Methods for Controlling Shrinkage in Metal Casting

Metal casting, like all industrial methods, requires a fair degree of accuracy and fidelity to the intended design. However, things are rarely as simple as executing a production method, as uncalculated variables can arise and disrupt the design, resulting in bumps, micro-gaps, and today’s topic: shrinkage.

To guarantee the accuracy of the final product, shrinkage must be kept within certain limits. The best one can do in this case is to control the amount of variation and take appropriate measures to prevent this variation from getting out of control. There are some basic protective measures one can take and a few things to keep in mind before beginning a die casting procedure.

Studying Shrinkage and Expansion Levels

The first thing to remember is the properties of the selected material. This may seem obvious, but things can quickly become complicated when one must consider the different strains, shrinkage and expansion levels of the alloy, as well as the properties of the materials within the alloy. Studying the material and how it changes when heated and cooled is a vital preparation.

For molten metals, there are 3 different types of material shrinkage: liquid shrinkage as it cools, solidification shrinkage, and solid shrinkage. While each material will experience different degrees of expansion/contraction and different boiling/solidification temperatures, there is a general rule for metals. Liquid shrinkage occurs near the liquidus temperature range and can change the rate of density change very quickly. In this part, it is important to use as little overheat as possible, thus reducing shrinkage and microcavities.

Solidification shrinkage is the next issue to focus on. Die casting alloys shrink a lot, but it is worth remembering that aluminum shrinks up to 6.5% during solidification, while magnesium and zinc only shrink about 4% or 3%, respectively. It is worth noting that aluminum with a higher silicon content can result in less overall part volume shrinkage. Solidification shrinkage is responsible for the high porosity in the final part, so it is important to understand the degree of dimensional change before selecting materials and controlling temperature. Solid shrinkage is the most difficult to calculate. In this regard, shrinkage is proportional to the volume used to make the material, that is, one inch of material volume is reduced for every inch.

①Tracking Metal Casting Temperatures

Again, this may sound obvious at first, but when you consider all the things you must keep in mind when properly monitoring temperature flow, there are some major industry tips you should keep in mind.

When die casting, the metal should be heated to achieve the appropriate melting characteristics, but not completely liquid. Overheating can cause unnecessary shrinkage and can also cause defects in the part. On the other hand, tracking cooling is also important for metal casting. For some casting methods, such as vacuum-assisted die casting, it is best to prepare the processing equipment for forming before the object is completely solidified, as the metal can cool up to 100°C per minute.

②Know Your Mold and Deposition Method

It is fair to say that your object is only as good as your mold. Therefore, there are some common defects that people may encounter during this part of the process. Depending on the shape of the object you want, some areas of the mold may be heavier. This will cause some differences in the solidification and shrinkage rate. The best solution is to use different gates.

Different gates have different characteristics in how the molten metal is deposited, which can be handy to remember. Connecting an appropriately sized gate directly to the heavier part of the mold can feed the material in a way that alleviates shrinkage when it cools. In addition, using round gates instead of flat or square gates on the gate can further reduce the risk of forming defects.

Narrow or tapered gates can eject metal instead of pouring it into the cavity. In this method, some parts of the object solidify before filling the entire mold. Therefore, a larger center gate or multiple gates will be more uniform when depositing the molten metal.