Possibly the number 1 question I get when people want to start working with Worbla is “Can I use a hairdryer?” The short answer is: “NO” The slightly longer answer is: “NO – because it will take you FOREVER to make even a small piece and you will likely burn out your hairdryer.” And the long answer is “NO – because you will either a) take forever, b) burn out your hairdryer, and c) not get the most out of the Worbla.”
The activation temperature for most types of Worbla is 90 degrees Celsius/194 Fahrenheit. Most hair dryers don't get that hot since your scalp will start burning at around 60 C/140 F. Depending on your hairdryer model it could possibly work with prolonged exposure on the “high” setting, softening a piece of Worbla enough to activate the adhesive and making it tacky enough to stick to a piece of foam. However that bond will likely be extremely weak – if the foam is flexed at all the Worbla will pop off. With a hairdryer you ARE NOT using Worbla to its full advantage.
If you don't get the Worbla to 90C you won't get it hot enough to stretch over curves, press into details, or roll into shapes – basically all the reasons you spent the money on the material in the first place. Technically there are other ways to get the Worbla to 90C. You can stick it in near-boiling water, hold it over the steam from a kettle, or put it in a hot oven. These methods, particularly the water and steam methods, all have the downside that, although they work initially, make it difficult to go back and heat just a small area in order to make adjustments to your piece. The easiest thing is just to go out and get yourself a heat gun. Trust me – you will find LOTS of things to do with it even if you don't end up doing a lot of Worbla work. I had my first heat gun for at least 10 years before I ever started working with thermoplastics and it was one of the most frequently used tools in my cosplay toolbox. Look for heat guns in the paint section of your local hardware store (they are most often sold as paint stripping tools). The “normal” price range currently seems to be $40+ with $60-$120 CDN being the most common but sales are frequent. Canadian Tire loves to put their 3-setting Mastercraft model (my current fave – it's even BLUE!) on for $30 three or four times a year. Daley Kreations also sells perfect “travel size” heat guns – these little guys aren't as powerful as the big models but DO get hot enough to fully activate your Worbla and are great for adjustment work. You can heat a smaller area than a bigger gun in order to pop air bubbles, re-stick an edge etc, without overheating your whole piece.