Last week we discussed wall thickness by resin types, where we learned the importance of uniform wall thickness and provided a guideline on thickness based on your material selection. Another very useful resource in selecting materials is a mold flow simulator, which tests different resins and how they fill using accurate molding pressure.
Melt Flow Index
All materials have a different melt flow index (MFI), but what does this exactly mean? MFI is a measurement taken on how well a material flows at a high temperature through a specified diameter during a 10 minute test. This measurement for MFI is calculated into grams per 10 mins. Typically, higher numbers mean you have a much better flowing material that can fill thin wall geometry easier. But that doesn’t tell the entire story as a higher MFI doesn’t always mean that you won’t encounter any issues on thin part geometries. All materials have varying melting temperatures, so compare MFI between different families of materials such as polyethylenes and polypropylenes, which have about a 40° difference in testing temperatures.
Knowing the Material
Work with Proto Labs’ customer service engineers (CSEs) as soon as you begin quoting your parts and tell them what material you are considering having the parts produced in. Having this information early allows them to properly analyze part geometry for appropriate wall thickness using the MFI of the selected material. Often times, a part that is too large or has features that are too thin for a selected material will require an increase in wall thickness or an alternative material to be chosen.
How does a mold manufacturer know the selected material will work? This is where the software takes over. Proto Labs uses a proprietary ProtoFlow® fill analysis program that is truly unique to our molding technique. We have several available materials that can be tested using a resin’s MFI and your CAD geometry.
After your CAD model has been uploaded, gate location and quantity of gates are selected based on your part’s geometry and material. A simulation is then run by our mold designers to review:
- gate location
- knit lines
- incomplete fill
- balanced fill
- and most importantly, fill pressure