TMF is not an election or an industry rating service. It is just a place to find stocks to invest in. They happen to have taken an early interest in DDD and SSYS ... too early, it turns out ... and their interest in Arcam has been much slower, but now seems to be building. So what? Arcam is not really a competitor to DDD and SSYS, so if you are analyzing either of these two there is no reason to really discuss small, specialized companies like Arcam. They recognize perfectly well that 3DP is not one market, but many, so it makes sense to consider them separately.
FWIW, the search box at the top of Favorite Boards I used is interesting because, if it recognizes a ticker, it gives a very different result than if it believes you are searching on a keyword. With a keyword, one gets to see the breakdown of results by article, comment, board post, etc. This is not available on the ticker results. There are, for example 2257 articles with the keyword "3D Printing".
Now, I happen to have picked Arcam as the most likely to succeed based on discussions on the TMF NPI board, back when it still talked about things like this for all the reasons I presume you picked them as well. But, I don't rely on TMF to do my picking for me.
AI
