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Assembly Methods for Die Cast Parts

The following assembly methods are used for magnesium, aluminum and zinc castings:

  1. Threaded Fasteners
  2. External Threads in Die Cast Parts
  3. Attachment systems (for use in selected applications)
  4. Injected Metal Assembly or IMA
  5. Inserts
  6. Forming or self cutting fasteners
  7. Interference fits

 

Threaded Fasteners

Threaded steel studs and bolts are commonly used in order to join aluminum, zinc and magnesium die castings. For magnesium die castings where a potential for galvanic corrosion is present, the use of aluminum bolts are needed.

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For attachments made at sections that have thin walls, clearance holes are needed in order to give the bolt a section to pass through. This bolt is then fastened and secured into place with a nut.

Bosses are also capable of being designed into the casting itself, in turn allowing for the receiving of bolts or studs into tapped holes. While coarser threads are preferred, fine treads can also be tapped.

Factors involving the interaction between the tapped holes in the cast alloy and the studs or bolts:

  • The stiffness of the thread.
  • The shearing of the thread.
  • Load distribution and the effect tolerances has on this load distribution.
  • Erratic friction / Seizing
  • Thread bending.
  • The dilation of the hole

 

Keep in mind that:

  • In order to minimize the porosity of the area where the thread is present, tapped holes have to be cored.
  • Variations in torque (used by the production tools) can lead to fasteners over-torquing.
  • A more practical option is tapping threads in a separate operation, but it is possible to cast internal threads. This is done by either using spin out cores or by unscrewing the casting itself from the cores.
  • The joint has to be tested and be able to maintain the clamping force required if the operating conditions of the part will be under the potential 'threat' of thread relaxation.
  • A more consistent relationship can be created between the clamping force and the torque when cold steel studs and bolts are used, as they have a lesser tendency to seize, unlike uncoated studs and bolts.
  • In order to minimize the porosity in the thread area, tapped holes should be cored.
  • The boss diameter has to be al the least twice the size of the bolt's diameter as die cast joints are more prone to dilation.

 

External Threads in Die Cast Parts

Forming threads with slides or across parting lines, flash, left in the threads, can occur and removal can be quite difficult. When flash is not severe and if the use and the assembly of the parts will permit it, the flash removal operation can include the installation of the first nut.

In cases where thread strength can be sacrificed, flats are able to be casted at the root of the thread on the parting line. Easily removable flash is then formed on the flats, making it a practical option for die casting manufacturers.

There are also certain conditions that enables the casting of external threads on features that are cylindrical in nature. Threads in these cases can be formed in one of two ways:

  1. With the use of solid die components, or
  2. Across the parting line of the die.

 

Attachment systems (for use in selected applications)

Improvements made in the technologies used, the strength of adhesive bonds as well as the improvements in the time it takes to created a joint that is bonded, has made the adhesive bonding of metals a popular and common practice among die casting manufacturers.

Where plastic deformation of a casting is required for joining purposes, alloys with a high ductility (such as the zinc alloys) are used. Though generally limited to zinc alloys, lower ductility alloys can also be joined in this manner, but only if a lower plastic deformation is required to accomplish this joining.

The ductility of alloy can also be increased. This is done by either the spinning of the tool used against the metal, or by employing a heating tool. Applications, such as electronic circuit boards, can be mounted to die castings by simply adapting the stand-off fasteners developed for sheet metal joining.

Injected Metal Assembly or IMA

The molten zinc alloy, with a predictable shrinkage of 0.07%, is used to join components in the IMA process very much in the same way as adhesive boding works.

In applications where two or more parts require assembly (or joining) IMA is an beneficial process. It has the following in common with adhesive bonding:

  • The ability to join dissimilar materials. (The types of materials that can be joined using IMA includes fibers, metals, plastics, ceramics, paper, glass and elastomers.)
  • A great distribution of stress.
  • The ability to join materials that have different thicknesses.

 

Where IMA has its advantages is in the fact that the molten alloy bond used for joining, requires no special surface preparations and, thermal degradation problems and peeling are not as common as they are with adhesive bonds.

IMA bonds also show a good performance in environments that are harsh, whereas compared to adhesive bonds only specialty adhesives were able to maintain its integrity in the same harsh environment.

The zinc alloy Zamak 3 is the material most commonly used for bonding in IMA and has the following characteristics:

  • A hardness of up to 82 BHN (Brinell).
  • A tensile strength of 41 kpsi.
  • A shear strength of 31 kpsi.
  • Comprises of Zinc, Aluminum, Magnesium and Copper.

 

Inserts

There are two types of inserts, namely inserts that can be either post installed or cast-in-place inserts. Choosing which insert will be used, is determined by the function of the insert as well as the economics associated with that insert. Both types of inserts cause residual stresses within the casting and a thorough analysis of these residual stresses and problems in terms of long term retention, is thus vital.

Cases where undercuts prevent installation, post-installation is not a viable option. Cast-in-place inserts are used because it allows, in terms of a securing method, a wide latitude. This is seen in cases where alloys are casted around the inserts - undercuts develop a mechanical lock.

The economics of the cycle, mentioned earlier, determines a choice between the two types of inserts. Post-installed inserts have to be installed or loaded into the die, which in turn causes an increases in both the cycle time as well as the cost of the process.

On the other hand, cast-in-place inserts are required to be as dimensionally accurate as the die into which it is inserted.

As the size of casting increase, so does the cycle times. This is why cast-in-place inserts are in many cases a more economical option for larger die cast parts. Inserts made of bonze, steel and brass are used with all die casting alloys, though it is important to note that zinc alloy inserts are also used in some cases for castings made of magnesium.

Forming or self cutting fasteners

This type of assembly includes fasteners that anchor into the die casting itself. This is done by either forming or cutting into the die cast part as they are installed. These types of fasteners include spring clips, self-tapping screws, stamped nuts and thread-forming screws.

Installed by either spinning on or pushing in, stamped nuts and spring clips usually have hard cutting edges and are made of steel with a high strength. Stamped nuts and spring clips that are attached to either posts or essential cast studs, is able to deliver a low cast fastening option.

With a greater retention capability, thread-forming screws (which deforms the metal without removing said metal) and self-tapping screws (which has a cutting action) also have a higher cost.

In turn, its cost is lower than that of tapped threads (seeing as the tapping operation is removed from the process), but it has a lower retention compared to tapped threads.

Interference fits

These types of fits are used in order to help retain certain components, such as dowel and bearing inserts, in die casting. In cases where the interference is light, the installation can take place with both members/parts at a room temperate.

Heavy interferences, which may cause metal removal which leads to a reduction in retention, will require one of the following:

  • The cooling of the internal member.
  • The heating of the external member.
  • Both the cooling of the internal member and the heating of the external member.

 

There is the capability to develop interference, using crush ribs, between an external member and an internal member in cases where cast alloys has the required ductility. The advantages of using crush ribs includes the fact that dimensionally tolerances that are very close are not required as they are in conventional interference fits.

Special Design Measures that can be taken

Die casting alloys have two characteristics that requires special design measures to be taken. These two characteristics are:

  • Galvanic Potential
  • Response to long-term loading

 

For galvanic corrosion to take place, the following must be present:

  • A continuous conducting liquid path, or an electrolyte.
  • A corroding metal, or an anode.
  • A dissimilar metal that is either more cathodic or less anodic, or a cathode.
  • Metals touching each other, or an electrical contact.

 

Because an electrolyte, anode, cathode and electrical contact must all be present for galvanic corrosion to occur in die cast parts, simply eliminating one factor is enough to break the galvanic circle. This can be effectively done in the early stages of the design process by:

  • Insulating against electrical contact,
  • Choosing metals that have the best compatibility, or
  • Either minimizing or excluding the accumulation of the electrolyte (the continuous conducting liquid path).

 

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