Nylon (PA) Injection Molding

Two moisture problems, not one

Most materials present a moulding job with one or two things to manage. Nylon presents two, and both come from the same property.

Before molding: the material must be dry. Polyamide chemistry attracts water, so pellets pick up moisture from the air and hold it. When that moisture reaches the melt it turns to steam, and the results are visible and structural — splay marks and silver streaks on the surface, internal bubbles, a weak part, and unreliable dimensions. Drying at around 80–90 °C for several hours, with the dried material kept sealed or in a dryer hopper, is the standard control.

This is not a refinement. On a nylon job it is the step that separates good parts from scrap, and it is the reason nylon wants a moulder who treats drying as part of the process rather than a warm-up.

After molding: the part grows. The part that leaves the tool is dry and at its smallest. In service it absorbs moisture from the air and the absorbed water pushes the polymer chains apart, so the dimensions increase. Over days or weeks a nylon part reaches equilibrium with its environment — and on a tight-tolerance part that growth is enough to matter on any close fit.

This is the same property that governs machined nylon. The difference is where the consequence lands: on a machined part it affects the tolerance you can hold, and on a moulded part it affects how you size the tool.

Sizing a tool for a part that will grow

The tool is not cut to produce the finished dimension. It is cut to produce a part that, after shrinkage at the mould and after moisture uptake in service, ends up at the finished dimension.

So the allowance in the cavity is two things added together:

Shrinkage at the mould, roughly 1.0–2.0% for unfilled nylon, depending on grade, wall thickness, gate location, packing pressure and mould temperature.

Dimensional change from moisture uptake, which depends on how humid the service environment is and how thick the part is — a thin part reaches equilibrium faster and a thick one more slowly, but both end up at broadly the same conditioned size for a given environment.

The practical approach, agreed per drawing in DFM, is to define the target dimensions as the conditioned ones and cut the tool so the dry as-moulded part is undersized by the amount it will grow. That is why a nylon tool gets more review than an ABS tool: the shrinkage figure alone is not enough.

Where a part is measured matters too. A part inspected straight off the machine, one inspected after a day on the bench, and one inspected after a month in service are three different sizes — and only the last one is the part the customer receives.

Anisotropy and warpage

Nylon’s shrinkage is not only large, it is directional.

The polymer chains — and any glass fibres — orient as the melt flows into the cavity. Shrinkage along the flow direction differs from shrinkage across it, and the difference produces differential contraction as the part cools. Differential contraction is what warps a part.

Grade	Shrinkage	Anisotropy	Dominant problem
Unfilled nylon	~1.0–2.0%	Moderate	Shrinkage and post-mould growth
Glass-filled nylon	Lower total	Severe	Warpage

On a glass-filled part, warpage rather than shrinkage becomes the design problem. It is managed through gate position, wall design and cooling — the flow pattern determines the fibre orientation, and the fibre orientation determines how the part moves.

That is why the grade has to be settled before the tool is cut. Unfilled and glass-filled nylon are different moulding problems, and a tool designed for one will not produce the other.

Design rules

The standard set, applied to nylon’s properties.

Keep the wall uniform. A thick section beside a thin one cools and shrinks differently, producing sink marks and warpage. Where local thickness is needed — a boss, a rib, a mounting pad — core it out or reduce it to roughly two-thirds of the nominal wall.

Draft the walls. 0.5–1° minimum; more where the surface is textured, because texture grips.

Radius the corners. An internal radius of at least half the wall thickness is the usual rule.

Place the gate for the functional surface. On a gear, a weld line running across a tooth is a weak tooth. On any part, gate position sets where the flow fronts meet and therefore where the weak line is — and on a glass-filled part it sets the fibre orientation and the warpage.

Design around creep. Nylon creeps under sustained load, more so when warm and conditioned. A moulded boss carrying a permanent bolt load, or a snap feature held permanently deflected, will change shape over time. Where a part is continuously loaded, that belongs on the design review.

Design around the growth. Where a fit is critical, the tool sizing decision has to be explicit rather than assumed. Saying which dimensions matter at the enquiry is what makes that possible.

Molding route

Plastic molding runs through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Mould tooling is quoted per drawing, and mould size, shot weight and gate count are reviewed before the tool is cut.

The insert arrangement suits nylon particularly well, because nylon’s toughness means the plastic around a moulded-in metal insert resists the stress of a tightened fastener better than a brittle material would. And because the mould and the inserts come from the same drawing, the plastic-to-metal fit is planned before the tool is cut rather than adjusted afterwards.

Moulding holds part-scale tolerances agreed per drawing in DFM; it does not match machined metal. A critical bore, face or thread is normally carried by a machined insert rather than by the plastic.

Where molding nylon is the right choice

• Gears, sprockets and mechanical parts where toughness and abrasion resistance matter — see custom gears
• Clips, latches and snap features, where nylon’s toughness is an advantage
• Housings and covers that take knocks or run in dirty conditions — see enclosures and housings
• Power-tool and machinery housings in the right grades
• Cable, hose and tube components
• Conveyor and material-handling parts
• Glass-filled structural parts where stiffness is needed and warpage can be managed
• Parts that combine nylon with a moulded-in metal insert

Where molding nylon is the wrong choice

The volume does not justify a tool. Below the break-even volume, machining from rod is cheaper and absorbs design changes.

The tolerance is tighter than moulding holds. Moulding has its own envelope, and it does not match machined metal. Where a fit is critical, it belongs in a machined insert — or the part belongs on a machine.

The service environment is uncontrolled and the fit is critical. A part that grows with humidity will not hold a nominal dimension across environments. Acetal is far more dimensionally stable and is usually the better material for a precision moulded part, and the grades we run across every process are listed on the materials index.

Strong acids, phenols or formic acid are involved. All attack nylon.

The part is outdoors without a UV-stabilised grade. Unstabilised nylon chalks and embrittles in sunlight.

Continuous load at temperature. Nylon creeps, more so when warm and conditioned.

A clean precision sliding surface is required. Nylon’s dimensional movement and its friction behaviour both work against a precision sliding fit; acetal or PTFE is the better family.

What to put on the drawing

Instead of	Write
Plastic	Nylon (PA), where toughness and abrasion resistance are the drivers
Nylon, grade	Name PA6 or PA66, and whether it is unfilled or glass-filled, with the percentage
Nylon, tolerance	Say which dimensions matter, and the humidity they will be used and measured in
Nylon, tight fit	Say so — the tool is sized for the conditioned part, not the dry one
Nylon, metal insert	Say so — the insert is machined on our floor and moulded in
Nylon, wall thickness	Specify the nominal wall and keep it uniform
Nylon, continuously loaded feature	Flag it — nylon creeps
Nylon, strong acid or phenol	Wrong material — nylon is attacked

The three lines that decide most nylon moldings are the grade and whether it is filled, which dimensions are functional, and the humidity the part will live in. The first decides the moulding problem. The second decides the tool sizing and the inspection point. The third is the one most often left off the drawing, and it is the one that decides whether the parts fit.

Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on drying, shrinkage, wall, draft and gate position, and a comparison against machining the same part where the volume is borderline.