Glass-filled nylon is one of the most specified and least understood materials in injection moulding. A buyer asks for nylon with 30 percent glass because a part has to be stiffer, and receives a material that is indeed stiffer, shrinks less, and behaves differently in the tool in three ways that were not on the drawing. This guide covers what GF30 actually changes, and how to write the specification so the moulder can quote against it.
What the glass actually changes
Short glass fibres lock into the nylon matrix and raise the stiffness dramatically. Tensile strength roughly doubles and flexural modulus can triple against unfilled PA66, which is why GF grades carry structural clips, brackets and housings that unfilled nylon would creep under. Heat deflection temperature rises as well, and the part holds shape at service temperatures where unfilled PA66 would soften.
| Property | Unfilled PA66 | PA66 GF30 | Direction |
|---|---|---|---|
| Tensile strength | 80 MPa class | 160 MPa class | Up, roughly double |
| Flexural modulus | 2.8 GPa class | 8 to 9 GPa class | Up, roughly triple |
| Mould shrinkage | 1.2 to 1.8 percent | 0.3 to 0.7 percent | Down, but directional |
| Impact toughness | Good, notched | Reduced, brittle at low temp | Down, the common surprise |
| Tool wear | Low | High, fibres are abrasive | Up, hardened steel needed |
Shrinkage: smaller but directional
The lower shrinkage of GF30 is real, and it is why the grade exists: a bracket that would warp in unfilled nylon often holds in GF30 because the material simply moves less as it cools. But the shrinkage that remains is directional. Glass fibres align with the flow, so the part shrinks less along the flow direction and more across it, and that anisotropy shows up as warp in parts with ribs, bosses or wall changes. A programme that switches from unfilled to GF30 without re-cutting the tool to the new shrinkage figures gets parts that are dimensionally different in the two axes.
What it changes in the tool
Three moulding consequences follow from the glass. First, wear: the fibres abrade the barrel, screw, nozzle and gate, so GF programmes need hardened tool steel and expect higher maintenance on the machine side, and a moulder who quotes a GF job on a standard screw is quoting a job that will drift. Second, weld lines: fibre orientation at a flow junction leaves the weld weaker than the surrounding part, and a GF part designed with a weld line in a load path fails at that line even though the tensile number on the datasheet looks adequate. Third, surface: glass fibres can bloom at the surface, leaving a streaky finish on cosmetic parts, which is why painted or textured parts usually stay unfilled and structural hidden parts take the glass.
When GF30 is the wrong answer
Glass raises stiffness and lowers toughness at the same time, and programmes that need impact resistance at low temperature are usually better served by an unfilled or impact-modified nylon. Parts that slide or wear, bushings and guides, want lubricating fillers such as molybdenum disulfide rather than abrasive glass, and food-contact parts need the specific grade declaration because not every GF nylon is made for food contact. If the requirement is stiffness at temperature, GF is usually right; if the requirement is drop survival, it is usually wrong.
Writing the specification
A GF specification a moulder can quote has five elements: the matrix, PA66 or PA6; the glass content with a verification method, typically thermogravimetric analysis on a lot basis; the test direction, because flow-direction and cross-flow numbers differ and the datasheet states which; the shrinkage figures per axis, taken from the moulder's own tool history rather than a generic table; and the surface requirement, which decides whether the part can take the bloom. Programmes that send all five get comparable quotes; programmes that send 30 percent glass filled nylon get parts that meet the datasheet and miss the drawing.
Related reading
The plastic materials category lists the reinforced grades we supply, and the POM versus PA66 wear parts guide covers the unfilled alternative for sliding applications.
