal.  This is the quickest way to shut off the flow of creativity and bring the brainstorming session to a screeching halt.  The idea is to generate as many ideas as possible to write on the board and then to decide which ones to include on the Fishbone Diagram.

In subsequent blogs, I will examine other structured problem solving methods that should be in every molder’s toolbox.

 

Brent Borgerson
Senior Process Engineer (Older Molder) 
Matrix Tooling Inc. /Matrix Plastic Products

Structured or Formal Problem Solving


There are times when informal or experienced-based problem solving alone may prove inadequate, a “band-aid” approach to a more complicated issue. Informal solutions that ignore the root cause of the problem are not likely to prevent it from rearing its ugly head again later.

Structured or formal problem solving methods and tools allow us to get to the root cause of the problem and solve it permanently. The myriad of analytical tools and structured methods available can befuddle even a trained statistician. A structured problem solving method can enable a molder to choose which problem solving tools best apply to a particular project. Many structured problem solving tools are also process improving tools. (I have always considered process improvement to be problem solving on a grander scale, but I will examine process improvement tools more specifically in a future blog entry.)

Almost all structured problem solving methods hark back to Drs. Deming’s and Shewhart’s PDCA (Plan – Do – Check – Act) Cycle, which can be traced back further to early 1600’s England and Francis Bacon’s Scientific Method. Today’s DMAIC (Define, Measure, Analyze, Improve, Control) from Six Sigma, and the various numbered (5-, 6- 7-) step methods of problem solving, all stem from Deming’s and Shewhart’s methods.

Before a problem can be solved, we must define and get to the root cause of the problem. A technique called The 5 Whys can be used. This technique was gleaned from the 1970’s Toyota Production System (TPS), and can be useful for quickly getting to the root of a problem.

For example, assume that a late delivery on a project has resulted in an unhappy customer.

Apply the 5 Why method:

1. Why is the customer unhappy? = Because the project didn’t get delivered as promised.

2. Why didn’t project get delivered as promised? = Because the job took longer than anticipated.

3. Why did it take longer than anticipated? = Because the complexity of project was underestimated.

4. Why was the complexity of the project underestimated? = Because we made a rough estimate, ignoring the complexity of separate stages of the project.

5. Why did we do this? = Because we were running late on other projects.

It is now apparent that we need to revise our time estimation methods.

We can also apply The 5 Whys to a more typical molding problem; say a reject from a good customer:

1. Why did the customer reject the shipment? = Because of short shots and small dimensions.

2. Why did we get small parts and shorts? = Because not enough plastic got packed into the mold cavities.

3. Why didn’t enough plastic get packed into the cavities? = Because the molding machine wasn’t capable of doing this.

4. Why wasn’t the machine capable of doing this? = Because the melt index of the resin lot in question was too low and the machine became pressure limited.

5. Why didn’t we know this? = Because we didn’t do incoming resin inspection and/or set process alarms on the molding machine.

Conclusion: We clearly need to revamp our incoming resin inspection procedures and assure that our process/machine is robust enough to avoid this mistake in the future.

(To be continued next month)

 

Brent Borgerson
Senior Process Engineer (Older Molder)
Matrix Tooling Inc. /Matrix Plastic Products

 

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