As manufacturing equipment ages, the maintenance and repair (MRO) of the equipment increases while the Overall Equipment Effectiveness (OEE) decreases. Given this, companies must decide while bidding on new projects: “Use the aged equipment and invest in MRO while accepting a lower OEE” or “Invest in new equipment using the latest technology and generate a higher OEE.”
Die casting is a metal casting process that injects molten metal through high pressure into a die. It is an economical process that can manufacture a high volume of parts. While die castings have a good surface finish and are dimensionally accurate, porosity inside the part is inevitable.
This blog will define die casting porosity, problems porosity causes, and how vacuum impregnation seals die casting porosity.
The goal of a foundry is to produce high quality die castings that meet or exceed the customer’s specifications at a competitive cost. In some die casting cases, those specifications require that the part must hold pressurized fluid or gasses. Companies use vacuum impregnation when the part must hold fluids or gasses under pressure. A common question asked about vacuum impregnation is “When Should I Impregnate a Casting?” This video addresses this question by answering if vacuum impregnation should be done before or after machining and finishing.
Hey, everyone. Welcome to “Casting Call” with Johnny Impreg. This is a premiere episode of a video blog series where we hope to answer all your compelling questions of everything impregnation. We’re going to start with the question I think I hear most often from folks is, “When should I impregnate a casting?”
Now, spoiler alert, I’m going to give you the answer now in case you’re short on time. But you want to do the impregnation after machining and prior to any kind of finishing like plating or painting.
Now, here’s why. Let’s consider the three different types of porosity you see in a raw casting—blind porosity, through porosity, wall to wall, and fully enclosed. If you impregnate a casting in this condition, you’ll get sealant in the blind. You’ll get it here, but you won’t get any sealant in this area. That becomes important when you do the machining, because when you machined from here, you’re going to have a leak path.
Now, if you do the impregnation after machining, you’ll still fill this and now you’ll fill this leak path as well. The reason you want to do it prior to finishing is you want to fill up all the porosity before you do the plating or painting. Otherwise, you could end up with out gassing or other blemishes that really don’t look so nice.
Let’s look at a real-world example on a cylinder block. Now, in this region here, we had a case where there was blind porosity going from here into the casting. It didn’t cause a problem until this machining cut was made. We have some other areas where we had enclosed porosity that was connected through two different machining passes. So, this is a real-world example of why you should do impregnation after machining.
So, I hope this little tutorial helped you. If you have any questions, please feel free to leave comments below and hit me up on WhatsApp or LinkedIn.
A commonly asked question is “What size of porosity can vacuum impregnation seal?” What seems like a simple, straightforward question is actually a complicated one. This blog will address the topic by describing the basics of die casting porosity, and what vacuum impregnation will seal.
While some refer to porosity as a defect, it occurs naturally and is found in most materials, both man-made and in nature. In metal castings, porosity is typically considered any void found in the casting. Casting porosity can be caused by gas formation or solidification while the metal is being moved from a liquid state to a solid state. This porosity can range in size, from sub-micron to voids greater than 10 mm, depending on the casting.
Metal casting porosity can affect the part’s structural integrity, creating a failure point. Porosity can also prevent the part from being pressure tight. This will impact performance if the part is designed to hold gases or fluids.
Vacuum Impregnation is a process that seals metal casting porosity. Specifically, it seals the internal, interconnecting path of porosity, which breaches the casting wall. The process is not a surface treatment, so it does not seal open pores found on the casting surface. Nor is it intended to seal casting structural defects such as cracks or open knit lines.
It’s difficult to pinpoint a generic porosity range that vacuum impregnation seals because, generally speaking, one pore does not cause a leak path. A leak path is created through a series of interconnected pores. For example, a breach caused by a 5mm pore interconnected with a series of smaller pores will be easily sealed (Figure 1).
Figure 1: This sectioned casting shows a 5 mm pore that is interconnected to a series of smaller pores. Vacuum impregnation can seal this leak path.
Conversely, if the same 5mm pore breaches a 5mm wall it will be difficult, if not impossible, to seal as there is little casting material for the sealant to adhere (Figure 2). A pore of that nature has characteristics similar to surface porosity which is not a candidate for sealing through vacuum impregnation. The large open pore breaches the both casting walls and is sometimes called “see through” porosity. One needs to view the porosity in three dimensions to see how it is interconnected, not simply analyze individual pores.
Figure 2: Vacuum impregnation will not seal this surface porosity. There is not enough casting material for the sealant to adhere.
The wide range of casting parameters creates a limitless array of shapes and sizes of porosity possibilities. Despite this, vacuum impregnation can seal porosity of any size. While vacuum impregnation can seal porosity of any size, it is important to realize that the leak path is the key characteristic to evaluate and not pore size. A leak path is created through a series of interconnect pores, and not a single pore. Instead of asking “What size of porosity can vacuum impregnation seal?” one should ask “Can vacuum impregnation seal the leak path?”
A future blog will discuss the topic of leak rates.
Vacuum impregnation seals porosity and leak paths in metal castings and powdered metal parts that form during the casting or molding process. The process is done by filling the pores with a sealant under pressure to stop fluids or gases from leaking under pressure. Vacuum impregnation stops casting porosity and allows manufacturers to use parts that would otherwise be scrapped.
