| Rotating-Core Technology
This technology by Solvay Advanced Polymers achieves thinner, stronger, rounder parts without a weak spot at the weld line. Driven by a servo motor, the cylindrical core turns during injection and packing. This rotation causes a turbulent shift of the fill pattern and distributes the knit line around the circumference of the part. Reinforcing fibers orient uniformly in the direction of rotation, reducing anisotropic shrinkage and dramatically improving roundness. The rotating mold cores are designed to minimize weld lines in circular parts. The patented technology is offered by Solvay Advanced Polymers for licensing which was initially introduced in 2004 holds some promise with filled or reinforced polymers, where weld lines can sacrifice up to 75% of the material's original strength. Another major problem in molding circular, fiber-reinforced parts is anisotropic shrinkage due to non-uniform fiber orientation. Hence, obtaining precise roundness is difficult, and dimensions can change if the part is exposed to elevated use temperatures.
In conventional injection molding, cylindrical or conical sections within complex parts are formed by flowing thermoplastic material around a stationary core. On the other hand, the rotating core injection molding process patented by Solvay S.A. improves the strength of such parts by injecting the material onto a rotating core. Two-fold and greater strength increases have been demonstrated in parts molded of semi-crystalline polymers reinforced with glass or minerals or both. The improved burst strength can be achieved, in part, by the ability of the process to produce a preferred orientation in the material itself. On a macromolecular level, the polymer chains become arranged around the circumference of a cylindrical or conical part ? like the reinforcing belts under the tread of a radial tire ? rather than simply along the flow path of the injected plastic. The reinforcing fibers are also arranged in this direction, greatly increasing the part's ability to withstand internal pressure. Rotating the core during injection also reduces the detrimental effect of knit lines or weld lines ? areas formed when flow fronts meet, which results in incomplete material or fiber interaction across the front and introduces mechanical weakness.
However, the known processes for injection moulding onto a rotating core do not make it possible to manufacture articles several millimetres thick exhibiting satisfactory mechanical properties; in particular, their resistance to pressure is virtually identical to that which they would have exhibited if they had been injection-moulded without the core being rotated. In rotating-core technology, the core speed and timing empirically for each application has to be worked out, which can take time on new jobs. Parts with through holes in the cylinder walls may also pose some problems, though not major ones. The through holes require side actions to shut off on the moving surface of the core. One approach under development is to design slides that don't completely shut off on the core; however, they would have some clearance that could be removed in a simple secondary operation.
Thermoplastic materials employed hitherto for injection moulding, and in particular for injection moulding onto a rotating core, have a low viscosity. It was generally considered that highly viscous thermoplastic materials are not suitable for injection moulding and that their use results in numerous disadvantages; in particular, it requires an apparatus which withstands high pressures, increases the length of the injection cycles and causes the formation of untimely stress fields that can be detrimental to the mechanical strength of the injection-moulded articles.
The rotating core process has been reported to produce cylindrical or conical sections that exhibit improved dimensional stability during operation over a wide range of temperature and humidity conditions. One of the examples of the part created by the process includes an automotive throttle body which should maintain its dimensions and its roundness while in operation. The dimensional design tolerance between the air-regulating valve and the bore is quite tight and ideally would not change with fluctuations in environmental conditions. The rotating core process improves bore's dimensional stability over the full range of operating conditions in turn improves engine performance, including efficiency and emission characteristics. Rotating the core during injection could have other benefits, including helping control the initial molded dimensions of the bore or improving the quality of the surface finish.
Speed and the duration of core rotation during injection are essential parameters in maximizing the strength and stability of a finished part. The advent of new development tools will provide further data to help in optimizing the rotating-core process for a variety of materials and applications. Parts molded in such advanced materials as IXEF® polyarylamide and AMODEL® polyphthalamide could benefit significantly from this process. Pressure required to achieve the desired injection velocity comes down with optimized rotational parameters. The roundness of the part enhances considerably with rotation. Increase in rotation also increases the impact strength at the weld area and the hydrostatic burst pressure. However, the impact strength of the part comes down at the gate area with the increase in rotation. Solvay's rotating core method does require minor tooling and molding machine provisions. Solvay has used both servo and hydraulic core actuation. Neither actuation method presents much of a problem for molders or toolmakers building and running tools with unscrewing cores for applications like threaded caps. Many experts believe that tooling modifications could even be retrofit to existing molds. |
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