Enlargement of Glass and Plastic Shells to 2 mm in Diameter by Redropping through a Short Heated Tower

Large glass shells (≥ 1200 μm diameter) made by the traditional very high temperature (1650°C) long drop tower are usually wrinkled. We have found that these shells soften at relatively low temperatures. We have enlarged these shells by filling them with a few atmospheres of helium and dropping them down a very short (few feet long) tower heated to 900 to 1100°C. The helium acts as a blowing agent as the shell goes through the heated zone and causes the shells to grow larger. We have been able to smooth out large wrinkled shells by this process, as well. Glass shells as large as 2 mm in diameter and less than 6 μm out-of-round that do not have any obvious wrinkles have been made. In addition, the same process can be applied to both poly-alpha-methylstyrene (PAMS) and glow discharge polymer (GDP) shells at lower tower temperatures. Roundness of the enlarged shells is very much dependent on the wall thickness uniformity of the initial mandrels.