Glass fibre reinforced compounds are bought by loading level — 10, 20, 30, 50 per cent — but that number is measured on an ideal test bar. What your part receives depends on how much fibre length survives plasticising and how the fibres lie after the gate. This page covers that gap, and does not repeat what a given loading changes inside a given resin.
Each step up buys stiffness, heat deflection and dimensional stability. It pays in elongation, surface finish and tool life. Ask not how much fibre, but the lowest loading that passes.
Both rise with loading. Ask for modulus and HDT at your wall thickness, not at a standard test bar.
Fibre earns its keep where load is continuous. Specify creep at service temperature, not short-term strength.
Elongation and impact fall as loading rises, and so does surface quality. Over-specifying is paid twice: in resin and in scrap.
| Loading band | Buys | Costs | Goes to |
|---|---|---|---|
| 10–15% | Dimensional stability | Little toughness loss | Clips, brackets |
| 20–30% | Structural stiffness | Warpage, fibre show | Housings, fans |
| 35–45% | Heat under load | Weak weld lines | Pumps, brackets |
| 50% and up | Maximum stiffness | Narrow window, wear | Metal replacement |
Fibre leaves the screw shorter than it entered. Back pressure, screw design and regrind all cut it further, and short fibre delivers less than the grade name implies. Ask what length each expects to retain.
Fibres align with flow, making shrinkage directional — less along it, more across it. That difference lifts flat parts off the tool, so gate position decides distortion as much as resin does.
Where flow fronts meet, fibres lie along the line rather than over it. The resin-rich joint is usually where a reinforced part fails first. Move the gate or the hole.
A single modulus figure misleads. Use flow and cross-flow values in every structural calculation.
Geometry carries no surface, wear or weld-line penalty. Exhaust rib and wall design before raising the loading.
Glass abrades steel. Ask which screw and barrel metallurgy the moulder runs for reinforced grades — treat it as a capability question.
Gates, cores and sharp corners erode first, and erosion changes fill over a long run. Budget for gate inserts and inspection.
A reinforced surface abrades whatever it slides on. Specify the counterface material and ask for wear data against it.
It is the common compromise, not a rule. Where stiffness is needed locally, ribs at a lower loading usually beat a higher one.
Orientation makes shrinkage directional, so flat parts lift. See warpage causes, then move gates before changing grade.
Yes, at a declared percentage, because regrind shortens fibre further. Put the limit in the specification and on the analysis certificate.
Expect hardened or coated steel, with gates and cores treated as consumable. The cost lands in tool maintenance over the run.
Choose the base polymer first, then the loading. See engineering plastics for the routing.
Deciding on a loading? Send the load case and service temperature to the plastic materials desk. Compare base polymers on PA6 and PA66, check modified compounds for tailored loadings, review the example GF30 PA6 compound, and raise the tooling question with custom molding services before freezing the drawing.
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