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)

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