Two-stage unit opens new era in injection technologyTwo-stage unit opens new era in injection technology

A two-stage Injection Control Unit (ICU) is said to provide levels of accuracy and reliability until now unavailable to injection molders using reciprocating screw technology. Launched by plastication systems specialist Xaloy at NPE 2003 in June, it updates the well-known concept of a continuously-running extruder feeding an injection pot that today is commonly used for production of PET bottle preforms (Husky is the best-known exponent), but little place else outside specialist applications. Battenfeld uses the principle on its micromolding machine, for example, and Sodick Plustech high-precision machines also feature it. Ferromatik Milacron has an all-electric two-stage unit intended for high-speed applications.

Xaloy says the ICU "opens up a new era in injection molding technology," in broad range of applications. Conversion kits for processors make use of existing drives on the injection unit, adding an extruder that is driven by the motor originally used to turn the injection screw, and adding a manifold at the front of the injection unit. The company also hopes to interest machine makers in offering a variant as original equipment—although it admits that prospects of a take-up in the current economic climate are slim.

The ICU is the product of cooperation between Xaloy (U.S. headquarters in Pulaski, VA) and R. Dray Mfg. (Dallas, TX). Technical design largely comes from Dray, with Xaloy looking after the control technology. Bob Dray, a leading exponent of extrusion screw design, says process technology in injection molding has hardly moved forward in 40 years. "If we have the technology available in extrusion, why don''t we use it in injection?" he says.

Dray notes that the ICU''s extruder, with a high (30:1) L/D ratio, delivers melt with a consistent viscosity, which is not possible with reciprocating-screw units, since the effective L/D ratio of the screw, less than 25:1 at best, falls as the screw retreats. In addition, the ICU design does not require a non-return valve, a potential cause of shot-to-shot dose variation.

Instead, the feed port from the extruder to the shooting pot is blocked by the nozzle of the injection unit, which extends into the manifold. When the injection unit is in the forward position, the port is open, and melt can flow from the extruder, ahead of the plunger, which applies back pressure in the normal way as it retreats. A torpedo at the front of the nozzle prevents melt flowing into the mold. Then, when the injection unit retracts just before the plunger moves forward, flow from the extruder is cut off and flow is opened to the mold.

Packing pressure is then applied by the injection unit moving forwards. This can be done very accurately because of the narrow cross section of the channel in front of the torpedo, so the injection unit has to travel a long way to pack a small amount of material. Shot weight accuracy is claimed to be ±.1%.

Xaloy will likely offer four sizes, with extruder diameters ranging from 25 to 60 mm. "The system has a much better throughput than a conventional injection unit, so we don''t need to match all current screw diameters," says Xaloy''s technical chief Hartmut Jahnke. A 100-mm screw could be matched by a unit with an extruder with just half the diameter, he claims.

Dray notes that the system can easily be configured for variants of the basic injection molding process, such as in-line compounding, while "coinjection is a no-brainer."

Jahnke says presentations the company has made so far to processors have been well received. In August, he said two conversion kits had been sold, although neither was yet operational, and further projects were under discussion.

Peter Mapleston [email protected]