Gypsum Board Production Problems: Blisters, Delamination, Soft Edges and More

Most gypsum board problems in a plant come from four places: the stucco, the slurry formulation, the forming station and the dryer. Blisters and delamination usually trace to the paper-to-core bond and drying temperature; soft or burnt edges to over-drying of the thin edges; warping to uneven drying or storage; low strength to foam, voids or poor stucco. Locate the defect before changing anything.

How to Diagnose Gypsum Board Production Problems

This guide covers defects made in the factory, not problems that appear during installation on site. Most of them look alike at the dry end, and a change made to the wrong part of the line can make things worse. Before adjusting the dryer or the formula, answer four questions:

1. Where across the width? Only at the edges, only in the center, or everywhere. Edge-only defects point to edge forming or edge drying.
2. Which dryer deck? Defects on one or two decks point to the dryer’s air distribution. Defects on all decks point upstream.
3. When did it start? Match the start to a change: a new paper roll, a stucco silo change, a calciner setting, a new foaming agent batch, a thickness change or a line speed change.
4. Which face? Face paper, back paper, or both. A defect on one face only often points to that paper or to the airflow on that side.

Keep a sample of every defect, labeled with time, deck and position. With those four answers, the table below narrows the search.

Blisters and Paper Blows

A blister is an area where the paper has lifted from the core and formed a raised bubble. Plants see two different kinds, and they have different causes.

Paper blows in the dryer. When the board enters the dryer, water near the paper turns to steam. If the paper is not yet bonded to the core, or the bond is weak, the steam lifts it. A 1977 USG patent describes the problem directly: failure to achieve a proper bond “produces paper ‘blows’ during the kiln heating, or ‘peelers’ after the heating” (US4009062A). The same patent found that where there was no wet bond between paper and slurry, the test paper lifted or “blew” off the sample.

Foam and air under the paper. Foamed slurry carries millions of small air bubbles. If they collect or merge at the paper surface, they leave a weak layer. A Gold Bond Building Products patent notes that foam cells at the paper-core interface can prevent a uniform paper-to-core bond (US11697618). A Yoshino Gypsum patent adds that air trapped in the slurry forms internal voids that cause bulging and denting (US10195765).

Causes and checks

• First dryer zone too hot for the board. Water flashes to steam before the bond is strong. Compare first-zone temperatures with the settings used when the line ran well, at the same thickness and speed.
• Unstable or coarse foam. Check foaming agent dose, foam generator settings and where the foam is added. A change of foaming agent supplier or water temperature often shows up here first. See our guide on the gypsum board forming machine for where denser slurry layers are applied against the paper.
• Paper. Paper that is very wet, poorly sized or of the wrong grade can bond badly to the core. Check the roll at unwinding when blisters start after a paper change.
• Read the blister. Cut it open. If gypsum crystals stick to the inside of the paper, the bond formed and then broke, which points toward drying. If the paper is clean and the core under it is smooth or full of bubbles, the bond never formed, which points toward foam, air or paper.

Paper Delamination and Poor Paper-to-Core Bond

Delamination is paper that peels off the core after drying, in large areas or at the edges. A good bond fails inside the paper: when the paper is peeled, the inner ply stays on the core. The USG patent defines 100% bond in exactly that way (US4009062A).

Two mechanisms explain most bond failures.

Calcination at the interface. The bond depends on fine gypsum crystals that grow into the paper fibers. Those crystals are the first to lose their water if the board overheats. The USG patent cites the theory that “the very thin crystals which form the bond are readily calcined and so lose their bond to the paper” (US4009062A). A newer dryer patent states that heating the gypsum layer above about 100 °C has to be avoided, because the dihydrate converts back to hemihydrate and liner adhesion weakens (US11644238). Late in the dryer, when the board is almost dry, the surface heats up quickly, so the last zones decide most bond problems.

Starch not reaching the paper. Starch in the slurry protects and strengthens the bond. It does that only if it dissolves, cooks and moves to the paper with the evaporating water. The USG patent found starch migration “directly related to the availability and presence of liquid water” (US4009062A). The Gold Bond patent explains that unmodified pearl starch “has a gelling temperature that is too high and is often not fully cooked during the drying process”, while acid-modified starch migrates to the paper-core interface with the evaporating water (US11697618). The USG patent also notes that precooked starch interferes with the wet bond (US4009062A).

Causes and checks

• Overdrying, especially at the dryer exit. Check last-zone and cooling-zone temperatures, and board weight by deck. Bond loss concentrated at the edges or on one deck points to the dryer.
• Starch type, dose or preparation. Check what changed in the starch. The USG patent used an iodine stain (TAPPI method) to see where starch ended up in the board, and the same approach shows whether starch is reaching the interface.
• Drying too fast early on. If water leaves the core too quickly, there is less time for starch to migrate.
• Paper. Compare bond on boards made with different paper lots on the same day.

Edge Defects: Soft, Hollow, Burnt and Cracked Edges

Edges are the weakest part of a gypsum board. They are thinner (on tapered board), more exposed to hot air in the dryer and, in foamed board, often lower in density than the core. A National Gypsum patent lists soft edges, wet edges and cupped tapers as typical edge problems from over-drying or under-drying (US4279673).

Overdried edges are the classic drying fault. A 1970 patent explains that raising dryer entry temperatures “frequently creates an over-dried condition in the thin tapered edges”, producing a soft edge, while lowering them leaves the center underdried (US3529357A). A CertainTeed patent adds that the outer edges of the outermost boards on each deck are the most at risk, because the wet boards cool the air in the middle of the deck but not at its sides (US7726040). A dryer supplier’s technical brochure describes nozzle layouts designed to keep the cut edges from burning (Grenzebach).

Low-density edges start at forming. National Gypsum’s solution was to feed denser slurry along each edge, with some foam knocked out. In the patent example, the edge strip came out about 17% denser than the core (US4279673).

Hollow edges also start at forming. A CertainTeed patent describes eddy currents near the edges of the forming table that stop slurry reaching the edges evenly, leaving voids and density variations (US8177541).

Warped Boards

A warped board is bowed along its length or cupped across its width. The paper and the core both shrink and swell with moisture. If one face dries more than the other, or the board leaves the dryer with uneven moisture, it bends as moisture evens out.

Causes to check, in order:

1. Uneven drying top to bottom. On jet dryers, air is blown onto the board from above and below (Grenzebach). If one side gets more air or hotter air, that face dries harder.
2. Deck-to-deck variation. If boards from one deck warp and others do not, the problem is air distribution in the dryer.
3. Paper mismatch. Face and back paper with very different weight or moisture response pull differently as they dry.
4. Stacking warm or uneven. Boards stacked straight from the dryer while still warm, or stacked on uneven supports, can take a set in storage. Check that stacks are flat and well supported, and that cooling is adequate before stacking.

Measure warp on boards taken by deck and by position, both straight from the dry end and after a day in storage. That separates dryer warp from storage warp.

Low Strength: Flexural, Nail Pull and Hardness Failures

When board fails a strength test, check the test first. EN 520 dries specimens to constant mass at 40 °C before the flexural test; testing damp board gives low results (EN 520 as reproduced in Rwanda Standards Board draft DRS 632). If the test is right, look at the core.

• Voids and density variation. Uneven slurry spread at the forming station leaves voids and density variations that “yield boards of reduced strength” (US8177541).
• Foam. Foam lowers core density, compressive strength and nail pull (US11697618). Too much foam, or a bubble structure that does not suit the formulation (uneven bubbles, or bubbles merging into large voids), weakens the core more than the weight saving justifies.
• Too much water. Extra mixing water leaves more pores in the set core and adds to the dryer load.
• Stucco quality. Under-calcined stucco still contains raw gypsum; over-calcined stucco contains anhydrite. Both change water demand and set. Check combined water (ASTM C471M) and setting time against the stucco that made good board. Calcination problems belong to the gypsum powder production line section of the plant.
• Not set at the knife. Board cut before it has set enough can be damaged in cutting and transfer. USG notes that set time depends on stucco age and impurities, surface area, pH, particle size and mixing temperature (US10677698).
• Calcined core. Severe overdrying weakens the core as well as the paper bond.

Cut a board across and look at the core: large holes, layers or a crumbly zone near one face each point to a different cause. Compare density with strength. Low strength at normal density points to core structure; low strength at low density points to foam or water.

Over-Drying and Under-Drying

Most of the problems above are symptoms of drying going wrong in one direction or the other. The dryer has to remove about 4 liters of water per m² of standard board (Eurogypsum) without letting the gypsum overheat.

Under-drying and over-drying can happen on the same board: dry, overheated edges and a wet center. That is the trade-off described in the 1970 patent (US3529357A). It points to air distribution rather than to the overall temperature. Water condensing in the dryer, when moist air cools below its dew point, also marks boards and corrodes the housing (US11644238).

For zone temperatures, residence time and how dryers are controlled, see our article on the gypsum board drying process. For the checks that catch these problems early, see gypsum board quality testing.

Start-up is when most of these problems appear. On our reference project in the Middle East, the line reached design capacity about three and a half months after start-up, with a product pass rate of around 95% (read the case). Most of the work in that period is this kind of fault-finding.

FAQ

What causes blisters on gypsum board during production?

Blisters form where the paper is not bonded to the core when the board enters the dryer, so steam lifts the paper. Weak wet bond, a first dryer zone that is too hot, unstable or coarse foam collecting under the paper, and paper problems are the usual causes. Cutting a blister open shows whether the bond formed and broke or never formed.

Why does the paper peel off gypsum board?

Usually because the gypsum crystals that bond the paper were overheated in the dryer and lost their water, or because starch did not reach the paper-core interface. A dryer patent states that heating the gypsum layer above about 100 °C has to be avoided, because the bond then weakens (US11644238). Check last-zone dryer temperatures, starch type and dose, and paper quality, and compare boards deck by deck.

What causes soft edges on gypsum board?

Soft edges are usually overdried edges. The thin tapered edges lose their water first and then overheat, especially on the outer boards of each dryer deck. Low-density edge slurry makes them weaker still. Plants control this with edge-zone temperature and nozzle settings in the dryer and with denser slurry fed along the edges at forming.

Why are my gypsum boards warping?

Warping comes from uneven moisture between the two faces or along the board. Check whether the airflow above and below the boards is balanced, whether warp is limited to certain dryer decks, whether the face and back paper are well matched, and whether boards are stacked warm or on uneven supports. Measure warp at the dry end and after storage to separate causes.

How do you find the cause of low gypsum board strength?

First confirm the test: specimens must be dried to constant mass before flexural testing. Then compare strength with density. Low strength at normal density points to voids or an uneven bubble structure; low strength at low density points to too much foam or water. Also check stucco combined water and setting time, and whether the core has been overheated in the dryer.

Planning a Gypsum Board Production Line?

Many of these problems are designed in or out when the line is specified: forming station, edge slurry, dryer zoning and laboratory. See what a complete gypsum board production line includes, and use our gypsum board plant setup guide to plan the plant around your raw materials and board types.

Sources

• US4009062A, Gypsum wallboard – making process and product, United States Gypsum (Google Patents) — https://patents.google.com/patent/US4009062A/en
• US11697618B2, Gypsum board with improved starch binder (Google Patents) — https://patents.google.com/patent/US11697618B2/en
• US10195765B2, Gypsum board manufacturing method and manufacturing device, Yoshino Gypsum (Google Patents) — https://patents.google.com/patent/US10195765
• US11644238B2, Method and device for drying gypsum board (Google Patents) — https://patents.google.com/patent/US11644238
• US4279673A, Hard-edge wallboard, National Gypsum (Google Patents) — https://patents.google.com/patent/US4279673
• US3529357A, Method and apparatus for high-speed drying of gypsum board (Google Patents) — https://patents.google.com/patent/US3529357A/en
• US7726040B2, Apparatus and method for the uniform drying of board materials, CertainTeed Gypsum (Google Patents) — https://patents.google.com/patent/US7726040
• US8177541B2, Gypsum board forming device with improved slurry spread, CertainTeed Gypsum (Google Patents) — https://patents.google.com/patent/US8177541
• US10677698B2, On-line slurry set measurement, United States Gypsum (Google Patents) — https://patents.google.com/patent/US10677698
• Grenzebach, Gypsum Solutions (technical brochure) — https://www.grenzebach.com/fileadmin/Grenzebach_Group/Gips/Grenzebach_Gypsum_Solutions_EN.pdf
• Eurogypsum, Water Roadmap — https://eurogypsum.org/wp-content/uploads/2025/11/Eurogypsums-Water-Roadmap-final.pdf
• Rwanda Standards Board, DRS 632 draft (reproducing EN 520 requirements and test methods) — https://www.rsb.gov.rw/fileadmin/user_upload/DRS_632_2026.pdf
• ASTM C471M, Standard Test Methods for Chemical Analysis of Gypsum and Gypsum Products — https://store.astm.org/c0471m-16a.html