Polycarbonate is the resin chosen when a part must be tough and transparent at the same time, and it is unforgiving of three specific mistakes: moulding it wet, notching it, and exposing it to the wrong chemical. Each of those produces a part that looks correct and fails later, which is what makes them worth naming explicitly.
What it offers
Very high impact strength across a wide temperature range, optical clarity, good dimensional stability and good heat resistance relative to commodity resins. It is why safety glazing, machine guards, headlamp lenses, electrical enclosures and a great deal of medical equipment are made from it. Flame retardant and glass filled grades extend the range further.
Drying is not optional
Polycarbonate absorbs moisture and hydrolyses at moulding temperature, which permanently reduces molecular weight and therefore the toughness that was the whole reason for choosing it. A part moulded from insufficiently dried material can look perfect and be brittle. Drying to the specified moisture level for the specified time is part of the process specification.
Notch sensitivity
Its impact strength collapses at a sharp notch, so a scratch, a sharp internal corner, a gate scar or a moulded in stress concentration can turn a very tough part into a brittle one. Generous radii everywhere, careful gate placement away from stressed regions, and protection of the surface in handling all matter more here than with most resins.
Chemical attack and stress cracking
Many solvents, some adhesives, some cleaning agents and certain lubricants cause environmental stress cracking in polycarbonate, particularly in the presence of moulded in stress. The failure appears days or weeks later as a network of cracks. Compatibility must be checked against the specific chemicals the part will meet, including whatever the customer will clean it with.
Questions people ask about polycarbonate injection molding
Why must polycarbonate be dried?
Because it hydrolyses at moulding temperature if wet, permanently lowering molecular weight and toughness. The part can look perfect and be brittle.
What makes polycarbonate crack?
Notch sensitivity and chemical stress cracking. A sharp corner, a scratch or an incompatible solvent combined with moulded in stress will crack it days or weeks later.
Is polycarbonate scratch resistant?
No, it scratches easily. Where surface durability matters, a hard coat is applied, which is standard on lenses and glazing.
Polycarbonate or acrylic?
Polycarbonate for impact strength and heat. Acrylic for optical clarity, scratch resistance, ultraviolet stability and lower cost. They fail in opposite ways.