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Rat-holing in a bulk bag is one of the most frustrating “it looks like it should be flowing” problems you’ll ever deal with.
Because from the outside, it can look like the bag is emptying… until you realize what’s happening:
The product is flowing only down the middle, forming a clean “tunnel”… while a thick ring of material clings to the sides like it’s glued there.
Then the tunnel collapses, the flow chokes, and your operator is back to doing the daily ritual: shake, slap, massage, vibrate, curse, repeat.
Here’s the good news:
Rat-holing is not random.
It’s a predictable flow pattern caused by a handful of conditions — and you can prevent it once you know which condition is driving it in your operation.
Let’s break it down like adults.
First: what rat-holing actually is (and why it’s different than bridging)
People confuse bridging and rat-holing because both end with “no flow.”
But they’re not the same.
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Bridging (arching): product forms an arch over the outlet and holds itself up. Flow stops abruptly.
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Rat-holing: product flows in a central channel while material at the walls stays put. Eventually the channel collapses or chokes.
Rat-holing is basically your bulk bag acting like a paper towel roll: empty in the center, full on the sides.
And rat-holing usually points to wall friction + cohesion + compaction problems more than outlet size problems.
The #1 reason rat-holing happens: your product is cohesive and “sticks” to itself (and/or the walls)
Rat-holing loves materials that:
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are fine powders
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have high moisture sensitivity
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are slightly tacky
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have high fines content
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compact and “cake”
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are irregular particle shapes (flakes, fibers)
In these products, the material doesn’t flow as a uniform mass. It forms stable “sidewalls” that can support themselves while the center drains.
Signs it’s product cohesion
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rat-holing is worse on humid days
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rat-holing is worse after storage
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product comes out in chunks or clumps
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it improves when you aggressively massage/vibrate the sides
Rat-holing is basically cohesion showing you its teeth.
The #2 reason: high wall friction (bag wall or liner interaction)
Even if your product isn’t extremely cohesive, rat-holing becomes more likely when wall friction is high.
Wall friction increases when:
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product has fines that cling
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static causes cling
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liner material grabs the product
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bag walls are deformed/creased (used bags)
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product is slightly damp and sticks to walls
Liner-related rat-holing (very common)
Liners can amplify rat-holing because:
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liner wrinkles create friction zones
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liners cling via static
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liners can “hold” product at the sides
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liner can collapse in a way that maintains a sidewall ring
If rat-holing is worse with liners, you’ve likely found your smoking gun.
The #3 reason: compaction (storage time and vibration)
Compaction is one of the strongest rat-holing drivers.
Bulk bags compact from:
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product weight itself
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stacked bags
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long storage time
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vibration during transport
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temperature swings that change moisture behavior
Compacted product creates stable “walls” around the bag. The center can still drain… but the compacted ring stays put.
Signs compaction is the driver
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rat-holing is worst on bags that sat the longest
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rat-holing is worse after shipping
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the first discharge after storage is the hardest
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product loosens after significant vibration/massaging
If compaction is involved, your bag isn’t holding “loose powder.” It’s holding a partially consolidated mass.
The #4 reason: uneven discharge conditions (flow is being encouraged in the center only)
Sometimes the discharge setup unintentionally encourages a center tunnel.
Examples:
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discharge spout is partially restricted, creating a narrow flow channel
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the spout is opening in a way that forms a “center path”
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the discharge interface pinches the spout so the product funnels in the middle
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downstream restriction causes intermittent flow
Rat-holing becomes more likely when flow is constrained into a narrow stream. Narrow stream = tunnel.
The #5 reason: the bag isn’t hanging square (tilt creates a preferred flow path)
If the bag hangs crooked:
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one side takes more pressure
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product settles unevenly
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the flow path forms in the easiest direction (usually center/downhill)
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sidewalls become more stable and stay put
This happens from:
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uneven loop engagement
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a bag caught on the frame
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unloader hooks misaligned
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bag geometry variability (used bags)
A bag that hangs crooked tends to discharge crooked, which encourages tunnel flow.
The #6 reason: product particle size distribution is “sticky” (too many fines)
Even granular materials can rat-hole if they have a lot of fines mixed in.
Fines act like glue:
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they increase cohesion
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they increase wall cling
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they increase compaction behavior
If rat-holing started after a supplier change or a grind change, check fines content. It matters more than most people think.
Call or Text us at 832.400.1394 for a Quote!
How to diagnose rat-holing quickly (without a PhD)
Answer these questions:
1) Is it worse after storage or after shipping?
If yes → compaction is driving it.
2) Is it worse on humid days or with certain lots?
If yes → moisture + cohesion is driving it.
3) Is it worse with liners than without?
If yes → liner friction/static/wrinkling is contributing.
4) Does aggressive massage fix it temporarily?
If yes → you’re breaking sidewall cohesion (classic rat-holing).
5) Does the bag hang crooked or vary bag-to-bag?
If yes → geometry and hanging alignment are contributing (especially used bags).
This is how you stop guessing and start diagnosing.
How to prevent rat-holing in bulk bags (high-ROI fixes)
Now for the practical prevention plan.
Fix #1: Reduce wall friction and liner issues
If liners are involved:
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ensure liner is properly sized (too much slack = wrinkles = friction zones)
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ensure liner is aligned and not twisted
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reduce static/cling conditions where possible
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avoid liners that wrinkle heavily if your product clings
If no liner:
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keep bag walls smooth (deformed used bags can increase friction)
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use consistent bag families (don’t mix random types)
Rat-holing is a wall friction problem as much as it is a product problem.
Fix #2: Prevent compaction
You can’t eliminate compaction entirely, but you can reduce it.
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minimize storage time when possible
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avoid stacking that crushes bags (or reduce stack height/pressure)
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reduce vibration during transport where possible
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store in stable humidity conditions if moisture affects your product
If your product cakes, you need a compaction plan.
Fix #3: Improve discharge setup so flow is more uniform
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ensure the discharge spout is fully open (no partial restriction)
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ensure the discharge interface is not pinching the spout into a narrow channel
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reduce downstream restriction that causes intermittent flow
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avoid tight bends right below the spout
Smooth, unrestricted discharge helps prevent tunnel formation.
Fix #4: Use controlled massage/vibration correctly
Massagers and vibrators can reduce rat-holing — but using them wrong can make it worse by compacting material in strange patterns.
Best practice:
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use controlled, consistent agitation
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don’t overdo it
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target the sidewall areas that are holding product
If rat-holing is chronic, controlled agitation should be part of the standard procedure, not “panic mode.”
Fix #5: Hang the bag square and centered
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ensure all loops are engaged evenly
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ensure the bag is centered in the unloader frame
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avoid angled hanging
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reduce variability by screening used bags for deformation
A crooked hang encourages a crooked flow path.
Fix #6: Address product variability (fines, moisture, lot changes)
If rat-holing started suddenly:
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check product moisture
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check particle size distribution
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check supplier or processing changes
Rat-holing is often your first symptom of a product change.
Call or Text us at 832.400.1394 for a Quote!
New vs used bulk bags: how rat-holing behavior changes
New bulk bags
New bags give you consistent geometry and consistent liner fit. Rat-holing in new bags usually points to:
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product cohesion/moisture
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compaction from storage/shipping
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liner friction/static
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discharge setup restriction
Used bulk bags
Used bags can increase rat-holing because:
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bag walls may be creased/deformed
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internal geometry isn’t consistent
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liners sit differently
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bags may not hang square every time
Used-bag programs can still work, but if rat-holing is inconsistent bag-to-bag, screen out deformed bags and keep bag styles consistent.
The simple operator checklist to reduce rat-holing today
Give your team this checklist:
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Confirm bag is hanging square and centered
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Fully open the discharge spout (no tie restrictions)
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Confirm discharge interface is not pinching the spout
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Ensure downstream equipment is accepting flow (no backup)
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If liners are used, confirm liner is centered and not collapsed into the outlet
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Use controlled massage/vibration early (don’t wait until it’s fully stuck)
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Note whether the bag sat in storage or was transported recently
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Log product lot and humidity conditions if variability exists
If they track this for a week, the pattern usually becomes obvious.
Bottom line
Product rat-holing happens because:
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the product is cohesive and wants to hold sidewalls,
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wall friction is high (often liners/static/wrinkles),
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compaction makes a stable ring of material,
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discharge restriction encourages tunnel flow,
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the bag hangs crooked,
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or fines/moisture changes make the product “stickier.”
Prevent it by reducing wall friction, reducing compaction, improving discharge setup, hanging the bag square, and using controlled agitation correctly.
If you want a precise recommendation, tell us:
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what product it is,
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whether you use liners,
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whether it’s worse after storage or shipping,
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and what discharge setup you’re using.
That’s enough to pinpoint the root cause fast.