By: Brent Borgerson
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 can't apply these methods to the molding of micro parts, many of which can't 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.
[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.