Effects of mold geometry on the surface finish of plastic medical instrument parts

Experiments are described which attempt to establish a relationship between the microgeometry of the plastic surface and the corresponding surface finish of the mold cavity for use under optimal casting conditions in order to arrive at a proper basis for choosing the surface finishes of the working parts. Such an investigation is of particular interest because strict specifications are made for the surface finishes of biomedical plastics parts, such as the valves in distributive systems, the parts of inhalation instruments, and so on. Some conclusions follow: the microgeometry of the plastic part is dependent on the finished class of the mold, the structure of the plastic (amorphous or crystalline), and the direction of the machining tracks on the mold. There is a considerable difference in the roughness profiles of plastics lengthwise and transverse to the machining direction, so one needs to have the direction of machining (higher finish) and that of the working motion of the part in coincidence, which considerably improves all the working characteristics of the rubbing components.