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LaCrosse Sales Magnesium Thixomolding Castings

Magnesium Thixomolding Castings

Thixomolding? is the injection molding of thixotropic metal alloys (magnesium to date) in a semi-solid or plastic-like state. The process takes place in a specially designed machine resembling a plastic injection-molding machine.

Chipped alloy magnesium is loaded at room temperature into a hopper on top of the machine. The granules are volumetrically loaded into a smaller hopper that mounts directly to the barrel. A rotating screw meters the material along the length of the barrel. The screw rotation produces a shearing action. When the granules are heated and shear forces applied, they form a thixotropic semi-solid slurry. The material is then forced past the shut-off valve and injected into a heated mold. Once the part solidifies, it is removed and trimmed. The use of robotics is utilized to ensure part consistency and quality. The Thixomolding? process adopts a laminar (plastic) flow similar to thermoplastics. Several advantages are realized: greater process control, part-to-part consistency; lower porosity; ability to mold complex features; better surface finish; net shape parts; thin wall molding; and reducing/eliminating the need for secondary operations.

Thixomolding? offers an excellent alternative to traditional die-casting and injection-molded plastics. Having great opportunity in a wide variety of applications, it is an attractive solution to part manufacturing.

Compared to die-casting, Thixomolding? superiority stems from its laminar flow and the use of solids. The Thixomolded? part can meet tighter dimensional tolerances with the aid of lower and more predictable shrinkage. Additionally, laminar flow allows for the elimination of trapped air particles in the mold thereby minimizing porosity.

Thixomolding? uniform fill, reduced shrinkage and low porosity allow manufacturers to obtain net shape parts where secondary operations can be eliminated (resulting in cost savings, ability to mold thin walls and intricate core shapes such as pins, holes, passages and slots, with improved mechanical properties. The process also allows for improvements in flatness and dimensional repeatability.

Thixomolding? also takes advantage properties inherent to Magnesium such as stiffness, heat management, EMI/RFI shielding, etc.

Variety of parts produced by the Thixomolding process

With Thixomolding, magnesium parts with walls as thin as 0.5 mm can be produced competitively for such end users as the electronics market where lightweight, stiff, strong, and heat-dissipating components that meet complex tight tolerances are required. Although Thixomolding has seen its greatest growth in the consumer electronics market, other applications are being found for notebook computers, mobile telephones, digital projectors, automotive parts, digital cameras, office equipment, and portable electric hand tools, among a host of others.

Thixomolding is a marriage of the die-casting and plastic injection molding processes to produce netshape components. It takes the best of each process and creates a totally new way of molding metal components. Though the process was developed mainly for magnesium alloys, several zinc alloys have also been run successfully and an aluminum development program is well underway. Thixomolding is a high-speed, semi-solid magnesium injection molding process that is environmentally friendly. In a single step, the process transforms room temperature magnesium chips?heated to a semi-solid slurry inside a barrel and screw?into precision-molded components. No sintering or de-binding steps are required as in the MIM (metal injection molding) process to complete component densification. Components after cooling in air are ready for trimming and assembly or secondary operations. They typically exhibit as-molded densities in the range of 98-99 percent. This low porosity level makes them good candidates for secondary operations such as coating or plating without blistering or out-gassing.

General design tips for Thixmolded components
Draft: Draft 0.5 to 3 degrees? Normal is 1 degree?Zero draft is possible if sufficient ejection surface is available.
Design or pattern shrink: This is the shrink factor that is applied to the print dimensions when cutting the tooling to account for dimensional changes during molding and cooling. +0.5 percent or +0.005" per inch of dimension. This shrink factor is consistent in all directions
Ribs: Use ribs to strengthen sections and reduce mass required. Ribs should be from 0.5 to 1.0 times the adjoining wall thickness.
Fillets and radaii: "T" junctions:
    R=wall thickness
"X" junctions:
    45 degree angle    R inside=0.7 x wall thickness
                                      R outside=1.5 x wall thickness
    30 degree angle    R inside=0.5 x wall thickness
                                      R outside=2.5 x wall thickness
"L" junctions:
    R inside=wall thickness
    R outside= 2 x wall thickness
Note: Too large of a fillet radius can cause porosity and a reduction in strength
Cross sections: Design wall thickness as uniform as possible. Generally, rapid changes in wall thickness cause porosity and internal shrink- age. The Thixomolding process is better equipped to handle changes in wall thickness because the process can vary the percentage of solids and reduce cross section porosity. Higher percentage of solids reduces porosity in heavy sections.
Ejector pin marks: The designer should consult with the customer to determine where ejector pin marks are allowed and where cosmetic surfaces are located.
Thixomolding Machine clamping forces: Machines sizes 75 to 650-ton:
   7 tons per square inch of component projected area
Machines over 650 tons:
   6 tons per square inch of component projected area
Component projected area is the total projected area of all parts in all cavities plus an allowance for the projected area of the gating.

How the process compares:
  Thixomolding Die-casting
Density 98% 95%
Elongation 6% 3%
Minimum wall thickness 0.02 inch 0.05 inch
Tolerance control NADCA Precision?0.002 inch (first inch) NADCA Standard?0.010 (first inch)
Source: Phillips Plastic Corp.

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