---
title: "Hopper and feed throat of an injection molding machine"
url: "https://www.imspares.com/injection-molding-machine-parts/barrel-screw/hopper-and-feed-throat/"
---

# Hopper and feed throat of an injection molding machine

How the hopper and feed throat of an injection molding machine feed the screw, why the throat is water-cooled, what a hopper magnet does, and feeding problems.

The injection molding machine hopper is the container on top of the injection unit that holds the plastic pellets, and the feed throat is the cooled opening below it through which the pellets fall onto the screw. Together they keep the screw supplied with material at a steady rate. A hopper magnet in the material path catches loose iron and steel before it reaches the screw, and cooling water in the feed throat keeps the pellets solid until the screw takes them in.

The hopper and feed throat are item 7 in the drawing, above the feed zone of the screw. For the rest of the plasticizing unit, see the [barrel and screw guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/).

## Parts of the hopper and feed throat

- **Hopper.** A container with a lid, often with a sight glass to show the level. Many machines use a hopper loader, which fills the hopper by vacuum, or a hopper dryer in its place.
- **Slide gate.** A plate under the hopper that shuts off the flow of pellets, so the hopper can be emptied or the barrel purged without more material falling in. Many hoppers also have a drain chute to empty them for a material change.
- **Hopper magnet.** A grate or a drawer of magnetic bars in the material path.
- **Feed throat.** A block between the hopper and the barrel's feed opening, with a channel for cooling water. On some machines it is part of the barrel; on others it is a separate block.

## Why the feed throat is water-cooled

The barrel is hot, and heat conducts back along it toward the hopper. Without cooling, the pellets in the throat would soften and stick to the walls and to each other before they reach the screw. The cooling water keeps the throat cool enough for the pellets to stay free-flowing, and keeps heat from rising into the hopper.

The water usually comes from the machine's cooling circuit; see the [cooling water manifold guide](https://www.imspares.com/injection-molding-machine-parts/hydraulics/cooling-water-manifold/). Set the flow so the throat stays cool but not cold: too much cooling wastes energy and, in humid conditions, can cause condensation on the cold steel. Some materials feed better with a slightly warmer throat; the material supplier gives a recommendation.

## The hopper magnet

The magnet catches **ferrous** metal: iron and steel such as bolts, washers, wire, chips from grinders and flakes of rust. Metal that reaches the screw can score the flights and the barrel, break or jam the [check ring](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/check-ring/), block the nozzle, and damage gates and hot runners in the mold.

It does not catch aluminum, brass, copper or the non-magnetic stainless steels. A [filter nozzle](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle-filter/) catches particles the magnet cannot. To work, the magnet must sit where all the material passes close to its bars, and it must be cleaned: a magnet covered in metal stops catching more.

## Feeding problems and their causes

- **Bridging.** The pellets form an arch in the throat or the hopper and stop falling. Causes: a hot throat (cooling water off, low or blocked by scale), sticky material, fine dust or long flakes in regrind.
- **Long or varying recovery time.** The screw is starved: a partial bridge, a low hopper level, a half-closed slide gate, or a magnet grate blocked with fines. A worn screw and barrel cause the same symptom.
- **The screw turns but does not move back.** No material is reaching the screw: an empty hopper, a closed slide gate or a complete bridge.
- **Metal in parts, scored screw flights.** No magnet, a full magnet, or metal that is not magnetic.
- **Specks of the previous color or material.** Pellets left in the hopper, the loader or the throat at a material change.

Starved feeding also affects [back pressure](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/back-pressure/) and shot consistency, and can lead to [short shots](https://www.imspares.com/blog/short-shots-injection-molding-causes-fixes/).

## Maintenance

1. Check that cooling water flows through the feed throat whenever the barrel heaters are on.
2. Watch for scale: if the throat runs warmer than usual with the water on, the channel may need cleaning.
3. Clean the hopper magnet regularly, and look at what it has caught: a sudden increase in metal points to a problem upstream, such as a worn grinder.
4. Empty and clean the hopper, loader, slide gate and throat at each material or color change.
5. Check that the slide gate moves freely and closes fully.
6. After any overheating, look into the throat for melted material stuck to the walls and remove it before restarting.

## What to send for a replacement

For a hopper, a feed throat block or a hopper magnet, send:

1. The machine brand and model, and the screw diameter.
2. The dimensions of the mounting flange and the feed opening.
3. The size of the cooling water connections, for a feed throat.
4. Photos of the part and its mounting.

[Request a quote](https://www.imspares.com/request-quote/) with these details.

## Questions

### Why is the feed throat of an injection molding machine water-cooled?

Cooling keeps the pellets solid until the screw takes them in. If the throat gets too hot, the pellets soften, stick together and bridge over the screw, so the screw cannot pick up material and recovery fails.

### What does a hopper magnet do?

A hopper magnet sits in the material path above the feed throat and catches loose iron and steel, such as bolts, washers and fragments from grinders. That metal would otherwise score the screw and barrel, break the check ring or block the nozzle or gates.

### What causes bridging in the feed throat?

Usually a throat that runs too hot because the cooling water is off, too low or blocked by scale, or material that is sticky, very fine or made of long flakes. The pellets form an arch above the screw and stop falling.

### How often should a hopper magnet be cleaned?

There is no fixed interval. It depends on how much metal the material carries; regrind usually carries more. Check it regularly, at least at each material change, and clean it more often if it collects metal quickly.