Why Must Certain Materials Be Dried Before Injection Molding?

On the injection molding floor, there is one step that seems like "unnecessary extra work": plastic pellets that look dry and feel dry to the touch must still be placed in a dryer for several hours before injection molding. This is especially true for materials like PET, PC, and PA, where drying times can run four to five hours, and sometimes a desiccant dryer must be used instead of an ordinary hot-air dryer.

If it were just "a little moisture on the surface," wouldn't a hot-air blow be enough? But things are not that simple. The moisture absorbed by these materials is not on the surface—it is inside the pellets. Drying is not about removing surface moisture; it is about forcing out the water that has already penetrated between the plastic's molecular chains. If this step is not done properly, the molded products will show silver streaks, bubbles, reduced strength, dimensional deviation, and even direct material degradation in the barrel.

To understand why drying is necessary, we first need to understand how water gets into plastic and what it does during injection molding.

Water Is Not "Stuck" on the Plastic—It Is "Dissolved" in It

Plastic pellets absorb moisture from the air, a process called hygroscopic absorption. The tendency to absorb moisture varies enormously between materials, depending on their molecular structure.

Highly hygroscopic materials have polar groups on their molecular chains (such as amide groups, ester groups, and hydroxyl groups), and these groups have a strong affinity for water molecules. Water molecules do not just stay on the pellet surface—they diffuse into the free volume between molecular chains and even form hydrogen bonds with polar groups. PA (nylon) is a typical example: its amide groups have a strong ability to form hydrogen bonds with water molecules, and its water absorption rate can reach 1%-2% or even higher. The ester groups of PC (polycarbonate) also have some hygroscopicity, and PET is the same. The butadiene component in ABS also readily absorbs water.

Weakly hygroscopic materials have non-polar molecular chains (such as PP, PE, and PS). They have no affinity for water molecules, and moisture can hardly diffuse into the interior. These materials may have a small amount of surface moisture, but generally do not require long drying—and in some cases may not need drying at all.

The key difference is: for highly hygroscopic materials, the water is inside the pellets, not on the surface. Ordinary hot-air drying can only blow away surface moisture and is powerless against internal moisture. Dry hot air must be continuously blown over the pellets so that moisture inside the pellets slowly diffuses to the surface and is then carried away by the hot air. This process takes time and also requires dew point control—the lower the dew point of the drying air, the greater the moisture concentration difference between the inside and outside of the pellets, and the faster the moisture diffusion. This is why materials like PET, PC, and PA require desiccant dryers rather than ordinary hot-air dryers.

What Does Water Do During Injection Molding?

When material that has not been properly dried enters the barrel, water undergoes several destructive processes.

The first process: hydrolytic degradation. This is the most serious problem. At high temperatures, water molecules react chemically with certain plastic molecular chains, cutting the long chains apart. PET and PC are typical hydrolysis-sensitive materials. In the molten state, if moisture is present in PET, the ester groups undergo hydrolysis, molecular weight drops rapidly, intrinsic viscosity decreases, and the product's mechanical properties—impact resistance and tensile strength—decline severely. The same is true for PC: hydrolysis causes molecular chain breakage, and the product becomes brittle and yellows. This degradation is irreversible—once it happens, the material is ruined.

The second process: silver streaks and bubbles. Even if hydrolysis does not occur, water vaporizes at high temperatures. Barrel temperatures are typically above 200°C, and water is in a gaseous state at this temperature. When the melt is injected into the cavity, water vapor is compressed inside or on the surface of the melt, forming silver streaks (splay) or bubbles. Silver streaks appear as radial silvery lines on the product surface, while bubbles are internal voids. These two defects are especially fatal on transparent products (such as PET preforms and PC lampshades) because they are directly visible.

The third process: degraded surface quality. Water vapor forms a gas film between the cavity surface and the melt, preventing the melt from replicating the mold surface's texture and finish. The product surface appears cloudy, rough, and uneven in gloss—unacceptable for high-gloss products.

The fourth process: unstable dimensions. Hydrolytic degradation causes molecular weight to drop, melt viscosity to decrease, flowability to change, and the product's shrinkage rate to change accordingly. With the same mold and the same set of parameters, products made from poorly dried material may have completely different dimensions from those made from properly dried material. For precision parts with strict tolerance requirements, this dimensional drift is fatal.

The "Drying Red Lines" for Different Materials

Drying requirements vary greatly between materials, depending on their hygroscopicity and hydrolysis sensitivity.

PET is one of the most demanding materials for drying. PET's ester groups are extremely sensitive to hydrolysis, and its processing temperature (around 280°C) is high, so hydrolysis proceeds rapidly. Before injection molding PET preforms, the material typically needs to be dried at 160°C in a desiccant dryer for 4-6 hours, with a dew point requirement below -40°C and moisture content controlled within 50 ppm (0.005%). If moisture content exceeds the limit, the preforms will show silver streaks and haze, intrinsic viscosity will drop, and bottle blowing may cause rupture.

PC has equally strict drying requirements. PC is sensitive to moisture in the molten state, and hydrolysis causes molecular weight reduction, yellowing, and decreased mechanical properties. PC is typically dried at 120°C for 3-4 hours, with moisture content controlled below 0.02%. If a PC product shows silver streaks or brittle cracking, the first thing to check is whether drying was adequate.

PA (nylon) has the strongest hygroscopicity, but its hydrolysis sensitivity is somewhat lower than PET and PC. PA typically needs to be dried at 80-100°C for 2-4 hours before processing, with moisture content controlled below 0.1%. The trouble with PA is that it absorbs moisture quickly; after drying, if exposed to air, it will soon absorb moisture again. Therefore, dried PA must be used immediately or stored sealed.

ABS has moderate hygroscopicity and is typically dried at 80-90°C for 2-3 hours. The butadiene component in ABS readily absorbs water, and poor drying leads to silver streaks and surface defects.

PP, PE, and PS, being non-polar materials, have very weak hygroscopicity and typically do not require drying, or require only a very short surface drying. However, if these materials are stored for long periods in humid environments, moisture may condense on the surface, and simple hot-air drying is sufficient.

Drying Is Not Just "Baking Dry"—It Is a Diffusion Process

Understanding the essence of drying helps explain why drying takes so long and why desiccant dryers are more effective than ordinary hot-air dryers.

The drying process is: moisture inside the pellets diffuses to the surface, and surface moisture is carried away by dry air. The rate of this process depends on two factors: the moisture concentration difference between the inside and surface of the pellets, and the moisture concentration difference between the surface and the air.

An ordinary hot-air dryer heats air and blows it over the pellets. The moisture content (dew point) of the air depends on ambient humidity. In humid seasons, the dew point of an ordinary hot-air dryer may only be around 10°C, the moisture concentration difference between the inside of the pellets and the air is not large, drying efficiency is low, and the moisture content of the dried pellets may still exceed the limit.

A desiccant dryer first cools the air below the dew point, condenses out the moisture, and then heats it and sends it to the drying hopper. This way, the dew point of the drying air can be lowered to -40°C or even lower, the moisture concentration difference between the inside of the pellets and the air is greatly increased, moisture diffusion speeds up, and a lower final moisture content can be achieved.

So for materials like PET, PC, and PA, it is not enough to just "bake them dry"—the moisture content must be reduced below a certain threshold. This threshold depends on the material's hydrolysis sensitivity and the product's quality requirements. With an ordinary hot-air dryer, this threshold may never be reached; with a desiccant dryer, it can be reached within a reasonable time.

The Cost of Poor Drying

The cost of poor drying is multifaceted.

Higher scrap rate. Silver streaks, bubbles, and out-of-tolerance dimensions directly cause product rejection. For transparent parts and high-gloss parts, the scrap rate can be astonishingly high.

Declining performance. Hydrolytic degradation causes molecular weight to drop, and the product's impact resistance, tensile strength, and aging resistance all decline across the board. This decline may not be visible during factory inspection, but it is exposed during customer use, leading to returns or claims.

Additional wear on molds and equipment. Acidic substances produced by hydrolysis may corrode the barrel and screw. Water vapor condensing in the cavity may cause rusting of the mold surface.

Energy and time costs. Drying itself requires energy and time. If repeated trial molding, parameter adjustment, and product sorting are needed because drying was inadequate, these hidden costs far exceed the cost of drying itself.

Ultimately, Drying Is About Protecting the Material's Molecular Chains

The mechanical properties of plastic essentially depend on its molecular chain length and the entanglement state between molecular chains. Hydrolytic degradation cuts long chains apart, molecular weight drops, and performance is ruined. For hydrolysis to occur, three conditions must be met simultaneously: water, high temperature, and sensitive groups. The barrel temperature during injection molding cannot be lowered, the sensitive groups are determined by the material itself, and the only thing that can be controlled is water.