Some features on moulded parts are machined afterwards, either because the tool cannot form them, because the tolerance is beyond what moulding holds, or because the geometry would need an expensive action. Deciding which of those applies is worth doing deliberately, because a secondary operation on every part forever is a permanent cost that a tooling change might remove once.
The routine operations
Degating, where the runner is separated and the vestige trimmed. Trimming flash from the parting line. Drilling or reaming holes to a tolerance moulding will not hold. Tapping threads where an insert is not used. Milling a sealing face flat. Machining an undercut that would have needed a side action. Each is a station, a fixture and a handling operation on every part.
When machining is the right answer
When one feature needs a tolerance far tighter than the rest of the part, machining it is much cheaper than building the whole tool to that standard. When an undercut would need an expensive mechanism used on a low volume part. When the same moulding serves several variants that differ only in a machined feature, which is a common and elegant way to reduce tooling.
When it is a design failure
When the feature could have been moulded and nobody asked. When flash trimming is needed on every part because the tool shutoff was never right. When a hole is drilled on every part because the core pin was omitted. These are costs paid forever to avoid a change made once, and they are worth revisiting whenever a tool comes off for maintenance.
Machining plastic parts well
Moulded parts carry internal stress, and machining releases it, so parts can distort after the cut. Heat is the enemy, so sharp tools and light cuts matter. Fixturing must not deform the part. And a machined feature on a moulded part has a different surface from the moulded surface around it, which matters cosmetically more often than people expect.
Questions people ask about injection molding machining
Why are moulded parts machined?
For tolerances beyond what moulding holds, for features the tool cannot form, and to avoid an expensive tool action on a low volume part.
Is a secondary operation always a design failure?
No. It is often the right economic answer, particularly where one feature needs a much tighter tolerance than the rest or where variants share one moulding.
Can moulded parts distort when machined?
Yes. Moulded parts carry internal stress and releasing it by removing material can move the part, so critical features may need machining after a stabilising period.
Does machining a moulded part show?
Usually. The machined surface differs from the moulded surface around it, which matters on cosmetic faces and should be planned rather than discovered.