Afew posts have mentioned "drying". Acrylics dry as the vehicle that carries them, mostly composed of water, leaves the film. As water evaporates or is absorbed by the substrate, tiny acrylic polymer spheres are forced into ever closer contact. Eventually they are crowded so tightly that the spaces between them create capillary forces, and water is pulled from the paint film. This capillary action packs the acrylic spheres against one another in a honeycomb-like pattern, and they begin to form a continuous, cohesive film. As this occurs, the polymer spheres, composed of long chains of acrylic, actually deform and partially combine with one another in a process of film formation called coalescence.
Thus the "drying" of acrylic paints occurs in two very different stages and drying times must be thought of in two different time frames. The first stage, a relatively short period of time, results in the formation of a skin over the surface of the paint. This is the time that it takes for acrylics to "dry to the touch". At this point, the flow of water towards the surface is no longer sufficient to keep the paint film wet. Very thin films can feel dry within seconds, while thick films may take a full day or more to skin over.
The second stage of drying is the time for the entire thickness of the film to be thoroughly dry. That is, the time required for all of the water and solvent to evaporate and leave the film. This is a most crucial time frame, as the ultimate physical properties, such as adhesion, hardness and clarity, do not fully develop until the film is near complete dryness. For very thin films, this time may be a few days, while films of 1/4 inch thickness or more will take months and even years to be completely dry.
Both the evaporation rate of the water/solvent and the chemical bonding rate of the acrylic polymer spheres depend on temperature and humidity. Ideally, the temperature should be around 70 to 90F(21C to 32C) during the drying/curing process. Temperatures below 49F (9C) will not allow the polymer solids to properly coalesce to form a continuous film, and may result in film failure (cracking, adhesion failure, powdered film, etc.). Higher temperatures, like those reached with a hair drier or heat lamp, can speed drying times up significantly, but overheating can cause bubbling or burn the acrylic film. Likewise, lower temperatures will slow down the drying process and can be used to one's advantage for increasing the working time of the acrylic paints.
Relative humidity in excess of 75% will slow the evaporation of water from the surface, slowing down the drying process. Temperatures of 70 to 85F(21C to 29C) and humidity under 75% are ideal for drying. So unless your work area is cold and damp extra heat is unecessary but you can use a gentle heat to 85F if you wish