Black Specks in Recycled ABS: Check Ash And Filtration First

Last winter a plant making TV back covers called me. Two tonnes of recycled ABS, a surface full of pitting, the first batch scrapped.

Their first instinct was to raise the flame retardant from 20% to 24%. I asked whether they had run an ash test. They had not. It came back at 4.8%, close to the 5% ceiling in the recycled-ABS standard. The flame retardant was never the problem.

Identify the defect before chasing the cause

They all get called black specks. Their origins are nothing alike, and a fingernail plus a loupe separates half of them:

Discrete particles with sharp edges, randomly scattered: foreign contamination from crushing and washing.

Fine, dense and patchy: agglomerated filler or pigment, sometimes poor additive dispersion.

Elongated along the flow, concentrated opposite the gate: carbon deposit in the screw or hot runner.

A small patch with a yellow rim: scorching from poor venting or excessive melt temperature.

The whole batch darker with no distinct particles: mixed recyclate colour, not specks at all.

Without sorting these five apart, every adjustment is a guess. My caller had two of them at once.

Why ash comes first

Recycled ABS comes from appliance shells, TV backs and office equipment housings. Those parts carry paint, plating, ink, labels and metal inserts. However well the material is washed, inorganic residue stays.

GB/T 40006.5-2021 allows up to 5% ash for recycled ABS, far wider than virgin resin. That fraction may contain calcium carbonate, talc, glass fibre, metal fines and pigment. None of it flame retards, and all of it adsorbs additive, diluting the system twice over.

The test is cheap: burn a sample in a muffle furnace, weigh the residue, half a day. Doing it first eliminates a long list of pointless adjustments.

Second, the filter screen

The screen on the pelletising line is the last gate before pellets. Too coarse a mesh, a torn screen, or a bypass to protect output, and contamination goes straight through.

The check is crude and effective: spread a handful on white paper and count under a light. Then pull the screen-change log for that batch. Many specks plus infrequent changes settles it.

The usual trick is a fine screen on day shift and a coarse one at night. The log will not show it. The screens pulled off the machine will.

Third, screw and hot runner

Incomplete purging leaves degraded material that bleeds into the next batch. Hot runners hold more dead spots, especially across colour or flame-system changes.

Use the right purging compound. Straight ABS beats PE, and pulling the screw beats not pulling it. It costs hours; a rejected batch costs more.

Fourth, drying

Moisture does not create specks, but it creates silver streaks and bubbles that get misread as pitting. Recyclate absorbs faster than virgin resin because the surface is rough and porous, so drying conditions copied from virgin material often fall short.

A quick test: bake one part at 80°C for two hours and look again. Fewer streaks means water. No change means something else.

Fifth, and only now, the flame retardant

Additives cause exactly two appearance problems: agglomerates from poor dispersion, and bloom migrating to the surface. The first shows as localised pitting, the second as a white haze.

Red phosphorus carries one more constraint: the masterbatch itself is black-red. Invisible in dark parts, disqualifying in light ones. That is the chemistry, not a formulation variable.

One point has to be stated plainly. A red phosphorus masterbatch does not make the surface smoother. Parts made with ABS-P-20M show a surface finish one step below virgin resin, and high-gloss or unpainted cosmetic parts are the wrong application. It is not a batch issue, it is the system. If the part must look good, paint it, or change route.

Temporary fixes and permanent fixes are different conversations

Temporary: finer filtration, lower recyclate ratio, tighter incoming inspection, downgrade the batch to dark non-visible parts. That ships the order; the cost does not disappear.

Permanent: sort painted and plated parts out before crushing, add a wash stage, change screens by batch instead of bypassing, and compensate flame performance with a masterbatch.

The recyclate ratio deserves its own line. Industry experience puts the risk line at 30%: above it, V-0 can fall to V-2. The number depends on source, contamination and flame system, so treat it as a warning line, not a law.

When not to gamble on recyclate

Export parts, parts with a yellow card requirement, parts whose drawing specifies a flame rating. Recycled material cannot inherit the virgin resin's yellow card, because a changed ratio is a changed formulation. Use virgin resin for those orders, or a stable recyclate plus a dedicated masterbatch, validated in a small batch.