Storage - A180

 

Storage
Document Type	Article
Document Identifier	180
Tags	Storage

Introduction

Storage and Inventory management are critical to running a profitable composites manufacturing business.

The storage step is the action of storing raw materials and parts prior to processing in order to preserve them. This page links to the individual equipment found within the storage cell. Examples include, freezers, shelving, and environmental monitoring equipment.

It is important to keep all areas clean, organized and to know where everything is stored via an MRP (material requirements planning), ERP (enterprise resource planning) or other (logistics, tracking, warehouse management) system.  

Significance

Supply chain issues plague manufacturing from a global issue down to a local issue. Understanding the storage requirements for these materials is key to ensure manufacturing timelines, part quality and efficiency are maintained. Some discussion of storage needs throughout the factory are also discussed here.

Scope

Storage requirements run throughout the factory, where WIP must be stored or held, pending the next step in the process, or where consumables and hand tools are needed locally. This article deals primarily with the dedicated storage areas in the factory.

Overview

Materials should be stored in controlled areas and managed carefully for inventory management, including asset management, cost control, avoiding shortages in the factory, shrinkage and wasted time locating the right item. Materials are usually entered into the ERP or MRP system in the receiving area and consumption is controlled through the storage system. The consumption point can be managed at a stores facility, by distributed “vending machines” (for tools, PPE typically) that the operator can remove by bar code on employee passes or program cards, min/max distribution to workstations for general consumables. A private company such as the Fastenal™ FastVEND™ system can take on the responsibility for end point use with a variety of vending style distribution machines that they replenish, freeing up labour, cash flow, ongoing inventory control for more off the shelf consumables.

Concentrating storage of similar use products and of similar environmental controls is important to manage inventory. Some of the separate areas include:

• Receiving and Shipping
• Prepreg Freezer Storage
• Consumables, Hand Tools, and special part storage
• Part Tool, Fixture and Handling Fixture Storage

Prepreg Freezer Storage

Prepreg materials typically need to remain at sub zero temperatures unless actively being prepared or used to make a part.

Prepreg materials and certain chemicals require freezing (typically <-18° C) to minimize the degradation of the material’s shelf Life.  Rolls of prepreg as well as cut prepreg “kits” are also stored in freezers to prolong overall material life. Careful tracking and logging of material time in and out of the freezer is required to track the material's out-time due to the limited shelf life of certain materials (typically prepreg).  Prepreg has a defined “lifetime” with a maximum “due date” for use.   

Mechanical Life:  

• Out Time (Mechanical Life)-Maximum time allowed from freezer to part cure 
• Once cured, prepreg is set and no longer dependent on life management  

Shop Life: 

• Shop Life (Work Life)-Maximum time until part is bagged for cure 
• Freezer time allows the life to be extended (analogous to fresh produce) 
• Production control, supervisors and operators must check remaining life to ensure there is enough mechanical life remaining until completion of the Cure cycle (precisely the achievement of the gelation temperature and period). 

Mechanical Life and Shop Life

Good material management is required for the following reasons:

• Minimize the material that must be scrapped due to out of life. 
• Material recertification that can be used to extend material life, however it is a situation to be avoided if possible and typically requires customer approvals. 
• Tight material life timing can affect autoclave and/or oven planning and can impact overall production schedules. 
• Make sure materials that will be close to remain within shop and mechanical life are expedited as a priority.  Place a visible note (bright sticker or other) highlighting the priority. 
• A tool such as JETCAM’s CrossTrack with RFID or barcode tracking can greatly improve the traceability, material usage and material scrap rate. 

Prepreg Fibre Material Rolls

• Prepreg glass, aramid (Kevlar), carbon fibre are typically stored in freezers at-18 C (to avoid life depletion).
• Prepreg is typically shipped on a centre roll, bagged, with the roll suspended on the ends to avoid/minimize any interaction/bonding/sticking or crushing of the material by resting on its own weight.

Roll Storage

• Materials often come in 1 m (42”) wide rolls. Other widths are available, however shelving that is 1 m plus ~ 10 cm (42” plus ~4”) for the box housing is recommended.   
• Boxes can typically be stacked only up to the material supplier’s allowable limit, so height between shelves should be sized according to typical materials 
• Box identification on the exposed end improves retrieval and minimizes errors in material selection. 
• Per above section on shop and mechanical life, freezer stored materials with life requirements must have life management system. 
• Material life starts at manufacture and must be tracked as to frozen and thawed life. The material life should be tracked as of entry in the factory, adding in the existing life from manufacture to arrival at the factory. Similar to food it has an expiry and an out of freezer time limit. 
• In a worse case scenario, a 're-lifeing' of the material may be performed to see if an out of date material can be used. This process would require testing to re-certify a new life time for the material. 

Prepreg Fibre Material Rolls

Kits are the plies and other components of a part or work package that are grouped and typically set in a pre-determined reverse order of assembly, ready to send to assembly. More information on kitting can be found here. Plies are pre-cut, stacked and stored in a sealed bag. Depending on production methods and needs, it is often beneficial to cut multiple kits at the same time. The unused kits should be hermetically bagged and returned to the freezer as quickly as possible. Careful identification and tracking of the kits should be taken to enable quick retrieval. Loss of kits in a freezer is a common issue that results in delays or recutting and kitting. The freezer should consider the following:

• Room to store kits that are bagged should be provided for. 
• Prepreg kits should not be stacked or only as high as the material supplier permits. Care must be taken to keep the plies flat.  The risk is that the prepreg starts to adhere to the backing plies if stacked too high.   
• Kits should be well marked as to their content. 
• Their location in the freezer should be well marked and in an organized and documented location to avoid lost time (a common occurrence) due to misplacement and time spent searching for kits. 

Prepreg Kits, Ready for freezer storage

Freezers should be controlled and monitored as to access and to ensure the freezer door is closed and conditions such as temperature and humidity, are maintained. Thawing warning alarms should be use to ensure the known thawed life of a part is known.

Non-Freezer Storage

Part Materials

Part material is any material that will be built into a composite part (hardware). Part materials pre cure include:

1. Honeycomb core (Nomex or metals among others)
2. Lightning strike
3. Dry fibre (tow, UD, weave, braid)
4. Adhesives (Film, Foam)
5. Other build materials (Tedlar)
6. Potting compounds
7. Inserts

Most materials including dry glass fibre, aramid and carbon rovings, woven and braided materials can typically be stored at room temperature. Note: best practice is to store dry fabric by suspending it on a rack with a pole through the centre of the material. Standing a roll of dry fibre on end can lead to the fibre sagging under gravity and damage the weave, potentially rendering it unusable.

Honeycomb materials should be carefully stored per the supplier/ manufacturer’s recommendations. Storage should be temperature and cleanliness controlled. Post cure materials, such as paint and other finishing coating, should also be stored following the manufacturing standards.

Consumables, Hand Tools and Special Part Storage

Consumable materials are materials used in the manufacturing of composite parts that are discarded or consumed in/after the process.  Some may be re-used a few times depending on requirements.  Some may be recycled.  Some examples and uses of consumables used in the manufacturing processes include: 

1. Vacuum bag and bagging material 
2. Tape (Tacky tape, and other tape type)
3. Peel ply 
4. Release film 
5. Mould release 
6. Thermocouples 
7. Parchment and other papers 
8. Tedlar
9. Breather material 
10. Flow mesh 
11. String bleeders 
12. Dam material

Consumables and hand tools that are used in the post-cure processes include:

1. Plastic or wood wedges (to separate the part from the tool)
2. Rubber or other hammer/mallet
3. Machine tools for trimming, drills
4. Abrasives for tool cleaning and part finish
5. Tool cleaning fluids

Part Tool, Fixture and Handling Fixture Storage

Hand Tools	Calibrated Tools	Personal protective equipment (PPE)
Razor knives (Olfa and Exacto), Scissors, Spatulas / scrapers, Tape Dispenser, Sharpies	Lasers, Vacuum gauges, Timers, Heat guns, Vernier Calipers	Hairnet, Protective Safety Glasses, Breathing apparatus (where required), Lab coats

PPE is an ideal item to automate through the use of vending machines distributed near to their end use.

Tool and Fixture Storage

Tool storage can be broken down into several parts. Tools are often made of Invar, steel, or aluminum, resulting in heavy tools. Tooling can quickly consume large amounts of floor space. Tool storage should consider shelving systems and inventory management systems to store and pick tools from shelving. The moving of tools in the factory should be considered, either by the use of wheels that are autoclave survivable, forklifts or Automated Ground Vehicles (AGVs).

Part Tooling (Moulds)

Part tools are high precision tooling to build parts against. The surface provides a part surface and must be of the highest quality finish. The tool must also be built, transported and stored in the final shape, such that no distortion such as twist or sag is input to the tool to result in an incorrect part.

Caul Plates

A caul plate is a separate piece of tooling to provide a second finished surface for the part external to the main mould tooling. It typically requires alignment tooling to ensure the finished surface provides the desired finish at the desired location.

Fixture Tooling

Part tooling (Moulds) can be large and heavy. In order to move the tools from storage to layup to the autoclave then to demould, cleaning, prep and back to storage, tool specific tooling and fixtures are often required to ensure the tool is not damaged or distorted in its activities. The support structure, in addition to the forklift lift points, or dedicated wheels are of critical importance and must be designed to not input any stresses that might distort the mould surface over the use cycle of the tool. This includes thermal cycling.

Factory floor, tooling, kit cart, and trolley

About	Help

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A tow is a bundle or yarn of individual fibres. The tow size is inherent to the fibre manufacturing process (i.e. a tow is manufactured in one process, rather than each fibre individually then bundled together after).

Typically, smaller tows are better because they result in a more homogeneous material.

The larger the tow:

• The faster it is to deposit material
• The easier is it for resin to flow between tows
• Harder for resin to saturate

Typical tow sizes:

• 1k (thousand)
• 3k
• 6k
• 12k
• 24k
• 50k

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.