Plastic Raw Materials and Additives

Plastik Raw Materials and Additives

Engineering plastics (PA, POM, PBT, etc.) and commodity plastics (PP, PE, PS). Additives: Glass fiber (for strength), colorants/masterbatch, UV stabilizers, flame retardants.

The importance of drying and dehumidification processes.

Raw Materials and Additives

  1. Introduction: Chemical Structure of Plastics
  • Polymerization Logic: Monomers forming polymers through a chain reaction.
  • Classification by Degree of Crystallization:
    • Amorphous Plastics: Irregular chain structure. They are transparent and exhibit low shrinkage (e.g., PS, ABS, PC).
    • Semi-Crystalline Plastics: Regular chain structure. They are opaque and possess high chemical and wear resistance (e.g., PE, PP, PA).
  1. Types and Classification of Thermoplastics

According to the raw material pyramid, plastics are divided into three categories based on performance and cost:

/ \

/   \       Engineering Plastics

/ PEEK\      High Performance (PEEK, PPS, PTFE)

/——-\

/ PA  PC  \    Technical Plastics

/ POM PBT   \   (PA, PC, POM, PBT, PET)

/————-\

/ PP  PE  PS    \ Commodity Plastics

/   PVC   ABS     \ (PP, PE, PS, ABS, PVC)

——————-

1. Commodity Plastics

High-volume, low-cost, general-purpose products.

  • PP (Polypropylene): The workhorse of the injection molding industry. Kitchenware, automotive interior trims, packaging caps.
  • PE (Polyethylene – HDPE / LDPE): Flexible or rigid packaging, pipes, pipe fittings.
  • PS (Polystyrene): Disposable cutlery, transparent boxes, rigid packaging.
  • ABS: High impact resistance, glossy surface. Toys (LEGO), home appliance housings, automotive parts.

2.Engineering and Technical Plastics

Used as an alternative to metals in parts requiring high mechanical, thermal, or chemical resistance.

  • PA (Polyamide / Nylon – PA6, PA66): High friction resistance and strength. Gears, engine covers, load-bearing brackets.
  • PC (Polycarbonate): Transparent, unbreakable, and heat-resistant. Automotive headlight lenses, protective eyewear, medical devices.
  • POM (Polyoxymethylene / Acetal): High slipperiness and dimensional stability. Precision mechanical parts, clips, pumps.
  • PBT / PET (Polyesters): Electrical insulation and heat resistance. Electrical sockets, relay housings.
  1. High-Performance Plastics

Special polymers for extreme temperatures (200°C+) and harsh conditions.

  • PEEK, PPS, PTFE (Teflon): Aerospace, defense industry, medical implants, and racing cars.

3.Additives

The raw polymer (neat resin) often cannot provide the targeted mechanical or visual properties on its own. Additives are added to enhance performance.

Additive Type Function Common Application Area
Glass Fiber (GF) Significantly increases mechanical strength, stiffness, and heat deflection temperature (HDT). Engine compartment parts, load-bearing plastic brackets (15%, 30%, 50% GF).
Masterbatch (Colorant) Concentrated pigment granules used to color the plastic. All colored plastic products.
Flame Retardants (FR) Delays burning or makes the material self-extinguishing. Electrical panels, sockets, electronic device housings.
UV Stabilizers Prevents sunlight from yellowing and embrittling the plastic. Garden furniture, stadium seats, outdoor automotive parts.
Impact Modifiers Reduces brittleness and increases impact resistance at low temperatures. Automotive bumpers, protective helmets.
Lubricants and Processing Aids Increases molten plastic flowability and facilitates mold release. Thin-wall injection molded products.
Calcite / Talc Fillers Lowers cost, reduces part shrinkage, and adds rigidity. Household items, automotive trims.

4.Moisture Control and Drying Process

Moisture control is the step that impacts injection quality the most after raw material selection.

  • Hygroscopic (Moisture-Absorbing) Plastics: They absorb moisture from the air into their structure. They must be dried using desiccant dryers prior to processing.
    • Examples: PA, PC, PBT, PET, POM.
    • What happens if not dried? The material undergoes hydrolysis inside the barrel (polymer chains break), leading to white silver streaks, loss of strength, and embrittlement in the final product.
  • Non-Hygroscopic Plastics: They retain moisture only on their surface. Simple hot-air dryers are sufficient.
    • Examples: PP, PE, PS.

 

1.Moisture-Induced Mold Defects and Hydrolytic Degradation

Inadequate drying of hygroscopic materials (PA, PC, PBT, PET, etc.) leads to massive both visual and structural defects in injection molding.

a.What is Hydrolysis?

Hydrolysis is the chemical breakage of polymer chains caused by the reaction of molten plastic with water molecules at high temperatures.

  • The molecular weight of the material drops.
  • Even if the product appears intact from the outside, it breaks like glass under the slightest load.

b.Symptoms and Diagnosis

  • Silver Streaks: During injection, moisture turns into gas and leaves silver, streak-like marks on the mold surface along with the material flow.
  • Bubbles and Voids: Trapped micro gas bubbles are observed on the part cross-section or surface.
  • Increased Flowability (Drop in Viscosity): Because polymer chains break inside the barrel, the material suddenly becomes highly fluid and begins to flash along the mold parting lines.

Inadequate Drying ➔ High Heat + Water in Barrel ➔ Hydrolysis (Chain Scission) ➔ Brittleness and Visual Defect

c.Remedial Actions

  • Proper Dryer Usage: Dehumidifying/desiccant dryers should be preferred over simple hot-air blowers (Dew point should be -40°C).
  • Moisture Measurement: Raw material should be checked with moisture analyzer devices before entering the barrel, reducing moisture content to the material’s threshold (typically <0.02% – 0.05%).
  • Drying Time and Temperature: The material’s Technical Data Sheet (TDS) must be strictly followed (e.g., 4–6 hours at 80°C for PA66).

Effects of Glass Fiber Reinforced (30% GF) Raw Materials

Glass fiber added to provide strength to plastics requires major adjustments in both process parameters and mold design.

Shrinkage and Anisotropy

  • Anisotropic Shrinkage: Glass fibers align along the flow direction inside the mold. The material shrinks very little parallel to the flow direction, while shrinking much more perpendicular to it.
  • Result: This uneven shrinkage leads to severe warpage and distortion in the part. Shrinkage allowances must be calculated differently along the X and Y axes during mold design.

Mold and Equipment Wear

  • Glass fiber is an extremely abrasive material.
  • Preventive Measures:
    • The injection screw and barrel must be selected from armored/bimetallic materials.
    • Mold steel must be hardened (steels with at least 52–54 HRC hardness, e.g., 1.2379 or CPM steels) or surface-coated (TiN, DLC coating).
    • Runners and gate areas, being most prone to wear, should be designed as replaceable inserts.

Weld Line Weakness

  • When molten plastic flows around a core inside the mold and re-knits (weld area), glass fibers fail to interlock across this re-knitting line.
  • Result: Mechanical strength at weld lines can decrease by up to 50%. This impact should be mitigated by maintaining high mold temperatures and strategically placing gates.