Metal casting, like all industrial methods, necessitates a fair amount of accuracy and fidelity towards the intended design. However, things are rarely as simple as executing a method of production because uncalculated variables can rear their heads and ruin a design with bumps, tiny gaps and today’s topic: shrinkage.
The rate of shrinkage has to be contained within a certain range for the end product to be accurate. The most one can do in these scenarios is keep a controlled amount of change and have suitable measures in place to prevent such changes getting out of hand. There are some basic safeguards one can have in place and certain things they should keep in mind before they set out with the die casting procedure.
Research Shrinkage and Expansion Levels
The first thing one has to keep in mind the chosen material’s properties. This may seem obvious but it gets complicated quickly when one has to consider different strains of alloys, shrinkage vs. contraction levels, and the properties of materials within the alloy. Research on the materials and how they change upon heating and cooling is vital preparation.
With molten metals there are 3 distinct types of material shrinkage: contraction during liquid cooling, solidification shrinkage, and solid contraction. While every material experiences these in different levels of expansion/contraction and different boiling/solidification temperatures, there is a general rule for metals. Liquid contractions occur near the liquefying temperature range and can change the rate of density change quite quickly. It’s important to use as little superheat as possible in this section, thus diminishing shrinkage and micro-cavities.
Solidification shrinkage is the next thing to keep track of. Die-cast alloys shrink a significant amount but it’s worth remembering that aluminum takes the cake with a whopping 6.5 % during solidification, while magnesium and zinc only shrink about 4% or 3% respectively. It should be noted that aluminum with a higher percentage of silicon can leads to a decrease of the overall part volume shrinkage. The solidification shrinkage is responsible for a lot of porosity in a final part, so it’s important to know the level of size shifting before choosing the material and while controlling the temperature.
Solid contraction is the most notoriously difficult to calculate. For this aspect, shrinkage happens commensurate to the volume of material used to make it, i.e. inch decrease in volume per inch of material. Diecasting.org has a great summary of the calculations.
Tracking the Temperatures
Again, it might sound obvious at first, but when you consider all the things that you have to keep in mind when properly monitoring the temperature flow, there are some major tricks of the trade one should keep in mind.
When die-casting, one should heat the metal to achieve appropriate molten characteristics, but do so without fully reaching its liquid state. Overheating can result in an unnecessary level of contraction that can lead to imperfections in the part as well. On the flip side, keeping track of cooling is also important for metal casting. With certain methods of casting such as (vacuum assisted die-casting), it’s best to keep treating equipment ready to form the object before it fully solidifies, as metals can cool up to 100 °C per minute.
Know Your Mold and Your Deposition Method
It’s fair to say that your object is only going to be as good as your mold. So there are a few common flaws one can run into with this area of the process. Some areas of a mold can be heavier depending on the shape of the object you want. This leads to some discrepancies in the rate of solidification and contraction. The best cure for this is to employ the use of a different casting sprue.
Different sprues have different properties when it comes to their style of depositing molten metal and it’s handy to keep these in mind. A properly sized sprue attached directly to heavier sections of the mold can feed the material in such a way so as to mitigate the shrinkage as cooling occurs. In addition, using a rounded, rather than a flat or square, gate on the sprue can further reduce the risk of forming defects.
Narrow or tapered sprues can spray the metal as opposed to pouring it into the cavity. In this method, some sections of the object solidify before the entire mold is filled. Larger center sprues or multiple sprues are therefore more uniform in depositing molten metal.