Exton, Pa. — When it comes to screws for injection molding machines, the term "general purpose" covers a wide range. Now one screw designer is calling for some standards.
"Everybody that's bought an injection molding machine has heard about general purpose screws. But every OEM in the world uses a different general purpose screw," said Michael Durina, president of Md Plastics Inc. "They may have their own ideas on what they're going do, what kind of compression? What kind of length. What kind of depths?"
Durina spoke April 10 at Boy Machine Inc's open house in Exton.
"Why doesn't the world establish a standard screw?" Durina asked.
The standard would cover every different screw diameter and every length-to-diameter ratio, he said.
Durina said industrial groups such as the Plastics Industry Association and Euromap would publish the standard for general purpose screws, making it an international standard.
Durina was vice president of sales at screw and barrel maker Spirex Corp., where he worked for 15 years. He started Md Plastics in 1999, in Columbiana, Ohio.
Durina also advocates making improvements on the traditional reciprocating screw for delivering melted plastic to an injection mold. One problem with reciprocating screw technology is the variation in the amount of shearing and residence time from the first pellet to the last pellet to enter the barrel, he said.
At the Boy open house, he listed four main deficiencies with the reciprocating screw:
• Viscosity variation due to poor melt homogenization.
• Variation in the amount of polymer downstream of the screw tip, due to constant pressure flow changes in decompression and material consistencies.
• Variation in the quantity of the melt displaced, due to inconsistent non-return valve performance.
• Variations caused by poor control of the shutoff points.