Moulding 2 - Post-moulding Flashcards
Different approaches for converting
* dry fibres and thermoset resins,
* thermoplastic prepregs,
* thermoset or thermoplastic compounds,
into composite components (continued from Lecture 5).
What are the next steps once the component is demoulded ?
* Non-destructive testing (NDT) and inspection
* Machining / trimming
* Joining and assembly
What is filament winding?
Filament winding is where continuous fibre reinforcement such as rovings or tows are wetted out in resin and deposited on a rotating mandrel. Once cured, the mandrel is removed to give a hollow product.
Winding patterns can be hoop, helical, or polar.
Advantages are that they have high fibre volume fraction suitable for structural parts, and metallic fasteners can be integrated. It can be auotmated and is highly reproducible, but there are geometric restrictions and poor dimensional accuracy on the outer surface.
Used in pipes, drive shafts, pressure vessels, wind turbine blades and railway carriages.
What is pultrusion?
Pultrusion is where composite profiles of constant cross-section are made in a continuous process, wherein layers of dry fibre bundles or fabrics are pulled through a resin bath for impregnation and through a heated die for curing.
There is high productivity and low labour content, with precise cross-sectional dimensions. However there are geometric limitations and complex die designs required, making it impractical for low volumes of production.
What is thermoplastic stamping?
Thermoplastic prepreg is heated to melt the polymer, pressed into a stiff tool then cooled to solidify. The blank is typically constrained within a spring-loaded frame to aid handling, annd also provide in-plane tension to reinforcement.
What is compression moulding?
This is where a moulding compound made from discontinuous fibres and thermoset resin is pressed into shape in a heated tool, then left in the tool to cure. It is an isothermal process and component is hot-demoulded.
It is a highly automated process with short cycle times and therefore high production volumes. Class-A surface finish and low material wastage.
However it is highly variable with high tooling costs.
What is injection moulding?
In injection moulding molten thermoplastic polymer is injected into a cavity where it is solidified before being ejected. Discontinuous fibres can be added to the polymer before injection to produce composite parts.
It has very high production rates and high levels of automation. Complex geometries can be produced with close tolerances.
High tooling costs, variable wall thicknesses.
What is non-destructive testing?
NDT is used to identify and characterise damage on the surface and within components without cutting them apart or altering the structure.
Not to be used as as a substitute for in-process control such as visual and dimensional inspections.
Should be a last line of defence to extract non-compliant components.
Used to detect:
* Delamination
* Cracks
* Voids/porosity
* Foreign objects or contaminants
* Ply misalignment
What are the different non-destructive testing methods?
- Surface finish is a good guide. Pitting suggests internal voidage. Coatings are used to improve contrast, and dye penetrants are used to apply to surface of component. The excess dye is removed, and a develloper added to draw the dried dye back to the surface to highlight defects.
- Dimensional inspection is used to compare dimensions of the component against the original CAD. Contact and non-contact methods are used, and method chosen depending on reflectivity of surface, structure, colour, scale
- Tap testing - every point of structure is tapped with a coin or hammer. The acoustic response differs based on local material stiffness, so a clear sound indicates stiffness and dull sound indicates reduced stiffness. Advanced versions of this test exist.
- Ultrasonic methods work in a similar way which use high frequency sound waves to detect internal flaws in a component. A-scan records amplitude of pulse vs time and provides information at a single point. B-scans work by reocrding A-scans whilst the transducer moves on specimen surface. C-scan gives attenuation as a function of 2D position.
- Radiography uses x-rays to scan materials. It easily detects voids and cracks but safety precautions are required.
- Thermography is the analysis of thermal flow originating from thermal excitation of an object. A externally applied thermal field can be used, or a stress generated thermal field.
- Structural health monitoring is used for in-service monitoring of structures. Optical fibres are embedded in composite structure to provide in-service strains.
What is composite machining? What issues are faced?
- Most composite processes are not net-shape, and components are manufactured to be slightly oversized to then cut off areas affected by variability. For small holes, they are not moulded but drilled post-moulding.
- Possible issues machining include: heat damage, delamination, cracking and matrix chipping. For milling, high speeds and low feed rates are recommended.
Diamond coated tools used and dust extraction is required as it is conductive. - Water jet trimming is therefore preferred, but delamination can occur if piercing. It is better to start from a free edge.
- Laser cutting is not acceptable in composite machining due to heat damage. Grinding and sanding is common to improve surface finish.
What joining and assembly methods are used for composites?
- Adhesive bonding: Complexity of preform and mould tool are limited so post-mould joining may be required. This type of bonding gives uniform stress distribution (increased fatigue life) stiffening over contact area (resistance to buckling) and is lighter than metallic fasteners. It is a permanent joint though, sensitive to surface preparation and chemical degradation.
- Mechanical fasteners: Lots of disadvantages, failure types that can occur include tension, shear-out, cleavage-tension, fastener failure, and fastener pull-out. Even a properly designed fastened joint exhibits only 20-50% of basic laminate strength.
- Welding: Thermoplastic matrix composites can be joined by welding by heating and melting bond surfaces, pressing surfaces together and then cooling. Result is a homogeneous polymer but no fibres connecting joined parts.
- Composite inserts: Welded stud/nut on perforated base plates, which can be bonded on after moulding or embedded in laminate during lay-up. Material or adhesive can flow through perforated holes to improve pull-out load.
