mobile with capacities from 100 to 750 tons. They are simple to transport and are easy to set up on site. These mobile silos generally come equipped with an electronic weighing system with digital display and printer. This allows any quantity of cement or powder discharged from the silo to be controlled and also provides an accurate indication of what remains inside the silo. The static upright silos have capacities from 200 to 800 tons. These are considered a low-maintenance option for the storage of cement or other powders. Cement silos can be used in conjunction with bin-fed batching plants. Sand and salt silos Sand and salt for winter road maintenance are stored in conical dome-shaped (clear truss roof) silos. These are more common in North America, namely in Canada and the United States. The shaped is based on natural shape formed when piling solids. The dome is made of prefabricated wood panels with singles installed on a circular reinforced concrete base. Open canopy entrance allows for front end loaders to fill and retrieve easily. These are usually found along major highway or key primary roads. Plastic Silos Plastic silos, also known as hopper bottom tanks, are manufactured through various processes such as: injection molding, rotational molding, and blow molding. They are constructed using a wide variety of polyethylene plastics. The silos are light weight and make for great small scale storage for farmers with livestock and grain operations. The light weight design and cost effective materials make plastic silos a great alternative to traditional steel bins. Unlike fabric silos, which "tend to be prone to grain rot and pests which have left many farmers frustrated", plastic silos are more safe and secure, keeping grain fresh and unspoiled. They can be designed to be stationary hopper bottom bins or portable pallet bins. Fabric silos Fabric silos are constructed of a fabric bag suspended within a rigid, structural frame. Polyester based fabrics are often used for fabrication of the bag material, with specific attention given to fabric pore size. Upper areas of silo fabric are often manufactured with slightly larger pore size, with the design intent of acting as a vent filter during silo filling. Some designs include metal thread within the fabric, providing a static conductive path from the surface of the fabric to ground. The frame of a fabric silo is typically constructed of steel. Fabric silos are an