Overview
This document will discuss pressure die casting processes and compare them to the Thixomolding with respect to process/machine configurations, metal pressures and the relative strengths and limitations of each process. A brief tabular summary will be presented at the end of the discussion that will help guide the part designers and technical purchasing decisions.
We have not included a wide range of “die casting” processes such as: sand casting, investment casting, low pressure (vacuum assisted casting), squeeze (semi-solid casting), lost foam or plaster casting in this comparison. We have focused solely on processes that utilize high pressure and high speed as the method of filling the cavity.
These “high pressure/high speed” process account for the vast majority of parts that are non-structural and non-powertrain in vehicles.
Similarities
All of the casting processes detailed: hot chamber, cold chamber and Thixomolding use high pressure and high speed to fill the die cavity. The physics of the injection process is very similar for all three processes. However, the melt method, melt conveyance and the process parameters allowed by the process/machine tool designs favor certain alloys, part designs, tolerances and the overall quality of the castings, depending on the process. A wide variation of outcomes is achievable depending on the process selected.
In all three processes the melt is conveyed into the cavity by “injecting” or “plunging” the molten (or semi-molten metal in the case of Thixomolding) into the die using a plunger and rings (seals). If you are familiar with the principles of a hypodermic needle than you have a complete understanding of the mechanical action used to inject the metal that is inside the barrel of a die cast or Thixomolding machine.
By moving the plunger forward on the syringe with your finger you “inject” the medicine into the tissue. The movement of the plunger in the case of high pressure die casting equipment is accomplished with high pressure hydraulic cylinders. The fluid is not allowed to go backwards into the syringe (barrel) by means of seals on the plunger.
The following image will graphically display the process of injection and is a good starting point before we present the machines and discuss the differences in the machines and process parameters.
The plunger moves forward at a speed of roughly 10 MPH, in this example, (the green line) and begins to develop cavity pressure (the blue line) as the fluid fills the cavity. The pressurization and pressure hold takes place at the end of the shot and is usually accomplished in 25-50 milliseconds. This is the spike the takes place in the blue line.
In order to achieve the high pressures the plunger velocity must also drop to zero as the metal in the die pushes back on the kinetic energy of the plunger causing it to come to an abrupt halt. In the example above the plunger is moving at roughly 10 MPH and comes to a stop in under 30 milliseconds and then holds the pressure against the metal so that it solidfies correctly.
NOTE: In Thixomolding the distance the screw travels before the pressure begins to build is MUCH less. This will be detailed below. The general profile of the chart above is still applicable.
Cold Chamber Die Casting
The name “Cold Chamber” comes from the machine design style where the plunger chamber is cold or unheated. The molten material is poured into the the shot sleeve (the barrel of the syringe) using a manual or automated ladle system.
The plunger moves forward slowly until the seals are past the ladle opening and then injects quickly to fill the cavity.
Cold chamber machines tend to be slower in cycle time than hot chamber machines (the ladling takes extra time) but are built with higher tonnages and can easily handle more alloy types than hot chamber or Thixomolding. The horizontal configuration of the machine allows for a more rigid design and therefore higher injection pressures and corresponding clamp tonnages.
Cold chamber machines can accommodate parts that are larger and have thicker wall sections than hot chambered machines. One of the draw backs to most cold chambered designs is that gas posrosity is higher as a large percentage of the air that is in the shot sleeve gets trapped in the melt and ends up in part(s).
Hot Chamber Die Casting
Hot chambered machines have the shot sleeve encapsulated in the melt pot and therefore are “hot” as opposed to the design of cold chambered machines. The machines also have a vertical (or slanted) shot sleeve arrangement and are less rigid, have lower metal pressures, lower clamp tonnages and generally make smaller components.
The primary advantage of hot chamber machines are cycle time (machines cycles can easily be 10 seconds or faster) and the ability to manufacture smaller more intricate parts with thinner walls then cold chamber processes. Hot chamber machines are generally used for Zinc and Tin based alloys although machines with magnesium specific melt pots are available.
Aluminum is typically not processed on hot chamber machines as aluminum alloys at melt temperatures have a high chemical affinity for the shot sleeve and plunger and erosion and maintenance issues arise quickly. Machines have been constructed with ceramic components for special applications but are not generally available in the industry for processing aluminum on hot chamber machines.
Thixomolding
As stated above all three process use a common injection sequence but differ greatly in the creation of the melt and the machine tool designs. Thixomolding is a hybrid process that uses a melt system similar to plastic injection molding (melt conveyance) and an injection sequence like die casting.
AZ91D chips are feed into the hopper system and augured like a plastic using a screw. In general the material is not fully liquid in Thixomolding. The material is usually 5-10% solidus in a slurry mixture at the time of injection.
Unlike cold chambered or hot chamber processes there is no slow movement forward of the plunger before the high speed portion of the injection takes place. Similar to hot chamber the plunger and rings are “hot” and in the melt. The melt is about 100 °C colder than hot chamber and the metal is not fully liquidius. This allows higher metal pressures in the cavity than hot chamber but slightly lower than cold chamber.
There are five primary advantages that result from the very different melt conditions used:
· Reduced porosity – less air is injected into the cavity then cold chamber and hot chamber processes lowering the porosity rates dramatically – see table.
· Reduced melt defects – ladling the material to the shot sleeve and open pot melting allows the material to oxidize and create defects and contamination in the melt. Thixomolding is a closed melt process.
· Thinner Wall Sections – injecting the material as a slurry (higher viscosity) and closer to the cavity allows 10-20% thinner walls than traditional Magnesium die casting and 20-30% thinner walls than Aluminum Die Casting (ADC).
· Better Tolerance Control – in die casting processes the melt is “superheated”. The metal is 50-100 °C hotter than the melting point. This causes the filling and solidification to have more variation. More variation means more tolerances are required.
· Longer Tool Life – Super heating the material also causes the tool life to suffer. In the case of ADC where the aluminum is chemically and physically attacking the die the tool life in Thixomolding can be 2X of ADC and 1.5X of Magnesium Die Casting, at a minimum.
With respect to cavity metal pressures Thixomolding is very similar to die casting however they are slightly lower than cold chamber but higher than hot chamber die casting, in general for similar size machinery (barrel size and clamp tonnage).
As a result of the lower metal pressures as compared to cold chambered processes the maximum practical wall thickness (general wall – not localized bosses, etc.) is lower in Thixomolding, about 5.5 mm versus 8.0 mm in cold chambered processes.
Please see the table below for a summary of the processes and the related attributes.
This is a generalized summary. Alloy, machine designs and options, end use criteria and other factors can affect the rankings. A score of (10) in a category is the best. Actual ranges of values are given where applicable.
Variable/Process / Hot Chamber / Cold Chamber / Thixomolding / NotesMetal Pressure* / 4 / 10 / 8.5 / See below
Clamp Tonnage / Max 500 / 3000 + / Max 850 / Larger tonnage = larger parts, bigger tools.
Gas Porosity / 2-3% / 2-4% / 1-1.5%
Melt Defects / 8 / 7 / 10 / Thixo has virtually zero melt defects
Tolerances / 8 / 6 / 10 / Typically ½ of cold chamber die cast
Alloys that can be cast / 5 / 10 / 1 / Thixo is limited to AZ91D ONLY
Wall sections Complexity / 8 / 6 / 10
Tool Life (alloy dependent) / 150,000 for Mag / 100,000 typical for ADC / 200,000 ++ / 150,000 tool life Magnesium Cold Chamber
Cycle Times / 10 / 7 / 8 / Hot chamber processes can have cycle times of less than 10 seconds
Component Sizes / Small / Wide Range of component sizes / Up to Medium size components
Metal pressures vary considerable depending on the barrel size and the process parameters. For an 800 Ton Cold Chamber machine 1000 Bar (14,700 PSI) is relatively standard. An 850 Ton Thixomolding machine will develop slightly less (10-15%) than its cold chambered counterpart in the range of 850 Bar or 12,500 PSI. By comparison a hot chamber machine of 200 Tons may produce only 200 Bar (2,870 PSI).