Hot-Melt Prepreg Machine Features
A UD machine will include a multi-end Creel system to unwind the individual bobbins of fiber under controlled tension and will included fiber management and fiber spreading.
- Creels are available in various levels of tension control – from basic friction tension holdback up to servo driven and load cell controlled positions
- Creels are designed to present the fiber sheet in a stable condition to the impregnation process
Fiber Spreading/Tow Management
In order to make the UD tape, the individual fiber tows requirement proper tension and guiding management as well as proper spreading to achieve the desired fiber areal weights. This process requires adjustable combs and fiber spreaders
The adjustable comb system segregates and aligns the tows before entering the fiber spreading system
- Combs feature interchangeable inserts for different spacing
- Adjustable angles to fine tune the width and areal weight
- Larger machines feature double comb systems
- Custom combs available for special applications and can include special coatings (DLC, Teflon) and special materials for abrasive tows (tungsten fingers)
The fiber spreading is critical for achieving the proper areal weights. The tows enter a series of fixed spreader bars. The friction of the small diameter bars enable the tow to spread. By varying the angle of the spreader bars, the amount of spreading is optimized.
The tow spreading is VERY dependent on the quality of the incoming fiber and the amount of sizing on the fiber.
Spreaders are offered with a variety of custom options to optimize the spreading process:
- Various spreader bar coating surface finishes (chrome plated, flame sprays, bead blasted finishes)
- Vibrating spreader bars
- Oscillating spreader bars or oscillating combs
- Heated spreader bars or IR heating of the spreading process
- IR preheaters to “de-size” the fiber prior to spreading
In-line filming is available where the resin is introduced to the fiber prior to compaction directly instead of with a pre coated paper.
- Precision filmer using the same style 0.0002” (0.005 mm) rollers as Litzler compaction rolls and Litzler metering rolls
- Resin is fed from the top of the rolls and forms a dam – the fiber and protective papers are fed through the precision nip gap to introduce the resin directly
Compaction rolls are the heart of the hot melt prepreg process. Heat and pressure are used to heat the resin from the carrier paper/film and transfer it into the spread fiber. The “flow” of the resin is dependent on the heat of the rollers.
The Compaction Rolls are configured to operate in Nip mode or S-Wrap mode. S-Wrap gives the manufacturer the option for higher speeds on some products by exposing the prepreg to 180 degree wraps around the rollers – this gives the prepreg more time under heat versus the traditional Nip configuration. All machines are designed for both configurations.
The combination of heat, precision TIR of the rolls, precision of the deflection of the rolls, and the pressure (Pounds per Linear inch – PLI) all contribute to the proper impregnation. The prepreg manufacturer needs a precise final thickness, a precise final areal weight, and a tape with little or no voids in the prepreg.
To achieve these results, multiple sets of compaction rolls are implemented. Because the resins are viscoelastic, the prepreg tends to spring back after compaction. To avoid this, multiple roll sets are used with progressively smaller gap settings.
- Litzler compaction rolls feature exclusive designs including
- True 0.0002” (0.005 mm) TIR rolls using sophisticated bearings and bearing housings – the rollers are finish ground in the bearings
- Large diameter rolls to minimize deflection – typically 24” (610 mm) for 50” (1270 mm) wide prepreg – deflections are 00xx at 150 PLI (need to fill in this number)
- Precision gap setting mechanism through a wedge block design
Heating and Cooling Plates
The UD machines include high heat transfer aluminum machined heating and cooling plates. Conduction is a very effective means to heat or cool a web and the curved plates keep the web in contact with the plate for maximum heat transfer.
The heating is achieved with fluids (water or oil) or by embedded electric heaters.
Plates feature +/- 1 C accuracy across the plate uniformity
Unwinds and Rewinds
The UD machines require unwinds and rewinds for various papers, films, scrim, and poly liners.
- Unwinds (or Let offs) are used throughout the process in order to introduce a web into the machine
- Unwinds are used for coated paper, fabric, scrims, and poly liners throughout the machine
- All unwinds feature tension control through brakes or AC drives and are normally load cell controlled through the PLC
- Unwinds feature heavy duty safety chucks and lightweight air shafts – for operator convenience and safety
- Fixed, turret and guided versions are available
- Turrets are available in two, three, four, and six position designs
- Cantilevered unwinds are available on narrow machines
- Controlled via Litzler Automation control system
- Poly liners, scrims, and paper liners are all part of the potential flexible processing requirements
- Each unwind features safety chucks and tension control using load cell or diameter sensor closed loop feedback
- Some positions feature removable carts for loading and unloading
- Drop splice designs are available
- Web guiding is available for scrims