Gypsum Powder Packing Machine: Valve Bag Packers and Bulk Loading

A gypsum powder packing machine fills fine plaster into valve bags through a spout, usually 25 kg each, and weighs every fill as it goes. Impeller, air and screw packers differ in how they push the powder into the bag. How well a line works depends on three things: weighing accuracy, the bag and its valve, and dust control at the spout.

The packer is the last machine the plaster touches and the first one the customer judges. An underweight bag, a dusty pallet or a burst bag in a truck all reach the buyer before any lab result does. This article covers the packer types, the weighing rules that apply to filled bags, the choice of bag, dust control, palletizing, and bulk loading. For the mixing section upstream, and for bag sizes and shelf life, see gypsum plaster manufacturing after calcination.

How a Gypsum Powder Packing Machine Works

Most bagged plaster goes into multi-wall pasted valve bags. A valve bag is closed on all sides except a small internal sleeve, the valve, at one corner. The operator or an automatic bag placer slides the valve over the fill spout. The packer then drives powder into the bag until the scale reaches the set weight, and the bag drops onto a conveyor. The weight and volume of the powder are meant to press the valve shut, so no stitching or sealing is needed (NIOSH, 2019).

Plaster is fine and holds air, so the machine has to drive it into the bag against back-pressure. The same air then has to get out through the bag wall without taking product with it. That air is behind most packing problems: blowback around the spout, a spray of powder (the “rooster tail”) as the bag is ejected, dusty bag surfaces and burst bags (NIOSH, 2019).

A complete packing section for a plaster plant usually has:

1. A product bin above the packer, sized so the packer runs continuously while the mixer runs in batches.
2. A screen or lump breaker, so set lumps or foreign matter do not block the spout.
3. The packer itself, with one or more spouts and a weighing system.
4. A bag placer (manual or automatic) and a bag discharge conveyor.
5. A check-weigher and a reject point for underweight or broken bags.
6. Bag cleaning, then a palletizer and pallet wrapping.
7. Dust extraction at the spouts, under the packer and at the palletizer, with the collected dust returned to the product where it cannot contaminate another grade.

Valve Bag Packer Types

Valve bag packers for fine powders fall into three families by how they move the product. The table is a summary of common designs, not drawn from one published source; the right choice depends on your products and output.

Two practical points follow from the table. First, a lightweight plaster changes the decision. Perlite and vermiculite are light and fragile (ASTM C35 sets 96–192 kg/m³ for perlite used in gypsum plaster; check the current edition, now C35-01(2019)), and a machine that breaks the aggregate removes part of what the customer is paying for. Run a trial on your own formula before you choose between an impeller and an air packer. Second, the number of products matters. NIOSH observes that plants bagging many different products find cleanout and changeover easier with semi-automatic systems (NIOSH, 2019). A plant making five plaster grades needs a packer that empties completely and cleans quickly, not only one that runs fast. With plaster, a poor cleanout may be a quality fault as well as a delay. The New Zealand Institute of Chemistry notes that ground gypsum provides sites for crystals to grow and accelerates setting; by inference, set residue or fast-setting stucco left in the packer could shorten the working time of a retarded plaster packed after it. Check setting time on the first bags after each changeover.

Weighing Accuracy Rules for 25 kg Bags

The rules that apply to filled bags. In many markets the legal metrology rules for prepackages follow OIML R 87. For nominal quantities above 15 kg, it allows a tolerable deficiency (T) of 1% of the nominal quantity, so 250 g on a 25 kg bag. The average content of a lot must be at least the nominal quantity, no more than 2.5% of packages may be short by more than T (a “T1 error”), and no package may be short by more than 2T (OIML R 87:2016). Check which rules your market applies and whether building materials fall within them.

The rules for the machine. OIML R 61 covers automatic gravimetric filling instruments. For class X(1), the maximum permissible deviation of each fill from the average of the fills is 0.8% at initial verification and 1% in service for fills above 15 kg. Other classes scale by a class designation factor of the form 1, 2 or 5 × 10ᵏ, not more than 2, so X(0.5) halves those figures (OIML R 61-1:2017, Table 2).

What this means in the plant. Because the average must reach the nominal weight, the target has to sit slightly above 25 kg. How far above depends on the spread of your packer: the tighter the spread, the closer the set point can be to 25.00 kg, and the less plaster you give away on every bag. When comparing packers, ask for the standard deviation measured on a powder similar to yours, at the output you will run, not a figure measured on a test material.

Plaster makes weighing harder in a specific way. Powder that has just been conveyed or fluidized holds more air and flows faster. A change in stucco fineness, ageing time or additive package changes how much air goes into the bag and how fast it fills. Recheck the packer settings when any of these change, and check-weigh bags downstream. NIOSH notes that some operations use a bag scale on the conveyor to divert underweight (broken) bags to a separate reject line (NIOSH, 2019).

Choosing the Valve Bag

The bag is part of the packing system, not a consumable bought separately. Write the bag specification in the terms of ISO 6590-1:2025, the international vocabulary for paper sacks, which names sack types, valve types, sack papers and air-permeability perforation methods; the packer supplier and the bag maker then read the same words. NIOSH describes the multi-wall pasted valve bag as by far the most common bag for mineral products, and sets out what changes its behavior (NIOSH, 2019):

• Paper grade. Natural kraft paper, in three or four plies for 50–100 lb bags, has low porosity, so bags need perforations to avoid bursting. Extensible kraft is stronger and can cut the number of plies by about 30%. Air-permeable extensible kraft lets air out without perforations and depressurizes the bag three to four times faster than standard kraft.
• Perforations. They let fluidizing air escape but can reduce bag strength by up to 20%. Very fine products, about 0.045 mm and finer, can sometimes escape through them.
• Valve type. In a study NIOSH cites, an extended polyethylene valve, about two inches longer than a standard paper valve, gave respirable dust levels roughly 45–65% lower than the standard paper valve.
• Bag size. Undersized bags create more blowback and rooster tail.

For plaster, add one requirement that the dust handbook does not cover: moisture. Hemihydrate reacts with water, so moisture taken up through the bag starts turning plaster back toward gypsum, which shows up as lumps and a changed set. A market with high humidity or long, slow distribution needs a bag with a moisture barrier, such as a polyethylene film ply, and still has to vent air during filling. Fix the bag specification with the packer supplier and the bag maker together, and test it on your own product before the order for the year’s bags is placed. The bag volume follows from the bulk density of your plaster, which we cover in gypsum powder bulk density.

Dust Control at the Spout

NIOSH reports that bagging and stacking workers typically have some of the highest dust exposures in mineral processing (NIOSH, 2019). For calcium sulfate, the US OSHA permissible exposure limit is 15 mg/m³ total dust and 5 mg/m³ respirable, and the NIOSH recommended limit is 10 mg/m³ total and 5 mg/m³ respirable. The OSHA limits are 8-hour time-weighted averages; the NIOSH limits are time-weighted averages for up to a 10-hour workday (NIOSH Pocket Guide). Use the limits that apply in your country.

The controls NIOSH documents, with the results it reports:

AP-42, the US EPA emission factor guide, states that gypsum plants most often control emissions with fabric filters, and lists plaster mixing and bagging among the controlled sources (US EPA, AP-42 section 11.16). Size the filter for the airflow at every pickup point at once, and keep the makeup air clean, because it passes over the operator before it reaches the hood (NIOSH, 2019).

NIOSH’s bagging best practices add two simple ones: slow the fill rate if blowback is heavy, and keep the spout cut-off valve in good order to stop dribbling.

Palletizing and Wrapping

Manual stacking of 25 kg bags is a dust exposure and a lifting load at once; HSE’s guideline for men is 25 kg at knuckle height close to the body, and it is not a safe limit (HSE, INDG143). Mechanizing palletizing is often the first automation step a growing plaster plant takes.

NIOSH describes the options (NIOSH, 2019):

• Semi-automatic. A lift table keeps the pallet at knuckle height, with push-pull ventilation across the top layer.
• High-level palletizers. Layers are formed on a stripper plate and dropped onto the pallet.
• Robot palletizers. NIOSH calls them the preferred method: simpler, lower maintenance, less floor space and less dust, because the robot places bags rather than dropping them. They need full guarding.
• Wrapping. Spiral stretch wrap leaves the top open unless a top sheet is added. Shrink wrap or stretch hoods cover the whole pallet, and NIOSH reports stretch-hooded pallets as more stable. Bags must be free of trapped air before wrapping, or the wrap goes slack; a pallet press can squeeze the air out.

For plaster, the wrap is also the moisture barrier while pallets wait in a yard or a container. Choose the method by your storage conditions and your climate, not only by line speed.

Big Bags and Bulk Loading

Not all plaster goes out in 25 kg bags. Board and block lines take stucco by conveyor, and contractors with machine-plaster silos take it in bulk.

Big bags (FIBCs). NIOSH lists an inflatable neoprene bladder in the fill spout as the most effective dust control: it seals against the bag’s inlet spout, with extraction to relieve the pressure as the bag fills (NIOSH, 2019).

Bulk tankers and site silos. Loading spouts range from fixed tubes to telescoping spouts with an outer shroud and dust extraction. For closed tankers with round hatches, a cone-shaped discharge seals against the loading port; only the ports being loaded should be open. NIOSH reports that dust release rises with loading rate, drop height and wind (NIOSH, 2019).

Bulk plaster also needs its own quality controls. Unlike bags, there is no printed date or batch number on the product, so record the silo, batch and tanker on every delivery note. Keep a retained sample from each load.

Sizing a Gypsum Powder Packing Line

Work from tonnes to bags a day, then hand that figure, with your shift pattern, to packer suppliers to size the spouts:

1. Bagged tonnage. As an illustrative calculation: a plant packing the same tonnage that our 3,000,000 m² a year reference board line consumes, about 21,000 t of plaster a year (at 7.0 kg of plaster per m² of 12 mm board), would fill about 840,000 bags of 25 kg a year. Over 300 days that is about 2,800 bags a day.
2. Shift pattern. Divide by the hours the packer actually runs, after changeovers, cleaning and bag-supply stops, to get the bags per hour the packer must sustain. Spout count follows from that figure and the rate each supplier can demonstrate on your plaster; we found no neutral published figure for output per spout, so ask for one measured on a powder like yours.
3. Peak demand. Plaster demand is seasonal in many markets; size for the peak month, not the average.
4. Product changes. Each grade change takes cleaning time. More grades favor a simpler, fully emptying packer.
5. Downstream balance. The palletizer, wrapper and forklift traffic must keep pace with the packer, or the packer waits.

For a board line, the same plaster goes to silos by screw conveyor or bucket elevator rather than to a packer, which AP-42 describes as the usual transfer method in gypsum plants. The packing section matters only for what you sell as powder.

FAQ

What machine is used to pack gypsum powder?

A valve bag packer: an impeller, air or screw machine that fills plaster through a spout into a multi-wall valve bag and weighs it during filling. Big bags use a separate FIBC filler, and bulk tankers use telescoping loading spouts. The packer is usually followed by a check-weigher, a bag cleaner and a palletizer, with dust extraction at each stage.

How accurate should a gypsum powder packing machine be?

Where prepackage rules follow OIML R 87, the lot average must reach 25 kg, no more than 2.5% of bags may be more than 250 g (1%) short, and no bag may be more than 500 g short. OIML R 61-1:2017 class X(1) allows each fill to deviate 0.8% from the average at initial verification. A tighter spread lets you set the target closer to 25 kg.

Impeller packer or air packer for plaster?

Impeller packers suit fine, free-flowing plaster and reach high outputs. Air packers fluidize the powder and push it with air, which is gentler on fragile ingredients such as perlite in lightweight plasters, but they put more air into the bag. Test both on your own formula, measuring weight spread, aggregate breakage and dust before you decide.

How do you control dust when bagging gypsum powder?

Fit an exhaust hood at every spout or a ventilated hopper under the packer, use bags with good venting and an extended polyethylene valve, and clean the bags before palletizing. NIOSH reports 90–100% reduction in dust leakage from spout hoods, and 78–90% less dust on bags after a bag and belt cleaner. Connect all pickups to a fabric filter.

Why do valve bags burst during filling?

Air goes into the bag with the powder and must escape faster than it enters. Low-porosity paper without enough perforations, undersized bags, damaged or badly stored empty bags and gluing defects all raise the risk. NIOSH notes that bags stored too long in high humidity or heat fail more often. Air-permeable paper or under-valve perforations relieve the pressure.

Planning a Plaster Packing Line?

The packer, bag and dust extraction should be specified together with the mixing section and the bulk storage, around your products and your market’s bag rules. See our gypsum powder production line for how the sections fit, the gypsum powder plant setup guide for the planning sequence, and gypsum plaster quality testing for what to test before bags leave the plant.

Sources

• Dust Control Handbook for Industrial Minerals Mining and Processing, Second Edition (Chapters 7 Bagging and 8 Bulk Loading) – NIOSH, 2019: https://cdc.gov/niosh/publications/numbered/2019-124.html
• OIML R 87, Quantity of Product in Prepackages – International Organization of Legal Metrology, 2016: https://www.oiml.org/en/files/pdf_r/r087-e16.pdf
• OIML R 61-1, Automatic Gravimetric Filling Instruments, Part 1: Metrological and Technical Requirements – International Organization of Legal Metrology, 2017: https://www.oiml.org/en/files/pdf_r/r061-1-e17.pdf
• ISO 6590-1:2025, Packaging – Vocabulary – Part 1: Paper Sacks – International Organization for Standardization, 2025: https://www.iso.org/standard/87607.html
• Plaster and Plaster Board Production – New Zealand Institute of Chemistry: https://nzic.org.nz/unsecure_files/book/9D.pdf
• NIOSH Pocket Guide to Chemical Hazards: Calcium Sulfate – NIOSH: https://www.cdc.gov/niosh/npg/npgd0095.html
• AP-42, Section 11.16 Gypsum Manufacturing – US EPA, 1993 (reformatted 1995): https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf
• Manual Handling at Work: A Brief Guide, INDG143 (rev4) – Health and Safety Executive (UK), 2020: https://hse.gov.uk/pubns/indg143.PDF
• ASTM C35-01(2019), Standard Specification for Inorganic Aggregates for Use in Gypsum Plaster – ASTM International, 2019: https://store.astm.org/c0035-01r19.html