I’ve been working a lot in Vanna's Choice yarn, which I really like, as acrylics go. But I do have one problem with it. See, acrylics don't respond much to blocking, because they're acrylic. Okay if you’re making an alligator, but not so great when you’re making something that should be flat. Like the peanut butter and jelly. I tried blocking the pb the usual way, wetting it and pinning it down until it dried. I figured it was worth a shot. It came out okay, if a little stiff. It's flat, but it also wants to curl back up given the slightest nudge. I tried something else on the jelly, and it's amazingly better. I steamed it (using steam setting on the iron, hovering over the jelly and giving it a few shots of steam) and then set a heavy book on it while it cooled. Left the book for at least ½ hour while I did other stuff. I wasn’t in a hurry anyway. And the jelly? Not only is it flat, with no inclination to curl back up, it’s also softer, and has a nice drape to it. So different, it’s hard to believe I used the same yarn. I rolled it into a tube, it fell back out flat. That’s what you want in a sandwich filling.
(one small note: this also flattens your stitches some, so test this before doing a whole afghan to make sure you like how it looks. To avoid flattening the stitches, you could place the piece between a couple of towels, which should allow some fluffiness to remain while still giving you soft, non-curling pieces.) I will post some pictures soon, so you can see the difference.
Got the idea from something I learned in my college days (I have a BS in Clothing & Textiles). This actually saved the day a few years back when mom was making bow ties for one of my brothers' weddings. We were using a polyester satin (of course, because that's what you can get) and she couldn't get any of them to iron flat. She was getting desperate, and had tried everything she could think of, including wetting them down completely, and then trying to flatten them with the iron. Nothing was working. Then I remembered this tip from my college days: when working with acrylics, polyesters, nylons, etc, iron them using correct heat settings, but then press down on them with your pressing block (or anything flat and smooth) and press down, or keep that weight on it, until it cools completely. See, the heat relaxes the fibers, but they'll go right back to their former shape if you don't do something to stop that. That's what a pressing block (smooth piece of wood, fits in your hand) is for. A heavy book will do nicely as a stand-in. Especially since you can leave it and walk away, instead of pressing down with your hands while it cools. Speaking of hands, your hand will work fine in a pinch for most things, if you don't mind a little heat.