I have been asked to build a 16 cav mold that will run 6 million parts per year. The customer requested an aluminum mold. Is this possible? Or should I recommend a steel mold for this volume of parts?
The molding compound will be PP or HDPE.

IBM did a study on aluminum molds and could run as many as a million parts. I've seen about a half million parts out of an aluminum tool with 15%gfpp.
This many parts annually needs to be steel.

Roger, I have 7075 or QC 7 Aluminum molds that run 375,000 cycles or more in a year. I keep spare cavities and cores. If cavities and cores wear or get damaged, I throw them into the scrap bin. If you are subgating, you will have problems. I make molds with replaceable subgate inserts. The first thing that goes is the cavities or cores that have ejector pins going through them. Gas erodes the ejector pin holes very quickly, even when the mold is vented properly. We run 350 or so molds here and 75% or more are Aluminum. I have lots of experience with Aluminum mold and you must keep then very clean. The water lines must be cleaned often and are a real problem if you have bad water. Cycle time is great and Aluminum mold cost less to make up front but are a trade off because of damage problems. If you have a first class molding operation, you can run Aluminum molds. Hope this helps.
Good Luck, L. Huckaba