Decrease in flex modulus with no loss of strength or toughness...a tough one! E-glass fibre at 70 GPa (10 Msi) is about as flexible as reinforcement fibres get and has good strength (maybe 2800 MPa/400 ksi). Natural fibres such as flax from
have a claimed E of about 50 GPa a claimed strength of about 500 MPa so will be a bit weak and and will also vary a lot, not to mention being affected by moisture ingress, but it should make a more flexible material.
Just reducing the percentage of glass will decrease both stiffness and strength (roughly pro rata the %) but may well increase toughness. Using something like C-glass (more corrosion resistant than E-glass and slightly lower modulus) might give a small reduction in both stiffness and strength. Possibly a pure silica glass might help; you could try contacting ASGLAWO (
Pure silica should be about 10% lower modulus and similar strength (E-glass is mostly silica but has a load of other oxides in it).
Adding ordinary carbon will
increase stiffness and strength markedly but will probably decrease toughness a bit. Ultra-low modulus (ULM) carbon such as XN-05 (from NGF, see
(
will be more flexible than E-glass but probably a bit weak and may well be expensive. It will also be somewhat conductive (depends on details of your use). MCA (Mitsubishi Chemical America) also do some ULM carbon (e.g.,
looks a bit weak cf. glass).
Do you need both the decrease in bending modulus and maintenance of strength and toughness in the same direction? There might be practical ways to control fibre directions in the final part. Since it's injection molded there's presumably some fiber directionalising in the product anyway. Decreasing modulus and maintaining strength are usually not very compatible, but if you need the properties in different directions something might be possible just by choosing the directionality of the fibers in the finished product.
Changing the fibre length and/or diameter is also fairly doable (glass diameters range from AA to U, 1 micrometer to about 25). However, changes to modulus will also change strength in the same direction, although not the same amount. It also will change the injection molding characteristics somewhat. Changing length should affect strength more than modulus and decreasing diameter should increase toughness for the same Vf. A lot depends on the fiber length/diamter ratio and how much the short fibres are deformed, or if they are too short and fat to do so.
Fibers are what contribute most to the strength and stiffness, even glass at 25% Vf; but changing resin will also have an effect, and at 75% resin volume it might be significant. However, there aren't many resins to choose from with a lower modulus than Nylon (all polymers tend to be very similar). Possibly polypropylene (PP), though it won't help much and may well weaken the material even if it's manufacturable. Changing to something like PA-66 would give a small increase in strength cf. PA-6 and might permit a reduction in glass and therefore stiffness for the same strength. These changes will affect manufacture quite a lot.
Hope there's ome foood for thought there. More detail on your use would probably help. Please note that I'm not an injection molding specialist.
