In some die casting applications, components must also be pressure-tight to hold pressurized fluid or gases. Companies use vacuum impregnation to meet these requirements by sealing the internal leak paths without impacting any other features of the casting. A commonly asked question is in addition to leak paths, can vacuum impregnation seal cracks?
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. Vacuum impregnation is a proven process that seals internal porosity without impacting any other features of the manufactured part. A common question asked about vacuum impregnation is “Should vacuum impregnation be done before or after die casting machining?”
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. Some metal casting porosity can affect the part’s structural integrity, creating a failure point. More commonly, porosity prevents the part from being pressure tight. This will impact performance if the part is designed to hold gases or fluids.
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.
In general, there are three casting porosity classifications:
Figure 1: There are three types of casting porosity: blind porosity, through porosity, and fully enclosed porosity.
Blind and through porosity cause immediate casting problems. Blind porosity can cause internal corrosion; while through porosity will create a leak path and allow gas and liquids to seep through the casting (Figure 2). In addition, blind porosity can cause defects on the part surface when secondary treatments, like powder coating or anodizing, are done. This is because solutions used to clean the castings prior to the treatment will leech out of the voids after the surface finish process.
Figure 2: Blind porosity can cause internal corrosion; while through porosity will create a leak path and allow gas and liquids to seep through the casting.
When a machine tool cuts into the surface or “skin” of a casting, it can expose or open porosity (Figure 3). The porosity may be either blind porosity or existing blind and enclosed porosity may be opened and become through porosity.
Figure 3: When a machine tool cuts into the surface or “skin” of a casting, it can expose or open porosity.
The new inter-connected porosity (highlighted in green) will create a leak path (Figure 4). The leak path will cause fluids and gases to leak from the casting, causing it to be non-conforming, and in many cases unusable. Unfortunately, this occurs precisely at the wrong time, since the non-conforming part has already been cast, cubed (pre-machined), washed, tested, shipped, fully machined, washed and tested again. All the value has been added to the non-conforming part. In the worst-case scenario if the problem occurs frequently, the manufacturer may have maxed-out their production and may be unable to replace the non-conforming castings with functional parts, delaying shipments and significantly increasing costs.
Figure 4: The new inter-connected porosity (highlighted in green) will create a leak path. This leak path will cause fluids and gases to leak from the casting, causing it to be non-conforming, and in many cases unusable.
Impregnating 100% of castings after final machining is the best way to insure leak free castings at build. If prior to assembly, a final leak test of individual parts is incorporated into production, leak testing fully machined casting and impregnating only the non-conforming parts (often referred to as fix-on-fail) is an excellent alternative.
Another approach is that manufacturers have chosen to increase the machining content at the pre-machine stage (cubing) thus maximizing the exposure of blind and through porosity prior to the parts reaching the final production line. After pre-machining 100% of all parts are impregnated and tested. Only conforming parts are sent through to production when the parts are fully machined in production only a reduced amount of material is removed. Since the impregnation at pre-machining has already sealed both the blind and through porosity, the opportunity to open an interconnected leak path is substantially reduced. In some cases, any non-conforming parts that make it to final test can be easily impregnated without disrupting production on a “fix-on-fail” basis.
Because machining may potentially uncover additional casting porosity, vacuum impregnation should be done after machining. Performing vacuum impregnation after machining is the only way to seal all leak paths. However, some castings may be pre-machined or cubed. Impregnating 100% of production castings after cubing will seal the exposed porosity. In this case, impregnation is still very effective in reducing non-conforming parts at final assembly.
A previous blog What Size of Porosity Can Vacuum Impregnation Seal? discussed that porosity occurs naturally and that the purpose vacuum impregnation is to seal leak paths created by interconnected pores. This follow up blog discusses how to define what leak paths should be sealed.
It is important to understand that all materials permit leakage over time. In order for vacuum impregnation to effectively seal the leak and maximize the amount of acceptable parts, the manufacturer needs to define the part’s performance requirements, and develop measurable and repeatable standards around those needs. Defining these standards is done through leak rate testing. Vacuum impregnation is then used to seal specific leak paths to achieve the pre-defined leak rate.
The purpose of leak rate testing is to confirm that the manufacturing process is performing to specification and making acceptable parts. Finding defective parts early in the manufacturing process will reduce field failures, minimize unforeseen costs, and improve customer satisfaction (Image 1).
Image 1: Identifying defective parts with leak paths will reduce field failures, minimize unforeseen costs, and improve customer satisfaction.
Inspecting die castings requires quantitative, measurable values that define what is and isn’t acceptable given a part’s intended use. The fact is even materials cast with careful processes will allow some leakage, given enough time. Casting manufacturers develop and adhere to leak rate standards. Such standards define the maximum tolerable leakage for a part, typically specified by cc/min at a specified pressure and time duration.
Most automotive components operate with liquid. However, air is primarily used in leak rate testing for the following reasons:
To save time and resources, most manufacturers use the industry-recognized leak rates that are available for many products. Figure 1 shows typical ranges for existing parts.
Figure 1
At times, industry recognized standards are not relevant for a part’s application. To establish new standards, those parts should be analyzed through the following steps:
It is important to note that even after a casting is sealed and passes the leak test criteria, the casting can still exhibit leaks under more aggressive test conditions. Recall that all materials will leak in varying degrees. If the test standards were established so that the casting would hold oil a 1 bar, it may still exhibit a leak if tested with air or helium at 1 bar. Gasses are thinner than liquids (e.x. sealant, oil, etc.) and will leak through a path that would not pass fluids.
Once a leak rate is defined, parts within that range can be sealed through vacuum impregnation. Parts outside of the leak rate parameters are typically scrapped.
Vacuum Impregnation is a process that seals metal casting porosity. Specifically, it seals the internal, interconnecting path of porosity, which breaches the casting wall (Image 2). 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.
Image 2: Vacuum impregnation seals this leak path (highlighted in green) so that fluids do not seep from the part.
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?”
Since 1891, The Bonnot Company has designed and manufactured extrusion equipment. One of the unique features of a Bonnot extruder is a hollow screw which facilitates additional process temperature control through a re-circulating liquid.
Although the screw was pressure tested as part of a standardized ISO quality procedure, a leak of the liquid medium was discovered as it was installed at the end customer’s facility. The start-up of this product to the customer was critical, so the weld porosity needed to be sealed quickly.
Learn how we helped The Bonnot Company solve this porosity problem.
