Dosing and shot-to-shot repeatability are enhanced when extrusion and injection are separated.
You can count on one hand the biggest technical innovations in injection molding over the last 50 or so years. Among the most notable was the invention of the inline reciprocating screw plasticating unit by William Willert in 1952 (patented in 1956). The reciprocating screw, of course, is still with us, and the molding process continues its dependence on the sometimes idiosyncratic injection unit.
|Injection Control Unit operation sequence:
Different injection units have been introduced over the years, some of which
attempted to segregate plastication from injection. The latest comes from Robert
Dray, president of R. Dray Mfg. (Dallas, TX). Itâs called the Injection Control
Unit (ICU), and Dray hopes the system represents a paradigm shift for molding,
bringing to the industry the accuracy and repeatability that CNC did to machining.
Limitations of Reciprocation
Dray notes that in molding, the basic injection unit requirements are as follows:
1.Â Plasticate at the required rate, quality, and melt temperature for the resin.
2.Â Accurately inject this resin into the mold cavity.
Dray additionally notes that injection units that use the reciprocating screw have some limitations:
1.Â The shot size is determined by the barrel ID and stroke of the injection cylinder. The screw must be the same diameter as the barrel ID, which ties the screw recovery time to shot size.
2.Â Reciprocation of the screw shortens an already too-small screw L/D ratio, and introduces instability into plasticating.
3.Â Plasticating technology is basically unchanged since the 1950s, and is nowhere near the level of the extrusion industry. This lack of technological maturation can result in poor melt quality and excessive melt temperatures, which reduce or destroy resin physical properties, increase cycle time and energy costs, and increase scrap rates and failures in the field.
4.Â Resin suppliers have addressed these problems with additives to improve flow properties and decrease the effect of excessive melt temperatures. These changes increase material and part cost. To meet required recovery rates, machinery manufacturers add rpm to the screw, which normally only increases melt temperature and reduces melt quality. Additionally, injection units do not normally display readouts in torque and melt temperature, and therefore the molder cannot see the damage to the resin. Physical properties are rarely checked in finished parts and are normally found in field failures.
5.Â Injection velocities and decompression are more often used to close or stabilize the nonreturn valve than to fill the mold.
6.Â Vented applications have been problematic in most cases.
7.Â Most coinjection operations are expensive and limited.
8.Â Inline compounding is an important cost-saving feature in the extrusion industry and is virtually nonexistent in molding.
9.Â Melt filtration is problematic in injection molding and standard in extrusion.
10.Â Regrind is minimally used in the injection molding industry and is standard in the extrusion industry.
Considering the limitations of reciprocation, Dray notes how the ICUâs design can improve the process:
1.Â Screw diameter is sized for recovery requirements.
2.Â The screw design uses proven extrusion technology that provides minimal melt temperature and pressure variations. Extrusion screw designs also provide larger windows of performance so that different resins can be plasticated on the same screw design.
3.Â Vented designs can run routinely without vent flow, eliminating expensive drying with most resins.
4.Â Coextrusion with multiple layers requires only additional extruders.
5.Â Addition of regrind is done with no sacrifice to melt quality or recovery performance.
6.Â Melt filtration can be added via a screen changer.
7.Â Inline compounding is easy with additional feeding systems for the additives.
8.Â A separate injection cylinder is sized for mold fill requirements.
9.Â With the ICS (Injection Control System), the screw can rotate at any time, except during injection. This allows for smaller screw and barrel sizes, with reduced horsepower and torque requirements.
10.Â Shutoff is integral with the ICS.
11.Â Decompression to avoid drool is integral with the ICS and is accomplished in the mold sprue bushing, not with the screw, thus eliminating air entrapment that can cause splay.
12.Â The injection cylinder can be pressurized up to 8000 psi, eliminating the need for accumulators.
13.Â Extrusion technology provides melt temperatures that are 50 to 100 deg F lower than normal injection molding temperatures.
14.Â The ICS eliminates the nonreturn valve. Pack, hold, shutoff, and decompression functions are now accurately controlled with a small-diameter ICS accumulation piston and cylinder.
15.Â By precisely controlling injection and the ICS, accumulated pressure density is thereby controlled, enabling the molder to select part weight while molding. This means that a weight tolerance can be selected and maintained. This feature not only reduces resin cost but also adds consistency to the physical properties.
The ICU (patent pending) is a departure from traditional machine design. Dray is working with several machinery manufacturers to commercialize the design. It is available as a retrofit from Xaloy for existing machines.
R. Dray Mfg. Inc., Dallas, TX
Bob Dray; (214) 368-5424