Types of Plastics

(1) Thermoplastic materials: After heating, they become soft and flowable resins. This process of cooling and solidification can be repeated multiple times, thus allowing for recycling.
(2) Thermosetting materials: After heating, they become hard products. Reheating does not soften them; process is irreversible. Therefore, once thermosetting products are molded, their shape cannot be changed, and they cannot be recycled.

 

★ ABS (Acrylonitrile Butadiene Styrene)
ABS is synthesized from three chemical monomers: acrylonitrile, butadiene, and styrene. Each monomer has different properties: acrylonitrile has high strength, thermal stability, and chemical stability; butadiene has toughness and impact resistance; styrene has easy processing, high gloss, and high strength. In terms of morphology, ABS is a non-crystalline material. Properties of ABS primarily depend on ratio of its three monomers, which allows for great flexibility in product design and results in hundreds of different qualities of ABS material on the market. These different qualities offer varying properties, such as medium to high impact resistance, low to high smoothness, high-temperature torsion resistance. ABS material boasts excellent processability, appearance characteristics, low creep, excellent dimensional stability, and high impact strength. Its injection molding temperature is recommended to be around 220-245℃.
★ PMMA (Polymethyl methacrylate)
Processing Characteristics: PMMA has high viscosity and slightly poor flowability, therefore, high material temperature and high injection pressure are required for injection molding. Injection temperature has a greater impact than injection pressure, but increasing injection pressure helps improve product's shrinkage rate. It has a wide injection temperature range, with a melt temperature of 180-200℃ and a decomposition temperature reaching 270℃, resulting in a wide temperature adjustment range and good processability. Therefore, improving flowability can be achieved by adjusting injection temperature. Poor impact resistance, poor wear resistance, easily scratched, and brittle. Therefore, mold temperature should be increased and cooling process improved to overcome these defects. It is commonly used in signal light lenses.
★ PC Polycarbonate
PC is a non-crystalline resin, light amber in color, with excellent mechanical strength, especially its impact resistance, which is second to none. Other advantages include low-temperature resistance, heat resistance, electrical properties, dimensional stability, and temperature stability. It is relatively sensitive to ultraviolet light.
(1) Flowability: High melting temperature, highly dependent on temperature, but this does not necessarily mean good flowability.
(2) Shrinkage: As a non-crystalline resin, shrinkage is relatively small (0.5~0.8%).
(3) Thermal stability: Thermal decomposition begins at 330~340℃ with acidification; addition of water has a significant impact on decomposition.
(4) Resin temperature: 250~290℃; above 300℃, thermal decomposition will occur, similar to PA.
(5) Mold temperature: 80~120℃. Mold temperature also needs to be determined based on required quality of molded product. High mold temperature results in a very good surface condition and lower molding stress, but a longer molding cycle can cause deformation.
(6) Injection pressure: Due to high melt temperature and poor fluidity, high-pressure injection will leave stress on the surface. Therefore, it is necessary to increase resin temperature, mold temperature, and injection speed, and to use low-pressure molding as much as possible.
(7) Injection speed: High-speed injection, but avoid the occurrence of adverse phenomena.
(8) Molding cycle: Low back pressure, slow screw rotation speed, high dependence on temperature, special attention should be paid to temperature changes.
★ PE (Polyethylene)
It does not lose its flexibility under low temperature conditions, is impact-resistant, corrosion-resistant, and has good high-frequency characteristics, making it widely used. Based on crystallinity, there are two types: low-density PE and high-density PE.
(1) Fluidity: Wide temperature range, low viscosity, good fluidity.
(2) Shrinkage: Very large molding shrinkage rate. Unlike ordinary crystalline resins, it shrinks very much in flow direction when mold temperature is below 80℃.
(3) Thermal stability: Wide temperature range, good thermal stability.
(4) Other characteristics: Resin is relatively soft, heat-resistant, moisture-resistant, and solvent-resistant. It has good electrical and high-frequency properties. However, its heat resistance, weather resistance, and rigidity are poor. Bonding and printing are relatively difficult.
(5) Pre-drying: Drying is generally unnecessary; excessive drying will cause the resin to yellow.
(6) Resin temperature: In temperature range of 150~300℃, resin's stagnation and flow difficulties in low-density PE can lead to resin combustion.
(7) Injection pressure: To prevent product deformation, high-speed injection is required. Since high-density PE is more difficult to mold than low-density PE, pressure should be slightly increased.
(8) Injection speed: Different cavity sizes require different speeds. Inject at the highest possible speed without causing resin combustion.
(9) Mold temperature: 30~80℃. PE has a high molding shrinkage rate, so mold temperature should be set lower.
★ PA (Polyamide), Nylon
PA has good heat resistance, toughness, and insulation. However, due to its water absorption, its acid resistance is poor, making molding relatively difficult.
(1) Flowability: Highly sensitive to temperature, with significant viscosity variations. Due to high viscosity change rate, increasing resin temperature is necessary to address poor flowability.
(2) Shrinkage: Shrinkage varies greatly depending on thickness of molded product.
(3) Thermal Stability: High water absorption. Insufficient drying can cause hydrolysis during molding. Drying at temperatures above 90℃ during hot air drying will result in acidification and a yellow color.
(4) Other Properties: Due to hygroscopicity, dimensional changes (increases) occur. Therefore, dimensional stability is achieved through moisture conditioning after molding.
(5) Pre-drying: High water absorption requires thorough drying. Dry at 120℃ for 5 hours.
(6) Resin Temperature: Nylon 6: 240℃~260℃; Nylon 66: 280℃~300℃; Nylon 12: 200℃~220℃. ℃, generally nozzle should be set higher
(7) Mold temperature: 60℃~80℃, 100℃~120℃ when glass fiber is added or appearance is very important.
(8) Injection pressure: 700~1750kg/cm2, 900~2500kg/cm2 when glass fiber is added. Melt viscosity is low, so there is no need to increase pressure. If nozzle temperature is low, it will cause blockage.
(9) Molding cycle: Back pressure of glass fiber should be low and rotation should be slow. If nozzle is blocked, nozzle temperature should be increased
★PBT/PET alloy
PET has a high molding temperature and a narrow material temperature adjustment range (245-300℃). However, after melting, it has good fluidity, so processability is poor, and anti-drip device is often added to nozzle. Mechanical strength and performance after injection are not high. It is necessary to improve performance through stretching and modification. Accurate control of mold temperature is an important factor in preventing warping and deformation. Therefore, it is recommended to use hot runner molds. Mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding.

 

★Compared to thermoplastics, thermosetting plastics have advantages such as low price, high temperature resistance, good flame retardancy, high strength, aging resistance, and good surface gloss.
BMC: Bulk Molding Compound, is an unsaturated polyester thermosetting molding compound. It is commonly used for injection molding of lamp reflectors.