What Is Cavity in Injection Molding?

I still remember looking at the first mold spec I was handed. There were terms I didn’t recognize such as runner layout, gate type, and then one line that stood out “Cavity count: 1.”

I nodded like I understood, but truthfully, I didn’t. And I didn’t ask. I figured it wasn’t that important.

That small moment ended up costing me. The project took longer than expected, the cost per part stayed high, and I found myself constantly going back and forth with the supplier trying to figure out what went wrong.

If you’ve ever been in that position staring at a mold quote or spec sheet, unsure of what cavity count really means, you’re not alone. That’s exactly why I wrote this article.

In the sections ahead, I’ll explain what a cavity is in injection molding, why it matters, and how it affects your production cost, part quality, and overall output. You’ll also learn about different mold types, and how choosing the right cavity count can help you.

By the end, you’ll have the tools to speak clearly with your supplier, make confident decisions, and avoid the costly mistakes I had to learn the hard way.

Let’s start!

1. What Is a Mold Cavity?

I remember sitting across from a supplier, reviewing my first mold quote.

They mentioned “single cavity” like it was obvious but I didn’t ask what that meant. I thought I could figure it out later. Truth is, that one term ended up affecting everything from price, lead time, and how fast I could scale production.

If you’re here, you probably want to avoid that kind of confusion. So let’s start with the basics.

A cavity is the empty space inside a mold where molten plastic is injected.

It’s shaped exactly like the part you’re trying to make, just in reverse. Think of it as the negative image of your product.

Every cavity makes one part per cycle.

If your mold has two cavities, it can make two parts at the same time. If it has eight, it makes eight. Simple, right?

Cavity vs Core

To fully understand this, you also need to know what a core is.

While the cavity shapes the outside of the part, the core forms the inside.

Picture an ice cube tray. The tray is like the cavity, it shapes the outer form of the ice. But if you stick a small object in the middle of each cube to create a hole, that’s like adding a core.

Here’s how they work together:

• Cavity: forms the outer walls
• Core: creates holes, threads, or hollow sections inside the part

Both are needed to make a functional plastic part.

Why It Matters for Production

Cavity count isn’t just a technical detail, it affects how your business runs.

Here’s what it impacts:

• How many parts you can make per cycle
• How much the mold costs upfront
• How fast you can deliver
• How consistent the parts are
• What your unit cost will look like over time

Let’s say you’re planning to produce 500,000 units. Would a single-cavity mold make sense? Or would a 4-cavity mold save you months and lower your per-part cost?

It depends on your goals. But if you don’t know what a cavity is, you can’t have that conversation.

Understanding it is the first step. Now let’s talk about how different cavity types can change your production plan.

2. Single-Cavity vs Multi-Cavity Molds

Now that you know what a mold cavity is, the next question is how many cavities do you actually need?

That decision can change everything about your production process. From how much you’ll spend on tooling to how fast your parts get made.

Let’s take a closer look.

Below is a table that compares single-cavity and multi-cavity molds side by side. This should help you visualize the trade-offs more clearly.

Mold Type	Part Output	Upfront Mold Cost	Unit Cost	Lead Time	Best For	Pros	Cons
Single-Cavity Mold	One part per cycle	Lower	Higher	Shorter mold build time	– Prototypes – Low-volume runs – Parts with tight tolerances	– Lower initial investment – Easier to set up and test	– Slower output – Higher cost per part over time
Multi-Cavity Mold	Multiple identical parts per cycle	Higher	Lower	Longer mold build time	– High-volume production – Established, stable product designs	– Faster output – Lower cost per part in bulk	– Higher tooling cost upfront – Requires precise mold balancing

As you can see, the choice depends on where you are in your production journey.

Just starting out? A single-cavity mold might help you move forward without a huge investment.

Ready to scale? Multi-cavity molds help you save time and money over the long run.

But there’s another type you might come across that often causes confusion.

3. Family Mold vs Multi-Cavity Mold

This is where many people get tripped up.

You’ll hear the terms “multi-cavity” and “family mold” used like they’re the same thing. They’re not.

Let’s make it clear.

A multi-cavity mold is built to make multiple of the same part in one cycle. For example, if you need 4 identical gear housings, a 4-cavity mold will give you 4 parts every time the mold closes and opens. It’s all about volume and speed.

A family mold is different. It makes different parts at the same time like the top and bottom halves of a product housing, or a cap and base that fit together. You’re still molding several parts per cycle, but they’re not identical.

Here’s a quick breakdown to compare them:

Mold Type	Parts Produced	Best For	Things to Watch
Multi-Cavity Mold	Multiple of the same part	– High-volume production – Single-part projects	– Must be perfectly balanced to avoid defects
Family Mold	Multiple different parts in one mold	– Assemblies with 2 or more parts – Projects where parts are used together	– Risk of uneven cooling or fill imbalance due to part size differences

So, when should you use a family mold?

• When your product has several components that always ship together
• When the parts are small and similar in volume
• When you’re okay with the added tuning time to get it working just right

I’ve used family molds for short runs where speed mattered more than perfection. It saved setup time. But I also remember one project where just one of the parts didn’t fill properly—and the whole batch had to be tossed. That’s the risk.

The benefit? You mold an entire assembly in one go.

The challenge? You have to be okay with trade-offs in balance, cycle time, and quality control.

Understanding this difference helps you avoid surprises. It’s the kind of detail that’s easy to overlook—but it can change how your production plays out, especially when deadlines or costs are tight.

4. Production Cost

Once you know your part design is ready, the next big question is cost. And one of the most important cost drivers? Cavity count.

It’s easy to focus on the price of the mold alone. But what really matters is how that choice plays out over time especially when you’re producing thousands or even hundreds of thousands of parts.

Let’s break it down.

Mold Cost vs Unit Cost

Adding more cavities will raise your mold cost upfront.

You’re paying for more tooling, more machining, and more design time. But once the mold is built, the payback can come quickly if your volumes are high enough.

Fewer cavities keep your initial spend low. That’s great for prototyping or small runs. But it also means your unit cost stays higher, and your production takes longer.

Here’s a basic example to show how this works.

Scenario	Cavity Count	Mold Cost (Example)	Parts per Hour	Unit Cost Estimate	Best For
Small Pilot Run	1 cavity	$6,000	50	$2.50	Prototypes, Low-volume testing
Medium Production	2 cavities	$9,500	100	$1.60	Initial launches, steady demand
High-Volume Production	4 cavities	$14,000	200	$1.00	Scaling up, stable designs
Large-Scale or Contract Manufacturing	8 cavities	$24,000	400	$0.60	Long-term production, high volume

Note: Numbers above are simplified for illustration only. Actual pricing will vary based on size, material, design, and region.

So how do you decide what makes sense for you?

Ask yourself:

• How many parts do I need in the next 6 to 12 months?
• Can I recover the higher mold cost with lower per-unit pricing?
• Is my design final, or could it still change?

If you’re producing 5,000 parts for a pilot launch, a single- or two-cavity mold might give you speed and flexibility without a big spend.

But if you’re running 500,000 units annually, the extra investment in a higher cavity mold could save you tens of thousands in the long run while also meeting demand faster.

Let’s now look at some hidden factors that shape mold pricing and how to think through them before you commit.

5. Lead Time and Delivery

So now you know how cavity count affects cost. But what about time?

In manufacturing, time is just as valuable as money. Maybe even more. If your delivery falls behind schedule, it doesn’t matter how much you saved on tooling.

That’s why understanding how cavity count impacts your timeline is just as important.

More Cavities = Faster Output

Let’s start with the obvious benefit.

The more cavities your mold has, the more parts you get with every cycle.

An 8-cavity mold gives you 8 finished parts in the time it takes a single-cavity mold to make one.

This matters a lot if:

• Your customers expect fast delivery
• You’re scaling a product that already has demand
• You have strict timelines from retailers or distributors

I’ve had projects where doubling cavity count shaved weeks off the delivery schedule. That speed made the difference between hitting a launch window and missing it.

But there’s a trade-off.

Multi-cavity molds are more complex.

Getting the pressure balanced, setting up the cooling system, and dialing in the shot size takes longer. Each cavity needs to be tested, not just one.

So yes, you get faster production. But it might take extra time up front to get it right.

6. Quality Considerations

By now, you’ve seen how more cavities can speed up production and lower costs. But here’s the part many people overlook which is quality.

Just because you’re making more parts per cycle doesn’t always mean they’ll all be consistent.

When you add more cavities, the mold becomes more complex. And that complexity can lead to issues if it’s not handled carefully.

Potential Risks

Here are some of the most common quality problems that come with high-cavity molds:

• Uneven cavity filling: If the molten plastic doesn’t reach each cavity with the same flow and pressure, some parts may be underfilled or malformed.
• Inconsistent cooling: This can lead to warped parts or small defects, especially if cavity spacing isn’t even or the cooling system isn’t well designed.
• Flash or sink marks: Poor mold design or bad alignment can cause extra material to leak at the edges (flash), or dents and dips in the surface (sink marks).

I’ve seen this happen firsthand. One mold looked perfect on paper, but in testing, only five out of eight parts were usable. The rest had tiny flaws that were just enough to get rejected.

So how do you avoid that?

How to Reduce Risk

Here are a few ways you can protect your product quality:

• Ask if the mold maker uses MoldFlow simulation: This software helps predict how plastic will fill the mold, so problems can be fixed before the mold is built.
• Check for proper mold balancing: Things like gate size and runner design affect how evenly plastic flows.
• Request samples from every cavity: Not just the first one that comes out perfect. You want to see consistency across the full set.

Quality issues in multi-cavity molds are preventable but only if you ask the right questions and review real results before production begins.

That’s why who you work with matters.

If you’re looking for a manufacturer who understands how to balance cavity count, mold flow, and production scale, MachMaster can help. We support every stage, from design feedback to high-volume delivery, with ISO 9001 systems and real-world experience across 500+ global projects.

7. Choosing the Right Number of Cavities

There’s no one-size-fits-all answer here. The “right” cavity count depends on your product, your goals, and your current stage of growth.

I’ve been in situations where a single cavity made perfect sense—and others where anything less than eight would’ve cost us time and money.

So how do you decide?

Start by thinking about where you are in your product cycle, and what kind of pressure you’re under from both customers and cashflow.

Factors to Consider

Here are the key things you should think through before choosing a cavity count:

• Your monthly or annual production volume How many parts do you actually need in the next 3, 6, or 12 months?
• Product lifespan and version stability Is this a long-term product, or are you still refining the design?
• Cashflow and budget for tooling Can you afford a higher upfront mold cost, or do you need to get started lean?
• Flexibility needed for revisions If the design might change, starting small can reduce waste.

Every one of these factors plays a role. You don’t want to overcommit too soon. But you also don’t want to bottleneck production when orders start coming in.

Common Scenarios

Here’s a quick breakdown based on where your business might be:

• Prototype stage: Start with a single cavity to test design and fit without spending too much.
• First batch or market testing: A 1–2 cavity mold gives you speed and flexibility while keeping cost under control.
• Stable product scaling: If your design is locked in, move to 4+ cavities to speed up output and drop your unit cost.
• Mass production: 8+ cavities work best when demand is high and consistent, and you’re focused on long-term savings.

The right choice is the one that fits your current reality and doesn’t hold you back later.

Conclusion

Now you know what a mold cavity is.

You’ve seen how it affects your cost, quality, lead time, and production strategy. You’ve learned the trade-offs—fewer cavities for flexibility, more for scale.

I’ve made mistakes by not understanding this early on, so you don’t have to.

Start by asking the right questions. Use what you’ve learned here to guide your next mold decision with clarity.

Are you planning your next production run? Or still weighing your options?

Let’s talk. Contact us today and we’ll help you choose the right path forward.