Mistakes in the world of metalcasting can kill a project, so you can’t take any chances from start to finish. Understanding the critical aspects of casting design and production will help you create the best process for your parts, giving you a leg up on less-informed competitors.
Metalcasting principles—including wall thickness requirements for a sound part, the importance of tooling on everything from cost to product lifecycle, and why secondary finishing impacts lead time and investment—are covered in the following 10 questions.

1. What is your annual usage?

It’s critical to create your production process around the goals for your final product. Quality standards and production volumes should be in the books before the planning phase.
The sweet spot for mold casting is 500-35,000 units produced annually. Low volume parts are better suited for a process like sand casting. If it is a high-volume part, it’s usually best to invest in a diecast tool, unless you have design criteria that diecasting cannot control, such as pressure tightness. Many permanent mold projects fall in the 3,000-12,000 range.

Permanent mold tooling must be built to keep pace with the long-term production goals. As the person specifying the production method and casting source, you need to know the return on investment for your tooling and how long your operation can wait to realize those investments.

Key considerations:

• Are you in the production sweet spot of 500 to 35,000 parts annually?
• Does your foundry have the right process to reliably meet tolerance requirements?
• Can you minimize setup costs and pass savings to the customer?

2. What is the end application?

The design and engineering team won’t know the best way to make a part unless they know how it will be used.

If the product ends up failing in the field, projects have to be abandoned or reimagined. Setup costs substantially contribute to the final piece price. Maintaining product quality across longer runs with greater quantities can also lower piece prices.

Project engineers without metalcasting experience may not know what to ask the foundry to ensure the parts are strong or aesthetically pleasing enough to serve their purpose. Examples of important information to provide to the foundry includes whether the part needs to have a clean finish, be leak-proof or meet certain industry regulations.

Ideally, the foundry will use drawing specifications to ensure the part is made to the dimensional and metallurgical quality standards. If the part is to be pressure-tight, the foundry should understand the quality standard or testing parameters.

To put this in perspective, consider a purchasing manager who came to Batesville Products Inc. after struggling with a couple of suppliers to produce a cover piece for the transportation industry. They couldn’t find a way to make the part structurally strong enough to meet requirements while maintaining a pleasing aesthetic. The part had to be structurally sound and leak-proof, but appearance was perhaps the most important aspect.
The foundry worked with the customer to find the perfect permanent mold solution for all of their problems, because they took the time to ask the client the right questions at the beginning of the process, such as their surface finishing and cast strength requirements.

Key considerations:

• Do you have experience in the field?
• Are you asking the right questions?
• Are there any applicable regulations?
• Are there any better design opportunities?

3. What is the size of the piece?

Permanent mold casting has a pretty big sweet spot for sizing. Parts ranging anywhere from 2 ounces to 110 lbs. can be made with this method, though a lot of products are in the 3- to 30-lb. range.
Large products can cause design constraints, and few permanent mold foundries have the equipment necessary for those jobs. Sand castings are more common for large castings, but the permanent mold process can make fairly heavy parts.

A healthcare provider needed a heavy base to provide a better center of gravity to its medical carts. The base was cast in zinc, which is twice as dense as aluminum, offering a smaller, but heavier product.

Key considerations:

• Is it within the sweet spot of 2 ounces 
• to 110 lbs.?
• Are there constraints on gating and 
• mold design?
• Is it manufacturable?

4. What’s the product’s history?

Background information, like if a product is completely new or already available for purchase, is key information to share with your casting supplier. If the part is already in production, how is it manufactured, and could the process be improved within budget?

Batesville Products Inc. works with fabrication manufacturers to rethink their existing products. A manufacturing company asked the foundry to turn a formerly fabricated part into a single permanent mold casting. By combining multiple parts into one casting, the foundry cut costs and created a stronger product.

The customer paid off its tooling in just a few months, and only had to use one part and one supplier to produce and control it.

Key considerations:

• Is it a new or existing product?
• Why are you looking to change the process, 
• and are there any existing problems?
• Can the deficiencies be improved upon?
• Can the designs be improved?
• Is your design flexible enough to adapt to 
• permanent mold casting?

5. Will you need engineering support?

Working with a dedicated metalcasting foundry during the design phase will allow you to make parts with better lead times. After all, this is what they do all day, every day.

While it’s best to approach a foundry before the part has been fully designed, customers sometimes look for a foundry after they have completed their specifications but don’t know how to make them a reality.
A good supplier will be able to take the product apart, run tests and perform finite element analysis on the casting process to help aid in designing the production system. 3D-printed prototypes help provide a sense of the structure before you invest in tooling.

A good supplier will also review the molten metal flow as it’s poured into the casting, watching it take shape. Analyzing the flow of molten metal for turbulence allows the foundry’s engineering team to identify possible weak spots in the casting. Once the process is perfected, you should be confident everything is ready for production.

Scanning technology can also be used to digitally create existing parts on the computer so engineers can look for new improvements to old designs. 

Key considerations:

• Does the foundry have in-house engineering?
• Is the lead time approved?
• Are all the critical design features understood?
• Are your engineers talking to the foundry’s 
• engineers?

6. What are the lead time and scheduling requirements?

Before a foundry can work on a project, it needs to know your lead time expectations and production schedule to decide if a permanent mold solution can fit your timeframes.
All expectations for the job need to be presented upfront, especially if you’re working with a tight production schedule.

For example, an engineering manager needed a part for the boating industry that was already designed for permanent mold. However, it was almost Christmas, and the company wanted parts in a very short time. The product had pressure tightness specifications and required internal cavities, which had to be created with a sand core. Permanent molding is great at creating pressure-tight castings but the foundry had to be creative and adjust the process to semi-permanent molding in which sand cores are used. The foundry went on to make 350 of the boating parts per week for 20 years, and still serves this customer today.

Key considerations:

• Are your expectations realistic?
• When does the clock start ticking?
• Do you understand key stage gates?

7. What are your finishing requirements?

Sometimes the appearance of a product is a major consideration. Castings should have a nice smooth surface right out of the mold if mold coatings and draft angles are used effectively.
Scabbing and blemishes can occur if the mold coating breaks down. Draft angles must be adjusted to maintain the coating throughout an entire shift. Insufficient drafting creates a greater chance for metal-on-metal interaction. Increasing draft lets applied coatings remain consistent across continued pours.

It’s also important to know if secondary processes like machining, finishing, polishing or coating will be required.

Key Considerations:

• Can you control the finish of your casting 
• coming out of the mold?
• Are processes controlled by one 
• source or multiple?
• Draft has a major influence on the finish.

8. What are your wall thickness and draft requirements?

The wall thickness of a casting has to be just right to limit weight while allowing metal flow.
Generally, the minimum wall thickness for a permanent mold is about 0.18 in. to ensure there’s enough space for the molten metal to settle as air escapes the mold. However, some permanent mold facilities can go down to 0.125 in. in select portions of a casting, and the walls also can be trimmed down in secondary operations.

The thicker a portion of the molding is, the longer it will take to cool. Parts that are consistently portioned throughout the mold will cool evenly, improving throughput.

The draft angle refers to the angle at which the part will smoothly eject from the mold with a high-quality finish. The minimum functional draft angle is three degrees, but it can be increased to five degrees to help maintain mold coatings.

Key considerations:

• Are the walls too thick or too thin?
• Is air evacuating the cavity?
• Is there any shrinking/porosity?
• The more draft, the better control of the mold coating.

9. What are your as-cast tolerances?

Foundry workers can occasionally take parts out of the mold, saw off the gate, trim it up and put it right into the assembly.

Understanding the secondary processes needed to turn the cast product into the end application should be taken into account at the beginning of the project.

Some foundries can perform all the work in their facilities, but not every foundry takes this approach. That’s why you need to know if the foundry can perform secondary machining in house or if they will need to outsource operations, adding time and money to the production.

Key considerations:

• Is the part near/net shape?
• Is any secondary machining required?

10. Have you budgeted for tooling?

A good portion of every project’s budget goes toward tooling, so you need to be aware of how long it will take to make a return on that investment compared to its expected lifetime, especially for long-term projects.
A cereal company had experience with a variety of casting methods, but their engineers didn’t have specialized experience in permanent mold casting. So, the engineers at the foundry helped them compare permanent molds to other forms of metalcasting and discussed what the process can consistently offer.

Moving to a permanent mold solved porosity and air entrapment issues experienced on other units made via sand casting and diecasting.    CS

Key considerations:

• What is your expected return on investment?
• What process is best? Permanent mold, 
• sand casting or die casting?
• What is the expected life of your tooling?

Click here to view the article in the digital edition of July/August 2021 Casting Source.