Nylon is chosen for strength, toughness and wear resistance, and it brings one property that dominates every practical decision about it: it absorbs water from the atmosphere. That affects the material before moulding, the part after moulding, and the dimensions and properties that anybody measures at any point in between.
Drying is not optional
Wet nylon hydrolyses at moulding temperature, which permanently reduces molecular weight and therefore strength and toughness. It also produces splay, bubbles and a poor surface. Nylon must be dried to a specified moisture level immediately before moulding and kept dry in the hopper, and a part moulded from wet material cannot be rescued by process adjustment.
Conditioning after moulding
A freshly moulded nylon part is dry, which makes it strong, stiff and comparatively brittle. As it absorbs moisture from the air it grows slightly, becomes tougher and becomes less stiff. Dimensions therefore change over days or weeks after moulding, and a part measured straight out of the tool is not the part the customer will receive. Parts are conditioned deliberately where dimensions matter.
Shrinkage and warp
Nylon is semi crystalline and shrinks appreciably, more than amorphous resins and more still in the direction of flow in glass filled grades. That directional shrinkage is a major cause of warp on flat parts. Uniform wall thickness, considered gate placement and, where necessary, a mineral rather than a glass filler are the design responses.
Grades and where nylon wins
Unfilled grades for tough, flexible parts such as clips, gears and cable ties. Glass filled grades for stiffness and heat resistance in structural and under bonnet parts. Impact modified grades where toughness at low temperature matters. Nylon wins wherever wear, fatigue in flexure and toughness matter more than dimensional precision, and loses where precision leads.
Questions people ask about nylon injection molding
Why must nylon be dried before moulding?
Because it hydrolyses at moulding temperature if wet, permanently reducing molecular weight, strength and toughness, and producing splay and bubbles.
Do nylon parts change size after moulding?
Yes. They absorb moisture from the air, growing slightly and becoming tougher and less stiff over days or weeks.
Why do nylon parts warp?
Because it shrinks a great deal and, in glass filled grades, shrinks differently along and across the flow. Uniform walls and considered gating are the responses.
When should I choose nylon?
For wear, fatigue in flexure and toughness. Choose acetal instead when dimensional stability is the leading requirement.