SiNguyen

Hi everyone, I had the same problem in my maples too, eventhough they were in full or partial shades at the time. I have to agree with Aldevaux and Soltan on this problem. The heat and dry winds and direct hot sun light are major contributing factors, but the main culprit is the poor root system, which is usually due to a combination of poor draining soil, salt build-up, and lack of nutrients and micronutrients. Perfectly healthy trees should not develope leaf burn in the sun.
Trident maples should be grown in full sun in order for them to develop quickly, especially if they are small seedlings or pre-bonsais, so putting them in the shade will slow them down unnecessarily. For "finished" bonsais, the full sun will allow for smaller leaves and shorter internodes and a deeper red foliage color in the fall.
To Soltan, I would suggest repotting the burnt trees in larger containers with a free-draining mix (like with 95% pumice), then water frequently and fertilize routinely and slowly expose them to full sun light outside of the hot house. Using filtered water is good too, like that used at the professional nurseries, but is not neccessary if the trees are rountine repotted every 3-5 years in order to remove the old soil and salt buildup. (By the way, I am not convinced that drenching or "flushing" the pot would remove the salt build-up in potted plants, although this has been taught for a long time.) I think repotting and replacing the old soil is the only way of removing salt- build-up in a pot. For me, when my trees start to get leaf scorch at the tips, I start to plan for repotting, whenever it is appropriate (usually from February to August, for me in Southern California). Keeping the pots shaded and cool in the summer is good too. Wish you Ozzies some rain soon!
Si