In injection molding production, process stability directly determines product quality, cost, and production efficiency. Degradation of polymer materials (plastics) during processing is one of the key factors that disrupt process stability, leading to product defects and performance degradation. For injection molding process engineers, a deep understanding and effective control of degradation is an essential course for achieving manufacturing excellence. Following analysis will examine definition, mechanism, impact, and improvement measures, aiming to provide systematic theoretical guidance and practical solutions for on-site process control.

 

Polymer degradation refers to process by which polymer chains, under influence of external factors such as heat, force, oxygen, and water, undergo chemical reactions such as chain breakage, side group elimination, or cross-linking, resulting in a decrease in average molecular weight, a change in molecular weight distribution, or a change in chemical structure.
In injection molding, degradation is primarily an unintended and destructive process. Essentially, it occurs when chemical stability of a material is compromised under high-temperature, high-shear-force processing conditions, resulting in irreversible damage to its original superior properties. Core of controlling degradation is ensuring that material remains within a "safe window" of its chemical structure throughout melting, plasticizing, and filling processes.

Degradation during injection molding is not caused by a single factor; it is usually result of multiple mechanisms working synergistically. These can be mainly categorized as follows:
1. Thermal Degradation:
Mechanism: During barrel heating and melt residence, polymer chains gain sufficient energy to overcome chemical bond energies, resulting in random breakage (main chain breakage) or depolymerization. This is the most common and primary form of degradation.
Causes: Barrel/nozzle temperature is too high, exceeding recommended processing temperature range for material. Melt residence time in barrel is too long, such as due to production interruptions, mold changes, machine adjustments, etc., leading to "material burning." Hot runner system temperature runaway or dead zones.
2. Shear (mechanical) degradation:
Mechanism: During screw rotation, metering, flow through narrow gates and runners, melt experiences extremely high shear rates and shear stresses. Powerful mechanical forces are sufficient to directly break polymer chains, especially in materials with high molecular weight or those sensitive to shear.
Causes: Excessively high screw speed. Excessively high injection speed. Inappropriate back pressure setting (too high). Inadequate mold gating system design, such as excessively small gate size or long, narrow runners.
3. Hydrolysis (for hygroscopic materials):
Mechanism: For materials such as polyesters (e.g., PET, PBT), polyamides (nylon PA), and polycarbonate (PC), trace amounts of moisture can react chemically with ester or amide bonds in polymer chains at high temperatures, leading to chain breakage.
Causes: Insufficient pre-drying of material is sole primary cause. Even with normal barrel temperature, materials containing moisture will undergo severe hydrolysis.
4. Oxidative Degradation:
Mechanism: In high-temperature region of barrel, oxygen in the air comes into contact with melt, triggering a free radical chain reaction, leading to molecular chain breakage and cross-linking.
Inducing Factors: Poor venting of barrel, excessive air mixing into material, and certain materials with poor thermal stability are more prone to this at high temperatures.
In actual production, these mechanisms are often intertwined. For example, high temperature (thermal degradation) accelerates oxidation and hydrolysis; mechanical energy generated by high shear is partially converted into heat energy (temperature rise), exacerbating thermal degradation.

 

Once degradation occurs, it will produce a series of chain reactions from process performance to final product performance:
1. Abnormal Melt Properties:
Significantly decreased melt viscosity: Molecular chain breakage leads to a decrease in molecular weight, resulting in an "abnormally" increased melt flowability. This may mislead process engineers into believing that material has good flowability, when in fact process window is narrowed and stability is poor.
1. Decreased melt strength: This leads to poor pressure holding effect, easy shrinkage of products, and susceptibility to blow-through or unstable filling in thin-walled or complex filling processes.
2. Severe deterioration of product mechanical properties:
A sharp increase in brittleness: This is the most fatal consequence of degradation. Decreased molecular weight leads to reduced entanglement between molecular chains, decreasing ability to bear and transfer stress. Products exhibit easy cracking, low impact strength, poor toughness, and may even break during demolding or subsequent assembly.
Decreased tensile and flexural strength.
3. Product appearance defects:
Silver streaks (silver threads): Small molecule volatiles (gases) produced by degradation are drawn into threads during mold filling.
Black spots, scorching (yellow streaks): Severe thermal/oxidative degradation leads to carbonization, forming black spots; mild degradation causes localized or overall yellowing of product.
Uneven surface gloss, turbidity: Degradation products alter surface tension and cooling behavior of melt.
Bubbles, voids: Small molecule gases produced by hydrolysis or degradation cannot be expelled in time.
4. Process Stability Degradation:
Large batch-to-batch and week-to-week fluctuations: As degradation accumulates or changes in barrel, melt viscosity continuously alters, causing drift in key process parameters such as filling time, injection pressure, and holding pressure effect, leading to unstable production.
Increased scrap rate and increased production costs.

Controlling degradation requires a systematic approach of "prevention first, comprehensive control," implementing all-round management from material preparation to process setting and equipment maintenance.
1. Material Pretreatment and Selection
Strict Drying: For hygroscopic materials, thorough drying must be carried out according to supplier's recommended temperature and time, using a dehumidifying dryer. Regularly check desiccant and drying air path. This is lifeline for controlling hydrolysis.
Material Screening: For heat-sensitive materials (such as PVC and POM), or in high-shear applications, consider selecting grades with a narrower molecular weight distribution and containing more efficient heat stabilizers or antioxidants.
2. Injection Molding Process Parameter Optimization
Temperature Control:
Adopt "lower-middle limit" principle: While ensuring complete material plasticization and smooth mold filling, use lower-middle limit of material's recommended processing temperature as much as possible.
Low-before-high or flat temperature control: For heat-sensitive materials, temperature of front section of barrel (near nozzle) can be set slightly lower than middle section to prevent overheating. Avoid steep temperature rises.
Precise hot runner control: Ensure uniform and accurate temperature at all points in hot runner, with no overheating dead zones.
Time and speed control:
Minimize dwell time: Optimize cycle time to avoid prolonged production interruptions. If interruption exceeds specified time (e.g., several minutes for PVC, slightly longer for ABS), barrel must be thoroughly cleaned or cooled to remove material.
Optimize screw speed and back pressure: Use a lower screw speed to reduce shear heat and shear force, while ensuring plasticization uniformity and venting.
Avoid using excessively high back pressure: Back pressure should be sufficient to compact material and expel air bubbles. Excessive back pressure generates significant shear heat and additional residence time, a hidden killer that triggers degradation.
Optimize Injection Speed: While avoiding jetting marks and ensuring filling, avoid using extreme high-speed injection, especially in narrow runners.
3. Mold and Equipment Maintenance
Mold Design: Optimize gating system, avoiding excessively small gate sizes and excessively large runner length-to-diameter ratios to reduce flow shear.
Equipment Status:
Check Screw and Barrel Wear: Excessive wear clearance can lead to material backflow, uneven residence time, and thermal degradation.
Calibrate Thermocouples: Ensure accurate temperature feedback.
Clean Barrel and Screw: Clean regularly to prevent accumulation of old, impurities, or carbonized material, which can induce degradation.
Check Check Ring: Ensure it seals well to prevent melt backflow and repeated shearing.
4. Operational Standards and Monitoring
Establish and adhere to Standard Operating Procedures (SOPs): including specific steps for start-up, shutdown, material change, and material cleaning.
Implement process monitoring: Monitor stability of key parameters such as peak injection pressure and filling time, as drift is often an early sign of degradation.
Establish a first-piece inspection and periodic sampling system: In addition to checking appearance, monitor changes in product's internal performance through simple mechanical tests (such as bending and drop tests).

 

For injection molding personnel, "controlling degradation" should be internalized as a core process philosophy. Please remember following action guidelines:
1. Mindset: Degradation is irreversible chemical damage, resulting in deterioration of internal performance; prevention is essential from source.
2. Golden Rule of Parameter Setting: While meeting production requirements, err on the side of lower temperature, shorter time, and slower speed and pressure.
3. Focus on Key Points: Drying is a prerequisite, temperature is core, time is critical, and shearing is a potential hazard.
4. Use your senses and common sense: Observe luster and smoke emitted from melt, smell its odor, check for silver streaks and yellowing on product. Any abnormality may be an alarm sign of degradation.
5. Keep your equipment "healthy": Good equipment condition is physical basis for a stable process.