Is metal molding in your future?

Thixomolding is the high-speed injection molding of thixotropic metal alloys like magnesium, zinc, or aluminum. Like plastics, they become more liquid when disturbed (sheared) by shaking or stirring. This process has a lot in common with standard injection molding - even more than it does with the standard die-casting process for making metal parts.

Thixomolding is the trade name for a process patented by Thixomat Inc. of Ann Arbor, Mich. Husky Injection Molding Systems is licensed to be the exclusive builder of Thixomolding machines in North America, South America, and Europe.

But why talk about a metal-molding process to plastics processors? Haven't plastics been hugely successful in replacing die-cast metals? The answer is yes, but now some of the business is going back the other way.

Consider some familiar, high-value parts such as laptop computer housings, cellular-phone components, and other consumer-electronics parts that have recently converted from plastic to metal. A variety of automotive interior parts, plumbing fixtures, hardware, and medical parts are not far behind. For molders who have an interest in these types of applications, Thixomolding may in the future be one of the molding skills needed to serve a customer base that demands both plastic and metal.

What's immediately noticeable about Thixomolding is the design flexibility it can offer. The process allows magnesium parts to be produced as thin as 0.025 in. (0.6 mm) and as thick as 0.750 in. (20 mm). And Thixomolding can impart details that would not be possible in die-cast or plastic parts. For example, holes with zero draft angle have been used in mass-production by Thixomolding.

Unlike die-casting, which isn't known for its worker-friendly operating environment, Thixomolding involves no melting pot or transfer of molten metal. Instead, the Thixomolding alloy partially melts by the time it reaches the barrel's halfway point.

For experienced molders, the transition to Thixomolding should be easy, since the machines look and operate just like traditional plastic molding machines. Here's a look at what plastics molders should know about the process, the machinery, and tooling needed to be successful at Thixomolding.

How it works

Thixomolding begins as room-temperature metal-alloy chips or pellets are fed into the molding machine barrel. The feed-throat area is blanketed with a small amount of argon gas (10 to 100 liters per hour) to keep air out of the barrel and protect the magnesium alloy from high-temperature oxidation.

As the screw carries the material forward, the magnesium alloy chips are heated to a temperature of about 560-590 C (1040-1095 F). The combination of heat and the shearing action of the screw transforms the metal chips into a semi-solid slurry, consisting of spherical solid particles suspended in a liquid-metal matrix. Unlike polymer processing, there is very little shear heat generated by the action of the screw. Instead, the barrel heater bands supply most of the energy for the process.

The semi-solid metal collects in the accumulation zone of the barrel, in front of either a ball-type or sliding-ring non-return valve. The metal is prevented from drooling into the mold by a frozen plug of magnesium formed at the tip of the machine nozzle at the end of each cycle.

At the start of injection, the frozen plug is blown out of the nozzle into a plug trap in the mold and the metal is injected into the cavities, which are heated to 200-250 C (390-480 F). To ease filling in tough applications, a vacuum can be drawn on the cavity prior to injection.

After injection, the part is cooled and removed from the machine. A mold-release agent is sprayed onto the mold before each cycle to prevent the alloy from sticking to the surface of the mold. Cycle time varies with the thickness of the part and also of the runner and sprue system necessary to fill out the part. Typical production cycles range from 15 to 60 seconds.

Finally, the part is allowed to air cool, and the gates and runners are trimmed off in a mechanical trim press. Further deflashing may be necessary, depending on the application and the quality of the tooling.

A mostly familiar machine

While Thixomolding machines look and operate much like the plastic injection molding machines they are derived from, there are a few significant differences on the injection end. For one thing, the process needs an ultra-high-speed injection unit with a maximum linear piston velocity around 3.8 meter/sec. The injection unit also requires a refractory metal barrel liner for corrosion and wear protection. The screw is typically of high-temperature tool steel, as are the non-return valve and nozzle.

Other equipment differences include optimized software for injection and temperature control, an integrated argon-gas shield system, and increased machine guarding for safety and cleanliness.

Thixomolding needs a clamp capacity of about 5 tons/sq in. of projected part area. Injection units must be sized to ensure sufficient injection capacity - runner and sprue material included - and sufficient delivery speed (g/sec).

Why magnesium?

Right now, only magnesium alloys are Thixomolded commercially, though zinc trials are proceeding well, and aluminum alloys suitable for Thixomolding are expected to be commercially viable in less than three years. Regardless of the availability of other alloys, magnesium has several mechanical and physical property advantages over engineering resins, steel, and other alloys:

* It's lightweight. Magnesium is 30% lighter than aluminum and 75% lighter than steel, making it an ideal material for automotive, electronics, and telecommunications applications.

* It's stiff Magnesium's stiffness is considerably higher than that of engineering resins. Parts with very thin walls that could not provide sufficient stiffness and strength if made of plastic become viable at the same or lower thickness in magnesium. For example, magnesium alloy AZ91D has a modulus of elasticity around 6.53 million psi, as compared with typical values of 435,000 to 1.16 million psi for engineering resins, even glass-filled types.

* It's conductive. One of the most critical requirements for an electronic device enclosure is the ability to act as a shield against electromagnetic and radio-frequency interference (EMI/RFI). Most metals are inherently conductive and therefore reflect and absorb EMI. Plastics, being insulators, are transparent to electromagnetic radiation and must rely on surface treatments or conductive fillers to satisfy shielding requirements. Both plating and conductive fillers for plastic parts increase the cost of the part significantly and prevent them from being easily recycled. Magnesium alloys are particularly suitable for EMI/RFI shielding, providing parts with superior levels of shielding that are stiffer, stronger, and fully recyclable than comparable plastic parts.

* It manages heat. For many consumer-electronic products, the ability to dissipate heat is critical. A good example is notebook computers. As processor speed increases and enclosure size decreases, the operating temperature inside the laptop enclosure can create reliability problems for the electronics. The use of magnesium allows designers to create enclosures that act as heat sinks to remove this excess heat. Thixomolding further improves the heat-transfer capability of metal by producing more intricate parts with thin heat-sink fins.

* It dampens vibration. Vibrations in a system are either transferred to other components, which can affect their durability, or into the air as sound, which can create excessive noise. Magnesium has the ability to absorb these vibrations and dissipate the energy as heat. Thus, magnesium can improve durability in electronic applications subjected to vibration or decrease the noise, vibration, and harshness (NVH) in automotive applications.

Mold design

Thixomolding tooling reflects technology from both plastics molding and die casting. The high temperatures and corrosive nature of the molten metals require tool steels typical in die casting, while overall tool design should be as close to plastic tools as possible.

In general, Thixomolding tools are like molds for technical thin-wall parts of engineering resin - i.e., parts less than 1 mm thick. These molds have stiffer supports to reduce flexing, plus large and numerous ejector pins to ease ejection and prevent part damage. There is also no looping of oil lines through the tool so as to provide a consistent and repeatable operating temperature.

Venting of Thixomolding tools is more critical than with plastics molds, and vents should be designed to existing die-casting standards. Adding vacuum draw on the tool can improve filling and reduce porosity of the metal parts.

Mold components that contact the molten metal should be constructed from hot-worked tool steels such as H-11 or H-13 types. H-13 tool steel, which includes 1% vanadium, helps prevent erosion of the gate area. Cavities should be hardened to about 44-46 Rockwell C. Cores and slides should be made harder than the cavity steel. They are generally constructed from H-11, 12, or 13 steel or AISI 440A alloys. Nitriding should be considered for areas of sliding contact. SAE 4140 is recommended for holder blocks, as it will hold its shape better in the higher temperature service.

One of the main benefits of Thixomolding is the laminar flow of the material into the mold (In die casting, the completely molten metal atomizes as it enters the cavity, which entraps air and creates internal porosity.) Therefore, Thixomold gates should be designed to inject as fast as possible while still maintaining laminar flow. Recommended gate velocities are 15 to 45 meter/sec. When adjusting a new tool, the gate is typically opened up until the die will fill out properly with acceptable internal part quality.

Runners should be made as short as possible to maximize material utilization and minimize flow lengths. Edge or fan gates are common in Thixomolding and are typically trimmed off in a mechanical press. Molds have been built with submarine gates that automatically degate as the tool opens, and there have been suggestions that three-plate molds could be used to strip off the runner and sprue.

Mold temperatures for Thixomolding magnesium alloys are similar to those temperatures used in die casting (200-250 C or 390-480 F). Hot-oil mold-temperature controllers are being used to achieve this operating temperature, although electric cartridge heaters are another option.

Tim Creasy is Thixomolding product manager at Husky Injection Molding Systems Ltd., Bolton, Ont.