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Injection Molding and Moldmaking
with Surgical Precision

Injection Molding and Moldmaking
with Surgical Precision

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

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

Of great interest to buyers, accountants, quality managers, toolmakers as well to, of course, molders, is the projected service life of an injection mold for thermoplastics.  Many people in the injection mold industry use the SPI Mold Classifications as guides for estimating the expected life of a mold. The common classifications are:

  • Class 101

For a life in excess of a million cycles, with a hardened mold base (minimum of 28 R/C), hard molding surfaces (minimum of 48 R/C) with other details of hardened steel. Guided ejection is mandated as are other features such as wear plates for slides. Parting line locks are mandated, and corrosion resistance is suggested for cooling channels. This is the highest quality of the SPI classifications, usually accompanied by the highest price.

  • Class 102

This is specified for a lifetime not to exceed 1 million cycles. This features the mold base hardness of class 101, molding surfaces (cavities and cores) also feature the hardness specified in 101, and functional details are heat treated. Parting line locks are recommended. Guided ejection, wear plates, and corrosion resistance of water passages are not mandatory, but dependent on expected total production quantities. If expected cycles approach the maximum, then these features should be specified.

  • Class 103

Aimed at molds intended for under 500,000 cycles. These are molds for low to medium production needs, and corresponding price ranges. Mold bases are at least 8 R/C and cavities and cores in excess of 28 R/C. Any extras must be agreed upon.

  • Class 104

For less than 100,000 cycles and limited production. These are lower priced molds. The base can be aluminum or mild steel. Cavities and cores can be of the same or a metal agreed upon.

  • Class 105

These are for cycles less than 500 (prototyping only) and are very inexpensive. They can be of cast metal or epoxy.

These SPI, or Society of the Plastics Industry (http://www.plasticsindustry.org), classifications should and do take much of the guesswork out of estimating the useful life of an injection mold, but not every class 101 mold is the same, and this is true in all the mold classifications. Classifications indicate, but don’t guarantee quality.

No matter the class of mold, how the molder treats the mold can determine the life of the tool. I have seen and heard of aluminum molds that have lasted for years, indeed decades, and conversely witnessed class 101 tools rapidly turned into junk.  Much of what the molder does, or how he treats the tool will determine the life of the mold.

Never over-clamp (use more than required clamp force) the mold not only will you wear, stress, or deform the steel prematurely, you will peen closed the vents, leading to a viscous circle of more injection pressure being dictated and then even more clamp force.

Don’t neglect preventive maintenance on the tool, devise a schedule or consult generic schedules, or better yet consult a reputable mold builder. Taking the mold down for a day or two for PM can add years of life to a mold. If you don’t have in-house tooling capabilities for this you can contact a mold builder such as Matrix Tooling Inc. A great part of mold PM is disassembly and cleaning and replacing components such as springs, o-rings, and pins. Many molding shops designate a person for these relatively simple but extremely important tasks.

Don’t skimp on mold protection, sometimes called low clamp pressure. You want to be set “fat” enough to stop the mold from clamping well before a possible stuck part is crushed by the mold faces. Your press maker can train you in this if there are any doubts. Many mold protection settings can be defeated by closing the mold too fast. Never slam a mold closed. Where there are slides or other actions and angle pins, you should slow the movement before they engage. The possibility of saving a half second on the cycle here could cost days of lost production while repairing the damage that a defeated mold protection could produce.

Daily cleaning of mold faces and lubing components such as pins and slides will extend the life of any class mold. Use the right lube for the job: FDA and medical grease where required and high temp grease for hot running tools such as those running PEEK, PEI, PPS and PSU, where mold temps can exceed 400°F. Remember it is the film of grease a few thousands of an inch thick that does the job, so don’t goop the grease on. It is counterproductive and can attract dirt.

Again the SPI classifications can give the molder a good idea of the potential lifetime of an injection mold, but not all molds in any one classification are made equally. One should always have their molds designed and built by a reputable mold builder. A mold builder such as Matrix Tooling Inc. will stand behind and care for every mold the build over its extended lifetime.

Brent Borgerson
Senior Process Engineer (Older Molder)

 

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