HK1161177A - Multi-component metal injection molding - Google Patents

Abstract

A metal alloy feedstock and method for metal injection molding is disclosed. The alloy includes at least two components, such as a first component and a second component. The first component has a first melting point and the second component has a second melting point higher than the first melting point. The first melting point and the second melting point match to the temperature gradient of the heated barrel of an injection molding machine whereby when fed into the injection molding machine the first component melts prior to the second component melts and enables the second component to solute into the first component. Additional components may also be used.

Description

Multi-component metal injection molding

Technical Field

The present invention relates generally to injection molding of metals and more particularly to compositions of metals suitable for processing in plastic injection molding machines.

Background

Conventional reciprocating screw injection molding machines are capable of handling/molding most commercial polymers as well as filled or reinforced polymers. While desirable, the machine has not been able to mold parts from metal alloys. Die casting or other variations of casting processes have become standard methods for making 3-dimensional near net shape parts from metal alloys. Thixomolding is a method of molding magnesium alloys using some of the characteristics of plastic injection molding equipment. The machines used in thixomolding are significantly different in design and size from conventional plastic injection molding machines.

Metal alloys, especially light alloys such as aluminum, zinc and magnesium, need to be processed and molded on conventional plastic injection molding equipment. There is a large installed base of injection molding machines worldwide and the operating cost of such machines is significantly less than that required for casting type operations.

Metal alloys typically have a relatively narrow temperature transition between the solid and liquid phases. Even semi-solid phases generally have a narrow temperature window.

Metal alloys cannot be processed in the solid phase or in the semi-solid phase above a certain fraction of solids on standard injection molding equipment because the machines are not strong enough to overcome the resistance of solids or semi-solids (with high solids content). Similarly, standard injection molding equipment is not well suited for handling any material with very low viscosity (e.g., water-like). Materials with too low a viscosity have little resistance to forces (a requirement in standard injection molding machine design) and exhibit flow patterns that are not ideal for filling the mold cavity (creating voids, difficult ejection, and poor mechanical properties). This allows only a narrow range of semi-solid areas (e.g., 5 to 30 solids), which is generally feasible for molding metals on injection molding equipment that requires a thermoplastic mold flow. This narrow range of the semi-solid region also corresponds to an acceptable viscosity range enabling injection molding.

In a conventional injection molding machine, plastic pellets enter the conveyor screw at or near room temperature. Depending on the type of plastic and the desired viscosity, it is typically heated to 450 ° F to 700 ° F (-232 ℃ to 372 ℃) down the length of the barrel. The barrel is heated from the outside to help heat the plastic. The induced shear created by the screw and viscous liquid is also a significant part of the heating of the plastic. Typically, the cartridge temperature is controlled in three zones (front, middle and back and feed). There is typically only a 100 ° F (-37 ℃) difference between the front and rear zone temperature set points. However, the material was heated from near room temperature to 500 ° F to 700 ° F (260 ℃ to 372 ℃) over the length of the barrel. The feed zone temperature is set above room temperature but below the temperature required to induce melting so that in this zone the pellets remain solid while being transported to the hotter zone. The material is continuously heated due to shear and residence time in the heated barrel. Thus, there is a continuous gradient of material temperature down the length of the barrel from RT to injection temperature (difference of 400 ℃ F. to 700 ℃ F. (-204 ℃ C. to 372 ℃ C.). The externally applied cartridge heat helps to increase the temperature of the material but does not control the material temperature.

In addition to the material temperature gradient down the length of the barrel, injection molding machines have other characteristics that hinder accurate temperature control. There is also a potential change in the temperature of the material as it moves back and forth, associated with rapid movement of the screw up or down the length of the barrel. The heating process is always transient due to the constant feeding and discharging of new material. The molding process is not always running or "on the fly". The down time for the adjustment or problem also changes the temperature profile of the material, since the material is typically not moving during these periods. All of these factors contribute to the inability to maintain the material temperature over a narrow range.

The temperature of the material in the process cannot be precisely controlled due to several factors:

a. continuously feeding and discharging material

b. Moulding is always transient (stop/start)

c. The material is heated from near room temperature to the injection temperature (e.g., 700F/372℃.) so that a temperature gradient exists down the length of the barrel

d. The cartridge setpoint temperature is only in the range of about 100 ° F/37 ℃ from front to back, but the material must be heated from 70 ° F/21 ℃ to, for example, 700 ° F/372 ℃ (so the cartridge setpoint can affect but not control the material temperature)

e. Significant material heat is generated from shear forces localized at the wall and not uniformly distributed throughout the material

f. When the machine stops circulating (and stops feeding/discharging material) for whatever reason, the thermal balance changes

All of these characteristics make it difficult to maintain the metal alloy in a manageable (narrow) temperature range. These properties are less obstructive when processing plastics, since the processable melting range occurs over a much larger temperature range and the resistance/strength of the cooled plastic is much less than that of metal and can often be overcome more easily by the forces of the machine/screw.

Disclosure of Invention

The present invention solves the problems of the prior art by providing a multicomponent composition having: at least a first component having a low melting point; and a second component having a higher melting point, the low and higher melting points being selected to match a temperature gradient of a barrel of a plastic injection molding machine. More than two components may be provided. Due to its lower melting point, the first component liquefies first and facilitates the transition of the second component to the liquidus mixture to reduce sticking in the injection molding machine. In particular, the first component becomes liquid and its temperature increases as it is moved forward along the length of the barrel by the injection molding machine screw. The second component becomes soluble in the liquid of the first composition. If additional components are used, the additional components also become soluble in the first composition. The additional component is selected to have a melting point greater than the melting point of the first component but less than the melting point of the second component. The process continues to increase the temperature up to the liquidus temperature of the second component. The composition of the liquid is constantly changing because it has a temperature dependent equilibrium solubility. As the composition changes, it also has an increasing liquidus temperature. Thus, the composition is to some extent self-regulating. With increasing temperature, there is more of the second (high melting component is soluble). The dissolution of the second component changes the liquid composition and raises its liquidus temperature, thereby requiring even high temperatures to incorporate more of the second composition. Similarly, if more than two components are used, a similar balance is achieved. This means that near liquid composition increases with increasing temperature (or down the length of the barrel of the injection molding machine) as the equilibrium liquidus is approached. Accordingly, the present invention provides a multi-component metallic composition that can be used in an injection molding machine to facilitate molding of metallic parts.

Drawings

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a binary phase diagram of a zinc-aluminum metal alloy made according to the method of the present invention;

FIG. 2 is a close-up view of inset A of FIG. 1;

FIG. 3 shows a close-up view of inset A of FIG. 1, where reference point B indicates a 95 wt% zinc/5 wt% aluminum eutectic;

FIG. 4 shows a close-up view of inset A of FIG. 1, wherein the vertical line with the designation C indicates an 85 wt% zinc/15 wt% aluminum single composition; and is

Fig. 5 shows a close-up view of inset a of fig. 1, wherein stepped line D indicates a multi-component composition having upper and lower limits defined by 85 wt% zinc/15 wt% aluminum and 95 wt% zinc/5 wt% aluminum.

Detailed Description

One approach is to define alloys with a wide range between the liquidus and solidus temperatures. This range is still wider than the manageable range. Generally, semi-solids with solids contents above about 30% to 35% are not handleable on conventional injection molding equipment. The handleability of the semi-solid metal of the homogeneous composition ranges from about 5 wt% to 30 wt% solids. The temperature range to maintain this% solids window is narrow. Even in alloys with a wide liquidus to solidus temperature Δ, the temperature window is narrow.

As an example of the present invention, an alloy having a range of about 130F between the solidus and liquidus (85 wt% zinc/15 wt% aluminum) would be a good candidate for injection molding due to the relatively large temperature difference. The range of 5% to 30% solids is significantly lower (about 70 ° F to 80 ° F). This material can be processed on standard injection molding equipment, but the window is not wide enough for acceptable routine processing. The material occasionally sticks.

To observe this example in limit, the Al/Zn eutectic approaches 95 wt% Zn/5 wt% Al. Referring to fig. 3, the composition transforms from a solid to a liquid without the appearance of a semi-solid phase. It is conceivable that this material is not feasible for injection molding. The liquid phase is too low in viscosity for processing (i.e., no resistance to flow and undesirable turbulence during mold filling). On the other hand, the solid phase will not flow and present too much resistance to the machine. Fig. 2 is a binary phase diagram of zinc-aluminum in the range of 80 wt% to 100 wt% zinc and between temperatures of about 600 ° F and 900 ° F.

The present invention relates to multi-component materials, such as two or more components, that provide a compositional gradient along the length of the cartridge that is parallel to the temperature gradient.

For the purpose of describing the present invention, the phase diagram of zinc/aluminum is shown with three different material compositions as seen in fig. 3, 4 and 5.

Referring to fig. 4, there is shown a phase diagram of a single composition of 85 wt% zinc/15 wt% aluminum of the present invention that can be processed without having a sufficient window for routine processing. In the phase diagram, it is clear that for this composition, the behavior can extend only up and down the vertical line. The range that will be processable is in a window that is only a part of this line. In addition, any temperature change will produce a change in the percent solids and thus a significant change in the rheological properties.

Referring to FIG. 5, a multi-component composition is depicted having upper and lower limits defined by 85 wt.% zinc/15 wt.% aluminum and 95 wt.% zinc/5 wt.% aluminum. As can be determined from fig. 5, the mixture of soluble compositions results in a composition gradient parallel to the temperature gradient in the barrel. This mixture ensures that the composition is always fairly close to the liquidus temperature (low% solids) and will maintain fairly consistent rheology down the barrel length of the injection molding machine.

The examples of the present invention use a mixture of two aluminum/zinc compositions (mixed beads with different compositions). In this case, both compositions are aluminum-zinc, but the ratio of each element is different. A specific example is 95 wt%/5 wt% zinc/aluminum as the first composition and 85 wt%/15 wt% zinc/aluminum as the second composition. The low temperature melting component will first form a liquid. As the first component becomes liquid and its temperature increases as it moves forward along the length of the cartridge, and the components of the second composition become soluble in the liquid. The process continues to increase the temperature up to the liquidus temperature of the second component. The composition of the liquid is constantly changing because it has a temperature dependent equilibrium solubility. As the composition changes, it also has an increasing liquidus temperature. Thus, the composition is to some extent self-regulating. With increasing temperature, there is more of the second (high melting component is soluble). The dissolution of the second component changes the liquid composition and raises its liquidus temperature, thereby requiring even high temperatures to incorporate more of the second composition. This means that near liquid composition increases with increasing temperature (or down the length of the barrel of the injection molding machine) as the equilibrium liquidus is approached.

This process is irreversible and therefore cooling of any given composition does not result in separation of the components. However, because of the composition gradient down the length of the barrel, any cooling effect (e.g., movement from the screw) is small relative to the critical temperature at which a particular composition will have too high a solids content to be mechanically moved or sheared by the machine.

This compositional variation provides the necessary window or latitude for processing metal alloys on conventional injection molding equipment.

The present invention has been shown to produce good molded parts on conventional injection molding equipment with modifications to the screw, i.e., 0 compression of the flight gap in the solid to melt transition zone. For simplicity, the examples listed below include two components. However, more than two components may be used. However, the additional component must be selected to have a melting point that falls between the first component and the second component on the phase change diagram of the alloy.

Three specific examples are listed below:

example 1)

10 wt% (+/-5 wt%) (95 wt% zinc/5 wt% aluminum)

90 wt% (+/-5 wt%) (85 wt% zinc/15 wt% aluminum)

More specifically, 15 wt% (95 wt% zinc/5 wt% aluminum) and 85 wt% (85 wt% zinc/15 wt% aluminum) have been found to be optimal.

Example 2)

85 wt% (+/-5 wt%) (85 wt% zinc/15 wt% aluminum)

15 wt% (+/-5 wt%) (86 wt% aluminum/10 wt% silicon/4 wt% copper)

More specifically, 88 wt% (85 wt% zinc/15 wt% aluminum) and 12 wt% (86 wt% aluminum/10 wt% silicon/4 wt% copper) have been found to be optimal.

Example 3)

50 wt% (85 wt% zinc/15 wt% aluminum)

50 wt% (86 wt% aluminum/10 wt% silicon/4 wt% copper)

In the examples, the first component of the 85 wt%/15 wt% zinc/aluminum single composition or 95wt/5 wt% zinc/aluminum single composition is generally not handleable without the second component.

An 86/10/4 wt% Al/Si/Cu single composition is generally not handleable without the first component.

However, by mixing the two compositions together, the mixed composition is typically handleable.

Although described herein with only three examples, the concepts are applicable to all metals. Of course, there will be limitations regarding the maximum temperatures achievable in conventional injection molding machines and the stability of machine components in the presence of hot metal alloys. In addition, non-alloyed reinforcing materials as known in the art, such as glass, cenospheres, fly ash, carbon fibers, mica, clay, silicon carbide, alumina fibers or particles, diamond, boron nitride or graphite or other reinforcing materials may be added to the feedstock. Additionally, the reinforcement material can be dry blended with the feedstock as it is fed into the injection molding machine to form the molded part and the metal-matrix composite.

Thus, it can be seen that the present invention provides a unique solution to the problem of molding metal parts using plastic injection molding machines by using a multi-component composition of two or more components or metal stock materials of varying composition.

It will be understood by those skilled in the art that various changes and modifications may be made to the illustrated embodiments without departing from the spirit of the invention. All such modifications and variations are intended to be within the scope of the present invention.

Claims (33)

1. A metal alloy feedstock for metal injection molding in an injection molding machine having a heated barrel with a temperature gradient, the composition comprising:

a first component having a first melting point;

a second component having a second melting point higher than the first melting point of the first component;

the first melting point and the second melting point match the temperature gradient of the heated cartridge;

whereby when fed into the injection molding machine, the first component melts before the second component melts and enables the second component to dissolve into the first component.

2. The metal alloy feedstock of claim 1, wherein the first component comprises from about 5 wt% to about 15 wt% of the composition and the second component comprises from about 85% to about 95% of the composition.

3. The metal alloy feedstock of claim 1, wherein the first component comprises from about 80 wt% to about 90 wt%.

4. The metal alloy feedstock of claim 1, wherein the first component and second component each comprise about 50 wt% of the metal alloy feedstock, respectively.

5. The metal alloy feedstock of claim 1, wherein the first component is a metal alloy comprising about 95% zinc and about 5% aluminum.

6. The metal alloy feedstock of claim 1, wherein the first component is a metal alloy including about 85% zinc and about 15% aluminum.

7. The metal alloy feedstock of claim 1, wherein the first component is a metal alloy formed from an element selected from the group consisting of aluminum, copper, silicon, and zinc.

8. The metal alloy feedstock of claim 1, wherein the second component is a metal alloy formed from an element selected from the group consisting of aluminum, copper, silicon, and zinc.

9. The metal alloy feedstock of claim 1, wherein the second component is a metal alloy comprising about 86 wt% aluminum, about 10 wt% silicon, and about 4 wt% copper.

10. The metal alloy feedstock of claim 1, further comprising at least one component having a melting point greater than the first melting point but less than the second melting point.

11. The metal alloy feedstock of claim 1, further comprising a non-alloyed reinforcement material.

12. A method of metal injection molding on an injection molding machine having a heated barrel with an increased temperature gradient, the method comprising:

providing a metal alloy feedstock comprising a first component having a first melting point and a second component having a second melting point higher than the first melting point, the first and second melting points selected to match the temperature gradient of the heated barrel of the injection molding machine;

feeding the metal alloy feedstock into the injection molding machine;

melting the metal alloy feedstock within the heated barrel of the injection molding machine; and

maintaining a percentage of solids to liquids in the metal alloy feedstock of the first and second components in a treatable range of about 5% to about 30%.

13. The method of claim 12, wherein the first component is selected to comprise about 5 wt% to about 15 wt% of the composition and the second component is selected to comprise about 85% to about 95% of the first component and the second component mixed together.

14. The method of claim 12, wherein the first component is selected to comprise about 80 wt% to about 90 wt% of the first component and the second component mixed together.

15. The method of claim 12, wherein the first and second components are selected to comprise about 50 wt% of the first and second components mixed together.

16. The method of claim 12, wherein the first component is selected to comprise a metal alloy comprising about 95% zinc and about 5% aluminum.

17. The method of claim 12, wherein the first component is selected to comprise a metal alloy comprising about 85% zinc and about 15% aluminum.

18. The method of claim 12, wherein the first component is selected to comprise a metal alloy formed from an element selected from the group consisting of aluminum, copper, silicon, and zinc.

19. The method of claim 12, wherein the second component is selected to comprise a metal alloy formed from an element selected from the group consisting of aluminum, copper, silicon, and zinc.

20. The method of claim 12, wherein the second component is selected to comprise a metal alloy comprising about 86 wt% aluminum, about 10 wt% silicon, and about 4 wt% copper.

21. The method of claim 12, further comprising selecting at least one component having a melting point greater than the first melting point but less than the second melting point in the metal alloy feedstock.

22. The method of claim 12, further comprising feeding a non-alloyed reinforcement material into the injection molding machine.

23. A method of selecting a metal alloy for use in a metal injection molding process, comprising:

selecting a first component having a first melting point;

selecting a second component having a second melting point higher than the first melting point;

selecting the first melting point and the second melting point to match a temperature gradient of a heated barrel of an injection molding machine; and is

Whereby when the first and second components are processed in the injection molding machine, the percentages of solids and liquids of the first and second components remain within a processable range of about 5% to about 30%.

24. The method of claim 23, wherein the first component is selected to comprise about 5 wt% to about 15 wt% of the composition and the second component is selected to comprise about 85% to about 95% of the first component and the second component mixed together.

25. The method of claim 23, wherein the first component is selected to comprise about 80 wt% to about 90 wt% of the first component and the second component mixed together.

26. The method of claim 23, wherein the first and second components are selected to comprise about 50 wt% of the first and second components mixed together.

27. The method of claim 23, wherein the first component is selected to comprise a metal alloy comprising about 95% zinc and about 5% aluminum.

28. The method of claim 23, wherein the first component is selected to comprise a metal alloy comprising about 85% zinc and about 15% aluminum.

29. The method of claim 23, wherein the first component is selected to comprise a metal alloy formed from an element selected from the group consisting of aluminum, copper, silicon, and zinc.

30. The method of claim 23, wherein the second component is selected to comprise a metal alloy formed from an element selected from the group consisting of aluminum, copper, silicon, and zinc.

31. The method of claim 23, wherein the second component is selected to comprise a metal alloy comprising about 86 wt% aluminum, about 10 wt% silicon, and about 4 wt% copper.

32. The method of claim 23, further comprising selecting at least one component having a melting point greater than the first melting point but less than the second melting point.

33. The method of claim 23, further comprising selecting at least one non-alloyed reinforcement material to add to the first component and the second component.