March 26, 2024

Polymer Degradation in Plastics & How to Prevent Them

Black specks on a molded part indicate significant degradation. Black specks, often known as carbon buildup, are sometimes confused with material contamination. Although contamination may be the cause, a closer inspection is necessary.

Black spots on plastic are a frequent processing issue for molders, extruders, and blow molders. Material degradation in the plastics manufacturing system is the typical cause of black specks.

Understanding polymer degradation helps improve plastic material lifespan. This exhaustive manual offers a thorough examination of the definition and different kinds of polymer degradation. You’ll also explore innovative polymer degradation prevention methods to improve polymer durability for regular use. This fundamental understanding is important in chemistry and substantially benefits various industries.

Polymer Degradation

The term “polymer degradation” refers to the gradual loss of a polymer’s original qualities, which typically results in a dramatic reduction of the material’s longevity. Heat, UV light, and chemicals cause polymer chain scissions and cross-linking. Degradation of polymers has a substantial impact on their thermal properties, physical and mechanical characteristics, and chemical structure. Car bumpers that fade over time, cracked garden hoses, and discolored window frames over time are all examples of plastic degradation. Determining the useful life of a polymer and its usefulness requires comprehending the factors contributing to its degradation and stability. Elements such as oxygen levels, temperature, moisture, pH, UV light exposure, and additional chemical reagents can accelerate the degradation of polymers.

Polymer Degradation Types

Degradation of polymers often begins on the surface and spreads gradually into the material’s interior. The list below shows polymer degradation by external factors.

  • Thermal-oxidative degradation
  • Photodegradation
  • Thermal degradation
  • Hydrolytic degradation
  • Biodegradation
  • Mechanical degradation

Degradation of Plastic Polymers during Injection Molding

Processing polymers involves high temperatures, shearing, etc. These powerful actions will break polymer chains and decrease molecular weight, causing plastic degradation. Injection molders of thermoplastic materials will likely ignore molecular weight when molding products. Processors must know the potential for material degradation inherent in the injection molding process. Heat slowly breaks thermoplastic molecules into carbon residues, creating black specks. This occurs frequently during system shutdowns and startup procedures.

Polymer degradation happens when the covalent connections along a molecular chain’s backbone are disrupted. This reduces the molecular weight (MW) of the polymer due to the shortening of the molecular chains. The degradation of the material’s performance properties occurs as the MW decreases. Polymer degradation may occur during molding due to wrong material preparation, plasticating, and first-stage injection.

Degraded residues can look like specks, material lumps, crust, or gels, and their accumulation during processing with the primary polymer results in the production of undesirable waste parts. Contamination of materials can cause serious issues, and black spots on plastic are a serious problem when it comes time to restart the machine. This might be caused by high temperatures, residence time, or screw or barrel contamination. Using a compound with a degradable ingredient or a multi-generation regrind are also prevalent causes.

Getting Rid of Black Spots

The particles’ source should be found and eliminated. It’s not easy to do this because there are many different sources. They are as follows:

  • Incoming resin
  • Resin degradation within the screw.
  • Resin degradation within the heated runner.
  • Screw tips and broken check valves due to poor construction.
  • Poor nozzle body design, connections, thermocouple placement, or temperature control.
  • Contamination.
  • Less than 25% barrel use causes long residence duration.
  • High melt temperature.

Preventing Black Spots during Injection Molding

1. Consider the residence time

The most common reason for black spots is time spent in the area, but don’t forget about mold-worn surfaces and dirt. Metal dust can be introduced into the mold cavity from miter locks, worn plates, and shutdown surfaces. When pressed into a light-colored component’s surface, the dust will appear as black specks injection molding. Keeping the mold clean is essential for preventing dust from causing problems with the item.

2. Clean up before leaving

Remove heat-sensitive resins and additives from the barrel. Any residual resin particle is a possible black speck. We always find that cleaning and changing production resin before the machine shutdown works best.

Filling the barrel with PP or PE before shutdown prevents carbonization during startup.

3. Choosing purge compound

Use the correct purge product to avoid machine residue accumulation. By softening degraded residues with sufficient frequency, a purge compound can eliminate material contamination concerns. To reduce the likelihood of material degradation, ensure that the melt temperature is not excessively high and that the machine is adequately purged during closure.

4. Check plastic backpressure

Verifying the correct amount of backpressure used to create a shot is essential. If you work with a degradable resin, apply a plastic backpressure of 800 to 1000 psi. Backpressure is essential for filling the screw’s flights to reduce space.

5. Check melt temperature

Inspect for incorrect placement of thermocouples, burnt-out heating elements, and improper temperature control. Use a hot probe or IR instrument to measure melt temperature.

Consider thermal expansion and hot tip depth relative to the gate surface. One way to detect temperature-control issues is to use infrared thermal images of the open hot runner.

6. Look at the flow path

Degradation is also commonly caused by the nozzle tip and the screw tip. Their construction and temperature regulation should be more credible. Cone-shaped, free-flowing, and uninterrupted, the flow path should start at the end cover and terminate at the nozzle tip. Sharp corners, limitations, and dead spots can cause polymer hangup and deterioration. Flights and nonreturnable valves/screw tip damage can cause black streaks and should be repaired or replaced.

7. Positioning of thermocouples

PID temperature regulation is preferred over percentage or variable controls—the critical thermocouple located on the nozzle body is frequently positioned incorrectly.

Placement of a thermocouple on the hex of a long nozzle will result in erroneous nozzle body and tip temperature management.

In conclusion, Preventative measures are always preferable. To mitigate the potential for plastic degradation, manufacturing personnel must possess a fundamental understanding of the following scientific concepts: plastic materials, polymer properties, material intrinsic interactions, mold design, and the plastic injection molding process. Once an understanding of these interactions has been achieved, suitable controls within the facility to prevent polymer degradation become quite simple.

A brand of Freudenberg.
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