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When you’re deep in a CAD model trying to finalize a design, it can be tricky to know if you truly need a Class A finish or not.����

In this article, I’ll explain what each surface class means, what they’re best for, and how to make the right call for your design. You’ll learn:  

• What Class A, B, and C surfacing are
• When to use each type and what it will cost you
• How surface classes affect manufacturing and tooling decisions
• Common mistakes engineers make when specifying finishes
• A practical framework for choosing the right surface class

What Is Class A, B, and C Surfacing?

Surface classifications are a shorthand way of communicating how visible or cosmetically important each area of your part is. They guide everyone from designers to toolmakers on where to focus effort.��

• Class A is the show surface. It’s what customers see and touch.��
• Class B is the secondary surface, or backside of the A surface. It can be partly visible.
• Class C is the mechanical and functional surface. It’s purely structural and rarely seen.

These classifications aren’t universal standards like ISO or ASME specs. They’re conventions that help align design intent and manufacturing reality. The exact definitions can vary slightly between industries (automotive, injection molding, consumer products, etc.), but the purpose is always the same: to match finish quality with visibility and function.����

Why Are Surface Classifications Important in Product Design?

A surface class affects far more than appearance. It impacts:����

• Tooling and polishing time��
• Cycle time in production��
• Perceived product quality��
• Cost of manufacturing and inspection��

When the right surfaces are prioritized, you end up with a product that looks great, functions well, and stays within budget. When they’re not, communication may break down and costs may increase.����

What Is a Class A Surface? And When Do You Need One?

A Class A surface is the part of your product the end user sees and touches. Think of the outer body of a car, the top cover of a laptop, or the handle of a high-end appliance.����

These surfaces must be visually flawless with no tool marks, no sink, and no visible parting lines. They typically require: 

• High-polish tooling (SPI A1-A3 finishes)��
• Tight cosmetic tolerances��
• Close collaboration with toolmakers to achieve uniform texture and color��

When to specify Class A:

• When the surface defines the product’s aesthetic or brand identity��
• When touch and reflection quality matter��
• When the surface is in constant user contact��

But it’s important to not use Class A if it’s not what’s truly needed because there’s a tradeoff: as the surface quality goes up, cost and time increase exponentially.��

What Is a Class B Surface?

Class B surfaces are secondary cosmetic areas, often visible but not the main focus. Think side panels, internal frame covers, or undercarriage areas that are occasionally seen. 

They typically receive a medium polish or textured finish and allow for minor imperfections. 

You might use Class B when:  

• The surface supports or frames Class A geometry��
• It’s occasionally visible but not customer-facing��
• You want a good look without excessive cost��

• You want a good look without excessive cost��

This class strikes a balance between appearance and manufacturability. 

What Is a Class C Surface? And Why It Matters Even If It’s Not Seen

Class C surfaces are hidden or functional, like the inside of a plastic housing, the underside of a chassis, or the ribs and bosses that support a structure.  

They don’t require polish or cosmetic attention, but they do matter. Poor surface choices here can cause problems like ejection issues during molding, poor paint adhesion (if later coated), and/or interference fits or flash.  

In other words, Class C may be “out of sight,” but it’s never out of spec.  

Class A vs. B vs. C Surfacing: Key Differences

Here’s a simple comparison table for you to reference.��

Feature	Class A	Class B	Class C
Visibility	Fully visible	Partially visible	Hidden/internal
Finish Quality	Mirror polish	Textured/matte	As molded
Tolerance	Tightest	Moderate	Functional
Cost Impact	Highest	Medium	Lowest
Typical Use	Exterior surfaces	Interior panels	Structural areas
Example	Car hood, laptop lid	Door jamb, appliance base	Mounting bracket, internal rib

How Surface Classifications Impact Manufacturing Cost and Tooling

Every upgrade in surface class adds polishing labor, inspection time, and maintenance (higher-polish tools often require gentler cleaning and more frequent reconditioning).  

If you’ve ever been surprised by a tooling quote, surface specs might be the reason. Toolmakers price based on the level of finish required for each region of the mold. A common rule of thumb is 10-20% of tooling cost is tied directly to surface finish requirements. 

How to Decide Which Surface Class to Use for Your Design

Here’s a quick framework.����

1. Identify what the customer will see and touch.
   • Those areas? Class A.
2. Look at the adjacent areas.
   • Likely Class B.
3. Everything else?
   • Class C, unless function demands tighter control.
4. Validate early with your manufacturer.
   • Toolmakers can usually recommend lower-cost finishes without affecting appearance.

Common Mistakes Designers Make When Specifying Surface Classes

• Over-specifying Class A surfaces on parts that won’t be seen��
• Failing to define clear boundaries between A/B/C zones��
• Skipping early collaboration with manufacturing partners��
• Using surface finish as a substitute for tolerance control��

Each mistake costs time and money—sometimes in the tooling, sometimes in rework. The fix is to define, document, and discuss surfaces early in your design reviews.����

Design with the Right Surface Class in Mind

When you match visibility, function, and finish, your design tells a clear story—one your manufacturer can deliver on confidently.  

If your team is developing a new product or refining an existing design, let’s connect. We can help you balance form, function, and finish to hit your goals faster and smarter.����