Custom PA+GF Injection Molding Factory

What PA+GF actually is

PA+GF is a polyamide - usually PA6 or PA66 - compounded with short glass fibres, most commonly at 30 % by weight. The fibre turns a flexible, moisture-sensitive engineering resin into a stiff, strong, heat-resistant material that can replace die-cast metal in brackets, housings and structural covers.

The reinforcement also changes the process. The melt is more viscous, the shrinkage becomes directional, the surface loses the resin's natural gloss, and the tool sees abrasive wear that an unfilled nylon would never cause. Those four consequences decide the tool design, the gate position and the steel, and they are the reason a PA+GF part should not be quoted as if it were an unfilled grade.

PA+GF is semi-crystalline, so it has a sharp melting point and a much higher heat deflection temperature than an amorphous resin. That is its advantage. The cost is a shrinkage range that depends on flow direction, wall thickness and fibre orientation, and a tolerance that moves with moisture after the part leaves the press.

Processing window

Run a 30 % glass-filled PA6 at a melt temperature of roughly 260–290 °C and a mould temperature of 80–100 °C. The melt window is narrower than ABS because the fibre raises viscosity and because polyamide degrades if it is held hot for too long. Above roughly 290 °C the signs are discoloration, a brittle part and a smell that is hard to mistake.

A hot mould is not optional. At 80–100 °C the resin crystallises properly, the surface is more uniform, and the part is less prone to warpage. A cold mould produces a part with a weak, chalky surface layer, poor weld-line strength and higher residual stress. It also makes the directional shrinkage worse, because the flow front freezes before the fibres have relaxed.

Drying is critical. Polyamide absorbs moisture from the air faster than most engineering resins, and at 280 °C that moisture hydrolyses the polymer. Two to four hours at around 80 °C is the normal instruction for PA6; the exact condition depends on the grade and on how it was stored. Keep the dryer and the hopper sealed, and do not assume that a part which looks acceptable is dry.

Injection speed is a quality variable on glass-filled parts. Fill fast enough to avoid a cold flow front, but not so fast that the fibre orientation becomes extreme at the gate. The surface will show a fibre-rich skin and a resin-rich core, and a gate placed on a visible face will leave a mark that cannot be polished out.

Shrinkage and dimensional tolerance

Design to 0.2–0.5 % in the flow direction and 0.6–1.0 % across it. Those two numbers are not an average to split: the difference is what warps a long part, bows a flat panel and pulls a round feature into an oval. The fibres align with the flow, restrain contraction along their length and leave the cross-flow direction comparatively free.

Wall thickness changes the balance. A thick section cools slowly, the fibre orientation is less constrained, and the shrinkage moves. A thin section freezes fast and holds a different value. Where a part mixes thick and thin walls, the transition is where the distortion appears, and no single shrink factor will fix it.

Gate position is a design variable, not a manufacturing detail. Moving the gate changes the flow direction, and therefore changes which dimension shrinks and by how much. On a long part, the gate belongs at one end if you want the length to shrink predictably, and in the middle if you want the two halves to balance. That decision should be made with the part designer before the tool is cut.

Moisture adds a second, slower dimensional change after moulding. Nylon absorbs water and swells, and a part that measures correctly when dry can grow after conditioning. For a precision part, specify the conditioned state and allow for the gain, or choose a grade with a lower moisture uptake. This is one of the reasons a PA+GF part is a poor candidate for a tight isotropic tolerance across a large dimension.

We cut the first sample with a nominal allowance, measure flow and cross-flow dimensions on a CMM, and correct the steel from that record. On a glass-filled part the correction is directional, so the measurement has to be too.

Where it wins, where it gets replaced

PA+GF wins where a part needs stiffness and strength at temperature, and where a metal part can be replaced without losing the structural function. Brackets, engine-adjacent covers, power-tool housings, pump components and structural frames are the classic cases. It also wins where the part has to be light: the density is around 1.35–1.40 g/cm³, well below aluminium.

The cheaper substitution is unfilled PA or POM for a part that does not need the stiffness, and PP or ABS for a part that does not need the heat. Those substitutions lower the material cost and remove most of the tool wear, but they also lower the heat deflection temperature and the stiffness.

The upgrade is a higher glass loading, a PA66 base for higher heat, or a high-temperature resin such as PPS or PEEK when the service temperature goes beyond what nylon can hold. Each step raises the melt temperature and the tooling demand again.

The common misuse is a cosmetic part. A glass-filled surface is never going to match an ABS or PC surface, and a painted PA+GF part is usually a sign that the material was specified for structure and the appearance requirement arrived later. If the part is seen, either accept a texture that hides the fibre pattern or choose a different resin.

The other misuse is a low-volume bridge tool in a soft block. The tool will produce good parts, and then the gate will wash out and the part will short-shot. If the design is still changing, cut the bridge tool in a softer grade but plan the gate as a replaceable insert from the start.

PA+GF is also a poor choice where the part has to hold a tight tolerance across a large flat dimension, because directional shrinkage and moisture movement work against it. A structural bracket with a local fit is a good candidate; a long precision rail is not.

What PA+GF asks of the mould

The first demand is wear resistance, and it is concentrated. Glass fibre erodes the gate, the sprue bush, the vent lands and any narrow restriction, because that is where melt velocity and impingement are highest. A 30 HRC pre-hardened block will show gate wash within a production campaign. The usual specification is a hardened grade at 48–52 HRC, with a replaceable gate insert so the wear item is a small block rather than the whole cavity.

Steel choice follows the resin chemistry as well as the wear. Polyamide at melt temperature can release corrosive products, particularly if it is processed wet or overheated, so a stainless grade such as 2316 is a reasonable choice where the tool also has to resist corrosion. Where the resin is dry and the campaign is long, a hardened hot-work grade such as H13 gives the wear resistance without the corrosion resistance.

Venting has to be designed for a viscous, fibre-filled melt. Air traps at the last-filled point cause burn marks and short shots, and the vent land is a wear surface because the fibre passes over it at speed. Vent the parting line generously, add vents at deep ribs and bosses, and expect to maintain the vent land as part of the tool's service schedule.

Gate design is the second wear decision. A small pin gate concentrates both shear and fibre impact, so it wears fastest and leaves the most visible mark. A fan gate or a wide edge gate spreads the flow and reduces the local velocity. For a structural part, place the gate where the weld line will fall in a low-stress region, because a glass-filled weld line is significantly weaker than the bulk material.

Cooling has to follow the mass, and the mould temperature has to be uniform. A hot spot changes crystallinity and shrinkage locally, which shows up as warpage rather than as a visible defect. Baffles and bubblers in the thick sections, and separate control for the core where the mass sits, are worth more than a lower overall mould temperature.

Ejection needs more draft than an unfilled resin because the fibre increases friction against the wall. Use at least 1° on smooth walls and 1.5–2° on textured walls, and add draft on any deep rib. A part that drags on ejection can be distorted even when the cavity is dimensionally correct.

Slides, lifters and inserts see the same abrasion as the gate. Harden the wear surfaces, and design the clearance so that glass that migrates into the slide does not bind it. This is the part of the tool that fails first on a glass-filled job, and it is cheaper to design for it than to repair it.

Cost and cycle-time effect

PA+GF costs more per kilogram than a commodity resin, and the tool costs more because of the hardened steel, the replaceable gate insert and the wear surfaces. The cycle is comparatively fast because the resin is semi-crystalline and the mould does not need to run as hot as a PC tool, which partly offsets the material price.

The dominant cost risk is tool maintenance. A glass-filled tool is a wear item, and a gate insert that is replaced on a schedule costs far less than a cavity that washes out mid-campaign. Budget for the insert from the start rather than treating it as a repair.

Yield is sensitive to drying and to gate condition. Wet resin embrittles the part; a worn gate short-shots it; a cold mould weakens the weld lines. All three produce scrap that costs more than the resin saving over a commodity grade, which is why the process discipline matters more on this material than on ABS.

For the right part - a structural bracket or a heat-resistant housing - PA+GF is usually the lowest total cost route that meets the function. For a cosmetic housing, it is the wrong resin at any price.

Design guidelines

• Wall thickness. Keep walls in the 1.5–3.0 mm range and change thickness gradually. A thick-to-thin step concentrates fibre orientation and distorts the part.
• Ribs. Rib thickness around 0.5–0.6 of the wall, with a generous fillet at the base. A thick rib pulls the face and creates a sink that the fibre makes harder to hide.
• Draft. At least 1° on smooth walls, 1.5–2° on textured walls. Glass increases friction, so err toward more draft than an unfilled resin would need.
• Corners. Internal fillet of at least 0.5 × wall. A sharp corner in a glass-filled part is a stress riser and a flow restriction at the same time.
• Weld lines. Place gates so weld lines fall away from load paths. A glass-filled weld line can lose a significant share of the bulk strength.
• Bosses. Core the boss to a wall around 0.6 of the nominal wall and add gussets. A solid boss is a shrink defect and a heat sink.
• Tolerances. Specify direction-sensitive tolerances and the conditioned state. Do not apply a single symmetric tolerance to a dimension that runs across the flow.

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PA+GF is chosen for structural parts that have to be stiff, strong and heat-resistant without the weight of metal: brackets, frames, power-tool housings, pump and valve components, and covers that sit close to a heat source. The common thread is a load path or a temperature that an unfilled commodity resin cannot carry.

It is also used where a metal part can be replaced without losing function, because the density is low and the part can be moulded into a shape that would need several metal pieces. That consolidation is often the real saving, not the material price.

Where the part is seen, or where a tight isotropic tolerance has to hold across a large dimension, PA+GF is usually the wrong specification. The fibre pattern, the directional shrinkage and the moisture movement are all working against those requirements, and a different resin or a different design is the honest answer.