Have you ever had a misrun casting defect while welding and wondered how you to avoid it in the future?
A misrun is a metal pouring defect. It happens when the liquid metal isn’t totally filling the cavity mold when it pours. This can be due to a lack of molten metal fluidity, and faulty designs and/or gating. Misrun remedies include using the right pouring temperature and adjusting the mold design and/or gating system.
Let’s start by looking at casting as a whole. Then, we’ll get into the various defects that can happen, before learning how to identify and prevent misruns.
So, What Is Casting?
People have been using casting to create sculpture, jewelry, tools, weapons, and more for thousands of years. The process may seem simple, but can be extremely dangerous for novices. This is mostly due to the extreme temperatures that most metals require to liquify. At its most basic level, casting is simply pouring liquid metal into a mold and letting it re-solidify.
What Are the Steps Involved In Casting?
Casting generally involves 5 different steps :
This involves making a replica of the outside of the casting. It is an important step, especially for industrial projects, as small errors in calculations can ruin a mold. This is also essential in preventing defects later. Most patterns are made from cast iron, wood, or aluminum. The vast majority of casters use a CAD (Computer-Assisted Design) program to help make their pattern. Pattern allowances should be made for shrinkage, machining (if applicable), shaking, draft, and distortion.
There are many different patterns available, including :
- Cope and drag
- Draw backs
- Loose piece
- Match plate
- Split Piece
After the design, material, and pattern are chosen, the caster will select their preferred sand or binder to prepare the mold. The most common are green sand, dry sand, silica sand, or another binder with additives. They should be chosen with a focus on their cohesiveness, collapsibility, permeability, and other factors depending on the final product. Once the pattern is removed from the aggregate material, the next step can begin.
For hollow castings, coremaking is the step that physically shapes the actual product. They should always be made from refractory (heat resistant) materials. Basically, while the mold is where the metal will go, the core keeps it from filling the entire area. This isn’t always necessary in every casting project.
Once the pattern and core are finalized, the molding can begin. Depending on whether the mold is expendable or non-expendable, casters will choose their material and begin! The material (sand, wax, metal, etc) fills the mold model before hardening. After it is hard, it is taken out of the mold to cast the actual component.
Melting and Pouring
This step is where things are at their most dangerous. It starts with liquifying the metal by applying heat prior to pouring it into the mold. Each different type of metal has its own unique melting point. For instance, copper melts at 1983 degrees Fahrenheit while stainless steel has to reach 2750 degrees Fahrenheit before liquifying. In the majority of casting processes, the metal is liquified in a crucible. A crucible is simply a container made of refractory, melt-proof material.
Once fully molten, the metal pours through the gating system and into the mold. It will stay there until it ultimately cools and solidifies.
Once you remove the casting from the mold, some of them may need to be manually finished. Common finishing techniques are buffing, grinding, and sanding. Once this is done, the casting is finalized.
What is a Casting Defect?
A casting defect is any unwanted irregularity created during the metal casting process. While all defects are annoying, not all of them will require starting completely over. The trouble with casting is that there is a risk of failure during the entire process, start to finish.
Below are the five categories and names of the specific casting defects. Today we will only be taking a closer look at one of them, however :
- Gas Porosity/Filling Related Defects – blisters, blowholes, open holes, and pinholes
- Metallurgical Defects – hot spots and hot tears
- Mold Material Defects – cuts/washes, drops, metal penetration, rat tail, and swells
- Pouring Metal Defects – cold shuts, inclusions, misruns, and slag inclusion
- Shrinkage Defects – closed shrinkage defects (shrinkage porosity – macroporosity and microporosity) and open shrinkage defects (caved surfaces and pipes)
What is A Casting Defect Analysis?
Casting defect analysis is essentially the process of finding the root cause of the defect. It involves taking a close look at every step. Ultimately, this will reduce the number of defects, increase yield, and also minimize frustration! Depending on the level of casting (personal versus industrial), performing a quick casting defect analysis can be life-changing. Defects mean increasing cost, and defects that not identified quickly can start to really add up financially.
What Exactly is a Misrun?
A misrun, as stated above, is a type of pouring defect that can happen during casting. It occurs when the molten metal doesn’t completely fill up the cavity of the mold. This leaves an unfilled area and leads to an incomplete casting (a not fully filled mold). Many misruns are identifiable by their smooth, round edges. Some people also call this type of casting defect a short pour.
Possible Causes of a Misrun
Misruns have several causes. In 90% of misruns, the design of the mold or gating system, or the fluidity of the molten metal is to blame.
With a mold or gating system with a poor design, the metal isn’t in its liquid state as long as it should be. This means that it isn’t able to flow into the furthest parts of the mold, which leaves the empty space.
More often than not, the problem will be with fluidity. An issue with fluidity can cause a significant difference in the thickness of the metal throughout the casting. This is due to the metal becoming too viscous near the end of the mold.
Misruns can also be the result of a broken mold, the mold’s temperature, or simply dirt.
Know Your Gating System
Since one of the main causes of a misrun is a bad gating system, let’s discuss exactly what that is. The gating system is the basic design of a casting process. A good gating system allows the molten metal to flow properly and smoothly from crucible to mold. A poor one is the cause of multiple casting defects.
There are multiple components of the gating system, and each one can cause issues if not properly designed. Knowing what each does can help identify potential problems.
- Sprue – This is the part responsible for minimizing heat loss and/or turbulence during the flow. It is a circular cross-section and the optimal design is big at the top and small at the bottom.
- Sprue Well – This part directs the metal at a proper angle through to the runner and helps limit free fall. It should be cylindrical, with a depth twice the size of the runner and a diameter twice the size of the exit from the sprue.
- Runner – Existing to slow down the molten metal flow speed from the well to the ingate, the runner should be much larger than the size of the sprue exit. However, it also has to allow for a full filling up before opening.
- Ingate – The ingate directs the liquid metal to the mold cavity. It should minimize velocity.
What Are the Remedies for a Misrun?
Once you see a misrun, the first thing to do is identify which of the causes is affecting your casting specifically. The best remedy for a misrun is having a quality mold design in the first place. It is important to know every part of the casting process (which we discussed above) as well.
Start by checking the actual mold for both temperature and cleanliness. If the mold starts out too cold, it won’t be able to maintain the temperature of the metal. If it is dirty, there will be an inconsistent flow of the metal.
Mold and gating system design issues will require a re-evaluation of both to figure out where the issue is. In most cases, the issue is that the mold filling time is too slow. Make sure to evaluate every portion of the process from start to finish, especially the runner. A runner that is too efficient will slow down the metal too much, leading to a misrun when it cools too fast.
If a metal fluidity issue exists, fixing the issue may be as simple as just increasing the temperature before pouring. Do your research on the specific metal you are using, since they all have their own melting temperatures. You may also need to change the actual chemical composition of the specific metal you decide to use.
Make Sure You Know The Difference Between a Misrun and a Cold Shut
In addition to misruns, cold shuts are another common pouring defect many people run into during casting. Many novice casters mistake them for misruns, even though they are different.
The difference between the two is how the metal pours into the mold. Instead of leaving a section of the mold empty, a cold shut leaves a noticeable crack. The crack occurs when two molten metal streams are pouring into a mold. If these streams aren’t hot enough, they will not fuse together correctly or fast enough, leaving the crack.
As you evaluate your casting system after identifying a casting misrun, it’s good to start at the very beginning. Thoroughly understanding the process of casting from start to finish, including each specific component, is essential. This will help you be able to more specifically find the cause of the misrun and how to remedy it.