Trim waste in blow molding large or complex industrial shapes.

Resin waste plagues blow molders who make either large parts like 55-gal drums or small parts with complex shapes like automotive air ducts. Two new accumulator-head machine developments from S.T. Soffiagio Tecnica S.r.l., a unit of Meico S.T. of Monza Italy, aim to cure the problem by controlling the shape and position of the parison in order to put the material just where you need it.

For large-part molders, S.T. developed a new accumulator-head system that controls the radial thickness of a parison at specific points on its circumference. Molders of small complex parts can use a new 3-D parison-manipulation system that produces nearly flashless parts. This machine also has a new extrusion head capable of either sequential or multi-layer coextrusion. Both new Meico technologies are available in North America through MoldPro Inc., Burlington, Ontario.

TO OVALIZE OR NOT TO OVALIZE

Varying the wall thickness of a parison helps to compensate for uneven stretching that occurs during the blowing cycle. Die-gap and die-lip adjustments address the longitudinal or overall thickness of the parison but cannot vary the radial thickness around the circumference of the parison - also known as ovalization. "On most blow molding machines, when you vary the thickness of the parison it is affected all the way around the parison" says Robert Agnew, v.p. of sales at MoldPro. On the other hand, ovalized tooling accomplishes the job of varying radial thickness, but generally must be tailored for producing a specific part.

A more flexible solution is S.T.'s new C.R.S.T. accumulator head, which can produce parts with either varied or uniform wall thickness from the same tool. "The C.R.S.T. turns your accumulator into an ovalized tool and then back again," Agnew says.

The special head design affects radial thickness by modifying the geometry of the flow passages in the head before the material reaches the die lips and pin. The material "remembers" the effect of this flow-passage modification. "From this point on, the radial thickness of the material is permanently modified, regardless of the die gap," Agnew explains.

As shown in the accompanying schematics, S.T.'s new head has two annular rings positioned near the die. The outer ring is fixed, while the inner ring moves up and down via a hydraulic cylinder. Movement of the inner cylinder is controlled by a 100-point parison programmer of the same type used for longitudinal parison-thickness control. The inner ring is not completely circular. A chamfer or ovalized shape is built into the inner ring. In the neutral position [ILLUSTRATION FOR FIGURE 1 OMITTED], the chamfer of the inner ring is opposite a machined impression in the outer ring that balances the geometry of the flow passages so that the radial thickness of the flow passage is uniform around its circumference. When actuated [ILLUSTRATION FOR FIGURE 2 OMITTED], the inner ring moves up into the accumulator head, and the chamfer, now opposite a part of the outer ring that does not mirror it, constricts the flow passage in an ovalized pattern. The ring systems can be designed with more than one set of 180 [degrees] offset chamfers, each chamfer built to varying dimensions.

MoldPro says this variable-ovalization approach is more versatile and less expensive than previous methods and is more effective at high push-out speeds because it takes better advantage of the resin's memory effect. S.T.'s system has no limit on the push-out speed of the accumulator. Ovalization control is independent of the longitudinal die-gap setting, making it possible to change the die and core to mold larger or smaller products using the same head configuration.

The new accumulator-head design is available in 11 standard sizes from 2 to 60 liters (approx. 4.5 to 98 lb). The heads operate on S.T.'s TA blow molding machines.

NEW 'FLASHLESS' 3-D SYSTEM

MoldPro is also offering a brand new 3-D blow molding system from S.T. that can produce complex parts such as auto air ducts with a minimum of flash. Standard machines generate up to 65% flash on such parts, says Beverley Agnew, MoldPro v.p. of sales and marketing. The new machine reportedly requires slight modification of standard molds to produce only a small amount of head and tail trim. This accumulator-head design can produce a parison with sequential (hard/soft/hard) or multilayer coextrusion. The new system operates on S.T.'s T 12/S3-D machine, which has a 12-ton vertical clamp and max. mold area of 500 x 1000 mm. The new 3-D technology is being used commercially in Europe to make air ducts for a German auto maker.

The cycle starts with the end of the parison pinched shut from the previous cycle. A parison-support air pin in the die head keeps the parison lightly inflated as it is extruded horizontally onto a special heat-resistant belt conveyor whose speed is matched precisely to the extrusion rate so as not to mar the parison surface. A clamping mechanism pinches the other end of the parison to separate the parison from the die.

Next, two halves of a pneumatically driven mold table come together horizontally to pre-shape the parison. When the mold-table halves close around the sides of the parison, they support it momentarily while the belt conveyor retracts to a stand-by position. The blow molds, arranged horizontally above and below the parison, move closer to the parison when the conveyor system retracts. The mold table then opens, dropping the pre-formed parison into the bottom mold half, and then it retracts to its stand-by position. The blow molds clamp together and blow the finished part.

Microprocessor controls coordinate all these movements. Overall cycle times appear to be unaffected by the parison pre-shaping steps.