Choosing the right edge gate vs sub gate mold design is not simply a matter of picking the smaller or cheaper gate. The right choice depends on part geometry, mold structure, appearance requirements, production volume, and how the part will be ejected and trimmed.
For mold buyers, the edge gate vs sub gate mold decision should be made before tooling is released for manufacturing. Once steel has been cut, changing the gate type may affect the parting line, runner layout, ejection system, and overall mold structure.
An edge gate is usually simpler and easier to manufacture. By contrast, a sub gate, also called a submarine or tunnel gate, can offer automatic degating and more freedom in gate placement.
However, it also requires more careful design and machining.
So, which one should you choose?
The answer depends on what the mold needs to achieve in production.
Edge Gate vs Sub Gate: Quick Comparison
Before choosing a gate, it helps to understand the main differences.
| Factor | Edge Gate | Sub Gate |
|---|---|---|
| Gate location | Usually at the parting line | Angled tunnel into the cavity |
| Mold structure | Simple | More complex |
| Initial tooling cost | Usually lower | Usually higher |
| Gate removal | Often needs trimming | Often automatic |
| Gate mark | Usually along the part edge | Can be placed in a less visible area |
| Filling ability | Good for many part designs | Good when correctly sized |
| Mold maintenance | Easier | More difficult |
| Automation | May need extra trimming | Often supports automatic degating |
| Typical use | General plastic parts | Suitable parts needing automatic degating |
The main difference is gate removal. An edge gate often needs trimming, while a sub gate can often break away during ejection.
However, neither gate is always better. The best choice depends on the complete mold design and production requirements.
What Is an Edge Gate in Injection Molding?
An edge gate is a common gate type in plastic injection molding.
It is usually placed along the parting line. Molten plastic flows from the runner through the gate and into the cavity.
The flow path is simple:
Runner → Edge Gate → Mold Cavity
Because the gate sits near the parting line, mold makers can machine and inspect it easily.
Advantages of an Edge Gate
An edge gate offers several benefits:
- Simple mold design
- Lower machining complexity
- Easy inspection and maintenance
- Flexible gate width and thickness
- Easy modification during mold trials
- Good filling ability for suitable parts
- Lower initial tooling cost in many cases
In addition, edge gates can provide a relatively large flow area. This can help reduce pressure loss for suitable thick-wall or long-flow parts.
Disadvantages of an Edge Gate
The main drawback is gate removal.
After molding, the runner may remain attached to the part. Therefore, operators or automation equipment must remove it.
Other limits include:
- Visible gate marks on the part edge
- Possible extra trimming labor
- Risk of damage during gate removal
- Limited gate positions
- Possible interference with assembly surfaces
However, these limits may not matter for hidden industrial components.
What Is a Sub Gate in Injection Molding?
A sub gate is also called a submarine gate or tunnel gate.
Unlike an edge gate, it passes through the mold steel at an angle.
The typical flow path is:
Runner → Angled Tunnel → Sub Gate → Mold Cavity
In many designs, the gate separates from the molded part during ejection.
Therefore, sub gates are often used when automatic production is important.
Advantages of a Sub Gate
A well-designed sub gate can provide several benefits:
- Automatic gate separation
- Reduced manual trimming
- Gate placement on less visible surfaces
- Good production efficiency
- Useful for suitable multi-cavity molds
- Less secondary handling
- Good integration with automated production
For example, a high-volume plastic clip project may benefit from automatic degating.
Disadvantages of a Sub Gate
A sub gate needs more careful design.
The angled tunnel must have suitable geometry. Otherwise, the runner may stick or fail to separate cleanly.
Common disadvantages include:
- More complex mold machining
- Higher initial tooling cost in some cases
- Greater sensitivity to gate geometry
- Possible gate breakage problems
- Higher local wear with abrasive materials
- More difficult repair and maintenance
Also, very small gates may create high shear and pressure loss.
Therefore, the mold designer must check the actual material and part geometry.
Edge Gate vs Sub Gate: Which Costs Less?
Cost is an important factor when comparing edge gate vs sub gate designs.
An edge gate usually costs less to machine because its structure is simple.
However, mold price is only part of the total manufacturing cost.
A sub gate may cost more at the tooling stage. Yet it can reduce labor if automatic degating works reliably.
Compare Total Production Cost
Consider two mold options:
Option A: Edge Gate
- Lower initial mold cost
- Simple maintenance
- Gate trimming required
- Possible extra labor or automation equipment
Option B: Sub Gate
- Higher initial tooling complexity
- Automatic gate separation may be possible
- Less manual trimming
- More attention to gate wear and ejection
For low-volume production, an edge gate may be the economical choice.
For high-volume production, the saved trimming cost may justify a sub gate.
However, production volume alone does not decide the result. Cycle time, scrap, maintenance, and gate quality also matter.
At Fentor Mold, we consider both tooling cost and production needs during the mold design review.
Edge Gate vs Sub Gate: Gate Marks and Surface Quality
Gate marks affect the appearance of injection molded parts.
An edge gate usually leaves a mark near the parting line.
This may be acceptable for hidden components. However, it can be a problem for visible automotive or consumer products.
A sub gate can often enter through a less visible area. As a result, the gate mark may be easier to hide.
Still, a sub gate does not guarantee a clean surface.
Gate marks depend on:
- Gate size
- Gate angle
- Plastic material
- Melt temperature
- Mold temperature
- Injection speed
- Gate separation
- Local part geometry
Therefore, appearance requirements should be reviewed before mold manufacturing begins.
How Gate Design Affects Mold Filling
Both edge gates and sub gates can fill plastic parts successfully.
However, filling performance depends on more than the gate type.
Engineers should check gate location, flow length, runner size, and wall thickness.
Filling Performance of an Edge Gate
An edge gate can offer a relatively large opening.
Therefore, it may be suitable for parts that need a higher melt flow rate or lower pressure loss.
In addition, the gate can often be enlarged during mold trials.
However, a poor location may still cause weld lines or uneven packing.
Filling Performance of a Sub Gate
A sub gate often has a more restricted flow path.
As a result, filling pressure and shear may increase if the gate is too small.
For high-viscosity materials, the design may need special attention.
Glass-filled plastics can also create more wear around small gate areas.
Therefore, gate dimensions should match the actual material grade.
For detailed engineering information, see our injection mold filling balance guide.
Edge Gate vs Sub Gate for Automatic Production
Automatic production is one of the main reasons to choose a sub gate.
With suitable geometry, the molded part and runner can separate during ejection.
Therefore, workers may not need to trim every part.
This can improve efficiency in high-volume production.
However, the automatic separation must remain reliable.
If the runner sticks inside the tunnel, production may stop.
An edge gate can also support automation. Yet it may need a separate trimming operation.
Which Gate Is Better for Multi-Cavity Molds?
Sub gates can be useful for multi-cavity molds that produce many small plastic parts.
For example, a mold producing plastic clips may benefit from automatic degating.
However, some parts still require an edge gate to provide stable filling.
Therefore, cavity balance, gate size, and part design should guide the choice.
Edge Gate vs Sub Gate: Ejection and Mold Maintenance
Gate design and ejection must work together.
An edge gate usually has a simple runner connection.
Therefore, maintenance and gate modification are often easier.
A sub gate needs careful control of the tunnel angle and gate connection.
During ejection, the runner must separate without damaging the molded part.
Potential problems include:
- Runner sticking
- Broken gates
- Gate residue
- Excessive wear
- Poor gate separation
- Part damage
To avoid these problems, mold designers should review the tunnel geometry and ejection movement during DFM.
In addition, the gate must have enough surrounding mold steel for strength and maintenance.
Our custom injection mold manufacturing services include DFM review, gate design, precision machining, and mold trials.
How Plastic Materials Affect Gate Selection
Different materials need different gate designs.
For example, ABS and PP may work well with either gate type when the part design is suitable.
However, some engineering plastics need more careful control.
ABS and PP
ABS and PP are common injection molding materials.
Both can use edge gates or sub gates.
The final choice depends on wall thickness, appearance, shrinkage, and gate location.
PC and Other High-Viscosity Materials
Some PC grades need careful gate sizing and filling control.
A gate that is too small may increase shear and pressure loss.
Therefore, engineers should review the material data before selecting a sub gate.
Glass-Filled Engineering Plastics
Glass-filled materials may cause more gate wear.
In addition, fiber orientation can affect shrinkage and part strength.
Therefore, both gate location and gate size are important.
The mold may also need suitable wear-resistant steel or replaceable gate inserts.
When Should You Choose an Edge Gate?
An edge gate is often suitable when:
- The part has an accessible edge at the parting line.
- The gate mark is acceptable.
- Tooling simplicity is important.
- The gate may need modification during trials.
- Manual or automated trimming is acceptable.
- A relatively large gate opening is useful.
- The part design does not require hidden gating.
For example, an internal plastic housing or industrial bracket may work well with an edge gate.
In these cases, a simple mold may offer better overall value.
When Should You Choose a Sub Gate?
A sub gate is worth considering when:
- Automatic gate separation is required.
- The gate should be hidden.
- Manual trimming should be reduced.
- The part geometry supports tunnel gating.
- The mold has enough steel for the gate tunnel.
- The ejection system can separate the runner reliably.
- The material can flow through the selected gate size.
For example, small plastic clips and selected automotive components may benefit from submarine gates.
However, the mold designer must confirm that the gate will separate consistently.
Common Mistakes When Choosing Edge Gate vs Sub Gate
Choosing the wrong gate can create quality and production problems.
Here are five common mistakes.
1. Choosing Only by Tooling Price
A lower mold price may lead to higher trimming labor.
Therefore, compare total manufacturing cost.
2. Ignoring Gate Marks
A gate may fill the cavity well but leave an unacceptable mark.
Check the gate position during DFM.
3. Making the Gate Too Small
A small gate can create high pressure loss and shear.
Instead, choose dimensions based on the material and flow requirements.
4. Ignoring Ejection
A sub gate must work with the runner and ejection system.
Otherwise, automatic separation may fail.
5. Changing Gate Type Too Late
Changing from an edge gate to a sub gate after mold machining may require major steel changes.
Therefore, confirm the gate concept before tooling starts.
For related design guidance, read our injection mold parting line design rules.
How to Choose the Right Gate for Your Mold
Before selecting the final gate type, answer these five questions:
- Where can the gate enter without affecting appearance?
- Can the gate provide stable filling and packing?
- Is automatic degating necessary?
- How much trimming is acceptable?
- Does the production benefit justify the tooling complexity?
Next, review the plastic material and part geometry.
For complex parts, Moldflow analysis can help compare gate locations and filling conditions.
Finally, confirm the gate design during mold trials.
A good gate should support part quality, reliable ejection, and stable production.
Conclusion
The edge gate vs sub gate decision depends on mold structure, material flow, appearance, and production requirements.
An edge gate is often the better choice for simple tooling and easy maintenance.
In contrast, a sub gate may offer better automation and less manual trimming.
However, neither option is always better.
The best gate design should balance tooling cost, part quality, production efficiency, and long-term reliability.
Fentor Mold provides injection molding production support alongside custom mold manufacturing. We can review your part drawings and recommend suitable gate locations during DFM.
Frequently Asked Questions
Is a Sub Gate Better Than an Edge Gate?
Not always. A sub gate is useful for automatic degating and hidden gate locations. However, an edge gate is often simpler and easier to maintain.
Which Gate Is Cheaper?
An edge gate usually has a lower initial machining cost. However, a sub gate may reduce trimming labor during large production runs.
Which Gate Leaves a Smaller Mark?
A sub gate can often place the mark in a less visible location. However, actual mark size depends on gate geometry, material, and the molding process.
Can Edge Gates Be Used for High-Volume Production?
Yes. Edge gates can work well in high-volume molds when gate trimming and automation are properly planned.
Can a Sub Gate Be Used for Every Plastic Material?
No. Material flow, shear sensitivity, gate wear, and part geometry must be considered.
Which Gate Is Easier to Maintain?
An edge gate is usually easier to inspect and modify because it is located near the parting line. A sub gate may require more work to repair its angled tunnel.