Actually, it is Great Planes, not Horizon that has GRP. Foam tires have been undervalued for years. To make a rubber tire you must first make a mold, then go to town running the tire. For foam you must:
1)Skin the sheets
2)Cut the sheets to the correct thickness
3)Laminate the sheets
4)Cut the donuts
5)Mount the donuts to the rim
6)True the tires
7)Clean the dust off from trueing.
There is much more labor involved in making a foam tire than a rubber tire. In addition, the scrap generated from a sheet of donuts is greater than the number of tires yielded. Thus, as a business owner you can understand the frustration of having to throw away 65% of your raw materials.