Near liquidus injection molding process

#This invention relates to an injection molding process for making near net-shape metal articles and in particular, relates to thin-walled metal articles made from metallic alloys, particularly light metals.

#In conventional casting, the metal is superheated above its liquidus temperature (i.e. the liquidus being the temperature above which the alloy is completely liquid). A minimum superheat is required to ensure that the metal does not solidify prematurely, particularly when molding thin-walled molded articles. Superheating metals which are prone to oxidation has attendant process control challenges to provide and maintain an inert atmosphere.

#Articles which are cast from superheated melts often are not sound in that shrinkage porosity and entrapped gases are not uncommon. In addition, their mechanical properties such as tensile strength, yield stress, and elongation suffer, and this is attributed to a microstructure characterized by coarse grains and dendrites.

#In order to better understand the invention, a preferred embodiment is described below with reference to the accompanying drawings, in which:

#FIG. 1 is a schematic showing an injection-molding apparatus used in an embodiment of the present invention;

#FIG. 2 is a graphical representation showing the near liquidus processing temperature range of alloys having a liquidus below 700° C.;

#FIG. 1 schematically shows an injection-molding apparatus 10 used to perform the process according to the present invention. The apparatus 10 includes a barrel assembly comprising a cylindrical barrel portion 12 with a barrel head portion 12a arranged at a distal end thereof, and a machine nozzle portion 16 opposite thereto, a contiguous melt passageway being arranged through said barrel assembly. The barrel portion 12 is configured with a diameter d of 70 mm and a length l of approximately 2 m. A temperature profile along the barrel assembly is maintained by electrical resistance heaters 14 grouped into independently controlled zones along the barrel portion 12, including along the barrel head portion 12a and the nozzle portion 16. According to a preferred embodiment, the apparatus 10 is a Husky™ TXM500-M70 system whereby the temperature of the alloy in the head portion 12a may be controlled within 2° C. of the liquidus temperature and even within 1° C. thereof.

#Solid chips of alloy material are supplied into the melt passageway of the barrel assembly through a feeder apparatus 18. The alloy chips may be produced by any known technique, including mechanical chipping or rapidly solidified granules. The size of the chips is approximately 1-3 mm. A rotary drive portion 20 turns a retractable screw portion 22 that is arranged in the melt passageway of the barrel portion 12 to transport the alloy material therealong.

#Experiments were conducted using two commercial die cast alloys AZ91D and AM60B whose nominal compositions are shown in Table 1. Another suitable alloy is AJ52 (Mg-5Al-1.5Sr) as described in U.S. Pat. No. 6,808,679 that has a nominal liquidus temperature of 616° C. It should be understood, however, that the present invention is not limited to the injection molding of magnesium alloys but is also applicable to injection molding of other alloys, including Al alloys and other alloys such as lead based alloys, zinc based alloys, and bismuth based alloys. FIG. 2 is a graphical representation showing the liquidus processing temperature range of several presently preferred alloys.