Why Does Material Decompose If It Stays in the Barrel Too Long?

In injection molding production, there is a type of defect that is very difficult to troubleshoot: the product intermittently shows black spots, burn marks, silver streaks, or darkened color and reduced toughness. The operator checks the mold, adjusts injection speed, and changes material batches, but the problem remains intermittent. In the end, it is often an easily overlooked factor at work—the material has stayed in the barrel too long.

The barrel is not a "in-and-out" tube. After plastic pellets enter the barrel from the feed opening, they must pass through the feed zone, compression zone, and metering zone, gradually melting, plasticizing, and accumulating as the screw rotates and retracts, waiting for the next injection. If the production pace is slow, the cycle is long, or the barrel capacity does not match the injection shot size, the residence time of the melt in the barrel is extended. And the longer the plastic stays at high temperature, the deeper the degree of thermal degradation.

To understand why "staying too long" causes decomposition, we first need to understand what the plastic experiences in the barrel and what the essence of thermal degradation is.

The Essence of Thermal Degradation: Molecular Chains "Break Themselves" at High Temperatures

Plastic molecular chains are stable at room temperature, but at high temperatures, the chemical bonds on the molecular chains gain enough energy to break. This process does not require oxygen—it is purely the effect of heat, hence the name thermal degradation.

The rate of thermal degradation depends on two factors: temperature and time. The higher the temperature, the greater the energy the molecular chains gain, and the higher the probability of chain scission; the longer the time, the more opportunities for chain scission accumulate. These two factors are multiplicative—if the temperature is a little higher, the time can be a little shorter; if the temperature is a little lower, the time can be a little longer. But if the temperature is already on the high side and the residence time is too long, degradation accelerates sharply.

Different plastics have very different "heat tolerance times." PP can stay at 280°C for over ten minutes without obvious degradation, but PVC begins to decompose within a few minutes at 180°C, releasing hydrogen chloride gas. PC at above 300°C for slightly too long will undergo ester bond scission, yellowing, and molecular weight reduction. PA at high temperature for too long will undergo amide bond scission and viscosity reduction. POM (polyoxymethylene) at above 200°C for a prolonged period will depolymerize and release formaldehyde, even generating gas that raises the pressure inside the barrel.

How Is "Residence Time" in the Barrel Prolonged?

Under ideal conditions, plastic pellets complete melting, plasticizing, and injection within tens of seconds to a minute or two after entering the barrel. But in actual production, several factors extend the residence time.

First, long injection molding cycle. If the product has a large wall thickness, long cooling time, or the mold structure itself results in a long molding cycle, the screw must wait a long time after each injection before the next injection can be made. During this waiting period, the melt at the front of the barrel remains at high temperature, continuously being heated.

Second, injection shot size far smaller than barrel capacity. This is the most easily overlooked situation. The barrel capacity is designed according to the maximum injection shot size. If the actual product's shot size is only a small fraction of the barrel capacity, the "waiting queue" for most of the melt in the barrel is very long. For example, if an injection molding machine's barrel capacity is 500 grams while the actual product only needs 50 grams, the average residence time of the melt in the barrel may be several times the normal situation.

Third, production interruptions or intermittent production. If the injection molding machine pauses due to debugging, mold changes, waiting for material, or troubleshooting, the melt in the barrel "waits idle" at high temperature. Some factories let the injection molding machine idle during lunch breaks or shift changes, and the melt in the barrel continues to be heated, posing a high degradation risk.

Fourth, low screw speed or excessive back pressure. Low screw speed prolongs plasticizing time; excessive back pressure increases the shear heat the melt experiences at the front of the screw, and the local temperature may exceed the set value. Both factors aggravate degradation.

Fifth, barrel temperature set too high. If the barrel temperature itself is set on the high side, the "tolerance" for residence time becomes shorter. The same residence time that is fine at normal temperature may cause problems at elevated temperature.

Degradation Products "Catalyze" More Degradation

Thermal degradation has a very troublesome characteristic: the products it generates often accelerate the degradation itself.

Taking PVC as an example, PVC loses hydrogen chloride when heated, forming carbon-carbon double bonds. These double bonds are very unstable and continue to trigger dehydrochlorination of adjacent chain segments, forming "zipper-like" degradation. The released hydrogen chloride gas also corrodes the barrel and screw, further worsening the processing environment.

POM thermal degradation releases formaldehyde, which at high temperatures further oxidizes to formic acid, and formic acid catalyzes POM's acidolysis. Once this process begins, it self-accelerates.

PC thermal degradation produces acidic substances, which catalyze the hydrolysis and alcoholysis of ester bonds. If moisture is also present, hydrolysis and thermal degradation aggravate each other.

This "autocatalytic" characteristic means degradation is not linearly cumulative—it has a "critical point." Before the critical point, degradation is slow, and the product may not show obvious signs; once the critical point is crossed, degradation suddenly accelerates, and the product shows obvious black spots, burn marks, and performance decline within a very short time.

Different Materials Have Different "Residence Time Red Lines"

Different plastics have very different sensitivities to residence time, depending on the stability of the chemical bonds on their molecular chains.

PVC is the most sensitive. PVC's processing temperature window is very narrow, typically between 160-190°C; above 200°C, it rapidly loses hydrogen chloride. If the residence time is slightly too long, it burns, yellows, and releases corrosive gas. Therefore, PVC injection molding must strictly control melt temperature and residence time, and the barrel must be emptied when the machine stops.

POM is equally sensitive. POM at above 200°C for a prolonged period will depolymerize, releasing formaldehyde gas. The pressure inside the barrel may rise, and even a "material blowout" accident may occur. POM's barrel temperature is typically controlled at 190-210°C, and the material must be purged if the machine stops for more than a certain time.

PC has better heat resistance than PVC and POM, but PC is sensitive to both hydrolysis and thermal oxidation at high temperatures. PC's processing temperature is 280-320°C. If the residence time is too long, ester bonds break, molecular weight decreases, and the product yellows and becomes brittle.

PA at high temperature for too long will undergo amide bond scission, viscosity reduction, and silver streaks and bubbles on the product. PA's processing temperature is 250-290°C, and residence time needs to be controlled within a reasonable range.

PET's ester bonds are sensitive to thermal degradation. The processing temperature is 270-290°C, and excessive residence time leads to decreased intrinsic viscosity and a brittle product.

PP and PE have relatively good resistance to thermal degradation. PP can stay at 280°C for a relatively long time without obvious degradation, but prolonged high-temperature residence still causes molecular chain scission and oxidation. PE's heat resistance is also good, but LDPE and LLDPE have lower processing temperatures, and overheating causes decomposition.

PS and ABS have moderate resistance to thermal degradation. ABS at above 270°C for a prolonged period will yellow, and the butadiene component is prone to oxidation.

When Degradation Surfaces: From "Occasional" to "Batch Defects"

Decomposition caused by material staying in the barrel too long has a very typical exposure pattern.

In the early stage, it is "occasional." At first, only a small portion of the melt at the front of the barrel has experienced excessive residence time. This degraded melt is injected into the product, manifesting as occasional black spots or silver streaks. Because the amount is small and dispersed in the product, it is easily ignored by the operator or attributed to other causes.

In the middle stage, it is "regular." As production continues, degraded melt accumulates at the front of the barrel, and black spots and burn marks begin to appear regularly on every shot or every few shots. At this point, the operator begins to notice the problem, but often first checks mold venting, adjusts injection speed, and changes material—rarely thinking of the barrel residence time issue.

In the late stage, it is "batch defects." If the problem is not solved, degradation products continue to accumulate, the autocatalytic effect kicks in, and degradation suddenly accelerates. The product shows patches of black spots, burn marks, overall darkened color, and significantly reduced mechanical properties. By this time, a large amount of scrap has already been produced.

There is an even more insidious situation: intermittent production. The injection molding machine stops and starts. Each time after a stop and restart, the already-degraded melt in the barrel is injected into the product, causing a batch of defective products. Then production returns to normal, and the problem "disappears." After the next stop, the problem appears again. This regularity makes it very difficult for the operator to connect the problem with "residence time."

How Can Excessive Residence Time Be Avoided?

Controlling residence time requires efforts at three levels: equipment selection, process settings, and production management.

In terms of equipment selection, the injection molding machine's barrel capacity should match the actual shot size. Generally speaking, the actual shot size should account for 20%-80% of the barrel capacity. If the shot size is too small, a smaller injection molding machine should be used, or a replaceable smaller barrel should be used.

In terms of process settings, barrel temperature should be as low as possible within the material's allowable range—do not set it high "just to be safe." Screw speed and back pressure should be reasonably matched to avoid excessive shear heating. For heat-sensitive materials, a shorter residence time limit should be set and clearly marked in the process documents.

In terms of production management, if the machine stops for more than a certain time (for example, 15-30 minutes, depending on the material), the barrel should be emptied or the barrel temperature lowered. When changing materials or molds, the barrel should be thoroughly purged according to procedures to avoid mixing different materials. For heat-sensitive materials (PVC, POM, etc.), the barrel must be purged when the machine stops—it cannot be "left to wait."

In terms of monitoring, the degree of degradation can be judged by observing changes in product color, measuring melt index, or regularly checking the state of the melt at the front of the barrel. If the product color gradually darkens, or the melt index gradually increases, degradation is already occurring.

Ultimately, "Time" and "Temperature" in the Barrel Are Multiplicative

Material decomposing in the barrel fundamentally comes down to molecular chains gaining enough energy at high temperature to break. The probability of this breakage depends both on how high the temperature is and on how long it stays at that temperature. The two are multiplicative, not additive. If the temperature is a little higher, the time must be shorter; if the time must be longer, the temperature must be lowered.