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
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
|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 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
||Use ribs to strengthen sections and reduce mass required. Ribs should be from 0.5 to 1.0 times the adjoining
|Fillets and radaii:
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
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
||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:
|Minimum wall thickness
||NADCA Precision?0.002 inch (first inch)
||NADCA Standard?0.010 (first inch)
|Source: Phillips Plastic Corp.