Black Specks and Contamination in Injection Molding: Causes and Fixes

Black or dark specks on molded parts usually come from one of three sources: carbonized material deposits that sat too long in dead zones of the barrel, nozzle or hot runners; degraded or contaminated regrind; or external dirt introduced through the resin, hopper or surrounding environment. Each has a different appearance pattern and a different fix.

TL;DR — check these first:

• Check whether specks appear randomly (carbonized deposits) or consistently (contaminated input material)
• Run a preventive barrel purge with a purging compound before suspecting anything else
• Check the regrind ratio and origin in the material blend
• Check the hopper, conveying lines and machine surroundings for foreign particles
• Rule out overheating from excessive residence time in the barrel

Are carbonized deposits (degradation in dead zones) the cause?

Material that sits too long in dead zones of the barrel, check ring, nozzle or hot runner manifold thermally degrades over time and carbonizes. The deposit occasionally breaks off and shows up in the part as a black or brown speck, randomly and with no clear pattern.

How to confirm: Specks appear intermittently on otherwise-identical cycles, unrelated to a material batch change; frequency increases after longer downtime or slower production (longer barrel residence time).

Fix: Preventive barrel purge with a purging compound before every color/material change and periodically during long runs; check barrel and mold design for dead zones (check ring, diameter transitions).

Is regrind the cause?

Regrind that has been thermally cycled multiple times loses molecular weight and becomes more prone to degradation; if mixed with foreign material or dust from the granulator, it introduces contamination directly into the blend.

How to confirm: Specks appear consistently in batches with a higher regrind ratio, and disappear or reduce with virgin material.

Fix: Limit the regrind ratio and number of reprocessing cycles per the material manufacturer's recommendation; check granulator and conveying-path cleanliness; store regrind separately from virgin resin.

Is external contamination the cause?

Foreign particles — dust, fibers, residue from conveying lines or the hopper — can enter the process with no connection to thermal degradation at all.

How to confirm: Specks have a different color or texture than the material (e.g. metallic sheen, fibrous structure) and appear from the start of production, not gradually over the run.

Fix: Check and clean the hopper, conveying lines and dosing system; check central material-conveying system seals; store resin sealed, away from dusty environments.

Cause overview

Perotec Systems supplies Asaclean (Asahi Kasei) purging compounds, the world's best-selling brand, for the regional market. Browse the full purging compounds range or read Color Changes Without Downtime: How Purging Compound Cuts Scrap. Contact us with a description of the problem — we'll help pin down the exact cause.