Fibre preforming

Question 1

What is a preform?

Answer 1

A preform is an assembly of materials that are cut and placed over a tool surface.
Preforming converts 2D intermediates into a 3D form.
Some routes use constituents directly, avoiding intermediates completely.

It can be manual or automated.

Question 2

What is 3D preforming?

Answer 2

It is a collection of preforming techniques that include hand layup, robotic layup, automated tape laying (ATL), automated fibre placement (AFP). Direct 3D layup is currently limited due to speed, hardware costs and process capability.
Creating a 2D preform and post forming is normally a more logical route.

Question 3

What is hand layup?

Answer 3

Hand layup is a 3D preforming technique that is only suitable for low production volumes. It is expensive at high volumes and requires skilled labour. There is also high material wastage.

Question 4

What are the advantages and disadvantages of humans in composite manufacture?

Answer 4

Advantages
* Highly dexterous
* Good at spotting errors
* Adaptive - can perform multiple roles
Disadvantages
* Expensive, skilled labour
* Variable - not consistent
* Take short cuts
* Require motivation

Question 5

What are the advantages and disadvantages of robots in composite manufacture?

Answer 5

Advantages
* Strong
* Repeatable and more consistent
* High precision
* Fast
* Cheap
* No tea breaks
* Able to work in harsh environments
Disadvantages
* Limited feedback
* Cannot take corrective action if there’s a problem
* Not as fast as expected
* Limited movement envelope
* Dedicated to one role
* Require skilled programmers

Question 6

What is automated tape laying?

Answer 6

ATL is exclusively used for aerospace. It is high cost and quality.
* Most effective over long courses
* Wide carbon fibre prepreg tapes robotically placed on tool surface
* Thermoplastic or thermoset
* Allows tailored fibre orientation
* Enables more intelligent use of materials

Question 7

What is automated fibre placement?

Answer 7

AFP is similar to ATL but uses individual tows rather than tapes (remember tow is a bundle of fibres). An AFP machine can build any part an ATL machine can, but not vice versa.
Tows are a lot narrower and give better resolution, but are more likely to fail in tension.

Question 8

What are the different 2D to 3D preforming methods?

Answer 8

• Press forming - matching male/female tools are located in a hydraulic press, with a punch, die and blank holder. As the punch is driven to deform the mould, it is heated to activate the binder and then the preform is ejected.
• Single diaphragm forming (SDF) - Also known as hot drape forming, the 2D preform is placed over the mould and heated, and a vacuum is used to deform it over the mould.
• Double diaphragm forming (DDF) - Same as SDF except a vacuum is drawn between two diaphragms to clamp fabric. It offers better control over fabric clamping/ forming relationship.

Question 9

What are the different direct textile preforming methods?

Answer 9

• Braiding - Using may-poling action to create prismatic sections or flat ribbons. Commonly classed as biaxial or triaxial. Can be used to create complex geometries, with displacers used to create holes. These holes are 1.8 times stronger than cut holes due to fibre continuity.
• 3D weaving - Large percentage of in-plane fibres like a 2D weave. There are three styles: angle interlock, orthogonal and layer-to-layer.
• Embroidery - Also known as tailored fibre placement, where reinforcement fibres are stitched onto a substrate such as fabric or polymer film.
• Filament winding - Typically a moulding process, but used as a preforming process for BMW bonnet to create bespoke prepreg.

Question 10

What are the advantages and disadvantages of random fibre products?

Answer 10

Random fibre products can be uniflo (continuous) or chopped strand mats (discontinuous). They are easy to make but historically used in low performance applications - marine in particular. They have a low fibre volume fraction due to high bulk factor, and difficult to form from sheet because of the risk of tearing (unable to shear uniformly in-plane).

Question 11

What is directed carbon fibre preforming (DCFP)?

Answer 11

As discontinuous fibres are easier to automate, and this method sprays the fibres and the binder robotically onto a perforated screen. Preforms are moulded using liquid resin methods.
A chopper gun is used which has servo-driven blades to chop the fibres and then dispense them.
There are many variables in DCFP like fibre orientation, tow size, fibre length, local fibre volume fraction (vf) changes.