Designing for the Sand Casting Process
CASTING DESIGN – THINGS YOU NEED TO KNOW
A situation we often encounter is our customers completing full design of their parts before contacting us. In their process they have locked in their design and assembly to the point where there is little flexibility to enhance the manufacturability of their part and the opportunities to gain valuable efficiencies are lost.
A simple step to improve manufacturability and experience all the benefits rolled into it, is to get us involved as early as possible in the design process. This will prevent a lot of potential headaches, delays, excess time or costs in your future. We can add value to your current process by utilizing our simulation software, allowing us to improve the part’s design and manufacturability for it to function to your expectations.
Solidification software enables us to predict defects.
We can simulate how a design will solidify while monitoring variables such as temperature or percentage of liquid versus solid material.
Looking for abrupt changes or watching how certain areas act in relation to their surroundings enables us to not only detect problem areas but test different design iterations to optimize the design prior to going into production.
Some of the most common and preventable issues impacting manufacturability relate to the following four topics.
THIN WALL SECTIONS CAN BE DIFFICULT TO FILL
Our rule of thumb for wall thickness, whether it’s the entire casting or just a wall section, is 3/16” and up. The issue with thinner walls is the aluminum cools and solidifies before it can fill the entire casting. This results in a void or hole in the casting.
ISOLATED HEAVY SECTIONS CAN GET STARVED FOR MATERIAL
This refers to parts designed with both thick and thin areas and how it’s oriented in the mold.The problem with isolated heavy sections and bosses is that the thinner material surrounding it solidifies first. This disrupts the flow of molten metal, and can result in a void or shrinkage defect in the heavy section.
Cooling the isolated heavy section quicker has given us the ability to control this problem. We can also design the mold to encourage the casting to solidify in a manner that minimizes these issues.
MINIMAL DRAFT ANGLE CAN IMPAIR RELEASE OF THE CASTING FROM THE MOLD
Draft angle is important because without it we can’t get the part out of the mold. Draft angle promotes a smooth release from the mold, resulting in high-quality surface finish. We’ll usually receive rough part designs from customers that include straight walls, so we add (our ideal) three-degree draft angle – without draft angle, it would just drag along the mold.
We’ve seen customers request as little as one-degree draft angle, which we can work with on a case-by-case basis, but we try to encourage three degrees whenever possible.
SHARP CORNERS MAY CAUSE DEFECTS AND INCREASE TOOLING COSTS
Generally a part designed with a sharp internal or external corner is a bad idea in casting because it concentrates heat and stress on the area, which can cause serious defects including cracking or tearing. Sharp external corners will also increase costs in mold manufacturing in order to achieve the desired result.
To avoid these issues, we suggest a minimum ⅛” radius for all corners. Implementing a radius reduces the heat concentration and eliminates stresses and will help ensure defect-free production of the part.
We always encourage customers to contact us directly if, at any time, there is any uncertainty around the manufacturability with a design.
Designing for the process can shorten lead time, reduce costs and eliminate pain. If your design needs to break some of the guidelines above, we might be able to find another way to achieve the results you are looking for in how we design the mold or specify an alloy suited to the application.
We welcome your call.