Trays, KLTs, Magazines: How a Mixed Storage System Brings Order to Material Flow
Managing different carrier formats in a single storage system sounds like a recipe for chaos, but it does not have to be. Structuring the differences deliberately instead of glossing over them gives you visibility, speed, and a foundation that grows with your operation.
In electronics manufacturing, a single uniform carrier format is a rare luxury. Reels sit next to trays, small load carriers (KLTs) next to sensitive PCB magazines, and all of them need to be supplied reliably from the same storage system. Such mixed storage is not an exception but the normal state of affairs in most SMT operations.
The question is therefore not whether mixed storage is feasible, but how to set it up so that it does not become a bottleneck in daily operations.
Where does mixed storage break down in daily operations?
Mixed storage rarely fails because of "too many formats"; it fails because of the ambiguity that arises when all carriers are treated the same, even though they are not. Four problem areas come up again and again:
Space utilization
Rigidly sized storage slots combined with a constantly shifting carrier mix lead to dead space and blocked areas, even when there is theoretically "enough room."
Traceability
When magazines and KLTs share the same return path despite having different states and typical return flows, misassignments build up, quietly and hard to see.
Access and staging
Differences in dimensions, grip points, and stackability make access and staging inconsistent when there are no clear handling rules per carrier class.
Operational side effects
Search time, restacking, unplanned temporary placements, and growing numbers of exceptions, because carriers are physically in storage but not clearly tracked in the right context.
Inventory appears to be there, but the carriers are not where the process expects them. The storage system is not empty, and the line stops anyway.
The core principle of mixed storage: structure the differences, don't paper over them
Mixed storage works when the differences between carrier types are treated not as a problem to minimize but as a fact to structure. In practice, this means managing carrier types as distinct classes with clear rules. Which zones are permitted? Which status changes are typical? Which handover and return processes apply? Answering these questions for each carrier class creates a structure that is easy to understand on the floor, without breaking the storage system into rigid silos.
Which carrier belongs in storage at all, and which is better kept at the line or in a buffer, is a prior decision. The criteria for it are covered in the article Material Carrier Placement.
Carrier types compared: what mixed storage needs to know
Each carrier type places its own demands on storage location, access, and returns, and the storage system has to know these differences to manage them properly. The comparison shows why PCB magazines cause the most special handling:
| Criterion | Trays | KLTs | PCB magazines |
|---|---|---|---|
| Unit of movement | Individual or stacked | Larger units | Individual, orientation-sensitive |
| Access frequency | Medium | Medium to low | High, line-side |
| Protection needs | Depends on contents | Low to medium | High, no warping |
| Typical storage location | Structured storage area | Storage or transfer point | Line-side or buffer |
| Return logic | Standard path possible | Standard path possible | Separate path recommended |
| Stackable | ✓ Yes | ✓ Yes | ✗ No or limited |
| Transport | Stable position, no content shifting | Stable position | Orientation-sensitive, no rotation |
What storage equipment and transport must deliver in mixed storage
With mixed carriers, storage equipment and transport must above all be robust against differences in dimensions and handling, otherwise every format change becomes a special case. A central storage system such as cts CUBE Storage is therefore designed for PCB magazines, KLTs, and tray stacks alike. Four requirements apply regardless of the system:
Racks and fixtures
Design fixtures and resting surfaces so that trays, KLTs, and magazines sit securely, with no tipping, misalignment, or jamming due to tolerances. At the same time, efficient access must remain possible.
Conveyors and AMRs
Carriers must be moved in a stable position, without content shifting and without rotating during acceleration or cornering. For magazines, this is a baseline requirement.
Handover points
The interfaces between transport and storage must absorb carrier variety: placing and picking work clearly and without collisions, whether an autonomous mobile robot (AMR) or a hand cart makes the delivery.
Special case: magazines
Magazines must not warp or rest incorrectly. What looks like a minor mechanical issue quickly causes follow-on problems at the next process point, the line.
How does traceability stay stable in mixed storage?
Traceability stays stable with mixed carriers when movements do not happen "in open space" but at process points with clear context. That sounds obvious, but in practice it often is not. Our solutions therefore record not only that a carrier was moved, but also which carrier type was involved, which zone it came from, and which status it moves into. This keeps magazines, trays, and KLTs from falling into the same "gray logic."
Misassignments often occur where similar carriers sit in similar areas and returns are improvised. That is why return and exception paths are kept separate, and plausibility checks catch obvious misplacements early. How to standardize the steps at handover points across all formats is covered in the article Standardizing Carrier Logistics Despite Format Variety.
How mixed storage scales without losing visibility
Mixed storage stays stable as carrier variety grows if it is designed to be modular from the start. Space for expansion is reserved in the layout early, and additional zones connect to the existing handover and return processes instead of creating new one-off paths.
The operating logic matters just as much: when training is built on principles, namely handover, return, and exception, rather than on exceptions for individual carrier types, the system stays understandable even as new formats are added.
What mixed storage does not solve
Well-structured mixed storage brings order to material flow, but it does not fix problems that arise before or after the storage system. Three limits should be clear before you build:
Missing rules
More storage slots do not help if zones, status changes, and returns are not defined per carrier class. Then only the area people search in gets bigger.
Improvised returns
If material is returned at the line without being booked, inventory in storage is wrong, no matter how well the storage itself is run.
Unclear placement
Mixed storage does not decide which carrier belongs at the line. Material needed within the running cycle is in the wrong place in storage.
As a supplier of storage systems, we have to be honest with ourselves here: the obvious answer to a full, confusing storage area is more capacity, and for us it is also the most convenient one. This article opens with the problem of "theoretically enough room" for a reason, because as long as the rules per carrier class are missing, additional capacity only moves the problem.
Three questions to answer before building mixed storage
Which carrier types do we have, and which rules apply to each?
Only once zones, status changes, and return processes are defined per carrier class can mixed storage run reliably. Skipping this step means building a storage system that reaches its limits under the pressure of the first changeover.
Where are our handover and return points, and do they work for all formats?
Even with different carriers, handovers, returns, and exceptions have to take place at clearly defined locations. Otherwise operational ambiguity builds up and shows itself as search time and resolution effort.
Is our storage built so that new carrier types can be added without reinventing operations?
Planning for growth today saves effort later: reserve space for expansion early and add new carrier classes as extensions of the existing model, without having to rebuild the interfaces to MES and ERP.
Conclusion: order in mixed storage is a design question
Mixed storage is not a compromise that has to be paid for with operational ambiguity, provided it is built thoughtfully. Managing carrier types as distinct classes, defining handover points clearly, separating return paths, and planning for growth from the start are not elaborate special measures; they are the fundamentals of a storage system that runs smoothly day to day.
How many carrier classes does your storage handle today?
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