Injection Mold Design Guide: Gates, Cooling & Ejection

Most “mold design” articles are really part design articles. Wall thickness, ribs, draft. That’s useful, and we cover it on our plastic part design page.

This one is about the tool itself. The steel. Where the plastic goes in, how the heat comes out, and how the part gets pushed off the core without a mark.

I’ve reviewed mold designs in Huangyan for over twenty years. What follows is the order we actually work in, with the numbers our designers use on real jobs.

1. What the mold designer needs before drawing anything

A designer who starts without full information will redraw the tool later. Usually after steel is cut.

Before we open the CAD software, we want these on the table:

If the part itself needs changes, this is the moment. A DFM review costs a day. A steel change after T1 costs a week and real money. Our polycarbonate enclosure case study shows what that review looks like in practice.

2. Choose the mold type: 2-plate, 3-plate or hot runner

This choice comes early because it decides where gates can go. Everything after depends on it.

Our rule of thumb: if the runner weighs more than about 30% of the shot and volume is above a few hundred thousand a year, price a hot runner. The resin savings often cover it in the first year. The full cost picture is in our mold cost guide.

3. Parting line, cavity and core

The cavity forms the outside of the part. The core forms the inside. Where they meet is the parting line.

Most of the time the cavity sits on the fixed half and the core on the moving half. Plastic shrinks onto the core. That’s what holds the part on the ejection side when the mold opens.

Where to put the parting line

• At the largest outline of the part, seen from the opening direction.
• Away from cosmetic faces. A parting line always leaves a faint witness line.
• On a flat or simple surface if possible. Stepped lines cost more to fit.
• Where shutoffs are needed, keep shutoff faces at 5–7° or more. Flatter shutoffs wear and flash.

Get the parting line right and many undercuts disappear. We’ve removed a slider more than once just by moving the line 2 mm.

Shrinkage goes into the steel

Cavity and core are cut oversize by the resin’s shrinkage. Pick the wrong value and every part comes out of tolerance.

These are starting values. For tight parts we use the resin supplier’s data for that exact grade, then leave steel-safe on critical dimensions. More on this in controlling injection molding dimensions.

“Steel-safe” means cutting so the fix is removing steel, never adding it. Adding steel means welding. Welding on a cavity is a last resort.

4. Cavity count and layout

Start from demand, then check the machine.

A simple way to estimate:

Cavities = (annual parts × cycle time in s) ÷ (production hours × 3600 × efficiency)

Example. One million parts a year. 30-second cycle. 6,000 machine hours at 85% efficiency.

1,000,000 × 30 ÷ (6,000 × 3,600 × 0.85) = 1.6. So two cavities.

Then check three limits:

• Clamp force. Projected area of all parts plus runner, times cavity pressure. For mid-viscosity resins we use about 0.3–0.5 t/cm². Add 10–20% margin.
• Shot size. The full shot should use roughly 20–80% of the barrel capacity.
• Tolerance. Every extra cavity adds variation. For tight parts, fewer cavities is often cheaper overall.

Layout matters as much as count. In a cold runner tool, every cavity should see the same flow length. An H-pattern layout does this. A row of cavities off one runner does not. The cavity nearest the sprue fills first and packs hardest.

Never mix different parts in one family mold if one of them has tight tolerances. You’ll end up tuning the process for one part and scrapping the other.

5. Sprue, runners and gates

The sprue takes melt from the nozzle. Runners carry it across the mold. The gate is the small opening into the cavity.

Runners

• Full-round runners flow best. They need machining on both mold halves.
• Trapezoidal runners sit on one half only. Cheaper, slightly less efficient.
• A common starting size: runner diameter ≥ part wall thickness + 1.5 mm.
• Add a cold slug well at the end of each runner branch. It catches the cold front of the melt.

Gates

The gate decides how the part fills, where weld lines form, and what mark is left behind. It is the single most argued-about detail in our design reviews.

Three gate rules we rarely break:

1. Gate into the thickest section. Plastic should flow thick to thin, so the thick area can still be packed.
2. Gate thickness at about 50–80% of the wall it enters. Too thin freezes early and causes sink.
3. Know where the weld lines will land before steel is cut. Move them off loaded areas and cosmetic faces.

We go deeper on each type in our gate design article. You can also try gate positions on the simulator below.

Gate and runner improvement case: A thick wall polycabonate cover,bubblbes were always found inside the thick wall,we made the runner,sprue and gate thicker,the problem was solved 

6. Venting: the cheapest feature, often forgotten

When plastic fills the cavity, the air inside has to leave. If it can’t, it compresses and heats up. You get burn marks, short shots and weak weld lines.

Vents are shallow grooves ground on the parting face. Deep enough for air. Too shallow for plastic.

Keep the vent land about 0.8–1.5 mm long. After that, open it to a deeper relief channel, around 0.3–0.5 mm, out to the mold edge.

Put vents at the end of fill and at every weld line. Ejector pins and inserts also vent, but don’t count on them alone.

7. Cooling: where the cycle time is won or lost

Cooling usually takes 60–70% of the whole cycle. Shave cooling time and you shave cost on every single part.

Poor cooling also causes warp. One side of the part runs hotter, shrinks more, and pulls the part out of shape.

Numbers our designers start from

• Channel diameter: 6 mm for small tools, 8–12 mm for mid-size, larger for big automotive tools.
• Channel center to molding surface: about 1.5–2× the channel diameter.
• Spacing between channels: about 3–5× the diameter.
• Inlet to outlet temperature difference: under 3 °C on precision tools.
• Flow should be turbulent. Laminar water carries far less heat.

Deep cores are the hard part

A straight drilled line can’t reach inside a deep core. We use baffles or bubblers to push water up into it. For thin cores where nothing fits, we use beryllium copper or other high-conductivity inserts.

Cooling should be designed before ejectors, not after. Once ejector pins are placed, the water lines have to snake around them. We’ve seen tools where cooling was drawn last, and the core ran 20 °C hotter than the cavity. The parts warped every time.

1. Map hot spotsThick walls, deep cores, gate area
2. Lay out channelsFollow the part shape, keep even distance
3. Add baffles/bubblersReach deep cores
4. Place ejectorsAround the water, not over it
5. Check by simulationSurface temperature within a few degrees

8. Ejection: getting the part off the core

The part shrinks onto the core and grips it. Ejection has to push it off evenly. Push one corner harder and the part bends, cracks or leaves a white stress mark.

Use the largest pins that fit. Small pins punch into the part. Set the ejector stroke to part depth plus about 5–10 mm.

Draft is what makes ejection possible. Start at 1° per side. Add roughly 1° for every 0.025 mm of texture depth. Polished cores in PC can go lower. Textured ABS cannot.

Deep buckets are a good example of everything at once. Our paint bucket mould guide shows how stripper ring and air ejection work together.

9. Undercuts: sliders and lifters

An undercut is any feature that blocks the part from coming straight off. Side holes, external clips, internal snap hooks.

First question: can the part be changed to remove it? A pass-through shutoff or a moved parting line often does the job. If not, the mold needs a side action.

• Slider: for external undercuts. An angled pin drives the slider sideways as the mold opens. Angle pins usually run 15–25°. The locking wedge is 2–3° steeper so it holds the slider under injection pressure.
• Lifter: for internal undercuts. It rides on the ejector plate and moves up and inward. Lifter angles typically sit around 5–12°. Steeper angles wear and stick.
• Collapsible core or unscrewing: for internal threads and full internal rings.

Every action adds cost, cycle time and a part that can wear. Each one also needs its own cooling if it forms much of the part.

One trap: if a slider moves back over ejector pins, the pins must retract first. That needs an early-return system. Miss it and the slider crashes into the pins on the first close.

10. Steel and mold base

Steel choice follows tool life, resin and finish. Not the other way around.

US buyers often specify tools by SPI class. Class 101 is built for over a million cycles. Class 103 is for under 500k. Class 104 is under 100k, usually softer steel. Ask your supplier which class they’re quoting. Two “identical” quotes can be two different classes.

The mold base holds everything: plates, guide pins, ejector system. We mostly use LKM standard bases. HASCO or DME parts are available when a US or EU customer’s maintenance team needs them.

11. Mold flow analysis and design review

Before steel is ordered, we simulate. A mold flow run shows fill pattern, weld lines, air traps, pressure, cooling and warp.

It doesn’t replace experience. It does catch the obvious mistakes cheaply.

Here is the sequence on a typical project with us:

1. DFM report1–3 days. Draft, walls, gate, undercut issues.
2. Mold flowFill, pack, cool, warp check.
3. 2D layoutCavity layout, mold size, gate, cooling.
4. 3D mold designFull tool with every component.
5. Customer sign-offSteel ordered only after approval.
6. T1 trialUsually 4–6 weeks after design approval.

Insist on seeing the 2D layout and mold flow before steel is cut. If a supplier won’t share them, you’re buying a tool you’ve never seen. More on how a mold comes together in what is a plastic mold and the full injection molding process.

12. Mistakes we still see every year

• Gate on a cosmetic face. Nobody marked the faces. Found at T1. Fixed by a new gate insert and a week’s delay.
• Cooling drawn last. Ejector pins took the space. Core ran hot. Parts warped.
• Datasheet shrinkage on a glass-filled part. Flow and cross-flow shrink differently. One value can’t cover both.
• Too many cavities for the tolerance. Eight cavities quoted cheap. Half of them out of spec.
• No vent at end of fill. Burn marks in the same corner on every shot.
• Slider over ejector pins, no early return. Damaged tool on the first close.
• Zero draft “because the drawing says so.” Parts scuff or stick. Ask for draft early, not after T1.

None of these are hard to avoid. They all come from deciding too late.

Want to check your own part first? Our interactive design tools cover wall thickness, ribs, draft and gates.

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