Ask the Engineer

Ask the Engineer: Micro-Molding – Material Selection

Don’t let material selection keep you from your ideal part design. In some cases, it might just not be a problem at all.

We continue our discussion on the five key components of micro-molding, with a focus on Material Selection this week.  

According to Prospector, there are around 86,000 data sheets from over 860 different resin suppliers worldwide. That’s a lot of plastic to choose from. You can also custom compound many of these base resins, and the lists gets longer. 
Material selection alone can be difficult enough when it comes to choosing what works best for your product’s needs. The challenge comes when you have finally decided which material will work and you now have to decide if it will be moldable into the shape you need. At the micro level, the more extreme you are looking to push the limits, the more difficult this task will be. 
So where do you begin? Most often, we hit our database of resins trying to match up what the part calls for and the choices available. We may contact our favorite supplier to get advice on what materials will be best suited for our needs. But this may be more difficult can it seems. 
To demonstrate what relation material selection has to pushing the limits in micro-molding, we decided to build a sample part and put it to the test. 
We built a mold to produce this part with a very thin section. The thin section is 76µm or .003” thick. The mold was built with an optimal thick to thin transition and good gate and ejection locations to maximize the opportunity each material we chose would have to fill to the end, a full 42:1 aspect. 
Next we set off as one might normally, with data sheets. The problem is that with most data sheets, information is based on much larger parts. In Fig.2 (left), we see a common “dog-bone” sample from a resin vendor. The data sheet’s recommended gate size, flow properties etc, are all based on this much larger part. You can clearly see the gate as it compares to an actual molded part, only 800 microns tall. 
This can be a very deceiving starting point. The frustrating thing is you could ultimately come to this barrier and decide your part is not moldable and either give up or alter the project in a non-ideal manner. While the design process can end at this step on some occasions, it doesn’t always have to. Remember when pushing the limits, you are bound to run into a few barriers. 
Next we decided to ask the resin experts themselves for their advice. Certainly they know their materials and what may or may not work. Here are a few of the responses we got back when we asked them what material they would recommend to mold this part: 
  • The wall thickness would require an increase of 5x to 10x the current .003″ in order to fill.  
  • Molding the part thicker and grinding it down to the desired thickness. 
  • In looking at the part, the .003″ section will not fill any of the listed materials. In fact, you will be hard pressed to find a thermoplastic material that would fill that .0127″ long, .003″ wall. 
  • I do believe you would need to be in the .015″ wall thickness zone. 
  • We would suggest increasing the .003″ wall thickness to at least .015″ or better yet, .030″ in order to improve moldability. 
  • Filling this geometry would not be possible in a production environment. Consider molding a PP at approximately .016″ – .018″ and an Acetal 9 melt at .025″. 
Again this can be a discouraging step if you are convinced that your part is not as moldable as is. 
We push on… 
We chose 11 different materials that we see every day and put them to the test to see how much material would fill the thin sections. Many of these are highly engineered resins, some with glass or other fillers as well. 
  1. Polyethylene (PE)
  2. Polypropylene (PP)
  3. Polyamide w/ 30% Glass Fill (Nylon)
  4. Polycarbonate (PC)
  5. Polysulfone (PSU)
  6. Polyoxymethylene (POM)
  7. Polybutylene Terephthalate (PBT)
  8. Polymethyl Methacrylate (PMMA)
  9. Polyether Ether Ketone (PEEK)
  10. Polyetherimide (PEI, Ultem)
  11. Liquid Crystal Polymer (LCP)

As you can see, there was a variety of results. Four of the chosen resins actually made it the entire 42:1 distance. Some of the others went well past of the guideline of 6:1 and some, like PEEK and Ultem, didn’t go terribly far at all. 
Now here’s where it can get difficult. This test wasn’t to see how far one could push PEEK at a .003″ thickness. It was simply to compare how material selection can greatly determine the moldable outcome all by itself. The test was also not a determination that in all situations, the materials will always perform this way. Your results are very much tied to your design. But you can clearly see where data sheets, resin suppliers and your part design can be at odds. 
This is why we recommend you work with your micro molder at the early concept stages of design. Don’t let material selection keep you from your ideal part design. In some cases, it might just not be a problem at all. 
For example, Fig.4 is an Ultem (PEI) part with a 70 μm through hole. As we saw Ultem can be tough when it comes to filling small areas. Part design can sometimes allow even stubborn materials a chance at creating small details. 

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