Generally it is necessary to preheat, the manual will also mention, you can look at. I use the bear, the manual is written in the use of the first 10 minutes to preheat, so that the temperature inside the oven can first reach the baking needs of the heat, the food will be easy to heat evenly. If you have a cake that doesn't bake well from time to time, it's possible that the preheating preparation is not done properly.