Your first sample parts look great until you flip one over. There’s a rough nub right on the face your customer sees first. That nub is where the plastic came in. It’s the mark left by the gate.
The types of gates in injection molding matter more than most people expect. That small opening affects how your part looks, how strong it is, and how much trimming it needs. The right gate can keep that mark off the faces that count. At Freeform Polymers, we settle gate type during mold design, before any steel is cut.
This guide breaks down each gate type, how it works, and which parts it suits. We’ll start with what a gate is. Then we’ll walk through cold runner and hot runner gates and compare them side by side. Last, we’ll show how gate location affects your part.
Injection molding gates fall into two main groups: cold runner gates and hot runner gates. Cold runner gates are fed by unheated channels. Each shot leaves a runner or sprue that has to be removed. Common types include:
Hot runner gates include thermal (hot tip) and valve gates. They leave little or no runner waste. The best gate for you depends on part shape, resin, looks, and volume.
A gate is the small opening where melted plastic enters the mold cavity. It’s the last stop before the plastic becomes your part. Every molded part has at least one.
In a standard cold runner mold, plastic travels this path:
These channels sit inside the molding system, one of the three main parts of an injection mold.
Gates are small on purpose. A narrow gate seals off as it cools, after the cavity is filled and packed. That keeps plastic from flowing back out. The thin spot also makes the gate easier to break or trim away.
The gate also leaves a small mark on your part, called the vestige. That’s why gate choice affects how your part looks. We set gate type and location during mold design, before any steel is cut.
Edge-fed gates are the most common gate family. They feed plastic into the side of your part at the parting line. That’s the seam where the two mold halves meet.
The tradeoff is trimming. Each of these gates has to be cut off after molding, by hand or as a secondary step. We can handle that step with our post-mold machining and trimming.
Edge gates are simple, but someone has to trim them. This next family removes itself. The gate breaks away from your part as the mold opens or the part ejects.
Upside:
Watch-outs:
Not every part is flat. Round parts, tubes, and large single parts need a different way in. These gates feed plastic evenly through the center or around the outside of the part.
Direct sprue and diaphragm gates suit single-cavity molds best. The tradeoff is cleanup. These gates have to be cut or machined off after molding.
Hot runner gates sit at the end of heated nozzles. The plastic stays melted all the way to the cavity. That means little or no runner to trim or throw away.
| Gate | How it works | The mark it leaves |
| Thermal (hot tip) | Heat at the tip controls the flow. Simpler design. | Can leave a small bump or string. |
| Valve | A pin moves forward to shut the gate. | Usually the cleanest, often just a faint ring. |
The tradeoff is cost. Hot runner tooling costs more up front, so it fits best at high volumes. For a closer look at that choice, see our guide to hot runner vs. cold runner molds.
Each gate type solves a different problem. This table shows how they stack up side by side.
| Gate type | Best for | Mark left | Trimming needed | Relative tooling cost |
| Edge | Most general parts, prototypes | Visible at the edge | Yes | Low |
| Tab | Low-stress parts, like clear parts | On the tab | Yes | Low |
| Fan | Wide parts | Wide mark at the edge | Yes | Low |
| Film | Flat, thin parts | Thin line along the edge | Yes | Low |
| Submarine (tunnel) | Higher-volume parts | Small, hidden | No | Medium |
| Cashew | Gating hidden spots | Small, hidden | No | Medium |
| Pin-point | Three-plate molds | Tiny | No | Medium |
| Direct sprue | Large single parts | Large | Yes, cut off | Low |
| Diaphragm | Round parts with an open center | At the center | Yes, cut or machined off | Low |
| Ring | Tubes | Around the outside | Yes | Low |
| Thermal (hot tip) | High-volume parts | Small bump or string | No | High |
| Valve | High-volume cosmetic parts | Cleanest, often a faint ring | No | Highest |
To narrow it down, answer these five questions:
Low volumes and prototypes usually lean toward simple edge gates. High-volume or cosmetic parts lean toward sub gates or valve gates. Bring your answers to our design for manufacturability (DFM) review, and we’ll help you weigh the options.
Picking the gate type is half the job. Where you put it matters just as much. The gate’s location shapes how plastic fills your part and where problems show up. Autodesk lists most of these checks in its Moldflow design rules for gate placement.
Gates also wear over time. Glass-filled and other filled resins wear them faster. A worn gate can change how your part fills.
Gate choice is easier when your mold maker is down the road. We’re an ISO 9001:2015 certified molder based in Logan. Since 2011, we’ve served businesses across Northern Utah and Southern Idaho.
Our engineers review gate locations, wall thickness, and parting lines at the start of each mold build. We work through gate type, cavity count, and cooling with you. You review and approve the mold design before any steel is cut.
Design, machining, and mold trials all happen in our building. Your part never gets handed off between outside shops.
Have a part that needs a gate plan? Contact us and request a free quote, or call us at (435) 774-9090. We’ll talk through your part and the options that fit it.
