Different Types of Gates in Injection Molding

I used to think gates were just… gates. A small feature. Nothing to worry about.

Until I got the bill for a mold rework.

It turned out that one small gate decision led to warping, poor venting, and hand-trimming every part. It slowed production to a crawl. After that, I made it my mission to actually understand how gate types affect part design and performance.

If you’re reading this, you’re probably asking:

What gate type should I use? And why does it even matter?

This article will give you a clear answer. You’ll learn the most common injection molding gate types, what they do, and how they affect flow, part appearance, and production results.

Whether you’re designing the part, sourcing the mold, or managing the output—this guide will help you choose the gate that works.

Let’s start!

Quick Injection Molding Gate Reference

I know gate selection can feel like a guessing game. So here’s a cheat sheet to help you sort it out fast before we dig into the specifics of each option.

Gate Type	Best For	Pros	Cons	Trimming Method	Visible Gate Mark	Works Well With	Avoid With
Edge Gate	Flat or wide parts	Simple design, easy to trim, low cost	Visible mark, weld lines possible	Manual	Yes	ABS, Polypropylene	Glass-filled resins
Submarine Gate	High-volume, hidden gates	Automatic trimming, minimal gate mark	Harder to machine, not for thick parts	Automatic	No	ABS, flexible plastics	Stiff or filled materials
Direct/Sprue Gate	Large, thick-walled parts	Strong flow, fewer fill issues	Large gate vestige, manual trim	Manual	Yes	Nylon, Polycarbonate	Brittle or cosmetic parts
Pin Gate	Hot runner systems	No visible mark, no runner waste	Expensive mold, more setup	Automatic (valve)	No	ABS, Polyethylene	Burn-prone or hard-to-control materials
Fan Gate	Large flat parts	Smooth flow, low stress, prevents warping	Visible mark, needs more space	Manual	Yes	HDPE, flexible plastics	Small or thick parts
Tab Gate	Weld line control, impact parts	Better weld lines, reduces stress	Larger gate, manual trim	Manual	Yes	ABS, impact plastics	Soft or cosmetic parts
Diaphragm Gate	Round, hollow parts (cups, tubes)	Even flow, strong welds, great balance	Hard to trim, large gate area	Manual or tool-assisted	Yes (inner surface)	Nylon, PP, glass-filled	Brittle plastics
Cashew Gate	Parts needing hidden gates	Hidden mark, automatic trimming	Mold wear, hard to clean	Automatic	No	ABS, HIPS	Glass-filled or high-viscosity resins
Ring Gate	Circular, load-bearing parts	Uniform fill, no weld lines	Gate ring visible, tricky to trim	Manual or tool-assisted	Yes	Acetal, Nylon, structural plastics	Thin-wall or brittle parts

1. Edge Gate

An edge gate is one of the most common gate types used in injection molding. It’s placed right on the edge of the part, letting molten plastic flow in from the side. The shape is simple: just a small rectangular channel between the runner and the part.

You’ll usually find this gate used on flat or wide parts where the flow needs to spread evenly. If you’re working on a design with a consistent wall thickness and no fancy features, this gate might be a good place to start.

But how do you know if it’s the right choice for your project?

That brings us to how it actually works.

How It Works

The edge gate connects directly to the part’s outer edge. During molding, melted plastic travels through the runner, enters the gate, and fills the part from the side. Once it cools, the gate is trimmed off usually by hand or with a simple tool.

It’s easy to set up, and it doesn’t need expensive tooling changes. That’s why mold makers often suggest it for prototypes or parts that don’t have strict cosmetic demands.

Still, it’s not always a perfect fit.

Pros & Cons

Pros

• Simple to machine
• Easy to trim
• Works well for basic shapes

Cons

• Leaves a visible mark on the part edge
• Might cause weld lines if the flow has to wrap around a feature
• Trimming may not be clean if the material is brittle

Compatible Materials

Edge gates work with a wide range of plastics. But in my experience:

• They work best with materials that are easy to trim, like ABS or polypropylene.
• They don’t do well with glass-filled materials or hard resins. You might get rough edges or cracks during trimming.

Things to Watch Out For

If you’re using this gate type, keep an eye on:

• Where the weld lines form
• Whether the part will be visible in a final product
• How much trimming is needed per part

The first time I used an edge gate on a visible housing, I didn’t think the mark would matter. The customer noticed. That mistake taught me to always think about gate marks up front.

2. Submarine Gate (a.k.a. Tunnel Gate)

A submarine gate, also called a tunnel gate, enters the part from underneath or at an angle. It’s hidden below the parting line, which means the gate vestige (or leftover mark) is out of sight.

This gate is great for automatic systems where you want the parts to drop clean with no manual trimming. If you’re producing high volumes and want a cleaner look, this gate might be the right fit.

But how does it really work behind the scenes?

How It Works

The gate travels at an angle under the part and breaks off as the mold opens. That makes trimming automatic. It’s shaped like a small tunnel narrow, angled, and tucked below the part.

You won’t need to trim it by hand, and the part can be ejected smoothly.

Pros & Cons

Pros

• Leaves little to no visible mark
• Trims automatically during ejection
• Speeds up cycle time in high-volume jobs

Cons

• Not ideal for thick parts
• Harder to machine into the mold
• Can clog with some materials

Compatible Materials

Best for

• ABS
• Polypropylene
• Most flexible thermoplastics

Can cause issues with:

• Glass-filled materials
• Stiff or brittle plastics

Things to Watch Out For

Here’s what I’ve learned the hard way:

• Don’t use this gate if your part is too thick
• Clogs happen fast if your material burns easily
• If your ejector system isn’t aligned right, parts may stick

Is the part cosmetic?

Do you need it to drop without touch?

If yes, the submarine gate could save you hours of trimming time.

3. Direct/Sprue Gate

The direct gate connects straight from the sprue to the part. No runners. No turns. Just direct flow. It’s often used in single-cavity molds or parts with thick walls.

It gives you a strong flow of material, perfect for big parts or large-volume shots.

Let’s look at how it functions inside the mold.

How It Works

The molten plastic travels through the sprue and enters the part without any runner system in between. This setup reduces resistance and allows for a high-pressure fill.

But once cooled, the gate can leave a noticeable mark. Trimming is often manual and needs care to avoid damaging the part.

Pros & Cons

Pros

• Great for large parts
• High-pressure fill with fewer flow issues
• Simple mold design

Cons

• Leaves a noticeable gate mark
• Slower cycle times due to larger mass
• Manual trimming required

Compatible Materials

Best for:

• Polycarbonate
• Nylon
• Materials that benefit from high-pressure fill

Use caution with:

• Brittle plastics
• Parts where cosmetics matter

Things to Watch Out For

I’ve used this gate on thick industrial parts, it worked well, but trimming always took time.

Watch for:

• Sink marks near the gate
• Uneven cooling
• Long cycle times

Is your part thick or built for strength? If so, the direct gate could give you better fill control.

4. Pin Gate (for Hot Runner Systems)

A pin gate is used in hot runner molds. It’s small, round, and placed directly on the part. This gate type allows for automatic, clean trimming and is usually invisible on finished parts.

If you’re building a hot runner system, this is one of the most common gates you’ll come across.

But what makes it work so cleanly?

How It Works

The molten plastic is kept hot all the way to the gate. A pin retracts just before injection, then pops back to seal the gate as the mold opens. No cold runners. No trimming.

The result: smooth parts with little waste.

Pros & Cons

Pros

• No visible gate vestige
• No runners to recycle
• Clean and consistent part finish

Cons

• Requires hot runner tooling
• Higher mold cost
• More setup and maintenance

Compatible Materials

Works well with:

• ABS
• Polyethylene
• Materials that run clean at high temps

Trouble with:

• Materials prone to stringing or burning
• Resins with narrow processing windows

Things to Watch Out For

One time, we ran this gate with the wrong temperature settings. The result? Stringing, flash, and stuck parts.

Check:

• Valve timing
• Temperature zones
• Parting line sealing

If you’re working with hot runner systems or need high-precision tooling for a project like this, you need a partner who’s done it before. MachMaster specializes in tight-tolerance injection molding, delivering precision parts with ±0.01mm accuracy for global industries.

5. Fan Gate

A fan gate spreads the molten plastic over a wide area. Instead of a single point, it creates a “fan-like” flow into the part. This helps reduce shear stress and gives better fill in large or flat parts.

If you’re molding something long and thin, like a panel or cover, this gate gives more control.

Let’s walk through how it actually spreads the flow.

How It Works

The gate gradually widens from the runner into the part. As plastic enters, it spreads evenly like a curtain. This reduces pressure spikes and avoids jetting or sudden flow bursts.

After molding, trimming is usually done by hand.

Pros & Cons

Pros

• Good for large, flat parts
• Reduces internal stress
• Helps prevent warping

Cons

• Visible gate vestige
• Slower fill in small parts
• Needs space on the part edge

Compatible Materials

Ideal for:

• Polypropylene
• HDPE
• Flexible materials

More difficult with:

• High-viscosity plastics
• Materials that cool too fast

Things to Watch Out For

I once used a fan gate on a small cap. Big mistake. The wide flow area made the part overflow and warp.

Avoid:

• Using this on tiny or thick parts
• Ignoring cooling times
• Placing it on parts that need a clean finish

Is your part wide and shallow?

Then this gate might give you the smoothest results.

6. Tab Gate

A tab gate is like an edge gate, but with a twist. It has a wider connection between the runner and the part. This “tab” helps absorb stress and control how the molten plastic flows in.

If your part has tricky weld lines or is prone to stress marks, this gate helps spread the flow better.

Next, let’s look at how that wider flow path works.

How It Works

The tab is a rectangular extension between the runner and the part. As the plastic enters, the gate slows down the flow just enough to reduce shear stress. That’s the force that builds up when plastic moves too fast and hits a wall or corner.

After molding, the gate is trimmed usually by hand.

Pros & Cons

Pros

• Helps reduce stress marks
• Better weld line strength
• Simple to add to an edge gate design

Cons

• Leaves a bigger mark than standard gates
• May require more space on the part edge
• Not suited for cosmetic surfaces

Compatible Materials

Works well with:

• ABS
• Polycarbonate
• High-impact resins

Not ideal for:

• Very soft or flexible materials

Things to Watch Out For

One time, I used a tab gate on a small part. It worked, but the tab left a visible chunk on the edge.

Think about:

• Will the gate mark bother your end-user?
• Can you trim the tab without cracking the part?

If your part needs strong weld lines and can hide the gate, the tab gate could be a smart pick.

7. Diaphragm Gate

The diaphragm gate is used when you need even, centered flow. It’s a circular gate that surrounds a core, often found in cup- or tube-shaped parts. The plastic enters all around at once.

If you need balanced filling without weld lines, this might be your answer.

Now let’s break down how it achieves that even flow.

How It Works

This gate surrounds the center of a part like a ring. During injection, the melt flows evenly from all directions toward the middle. That reduces internal stress and avoids weld lines that could weaken the part.

Trimming usually needs a blade or special tool.

Pros & Cons

Pros

• Great for symmetrical, hollow parts
• Excellent weld line control
• Balanced fill from all sides

Cons

• Leaves a gate mark on the entire inner surface
• Not for small or cosmetic parts
• Harder to trim

Compatible Materials

Best with:

• Polypropylene
• Nylon
• Glass-filled plastics

Can be tricky with:

• Brittle materials that crack easily during trimming

Things to Watch Out For

I once ran this gate on a short-run project. The balance was great, but trimming took way more time than I expected.

Ask yourself:

• Is even flow more important than fast trimming?
• Will the gate mark affect your part’s function?

If flow control matters more than finish, the diaphragm gate delivers.

8. Cashew Gate

The cashew gate has a curved path like the shape of a cashew nut. It connects below the parting line and curves up into the part.

This gate is useful when you need the gate to be hidden, but the layout doesn’t allow a straight tunnel like the submarine gate.

Let’s look at how this curve changes the way it flows.

How It Works

The plastic travels in a curved path under the part and pops up into it. This keeps the gate out of sight. It’s often used when there’s limited space for a gate on the part face.

Trimming is automatic during ejection, but mold machining is more complex.

Pros & Cons

Pros

• Hidden gate mark
• Automatic trimming
• Ideal for complex or angled parts

Cons

• Difficult to machine into the mold
• Harder to clean and maintain
• May limit material flow

Compatible Materials

Works with:

• ABS
• HIPS (High-Impact Polystyrene)
• Other mid-viscosity thermoplastics

Can struggle with:

• Glass-filled or stiff materials

Things to Watch Out For: One mold I used had cashew gates. The parts looked great—but cleaning those curved channels between runs was a pain.

Consider:

• Will the flow path support your material choice?
• Are you okay with a more expensive mold build?

For parts that need clean looks and tight space, the cashew gate works—if you’re ready for the extra tooling cost.

9. Ring Gate

The ring gate is exactly what it sounds like: a ring-shaped gate that surrounds a core pin or circular feature. It’s designed for parts like cylinders or gears where you need even flow from all sides.

It creates a uniform fill, especially in symmetrical, round parts.

Let’s look at how this ring helps balance flow.

How It Works

Plastic flows evenly through the ring and into the part. This helps avoid weld lines and stress that can form when flow comes from a single direction. It’s great for structural parts that need even strength.

The gate is trimmed later, often requiring special tools.

Pros & Cons

Pros

• Uniform flow
• No weld lines
• Strong center part fill

Cons

• Leaves a ring mark
• Requires trimming all around
• Needs careful mold setup

Compatible Materials

Best with:

• Nylon
• Acetal
• Structural plastics

Less effective with:

• Brittle resins
• Thin-wall parts

Things to Watch Out For

I used a ring gate on a gear mold once. The strength was solid but the trimming took two steps, and alignment had to be perfect.

Ask yourself:

• Does your part need even strength from all sides?
• Can you trim the gate cleanly?

If so, the ring gate gives you balanced performance just be ready for a bit more cleanup.

Conclusion

Gate selection used to confuse me.

Now, it’s one of the first things I look at because it affects everything: flow, cooling, part quality, and even your timeline.

In this guide, you’ve seen what each injection molding gate type does, where it fits best, and what can go wrong.

So what’s next?

Use this knowledge, build smarter, and ask better questions.

Contact us today! We’ll help you make the right choice from the start.