Temperature can definitely be a factor, but so can humidity. Hot, humid days make molding chocolate difficult. It could be that the molds aren't cold enough, though. If necessary, after you have poured the chocolate, put them briefly in a temperature-controlled refrigerator to harden completely, and don't try to remove them before they are fully set. The larger bars mean you need more cooling time. Don't freeze them, though! You should be able to keep the trays at room temperature while filling them (as long as the room isn't too hot!) Then cool them, so that the chocolate can pull away from the edges.You don't want the trays cold from the start, because the chocolate will harden too quickly at the bottom, and warm chocolate will spill over and solidify around the edges making the bars stick. You want the bars to cool evenly.
I mould 75% of our products and our moulds are relatively deep. I often have the same problem as you. In my experience almost all imperfections are caused by inability to cool properly. Crystallization created heat, and that heat gets trapped by the mould. At least this is my current understanding. I'm always wondering if i should heat or cool the moulds before filling.Both seem to work, but not always...
According to Callebaut, moulds should be as close to room temp (20 C) as possible, but they also recommend slight preheating. So, around 25 C?
They go on to say that cooling is best done at 10-12 C, followed by time in the fridge. Circulation during the cooling phase is important, although this winter when it was between 10-15 C in my workshop (there is no heating in this country lol) I rarely had blemishes even though I had no special ventilation so I'm thinking at that temp as long your 10 C space is not too small, you'll be fine.
Source: http://goo.gl/BIjVFC (first result, PDF format)
From my limited experience...
Flexibility of the mould can also lead to uneven release and cooling marks. I noticed this especially with large moulds with flat areas without much detail. As the chocolate shrinks back from the mould it releases on the edges and corners ok: there it cools first and the mould is more rigid due to it's shape. The middle of the chocolate is the warmest and solidifies the last, but as the mould flexes with the shrinking chocolate it doesn't release well or stick. When this happens to me, the chocolate will still release hard and shiny (sometimes I have to give the mould a quick tap to release), but you see visible cooling marks where the chocolate released in stages.
I can imagine better cooling will help too. But as I can't control my cooling very well, I don't know to what extent.
Do you know how your moulds are made: injection moulded or vacuumform and what is the material thickness? How complicated/intricate is your design?
I prewarm my moulds just above room temperature, that seems to give better results. I cool at 8-10degC with the moulds slightly raised, so air can circulate all around. On the 'to-do' list is to put a fan inside the cooling fridge, to see if forced air circulation gives better results. Now it can take some time (hours) before the whole piece has cooled sufficiently and the middle finally releases from the mould.
Thanks all for the response. It definitely seems to be a temperature issue we are battling with. Unfortunately, we work in a shared kitchen so there are a lot of variables out of our control.
What we are finding out though, is if we put a wax paper liner down on the metal sheetpans just before placing the mold on it that it seems to give enough of a buffer that the mold fully releases. Don't ask me why that's working so well but it has thus far!
You need to pre heat the molds to your usage temp of your chocolate to avoid shock then you should get contraction and release if you are in temper.
What's the best way to pre heat molds? We haven't tried that approach yet.
The least expensive way (depending on the number of molds you have) is to use a dehydrator, which will also make sure your molds are perfectly dry. Believe it or not, Cabela's (the outdoor gear store) offers some good-sized ones that are remarkably inexpensive. Set the temperature as close as you can to the working temperature of the chocolate as Jim mentions.
Another option is a used proofing cabinet - just make sure that you don't hook up the water. You can usually find used proofing cabinets in a local restaurant equipment supply house that deals in used equipment.
I'm suspicious about the "need" to preheat moulds. I moulded chocolate all Dec and Jan at 10-15C temperatures, never preheated any moulds, and it worked great. Using Belcolade 73% dark.
Now that it's 25C+ where I'm working I do one mould, then watch it as I start the next. Once it starts to look like it's firming up in spots, I put it in a bag, and then into the fridge for 10 minutes. Then I take it out again and let it sit for at least another 10 minutes. Again, no preheating.
Another thing I've found is that moulds have to be 100% clean before you put anything in them. I use a high cocoa butter chocolate which is probably not the best for moulding and it makes a mess of the mould no matter how well tempered it is. Chocolate comes right out looking great but the butter it leaves behind will ruin the next batch if I don't reclean it.
I would also say you can make your own with an old fridge, or even a cabinet/box/etc. Put a lamp or other small heat source inside. Get yourself an STC-1000 to control the temperature. There are lots of people using these for beer making, etc. Google it and you'll find how-tos for using temperature controllers. It's like a 20$ device :D
That's an interesting thought and certainly not beyond the realm of possibility that our molds aren't squeaky clean. I agree too that I dont think it's the temp of our molds as we've made chocolate in various temps and it always comes out fine in our first round of batches.
It's odd though. We set our molds on metal baking sheets on a rack in a walk-in fridge and when we pull them out they've fully released. We generally do two rounds and the first round always goes perfect. The second round is the one that gives us issue and the only variable is that we re-use the metal baking sheets. We've tried letting them sit out for a half an hour or sometimes an hour to get back to room temp. We've also tried warming them back up on the stove tops ever so slightly. But the only thing that's consistantly prevented the second round from sticking is putting a sheet of wax paper between the molds and the metal trays on the second round.
Must be an issue with the temperature of the metal trays? Maybe the wax paper creates enough of a barrier to prevent the chocolate from setting unevenly....
Do people ever use proofing cabinets to set their chocolate? Right now we use a walk in fridge but would be curious if a proofing cabinet would do the same thing....
Since poo-pooing the notion of heating molds I have read that not doing so may cause the chocolate to 'stick' to the molds in such a way that they release nicely and look good but leave butter behind which is sufficient to mar the next round. Related experiment currently in progress...