Home › Field notes › How to Write a Tech Pack for Modular Bags: Pages and Version Control

Modular Bag Pro · 2026-10-11 · 2,752 words · 13 min read

A tech pack for modular bags is a page-indexed document set that lets a production line build the same pack twice: dimensional drawing, bill of materials, process sheet, hardware schedule, modular interface page and packing specification, each page carrying its own revision letter. With that set approved, samples take 6-10 working days, or 12-15 where new tooling or print screens are cut, and volume follows in 35-50 days from a 500-unit minimum per reference, released against attribute inspection at AQL 2.5. The coverage is civilian load carriage: hiking packs, commuter bags, worksite tool rolls and first-aid modules. Anything touching weapon carriage, ballistic protection or a military certification claim sits outside it.

The Nine-Page Index Every Modular Bag Tech Pack Needs

A tech pack is not a single file. It is an indexed set in which every page has one job and one revision letter, and the set is only complete when all nine pages carry the same letter. A brand that sends a sketch plus a reference photograph is asking the line to guess; a brand that sends the indexed set is asking it to build. The distance between those two requests is measured in sample rounds rather than in days.

Page order follows the reader. Cutting opens the dimensional drawing and the bill of materials. The sewing floor opens the process sheet and the hardware schedule. Quality opens the test page and the packing specification. Every reader who has to stop and ask a question pushes the programme back into the queue behind the other jobs waiting on the same answer.

Modular programmes carry one page a conventional bag does not need: the interface page, which defines the attachment field independently of the bag that holds it. Pouches, EDC organisers and tool rolls are frequently made in a separate programme, sometimes a season apart, and they still have to mount on the same grid. Without that page the field is reverse-engineered from a photograph and two production lots quietly stop interchanging.

The cost of a missing page is never abstract. A blank hardware schedule means the line picks a slider from stock. A blank test page means nobody knows what acceptable meant until goods-in, which is the point at which the argument becomes expensive.

Spec rule: Release a tech pack for modular bags only when all nine pages carry the same revision letter and the interface page states 25 mm webbing with 38 mm row pitch, because one stale page puts back exactly the guesswork the document was written to remove.

Dimensional Drawings: Datums, Tolerances and the Grid Origin

Orthographic views are the minimum: front, back, side, plus a detail view of the attachment field. Every dimension should run from a stated datum, normally the bottom-left corner of the front panel together with the vertical centreline of the pack. Chained dimensions accumulate error, and six chained 25 mm segments drift further from truth than a single 150 mm dimension taken from a fixed origin.

Tolerances on soft goods have their own scale. A workable set is plus or minus 3 mm on cut panels, plus or minus 5 mm on a finished external dimension, plus or minus 1.5 mm on webbing placement and plus or minus 1 mm on grid pitch. Anything tighter raises the reject rate without improving the way a pouch seats; anything looser than plus or minus 5 mm on an outside dimension makes alignment between two bags visibly inconsistent.

The grid origin is where modular drawings fail most often. The geometry widely published for PALS-type fields uses 25 mm webbing, 38 mm vertical row spacing and a 50 mm horizontal repeat, and all three figures have to be dimensioned alongside row count, column count and the offset from datum to the first row. Leave that offset to the operator and every panel starts in a different place, so a pouch that mounts cleanly on one bag sits a few millimetres off on the next.

Bar-tacks deserve their own callout. State length, width, stitch pattern and the distance from the webbing end, since the pouch load path runs through those tacks and through nothing else.

The middle column deserves a note: a reference sample is excellent at communicating colour, hand feel and trim, and useless at communicating tolerance. Use both, but let the drawing govern, and apply the same rule on any custom modular backpack programme.

Verdict: Choose a datum-based orthographic drawing over sketch annotation or sample photography whenever a pack carries an attachment field, because only a stated datum plus an explicit grid offset keeps two production lots interchangeable.

Bill of Materials Page: Fields That Stop Sourcing Drift

The bill of materials page inside a tech pack records consumption per unit, not purchase quantity. Each row carries a component code, description, material specification, colour reference, an approved-alternative column, a unit of measure, consumption per unit and a waste factor. Those fields are what stop the line from improvising when the named material is out of stock.

The alternative column is the one left blank most often, and it is the one that decides what happens during a shortage. Blank means silent substitution: the line uses what is on the shelf, the pack looks the same, and the difference surfaces at goods-in or, worse, in the field. A named alternative with a stated trigger converts that moment into a phone call instead of a claim.

Material properties are stated by method, never by adjective. Abrasion resistance is reported against ISO 12947; tearing strength against ASTM D5034; coating behaviour against ASTM D751. The phrase heavy duty cannot be tested, so it cannot be rejected either, and a specification that cannot be rejected is not a specification.

The waste factor on a tech pack is a planning figure and should be stated with the roll width it assumes. Nesting efficiency on a 1,500 mm roll is not the same as on a narrower one, and a factor quoted without a width is a factor nobody can check.

Takeaway: Fill the approved-alternative column on every bill of materials row before the pack is released, because an empty column converts a material shortage into an unrecorded substitution that only becomes visible after 500 units are cut.

Process Sheet: Seam Type, Stitch Density and Assembly Sequence

The process sheet is an ordered list of operations. Each step names the seam type, stitch density in stitches per inch, thread ticket, needle size and any jig or template required. Operators read it from the top down, and if the order is wrong the pack can still be assembled, just badly, with the defect appearing only at the pull test.

Stitch density is the line most often omitted. A seam can be the correct type and still fail because it was sewn at the wrong density, and unlike a wrong panel this is invisible until load is applied. Structural seams commonly sit in the range of 6 to 8 stitches per inch; the exact figure belongs to whoever specified the thread, and it belongs on the sheet.

Thread and needle have to be specified together. A ticket number without a needle size leaves the operator to choose, and an oversized needle perforates a coated face fabric enough to change how the panel behaves in water even though the seam itself holds.

For a modular pack the attachment operation is the critical step. State bar-tack length, width, pattern and count per webbing end, and require a bar-tack rather than a back-tack at every termination. Where pouches carry real load, the attachment is the component under test, not the fabric around it.

Any operation that needs a template should say so. A grid field sewn freehand will not hold the 50 mm horizontal repeat across ten rows, however skilled the operator.

Bottom line: Write stitch density, thread ticket and needle size on every process sheet line, because a seam built at the wrong density passes visual inspection and fails only when the pouch is loaded.

Hardware Schedule: Finish, Pull Strength and Substitution Rules

Hardware is where a modular pack diverges most from an ordinary one. A single programme can carry 15 to 25 distinct hardware items across buckles, ladder locks, D-rings, snap hooks, sliders, cord ends and hook-and-loop tab tips. Each needs a code, a base material, a finish, a colour, a supplier and a stated acceptance method.

Zippers are specified by chain size. YKK #5, #8 and #10 cover most pack applications, and the specification should also name the slider type, pull style and tape colour. Stating only the brand leaves the size open, and size is what decides how the slider behaves under side load on a full pack.

Metal parts need a corrosion method attached to them. Salt spray exposure to ASTM B117 with a stated duration gives the plater something to aim at; the words rust resistant give nothing to anyone.

Then comes the substitution rule: one line stating whether a functional equivalent is acceptable or the named brand is mandatory. That single line moves a quotation more than most buyers expect. When branded components are specified they are frequently the second-largest driver after fabric, and an unspecified brand forces the estimate to assume the most expensive reading.

Plastic hardware deserves the same treatment on cycle life. A ladder lock that survives 2,000 adjustments and one that survives 200 look identical in a photograph, and only a stated cycle figure separates them before bulk.

Modular Interface Page: Webbing Width, Spacing and Repeat

The interface page is the contract between the bag and everything that mounts to it. Baseline geometry is 25 mm webbing, 38 mm vertical row spacing and a 50 mm horizontal repeat. Those three figures plus row count, column count, offset from datum and bar-tack count per end define the field completely, and they let a pouch made in a different programme mount without modification.

Two tolerances matter and only one is usually written. Pitch tolerance on a single row is the easy one. Cumulative tolerance across the field is the one that bites: a pitch held to plus or minus 1 mm per row can still accumulate past plus or minus 2 mm over ten rows if each row is measured from the one before it rather than from the datum.

Attachment life belongs on this page even where no published standard governs the assembled field. Define the number of attach and detach cycles the field must survive, commonly 200 to 500 depending on the use case, and define the pull figure a single webbing channel must hold. Then test the finished panel rather than a swatch, because a swatch has no bar-tacks in it.

The interface page also settles a question that otherwise recurs every season: whether the field is sewn or cut from laminate. Both can meet the same geometry, and the MOLLE attachment grid behaves differently in each, so the construction has to be named rather than implied.

Judgement: Dimension the grid from a fixed datum with cumulative tolerance stated across the whole field, because per-row tolerance alone permits drift that stops interchangeable pouches mounting after the fifth row.

Packing Specification: Polybag, Carton and Pallet Arithmetic

Packing data is the last page written and the first one needed when freight is booked. It states polybag thickness and print, whether vent holes are required, carton construction and internal dimensions in millimetres, pieces per carton, net and gross weight, and the pallet pattern with layer count.

Cube is arithmetic: packed length × width × height in millimetres, ÷ 1,000,000, gives cubic metres for one carton, and the shipment figure is that number scaled by carton count. A 20GP takes roughly 28 CBM and a 40HQ roughly 68 CBM, so a carton dimension guessed late in the day can waste a meaningful share of space that has already been paid for.

Transit testing sits on this page as well. ISTA 3A is the usual reference for packaged products moving through parcel networks, and it is a different exercise from the single drop test most people assume it to be.

Transit mode then sets the calendar: sea freight 25-35 days, air 5-8 days, courier 3-5 days. A programme that discovers its carton cube only at booking usually also discovers that the cheaper mode no longer fits the launch date.

Marking matters more than it appears to. Carton marks, side marks and any handling icons should be drawn, not described, since the warehouse reads marks rather than specification prose.

Version Control: Revision Letters, Change Logs and Approval Gates

Every page carries a revision letter, and the cover carries a change log with five columns: date, description, pages affected, who requested it and who approved it. Verbal changes are not accepted, on the plain grounds that a verbal change is a change nobody can date.

The pre-production sample is tagged with the revision it was built to. Without that tag the approved sample and the current document drift apart, and the bulk run ends up judged against a physical reference that no longer matches any drawing in the set.

The freeze point is PP approval. Before it, changes cost nothing but time. After it, drawings, bill of materials and packing data are locked, and any change issues a new revision. If that change touches tooling or print screens the cost lands in the USD 300-2,500 band, which is one reason the freeze is worth defending.

The SGS-verified production base we work with holds a 4,950 m² production floor with 7 production lines, 149 machines and 137 people, rated at 200,000 units per month. The founder has worked in bag production since 2004 and the company was established in 2014. Programmes move through sampling, PP sample, AQL 2.5 inspection and shipment in that order, and each gate carries its own revision reference.

Selection rule: Freeze drawings, bill of materials and packing data at PP approval and charge any later revision to a new letter, because post-freeze changes that reach tooling carry a separate charge in the USD 300-2,500 range.

The Missing Items List: What Comes Back Wrong When a Page Is Absent

Most first-sample failures trace to one of five omissions, and all five are cheap to fix on paper. The table below maps each omission to the symptom it produces and to the check that catches it before the sample is cut.

Two of those five cannot be corrected after bulk starts. A missing tolerance block is unfixable once 500 units are cut, and a missing packing specification is unfixable once the container is booked.

The remaining three are recoverable but never free, because each one costs a sample round, and sample rounds sit inside the 6-10 working day window that the launch calendar was built around.