Abstract: Composite particles formed from resin particles and silica particles (amount of silica particles: 0.2 to 10 wt. % on the basis of the resin particles) in an apparatus for formation of composite particles are placed in a PTFE mold and heat treated to weld the resin particles to one another, thereby forming a structural element, which is dipped in an ethyl silicate solution containing monodispersed silica particles, then taken out of the solution and subjected to further heat treatment to form an ultimate structural element, which is sealed in a tip case, whereby a tip can be produced at a low cost and used to separate nucleic acid from a liquid sample for a short time at a high recovery rate without any defects such as breaking, etc.

Abstract: A magnetic disk unit comprising a one-piece carriage structure having a small thermal off-tracking amount, with the carriage structure including a rotary shaft; guide arms integrally supported on a guide-arm retainer at their proximal ends and extending independently and parallely; bearing retainers formed on both ends of the guide-arm retainer; and bearings fixed on the bearing retainers rotatably connecting the guide-arm retainer to the rotary shaft. A center line in a direction of thickness of the outermost guide arm is located outside of that of the bearing in a direction of height thereof, and a concave portion is continuously formed on the bearing retainer. Thermal coefficients of expansion of the guide arms and the guide-arm retainer are substantially equal, with the carriage structure being fashioned of an alloy mainly containing Al and Si, eutectic crystal Si particles in the guide arm and/or the guide-arm retainer. The average value of the longest eutectic crystal Si particles therein is 1.6 .mu.