A part that comes off the press yellow, brown or darker than the standard has usually seen too much heat for too long. The resin, its additives or the pigment have started to break down in the barrel, the hot runner or the dryer. Look first at the real melt temperature and at how long the resin sits hot. This is usually a process or equipment problem and not a bad lot of resin.
Also described as: yellowing at the press, browning, darkened parts, off-shade from heat.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Every part is evenly yellow, or darker than the standard, shot after shot. | The melt is too hot, or it stays in the barrel too long for the temperature you run. | Melt temperature with a probe in an air shot, against the data sheet range |
| The first shots after a stop are dark. The color then clears. | Resin sat in the barrel or hot runner at full heat during the stop. | Length of the stop; whether the barrel was purged before restart |
| A small part on a large machine discolors even at mid-range settings. | The shot uses a small share of the barrel, so the resin waits through many cycles before it is molded. | Shot size as a share of barrel capacity; estimated residence time |
| Brown or yellow streaks, not an even shade. Often from the same cavity or the same area of the part. | A hot spot or a hang-up: a heater band stuck on, a failed thermocouple, the nozzle, one hot runner drop or a worn check ring where resin sits and cooks. | Actual zone temperatures against setpoints; which cavity; nozzle and hot runner temperatures |
| The shade is darkest near the gate, or changes when you change injection speed. | Shear heat. Fast injection through a small gate or nozzle, high screw speed or high back pressure heats the melt beyond the barrel settings. | Injection speed, screw speed and back pressure |
| The color drifts darker as the regrind share goes up. | Each pass through the barrel adds heat history, and regrind has already had at least one. | A short run on virgin resin only |
| The pellets are already yellow when they leave the dryer. | The resin was dried too hot or too long. Nylon is the usual case. | Dryer temperature, and how long the resin has been in it |
Checks in order, cheapest first
- Measure the real melt temperature. Take an air shot and put a probe in the purge. Barrel setpoints are not the melt temperature, because screw speed and back pressure add heat of their own. Purge is hot and can spit, so keep the guard closed and wear a face shield.
- Estimate the residence time, and count the stops. Divide the barrel capacity by the shot size and multiply by the cycle time for a rough figure. Add the hot runner volume if there is one. Compare the result with the limit for your resin, and remember that every unplanned stop adds to it.
- Bring the heat down in steps. Lower the barrel zones toward the low end of the range, then reduce back pressure and screw speed. Change one thing at a time and give the barrel time to settle before you judge the color.
- Check the dryer. Confirm the temperature and the time against the data sheet. Resin left in a hot dryer overnight or over a weekend can discolor before it reaches the screw. Lower the dryer temperature when a machine will be down for a long period.
- Purge after every stop, and drop the heats for a long one. Resin held at full temperature keeps degrading. A few purge shots before restart cost less than a box of off-color parts.
- Run without regrind and without color. A short trial on natural virgin resin shows whether the shift comes from the base resin, from the regrind or from the colorant. Some pigments and dyes change shade at the top of the melt range.
- Look for hot spots and hang-ups. Check each heater band and thermocouple, the nozzle tip, the end cap, the check ring and each hot runner drop. One zone that reads normal and runs hot can discolor every shot.
- Change the resin last. If the melt temperature and residence time are inside the supplier’s limits and the hardware is sound, compare the COA of the good lot with the bad lot, and then look at a grade with more heat stability.
The numbers
Typical melt temperature ranges and the longest time the resin should stay in the barrel. A maximum residence figure only holds at the low end of the melt range. The hotter you run, the shorter the time you have.
| Resin | Melt temperature | Maximum residence |
|---|---|---|
| Nylon 6 | 482–536°F / 250–280°C | 10 min |
| Nylon 66 | 527–580°F / 275–305°C | 10 min |
| Polycarbonate | 536–653°F / 280–345°C | 4–8 min |
| ABS | 425–525°F / 218–274°C. Flame-retardant grades never above 475°F / 246°C. | Not published |
| PBT, unfilled | 460–536°F / 238–280°C. Above 536°F it decomposes. | 5–10 min |
| Acetal copolymer | 370–446°F / 188–230°C. Never above 460°F / 238°C. | 15–20 min above 380°F |
| Rigid PVC | 380–410°F / 193–210°C | 15 min absolute |
| TPU | 374–455°F / 190–235°C, depending on hardness | Purge if the cycle stops for more than 10 min |
| Polypropylene homopolymer | 374–525°F / 190–274°C | Purge if idle for more than 30 min |
Typical values from published processing guides. Limits depend on the grade, so confirm against the data sheet for the grade you run. The full list is in the melt temp, mold temp and shrinkage chart. Overheated PVC gives off hydrogen chloride and overheated acetal gives off formaldehyde. If either is discoloring in the barrel, purge it out, ventilate the area and keep clear of the nozzle.
Why heat changes the color
- Heat and time act together. A resin can take a high temperature for a short time or a moderate temperature for longer. Discoloration starts when the two together use up the stabilizer in the resin. After that the polymer itself begins to oxidize and break down, and the breakdown products are yellow to brown.
- A small shot in a big barrel is a long residence time. When each shot empties only a small share of the barrel, the resin waits through many cycles. For PPA, the site’s processing chart gives a shot of 30–70% of barrel capacity as the target. Check the data sheet for the figure for your resin.
- Shear is heat. Screw speed, back pressure and fast injection through small gates all raise the melt temperature above the barrel settings. That is why the probe reading matters more than the setpoints.
- Hang-up points cook a small amount of resin for a long time. Resin that sits in a dead spot degrades and bleeds color into the melt passing it. Left longer it chars and comes off as black specks.
- Regrind arrives with heat history. It has already been through the barrel once, and sometimes through the dryer twice. See how much regrind to use.
- Some resins and additives have less margin. PVC, acetal, TPU and flame-retardant grades discolor soon after their limits are passed. Nylon yellows in a dryer that is too hot. Natural, white and light colors show a shift that a dark color hides.
- The color is a warning about the properties. The same breakdown that changes the color often shortens the polymer chains. Check impact strength on badly discolored parts before you ship them on appearance alone.
Material and equipment changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A heat-stabilized grade | More stabilizer gives the resin more time at temperature before the color moves. | Usually a higher price, and a new grade for your customer to approve. It will not fix a hot spot or a hang-up. |
| A higher-flow grade | It fills the part at a lower melt temperature and with less shear. | Higher flow usually means lower impact strength and lower stress-crack resistance. It is a new grade to approve, and it may pack and shrink differently. |
| A colorant rated for your melt temperature | Some pigments, dyes and concentrate carriers shift shade at high melt temperatures. A more heat-stable colorant holds its shade. | A new color match to approve, and often a higher colorant cost. |
| Less regrind, or none | Less heat history going back into the barrel. | More virgin resin used, and regrind to store or sell. |
| A machine with a barrel sized to the shot | A shorter residence time at the same melt temperature. | Scheduling limits, or the cost of a smaller barrel and screw. No change to the part. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Yellowing outdoors. Parts that are the right color off the press and turn yellow or chalky after months in sunlight are weathering, which is a different problem with different fixes. See UV fading and yellowing.
- Black specks. Specks are separate dark particles in a part that is otherwise the right color. Heat discoloration is a change in the shade of the whole part or a streak. The two often share a cause. See black specks and contamination.
- Burn marks. A burn mark is a brown or black mark at the end of fill or in a blind corner, caused by trapped air that is compressed and heated. The rest of the part is the right color. See burn marks.
- Color that varies by lot or by mix. A shade that moves with the colorant lot, the let-down ratio or the mixing, and does not respond to melt temperature or residence time, is a color control problem. See color inconsistency.
Quick answers
Why are my molded parts turning yellow?
Parts that come off the press yellow have usually seen too much heat for too long. The resin, its additives or the pigment have started to break down in the barrel, the hot runner or the dryer. Look first at the real melt temperature and how long the resin sits hot. This is usually not a bad lot of resin.
What causes brown discoloration in plastic parts?
Brown or yellow streaks most often come from a hot spot or a hang-up where resin sits and cooks: a heater band stuck on, a failed thermocouple, the nozzle, one hot runner drop or a worn check ring. An even, darker shade on every part points to a melt that is too hot or stays in the barrel too long.
How do I stop resin from degrading in the barrel?
Measure the real melt temperature with a probe in an air shot, then estimate residence time and compare it with the limit for your resin. Bring the heat down in steps: barrel zones first, then back pressure and screw speed. Purge after every stop. Purge is hot and can spit, so keep the guard closed and wear a face shield.
Is yellowing at the press caused by a bad lot of resin?
Usually not. Discoloration at the press is most often a process or equipment problem: melt temperature, residence time, shear heat, a hot spot or a hot dryer. Change the resin last. If melt temperature and residence time are inside the supplier’s limits and the hardware is sound, compare the COA of the good lot with the bad lot.
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