# Bulk Bag Fillers and Unloaders: How to Match Equipment and Bags

> How a bulk bag filler and bulk bag unloader work, which filling frames and discharge stations suit your product, and how to match spouts and loops.

Published 2026-10-01 by Evergreen Packaging (Yantai Evergreen Packaging Co., Ltd.). Canonical: https://www.egpbags.com/blog/bulk-bag-filling-and-discharging-equipment/

**Key takeaways:** - A **bulk bag filler** holds the bag by its loops, seals the fill spout to a fill head, weighs and often vibrates the product to settle it. ;; A **bulk bag unloader** (discharger) suspends the full bag over a hopper and controls flow through the discharge spout, often with an **iris valve** and bag massagers. ;; Most handling problems are **mismatches**: spout diameter, spout length, loop height or loop strength that do not fit the machine. ;; Send your equipment dimensions with your bag inquiry, and **never stand under a suspended bag**.

A bulk bag filler is a steel frame that hangs an empty FIBC by its lifting loops, seals the bag's fill spout to a fill head, and weighs (and often vibrates) the product as it flows in; a bulk bag unloader does the reverse, suspending a full bag over a hopper and controlling discharge through the bottom spout. The equipment and the bag work as one system: if the spout diameter, spout length, loop height or bag height do not match the machine, you get dust, slow filling, hang-ups and unsafe workarounds.

EGP manufactures the bags, not the machines. This guide explains how filling and discharging equipment works so you can specify bulk bags that fit what you already have — or know what to ask for when you buy new equipment. For the basics of bag design, see [how to choose an FIBC bulk bag](/blog/how-to-choose-fibc-bulk-bag/).

## How does a bulk bag filling machine work?

Every bulk bag filling machine, from a simple frame to an automated line, has the same core parts:

- **Frame and loop hooks.** Four hooks (or two, for two-loop bags) hold the bag open by its loops. Better machines release the loops automatically when the fill is complete.
- **Fill head.** The product enters through a fill head; the bag's fill spout is pulled over it and clamped, often by an **inflatable seal** that grips the spout evenly.
- **Height adjustment.** The fill head or hooks move up and down to suit different bag heights.
- **Dust vent.** Displaced air leaves through a vent connected to a dust collector, so dust does not blow out around the spout.
- **Inflation blower.** A short blast of air opens the bag and the liner before product flows, so corners fill and liners do not twist.
- **Weighing.** Load cells under the platform or in the frame measure the fill, either gross (bag plus pallet) or net (after taring).
- **Densification.** A vibrating deck settles powders during filling, so the bag reaches its target weight within its volume and stands square.
- **Grounding.** For Type C bags, a grounding clamp and, ideally, an interlock that stops filling if the ground is lost.

### Types of bulk bag fillers

| Type | How it fills | Typical use |
|---|---|---|
| Basic bulk bag filling frame | Gravity feed from silo or hopper; weight checked on a floor scale | Low volumes, coarse products, farms, aggregates |
| Gravimetric super sack filler | Load cells control a feed gate or screw to a set weight | Most powder and granule plants |
| Filler with densification | Load cells plus vibrating deck | Fine, fluffy or aerated powders |
| Automated line | Roller conveyor in and out, pallet dispenser, automatic loop release | High throughput, fewer operators |
| Loader or conveyor filling into open-top bags | Bag held in a simple stand or on the ground with loops supported | Sand, gravel, firewood, wood chips |

For open-top or duffle-top bags filled from a loader bucket or conveyor, the "machine" is often just a stand that holds the loops up. See [bulk bags for sand, gravel and aggregates](/blog/bulk-bags-for-sand-gravel-aggregates/).

## What bag features does a bulk bag filler need?

| Bag feature | What the filler needs | Common problem if wrong |
|---|---|---|
| Fill spout diameter | Slightly larger than the fill head (commonly 35–50 cm / 14–20 in spouts) | Spout too tight to fit, or so loose it leaks dust |
| Fill spout length | Long enough to clamp with the bag hanging (often 45–60 cm / 18–24 in) | Spout pulls off the fill head as the bag fills |
| Loop height | Matches the hook spacing and frame height; loops commonly 25–30 cm | Bag hangs too low or too high; loops twist on hooks |
| Bag height | Fits between fill head and platform when full | Bag tears at spout or cannot be removed |
| Liner | Liner spout separate from bag spout; tabbed so it stays in place | Liner twists or blocks the inlet |
| Grounding tab (Type C) | Accessible to the clamp | Clamp left off; static hazard |

Dimensional consistency matters as much as the numbers. A filler set up for one supplier's bags can struggle with a "same size" bag from another supplier whose spout is 3 cm narrower. Put the spout and loop dimensions, with tolerances, on your specification sheet; our [bulk bag dimensions guide](/blog/bulk-bag-sizes-dimensions-calculator/) has a template.

## How does a bulk bag unloader work?

A bulk bag unloader — also called a bulk bag discharger, super sack unloader or bulk bag discharge station — supports the full bag over a hopper and releases the product in a controlled, dust-tight way. Main parts:

- **Frame and lifting method.** Either a forklift loads the bag into the frame on a lifting bar or cradle, or an electric hoist on a trolley lifts it into position.
- **Spout access chamber.** A box with a door (sometimes a glove box) where the operator unties the discharge spout without exposure to dust.
- **Spout seal or clamp.** Seals the bag spout to the hopper inlet.
- **Iris valve or spout clamp.** Pinches the spout closed so flow can be stopped and restarted.
- **Flow promotion.** Bag massagers (pneumatic paddles pressing on the bag base) or vibrators break up bridged or compacted powder.
- **Bag tensioner.** Pulls the loops upward as the bag empties, keeping the bag taut so the last product drains out.
- **Outlet.** A hopper feeding a screw conveyor, rotary valve, pneumatic conveying line, or a weigh hopper for batching.
- **Dust collection.** A vent or collector at the hopper.

### Choosing a bulk bag discharger by product

| Product behavior | Discharger features |
|---|---|
| Free-flowing granules (pellets, sugar, salt) | Basic frame, spout clamp, hopper; minimal flow aids |
| Cohesive or bridging powders (flour, pigments, starch) | Bag massagers, bag tensioner, form-fit liner, sift-proof spout |
| Hygroscopic product that cakes in storage | Massagers or lump breaker; consider a better liner upstream |
| Hazardous or toxic dust | Glove-box spout access, sealed connections, dust collector |
| Combustible dust | Type B, C or D bag, grounding where required, explosion-protected equipment |
| Batching small quantities | Iris valve and loss-in-weight feeder |

Many cohesive-powder problems are solved more cheaply at the bag than at the machine. A **form-fit liner** discharges more cleanly than a loose tubular liner (see our [bulk bag liners guide](/blog/bulk-bag-liners-guide/)), and a **tubular or baffle bag** without inside corners leaves less residue. Our [PP Food Grade Tubular Woven Bulk Bag 1 Ton Antistatic](https://www.egpbags.com/products/fibc-bulk-bags/pp-food-grade-tubular-woven-bulk-bag-1-ton-antistatic-with-spout-for/) is a tubular body with a spout for this kind of duty.

## What is a super sack valve or spout closure?

The words "valve" and "closure" cover two different parts:

**On the bag (spout closures):**

- **Tie cord.** The spout is gathered and tied with a cord sewn to it. Simple and universal.
- **Petal (star) closure.** Four or more fabric petals fold over the tied spout and are tied together. It protects the spout from forks and dust and gives a second barrier against leaks. The standard choice for discharge spouts.
- **Full-open (dump) bottom.** The whole base opens for fast emptying of coarse products; the bag is usually not reclosable.

**On the equipment (valves):**

- **Iris valve.** A ring of overlapping blades or a twisted sleeve that closes around the bag spout like a camera aperture. It lets the operator untie the spout, then open, throttle and close the flow. This is what most people mean by a super sack valve.
- **Spout clamp or pinch bar.** Two bars that squeeze the spout shut — simpler than an iris valve, used for stop-start discharge.

If you plan to stop and restart discharge, the spout must be long enough to pass through the iris valve and reach the hopper — usually at least 45 cm (18 in). Our [Large Capacity Jumbo Bag Integrated Discharge Outlet](https://www.egpbags.com/products/fibc-bulk-bags/large-capacity-jumbo-bag-integrated-discharge-outlet-for-bulk/) shows a standard discharge spout layout.

## What is a bulk bag dispenser?

"Bulk bag dispenser" is used for equipment that releases part of a bag at a time rather than emptying it in one go:

- **In plants,** a discharge station with an iris valve or a feeder dispenses measured portions into mixers or batches.
- **On farms,** a frame mounted on a forklift, telehandler or loader suspends a seed or fertilizer bag over a seed tender, planter or spreader, and the operator opens the spout to dispense.

In both cases the bag must have a discharge spout with a reclosable tie or petal closure, and the person opening the spout must stay outside the zone under the load, using a pole, a cord or the chamber door rather than reaching underneath.

## How to handle bulk bags safely around equipment

- **Never stand or reach under a suspended bag.** Use spout access chambers, tie cords or long-handled tools.
- **Use all the loops** and lift vertically. Do not drag bags or lift by two loops of a four-loop bag.
- **Use rounded hooks and forks** of adequate size; sharp edges cut webbing.
- **Fill to SWL, not to the top.** Weighing on the filler is your control; see our [5:1 vs 6:1 safety factor guide](/blog/fibc-safety-factor-5-1-vs-6-1/).
- **Connect the ground** on Type C bags every time; consider interlocks. See [types of FIBC bags](/blog/types-of-fibc-bags-type-a-b-c-d/).
- **Do not slash bags open** under a hopper with a knife; use the discharge spout or a purpose-built cutting station.

## What to send with your bag inquiry

To make bags that run cleanly on your equipment, send:

1. Filler fill-head outside diameter and the length available for clamping.
2. Hook spacing and the distance from hooks to filling platform.
3. Discharger inlet or iris valve size and the spout length needed.
4. Maximum bag height under the filler and over the discharger.
5. Whether you use liners, and whether the filler has liner inflation.
6. Product, bulk density, target fill weight and electrostatic requirements.

## Bags that fit your line, from EGP

EGP manufactures FIBC bulk bags in Yantai, Shandong, with custom spout diameters and lengths, loop heights, closures and liners — for example our cross-corner-loop [Durable PP Woven FIBC Jumbo Bulk Bags 500-2000kg](https://www.egpbags.com/products/fibc-bulk-bags/durable-pp-woven-fibc-jumbo-bulk-bags-500-2000kg-capacity-cross/) and spout-top, spout-bottom designs such as the [5000kg Breathable FIBC Big Bag Bulk Cement Jumbo Bag](https://www.egpbags.com/products/fibc-bulk-bags/5000kg-breathable-fibc-big-bag-bulk-cement-jumbo-bag-with-spout-and/). We do not sell filling or discharging machines, but we will build the bag drawing around your equipment's dimensions. Browse our [FIBC Bulk Bags](https://www.egpbags.com/products/fibc-bulk-bags/) or [send your equipment data](/contact/) for a drawing and quotation.

## Frequently asked questions

### What is the difference between a bulk bag filler and a filling frame?
A filling frame is the basic steel structure that holds the bag by its loops under a feed point, usually with weighing done separately. A bulk bag filling machine adds load cells, an inflatable spout seal, dust venting, densification and often conveyors and automatic loop release.

### Can I fill bulk bags without a filling machine?
Yes, for coarse products such as sand, gravel or firewood. The loops are held up by a simple stand, forklift tines or a loader, and the bag is filled through an open or duffle top. Weigh the bag or measure the volume to stay below the SWL.

### What is an iris valve on a bulk bag unloader?
An iris valve closes around the bag's discharge spout like a camera aperture, so the operator can untie the spout and then start, throttle and stop the flow. It allows partial discharge and dust-tight reclosing of the bag.

### Why does powder hang up in a bulk bag during discharge?
Cohesive powders bridge over the spout and pack into the bag's corners. Bag massagers, bag tensioners, form-fit liners, tubular or baffle bodies and larger discharge spouts all reduce hang-ups.

### What spout size fits a standard super sack unloader?
Many unloaders are set up for discharge spouts around 35–45 cm (14–18 in) in diameter and 45–60 cm (18–24 in) long. Measure your iris valve or hopper inlet and specify the spout to match with a stated tolerance.

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Contact: doristeng2013@yt-evergreen.com · +86 186 6389 6305