Glass Bottle Layer Pad: Gauge, Material, Load Weight

The Three Duties a Layer Pad Performs Between Two Layers of Glass

A palletized load of glass is not a solid block. It is a stack of rigid, smooth, individually unfixed objects, and every interface between two layers is a place where something can go wrong. That is why a pad is not simply a sheet of cardboard put in to look tidy. It performs three separate duties, and the confusion that costs money is the assumption that a pad chosen for one of them will automatically be good at the other two.

The first duty is to spread the contact stress between the layers. Bottles do not sit flat on bottles. A bottle base, or a crown, or a shoulder, rests on the shoulders of the layer underneath at a small number of contact points, and the entire weight of everything above comes down through those points. Glass is strong in compression over an area and weak against a concentrated point load, so the failure modes are a chipped finish, a scuffed shoulder, a star crack at the heel of the base, or a hairline that only opens when the bottle is filled and warmed. A pad distributes that load across its own area instead of leaving it on the contact points. It also compensates for the height tolerance between one bottle and the next, which is a real tolerance rather than a theoretical one, so the layer above sits on a plane rather than rocking on whichever bottles happen to be tallest. This is the duty most buyers already know about, and it is the one that paperboard handles adequately on a light load.

The second duty is to break the vacuum effect at the point where the load is taken apart. On a line that unloads with a vacuum head, the cups seal against the top surface of the load, lift the layer, and the layer below stays where it is because it is heavy. That works until the top layer and the layer beneath it are held together by something. A film of condensation between two smooth glass surfaces, a stretch film that has been drawn over the top, or a plastic sheet laid directly on glass can create enough of a seal that the head lifts two layers instead of one. The consequence is a double pick: either the head detects the extra weight and stops, which costs line time, or it moves with both layers, and the second layer is dropped or dragged across the stack. A pad makes the separation predictable, gives the head a defined, flat surface to grip, and equalises the lift across the layer. If the line unloads automatically, the pad is not a nicety, it is part of the handling system. The machine side of that operation is set out on the glass bottle depalletizer page, and the pad specification has to be read together with the head design rather than separately from it.

The third duty is to carry the shear. A stack resists horizontal force through friction between its layers, and glass on glass, particularly with condensation or a smooth liner between them, behaves almost like a lubricated interface. During road transport, braking and cornering put horizontal load into the stack, and so does the moment a container is lifted and swung on a crane. If the layers can slide, they do. Once a layer moves a few millimetres, the edge bottles lose the support of the layer above, the stack picks up a lean, and from that point the tie-down or the strapping is holding a leaning load rather than a square one. A pad with a matte or mechanically textured surface raises the friction between layers and interlocks them, which keeps the stack square all the way to the destination. This is the duty that is hardest to see at the plant and easiest to see at the port, because a load that has shifted is obvious on the photographs.

Which duty binds depends on the load. A short stack of heavy bottles in a tray, unloaded by hand, is dominated by the stress duty. The same bottles in a tall stack unloaded by a vacuum head are dominated by the vacuum duty and the shear duty at once. A light bottle in a deep, tall stack with a tight strap around it is dominated by shear and by the compression at the strap path. The practical consequence is that the first question to settle before any material is discussed is which of the three is the binding constraint, because that answer decides the gauge, and the gauge decides everything else.

Pad Gauge, Bottle Weight, Layer Count and Stack Height: How the Numbers Move Together

A pad gauge is not chosen from a table of bottle weights alone, and the reason is worth stating plainly: the pad under the bottom layer does not carry the weight of the layer above it, it carries the weight of almost everything above it, and it carries it concentrated over the support points of the layer under it. A stack of eight layers has eight pads with eight different loads on them, and the top pad is the one under the least load. That is why a graduated pad plan, with a thinner pad at the top and a thicker one at the bottom, is often a better specification than a single gauge used throughout, and why a single gauge chosen from the top-layer duty is the most common cause of failure at the bottom layer.

The second variable is the span between the support points, and it is the one buyers most often leave out of the enquiry. The pad bends between the crowns or shoulders that hold it up. If the bottles are large in diameter, there are fewer support points under each square metre of pad and the distance between them is longer, so the pad has to be stiffer to avoid sagging into the gaps. If the bottles are small and the layer pattern is dense, there are many support points close together, the spans are short, and a thinner pad can carry the same load without deflecting far enough to matter. This is why a pad specification developed on a small bottle and copied onto a large one usually disappoints, and why a pad specification is normally written against a bottle family and a layer pattern rather than against a single bottle. The layer pattern itself is set at the stacking station, and the way it is decided is described on the glass bottle palletizer page.

The third variable is stack height, and it acts as an amplifier rather than as an addition. If the bottom pad creeps, meaning it takes a permanent set and no longer returns to flat, the stack above it tilts. A small tilt at the base becomes a large horizontal offset at the top of a tall load, and on a stack approaching two metres a degree of lean is enough to put the upper corner of the load outside the pallet footprint. A load outside its footprint is a load that will not go through a container door cleanly and that will not fit the wrapper. So the taller the stack, the more important the flatness of the lowest pads becomes, and the more a cheap pad in the bottom position is a false economy.

The fourth variable is the individual bottle weight. Heavier bottles raise the point loads at the contact points, so the spreading duty becomes more important, and a heavier load also generates more horizontal force when the truck brakes, so the friction duty becomes more important at the same time. The fifth variable is compression along the strap path. When the load is strapped, the strap bears on the pad at the corners of each layer, and a pad that crushes at the corner lets the strap go slack even though the tension was set correctly at the machine. A thin corrugated pad under a high strap tension is a slack strap waiting to happen, which means the pad gauge and the strapping tension are one decision rather than two.

The last variable is reuse, and it is the one that is most often left out of the original calculation. A pad does not behave the same on its tenth trip as on its first. It flattens, it picks up moisture, its surface burnishes, its edges fray and the cell structure of a honeycomb starts to separate. A gauge chosen for the first trip is therefore not the gauge that will be in service at the end of the pad’s life, and the honest way to specify is to choose the gauge that still does all three duties on the last trip for which the pad is intended. The direction of movement in every one of these variables is the same: the larger the bottle, the higher the layer count, the taller the stack, the higher the strap tension, the more automatic the unloading and the longer the reuse life, the further the specification has to move towards a thicker, stiffer and flatter pad.

Pad Gauge and Load Weight: Reading the Conversion Table

The table below sets load descriptions against pad constructions and shows the direction in which the specification should move. It is a starting point for a conversation with a pad supplier, not a substitute for a handling test, and the row that matches the load in question will normally point at one or two candidate constructions rather than at a single answer. The band descriptions in the first column combine the four variables just described, so a load can be light in bottle weight and still land in a heavier band because of the layer count, the stack height or an automatic unloading head.

Four reading notes belong under that table. First, the load bands are indicative and the real answer depends on the layer pattern, so the same bottle weight can move a load up or down a band. Second, the pad is verified by a handling test rather than by a datasheet: build one pallet, strap and wrap it if that is what the shipment does, run it through the actual route including lifting, road movement and container loading, and then take the top layers off and inspect the bottom pad for permanent set. A single trial pallet answers more questions than several sheets of specification. Third, a pad that is adequate for a hand-unloaded load can be inadequate for the same load unloaded automatically, because the vacuum duty adds a requirement that has nothing to do with weight. Fourth, the pad affects the container arithmetic, because every pad is a slice of height in the load. That interaction is set out on the glass bottle container loading page, and the pad thickness should be fixed before the loading plan is drawn rather than after it.

Four Pad Constructions and What Each One Costs in Reuse Cycles

Almost every pad in a glass bottle supply chain is one of four constructions, and the differences that matter commercially are stiffness for a given weight, strength at the edge, behaviour when wet, and how many trips the board can survive before it stops being flat.

A polypropylene honeycomb board is a cellular core bonded between two skins. It is the stiffest of the common plastic boards for its weight, it stays flat over a long service life, and it can be washed. It is also the most expensive of the four, so it is normally chosen where the pad has to work hard, which means at the bottom of a tall stack, in front of an automatic depalletizer, or inside a returnable programme where the same pad is used many times. Its failure sign is separation between the core and the skin, which shows first at the cut edge and at the corners.

A solid polypropylene sheet is a single homogeneous board. It is heavier than a honeycomb of comparable stiffness, it resists puncture and edge damage well, and it is washable. Because it is less stiff than a honeycomb at the same thickness, it is usually chosen for the middle of a stack or for a load where the edge condition matters more than the deflection. In a hot container or a hot yard it can bow slightly, and a bowed pad under a vacuum head is the classic cause of a mispick, so a solid sheet in front of an automatic head has to be checked for flatness rather than assumed flat.

Corrugated plastic sheet, often described as twin-wall, is the cheapest of the plastic options. It is a good single-trip pad and a usable pad for a small number of trips, and it is the usual choice for a one-way export load where the pad stays at the destination because the cost of bringing it back is larger than the cost of the pad. Its weak point is the edge, which crushes and delaminates, and once the edge has gone the pad no longer presents a square face to the strap or to the wrapper. Its stiffness also falls away after a few cycles, so it should not be carried into a reusable programme on the assumption that plastic means long life.

A paperboard pad, usually a kraft liner or a solid fibre board, is the cheapest per trip. It takes printing, it presents very well on an opened load, it is easy to recycle at the destination and it is often the form the customer’s own packaging specification was written around. Against that, it is single trip in any honest sense, it loses stiffness quickly as it takes on moisture, it sheds fibre dust, and it cannot be washed, so it can never be part of a programme with a wash step. Where the load is empty bottles going to a filler, the dust question deserves a specific answer rather than an assumption, because fibre can settle on a finish or into a bottle mouth, and some fillers refuse a fibre pad on that ground alone.

One further distinction is worth making because it is a frequent source of confusion. A slip sheet is not a layer pad. A slip sheet is a load handling device, a sheet that a push-pull attachment grips underneath so that a whole load can be moved without a pallet. A pad is a separator inside a stacked load. The two are different products serving different operations, and a pad cannot be used as a slip sheet or the reverse.

Pad Footprint, Overhang and the Strap Path: Sizing the Pad to the Pallet

Pad size is a separate decision from pad gauge, and it is decided by the pallet rather than by the bottles. The pad should cover the load bearing surface of the pallet deck, because its job is to transfer the weight of the load into the pallet and to keep the stack square. A pad cut to the outline of the bottle layer instead, which is the size a buyer often orders because it looks neat, leaves the outer bottles partly unsupported and puts the compression of the strap directly onto the glass at the edge, which is one of the ways edge bottles chip.

Overhang is the more common error and the more expensive one. A pad larger than the pallet deck hangs over the edge. That overhanging lip is the first thing crushed when the strap is tensioned, it holds the layer above off the layer below at the perimeter, it catches on the frame of a depalletizer or the film of a wrapping machine, and on a pallet with a chamfered or rounded corner it means the effective bearing area of the pad is smaller than its nominal size. The rule is straightforward: the pad footprint follows the pallet deck footprint, with a small deliberate clearance where the strap runs, and nothing more.

The format itself varies by market, and the pad has to match the format the load actually uses. Deck footprints in common circulation include 1200 by 1000 mm, 1200 by 800 mm, 1140 by 1140 mm, 1100 by 1100 mm and the 48 by 40 inch footprint used in North America, and a pad bought for one format will not fit another. There is a second detail inside the format: the pallet deck is made of boards with gaps between them, and the support surface is the boards rather than the full rectangle. Where a bottle base lands over a gap, the pad has to be stiff enough to span that gap, which is a reason why a thin sheet can perform differently on two pallets of the same nominal size but a different deck design.

The strap path has to be allowed for at the design stage. If the strap runs over the pad, the pad edge takes the compression, and a soft pad edge is where the tension disappears. The practical options are to keep a modest clearance at the pad edge so the strap bears on a corner protector rather than on the pad, or to accept that the pad is the compressible element and to size its edge strength for that duty. Whichever route is taken, the pad and the strapping should be specified together, because the strap tension that is correct for a hard pad is not correct for a soft one. That interaction is the subject of the glass bottle strapping page, which treats the load retaining method rather than the interlayer separator.

Finally, thickness has to be counted. A five layer stack with a pad under every layer contains five pads, and at six millimetres each that is thirty millimetres of load height that is not glass. On a load that is already against a container door height or a wrapper limit, that slice can be the difference between one tier fitting and not fitting, and it is the sort of number that should be known at quotation stage rather than discovered on the loading dock. The same arithmetic applies to the weight of the pads when the gross weight is checked against a payload limit.

Where a Layer Pad Stops and the Tray or Divider Takes Over

A packed pallet of glass can contain three different separating components, and they solve three different problems. The layer pad is a flat sheet between one tier and the next, and it controls what happens between layers. A divider, sometimes called a cell partition or an internal fitment, is a set of vertical walls that create cells inside a single layer, and it controls what happens between one bottle and the next inside that layer. A tray is a formed corrugated shallow container that holds one layer, or a group of bottles, and turns it into a single unit that can be handled and stacked. Each of the three can be present without the other two, and the useful diagnostic rule is that if the problem is bottles moving inside a layer, a pad will not fix it, and if the problem is layers sliding or a vacuum head picking up two layers, a divider will not fix it.

The tray is the component with the most engineering behind it, because a tray is formed from a board grade, folded, glued and squared on a machine to a tolerance, and the board grade has to suit the bottle weight it carries. That work, and the difference between a tray with a shrink film, a tray with a paper lid and a full carton, belongs to the tray former page and to the secondary packaging page, and this page deliberately assumes the tray already exists. What belongs here is the interaction: a tray raises the effective stiffness of the layer it holds, so a load of trays usually needs less pad than the same load of loose bottles, and the tray also raises the layer height, which changes the pad count and the stack height. A buyer moving a load from loose bottles to trays, or the reverse, should expect the pad specification to move with it.

A returnable crate is a different pack form again, and it changes the pad question rather than merely shifting it. A crate is a rigid container with cells and a stacking system of its own, so the separation and the shear resistance are built into the crate rather than being supplied by a sheet. Most crate programmes therefore use no layer pad between crates, and the crate’s own stacking feature governs the load. Where a crate programme does use pads, it is usually for a specific reason such as an uneven crate top or a mixed load combining crates and non crate units. The economics of a returnable crate programme, including the return loop, are set out on the bottle crates page, and that page owns the returnable question rather than this one.

The last boundary to state is the one with the outer pack form. Whether the shipment is a tray with film, a full carton, a crate or a floor stacked load is a decision about how the goods are presented and protected, and it belongs to the secondary packaging page. This page starts one level further in and asks only what lies between two layers once the outer form has been chosen.

Reusable Against Single-Use Pads, Including the Return Leg

The comparison between a reusable pad and a single-use pad is usually made on purchase price alone, and that is the smallest term in the real cost. The figure that decides the question is the cost per trip, and the cost per trip has five components: the purchase price divided by the number of trips actually achieved, the cost of the return leg where the pad travels back, the cost of washing or cleaning, the cost of handling and inspecting between trips, and the cost of the pads that are lost, damaged or removed from service earlier than planned.

For a one-way export load the return leg is usually the decisive item. A pad that leaves the plant and stays at the destination has made one trip, and bringing it back means moving a low-value, bulky item against the direction of the freight. On a lane that has a genuine backhaul, and where the pad can be consolidated into a partial load that is travelling anyway, the return leg can be cheap enough to make a reusable board rational. On a lane with no backhaul, the return freight will usually exceed the pad, and the honest specification is a single-use pad that the destination can recycle or discard. This is a freight question as much as a packaging question, and it should be answered for the actual lane rather than assumed.

Where the loop is genuine, reuse pays, and there are three common structures. The first is a closed factory-to-factory or plant-to-warehouse shuttle inside one country or one region, where the pads come back on the same trucks that deliver. The second is a pool, where a third party operator owns the pad and the customer simply draws from and returns to the pool, which converts the pad into a service rather than an asset. The third is a customer who reuses the pads internally, typically a large bottler or co-packer that stacks empty bottles in its own plant, where the pad is collected and reissued on site rather than shipped. In all three, the pad becomes an asset with a specification, a wash regime and a stock count, and the specification should be written with that in mind.

The hidden costs of reuse are real and are worth listing, because they are the reason a reuse programme that looks good on paper can fail in practice. A returned pad carries dust, moisture and sometimes product residue, and if it is going back into a food or beverage area the cleaning step is not optional, which means either a wash cycle in an existing crate washer or a separate unit, plus drying space. A pad that is washed and stacked wet will transfer moisture to the layer below and can develop mould between layers during storage. Reusable pads need dry storage, because a stack left outdoors has lost most of the benefit it was bought for. They also need a rejection rule and someone to apply it, because a pad that is bowed, delaminated or edge damaged has to be taken out of circulation rather than put back under a load. And they need a tracking system, because a pad that goes missing from a reusable pool is both a lost asset and a missing pad in the next load. None of these costs appear on a pad invoice, and all of them belong in the comparison.

Cleanliness, Climate and How a Pad Ages in Service

A layer pad is not a food contact surface, and it should not be described as one, but it sits directly above the layer beneath it and, in a load of empty bottles, directly over open finishes and mouths. That is enough to make cleanliness a real requirement rather than a cosmetic one. A paperboard pad sheds fibre and can deposit dust on a finish, which some fillers treat as a contamination risk and some do not, and the answer differs by customer rather than by industry. A plastic pad does not shed fibre, but it can carry dust, glass fragments and product residue from one load to the next if it is only brushed. Where the load is a food, beverage or pharmaceutical product, the pad material and the cleaning regime should be confirmed with the buyer’s quality team, and the general food contact frameworks that govern packaging materials are relevant to the pack rather than to the pad itself.

The cleaning regime follows from the material. A plastic pad can be brushed, air blown or washed, and where it is washed it must be dried before stacking, because a wet pad transfers moisture downwards and creates the conditions for mould. A paperboard pad cannot be washed at all, which is why it cannot join a programme with a wash step. A pad that is only brushed should be inspected at the same time, because brushing removes the visible dust and does nothing about a crushed edge, a permanent bow or a burnished face.

Climate is where the two material families separate most clearly. Paperboard is hygroscopic, so it takes on moisture from humid air and loses stiffness in the process. A container standing on a deck in strong sun with humid air inside is the worst case, and the combination of heat, humidity and time is what turns a paperboard pad from a stiff separator into a soft sheet that marks the load. Humid air cycling between day and night inside a container also creates a damp interface between layers, which reduces the friction that the pad was there to provide, so a load can lose its shear resistance over a long sea leg without anything being done differently. Plastic boards take on essentially no water, which is their main climate advantage, but polypropylene has a higher thermal expansion and a pad can cup or bow when it is stored flat in the sun or when the load moves from a cold warehouse to a hot yard. A bowed top pad is exactly what defeats a vacuum head, so where an automatic depalletizer is in use the flatness of the top pad is worth checking as an everyday routine rather than as a one-off.

Ageing is decided by three signs rather than by appearance. The first is delamination, most visible at the cut edge of a honeycomb board, where the skin lifts from the core. The second is edge damage, meaning crushed, frayed or torn edges, which reduce the bearing area, catch on handling equipment and allow the strap to dig into the board. The third is a burnished or polished face, which is the sign that the surface has been abraded against glass for many cycles and has lost the friction that the shear duty depends on. A fourth sign, a permanent bow, is the one that matters most in front of an automatic head. Because a pad is a low cost item and a rejected load is not, the practical approach is to write a short rejection rule into the packing specification for the pads, covering flatness, edge condition, cleanliness and dryness, so that the decision is made by a rule at the plant rather than by an opinion at the destination.

Misuses That Cost a Layer of Bottles

The following failure patterns account for most of the pad related damage that shows up at the far end of a shipment. They are listed together because they share a common shape: each one is cheap to prevent at the plant and expensive to correct after the container has sailed.

• Running an automatic depalletizer with no pad, on the assumption that the vacuum head will separate the layers on its own. It will, until a film of condensation or a smooth interface makes the top two layers behave as one, and then it lifts two.
• Using a paperboard pad for more than one trip. The second trip is where the stiffness has gone, the corners have gone soft and the fibre dust has started to work loose, and the load is now relying on a pad that is no longer doing its job.
• Using a thicker pad to correct a bad layer pattern. A pad cannot hold a poorly interlocked stack square through a road leg; the pattern has to be corrected where the layers are built.
• Choosing one gauge from the top-layer duty and using it at every layer. The bottom pad carries a multiple of the load of the top pad and is the one that creeps.
• Ordering a pad that overhangs the pallet deck. The lip is crushed at the strap corner, it lifts the perimeter of the layer above, and it catches on the handling equipment.
• Ordering a pad smaller than the layer pattern, so that the outer bottles bear the strap load directly on the glass rather than through the pad.
• Loading a pad that has been stored outdoors. A damp pad has lost stiffness before the load has even started moving.
• Mixing two pad sizes or two gauges inside one pallet, which produces an uneven stack and defeats the purpose of the pad at the position where it matters.
• Keeping a pad in service past visible edge or face damage, because it still looks like a pad and the damage is judged by appearance rather than by the flatness and friction it needs to deliver.
• Expecting the pad to stop bottle to bottle contact inside a layer. That is the divider’s job, and a pad cannot do it.

All of these are decisions rather than accidents, and all of them are made before the container is loaded. That is the argument for putting the pad rule into the packing specification at the point the order is placed, alongside the layer pattern and the pack form, rather than leaving it to the packing hall on the day.

When the Pad Question Becomes a Purchasing Question

For a small exporter with a handful of lanes, the pad is a consumable bought at short notice in a size that happens to be available. That is a reasonable position until one of three things changes, and when it changes the pad stops being a consumable and becomes a specified component.

The first trigger is volume and repetition. When the same bottle, the same layer pattern and the same pallet format are being shipped often enough that the pad count is a recurring line rather than an occasional purchase, it becomes worth cutting the pad to the pallet footprint rather than to the nearest stock size, and worth writing a short specification with a gauge, a footprint, a tolerance and a marking. The saving is not only in the pad; it is in the elimination of the trimming and improvising that happens when the pad does not fit.

The second trigger is a return loop. As soon as pads come back, they are an asset rather than a consumable, and an asset needs a specification, a wash regime, a storage arrangement and a stock control. At that point the purchase decision is really a decision about the whole loop, and it should be evaluated on cost per trip rather than on purchase price.

The third trigger is automation. Once the unloading head is a vacuum head working at line speed, a pad that is not flat stops being a quality issue and becomes a line stoppage issue. The pad then needs a flatness requirement, a rejection rule and a replacement trigger, which means it has to be bought to a specification rather than bought as a generic sheet.

Two further situations push the same way. Where a customer’s own packaging specification names a pad, the buyer has to quote against that specification, in that size, with that marking, and the freedom to substitute a convenient stock size has gone. And where a stack has been made taller or a bottle made heavier since the pad was chosen, the old pad is now marginal even though nothing about it has changed, which is a reason to re-check the pad whenever the pack changes rather than only when it fails.

The enquiry that gets a usable answer contains the weight of one bottle, the weight of the load above the bottom pad, the layer pattern with the number and spacing of the support points, the layer count and the total stack height, the pallet footprint and its deck board layout, whether the unloading head is a vacuum head, whether the load is strapped or wrapped and at roughly what tension, whether the pad travels once or comes back, the storage climate at both ends, the hygiene requirement and the expected annual quantity. With those in hand, a supplier can answer in terms of material and construction, the direction for the gauge, the footprint with the strap clearance, a reuse and inspection rule, and the checks to run on the first pallet.

Where the Layer Pad Stops and the Packing Document Takes Over

This page has stayed with one component. It has covered the three duties a pad performs between layers, the way gauge moves with bottle weight, layer count, stack height, span and strap tension, the conversion table that maps load bands to pad constructions, the four common constructions and their reuse behaviour, the fit between pad footprint and pallet format including overhang and strap paths, the boundary between a pad and a tray, a divider and a crate, the comparison between reusable and single-use pads including the return leg, the cleanliness and climate behaviour of the materials, and the failure patterns to write out of the process.

The adjacent subjects are handled elsewhere. The written packaging document that this decision belongs in, together with the layer pattern and stacking rules, is on the glass bottle packing specification page, and the pad rule is best read as one line item of that document. The machine that builds the layers, and the way the pattern is decided, belongs to the palletizer page. The machine that takes the load apart, and the vacuum head that makes the pad necessary, belongs to the depalletizer page. The height and weight arithmetic that the pad thickness feeds into belongs to the container loading page. The returnable pack form, where separation is built into the container, belongs to the bottle crates page. The way the load is held together, which interacts with the pad at the strap corner, belongs to the strapping page. The forming of the tray, and the board grade it is made from, belongs to the tray former page under the slug glass-bottle-tray-former. And the choice between a tray with film, a full carton, a crate and a floor loaded load belongs to the secondary packaging page. What is left here is the interlayer pad itself.

Questions Buyers Ask About Glass Bottle Layer Pads

Does a load of glass bottles really need layer pads if the bottles already sit in trays?

A tray changes the requirement rather than removing it. A tray holds one layer as a unit, so it raises the effective stiffness of that layer, provides its own friction surface and usually carries a handle point that a vacuum head can grip. Many tray based loads therefore run with fewer pads than the equivalent load of loose bottles, and some run with none between trays. What has to be checked is the interface between one tray and the top of the layer below it: if the tray base is smooth, if condensation forms between the film on one layer and the tray on the next, or if the stack is tall enough that the bottom tray carries a large cumulative load, a pad is still the cheapest way to keep the stack flat and to give the handling head a predictable surface. The right answer comes from the trial pallet rather than from the pack form alone.

How is the thickness of a layer pad actually chosen?

Work from the load, not from the material. Establish how much weight sits above the lowest pad, how far apart the support points are under each layer, how tall the finished stack is, how much compression the strap puts into the pad corners, whether the load is taken apart by a vacuum head and how many trips the pad has to survive. Each of those moves the answer in the same direction: more weight, longer spans, a taller stack, a higher strap tension, an automatic head and a longer service life all push towards a thicker and stiffer board. Then confirm the choice on a trial pallet rather than accepting a table, because the layer pattern and the deck design of the actual pallet both affect the result.

Why does a pallet of bottles sometimes lose a layer at the depalletizer?

Almost always because two layers have been lifted as one. A smooth glass surface, a film of condensation, a stretch film drawn over the layer or a plastic sheet lying flat can produce enough of a seal between layers that the vacuum cups do not separate them. The symptom is a double pick, which either stops the machine on an overload or drags and drops a layer. A pad breaks that continuous surface, gives the head a defined top surface to seal against and equalises the lift across the layer, so the separation becomes predictable. A bowed or damp top pad can reintroduce the same problem by preventing the cups from sealing consistently.

Can a paperboard pad be reused for a second shipment?

It can be physically reused, but it should not be relied on. Paperboard takes on moisture and loses stiffness, its corners soften and tear, and it sheds fibre. On the second trip the pad is no longer a specified component, it is a sheet of board of unknown stiffness, and the failure it produces is normally a stack that leans rather than a pad that breaks. Where a second or third trip is genuinely intended, the pad should be a plastic board specified for that service life, and it should carry an inspection rule so that a bowed, delaminated or edge damaged pad is taken out of circulation rather than put back under a load.

Does the pad have to match the pallet size exactly?

It should match the load bearing footprint of the pallet deck, which is the surface the stack actually stands on. A pad larger than the deck overhangs, and the overhanging lip is crushed at the strap corner, lifts the perimeter of the layer above and catches on handling equipment. A pad smaller than the deck, and in particular one cut to the outline of the bottle layer, leaves the outer bottles partly unsupported and puts strap compression straight onto the glass at the edge. The practical specification is the deck footprint with a small deliberate clearance where the strap runs, so the strap bears on a corner protector rather than cutting into the pad.

What fails first in a pad during a real shipment?

In practice, flatness and edge condition fail before the board tears. A pad that has taken a permanent set no longer returns to flat, so the stack above it tilts, and the tilt grows with height until the upper corner of the load is outside the pallet footprint. At the same time the edges crush wherever the strap bears on them, which lets the strap tension relax even though it was set correctly at the machine. On a paperboard pad the moisture comes first and the stiffness goes with it, and on a plastic pad the face burnishes against the glass until the friction that resists shear has gone.

How many pads does one pallet need?

As a rule, one between each pair of layers, which on a stack of eight layers means eight pads including the one under the bottom layer. Two qualifications follow. The bottom position is the one doing the most work, because it carries the cumulative load of everything above it, so a graduated plan with a stiffer board at the bottom and lighter boards higher up is often better value than one gauge throughout. And where a layer is already held as a rigid unit, such as a tray or a crate with its own stacking feature, the interface between that unit and the layer above may not need a pad at all. The count follows the layer interfaces, and the gauge follows the position in the stack.

Send the Bottle Weight, the Layer Count and Whether the Line Depalletizes Automatically

To get a pad specification that can be acted on rather than a general answer, send four numbers and two conditions. The numbers are the weight of one bottle, the number of layers in the finished stack, the total stack height including pads, and the pallet footprint with its deck board layout. The two conditions are whether the load is taken apart by hand or by a machine with a vacuum head, and whether the pad travels once or comes back in a return loop. Add the strap or wrap method if the load is held together mechanically, because the strap corner is where the pad gauge and the tension have to be decided together, and add the storage climate at both ends if the route involves humid heat or a long sea leg.

With those in hand, the reply can set out the material and construction that suit the load, the direction in which the gauge should move including whether a graduated plan is worth it, the pad footprint with the strap clearance, a reuse and inspection rule for the pads, and the checks to run on a first pallet before the specification is frozen. The bands shown in the table on this page are orientation for planning rather than a quotation, and the specification that governs an order is the one confirmed against the actual bottle, the actual layer pattern and the actual pallet.