Different plastic materials have drastically different physical and chemical properties (such as melting point, viscosity, hygroscopicity, crystallinity, shrinkage, etc.), therefore, targeted machine adjustment strategies must be adopted.

 

This chapter will provide detailed parameter setting guidelines using polypropylene (PP), nylon (PA), and polycarbonate (PC) as examples for reference only!

PP is a semi-crystalline plastic with excellent flowability, good chemical resistance, and low cost, but it has a relatively high shrinkage rate.
Material Properties and Pretreatment
PP has extremely low hygroscopicity and usually does not require drying.
However, to ensure consistent quality, especially when using recycled materials or fillers, it is recommended to dry at 80℃ for 1-2 hours.
Its melting point is approximately 160-175℃, and it has a wide processing temperature range.
Temperature Settings:
Barrel Temperature: Due to PP's good flowability, processing temperature range is wide.
Generally set between 180℃ and 240℃.
A typical temperature profile might be: 180℃ at the back, 200℃ in the middle, and 220℃ at the front.
For high melt flow index (MFR) PP, temperature can be appropriately lowered; for reinforced or filled PP, temperature needs to be increased.
Nose Temperature: Set 5-10℃ lower than front, approximately 210℃ - 215℃, to prevent drooling.
Mold Temperature: Mold temperature significantly affects crystallinity and shrinkage of PP.
To obtain good surface gloss and dimensional stability, mold temperature is typically set between 40℃ and 80℃.
Lower mold temperatures can shorten cycle time but may lead to uneven shrinkage and warping; higher mold temperatures help reduce internal stress and improve product dimensional stability.
Pressure and Speed Settings:
Injection Pressure: Due to PP's good flowability, high injection pressure is generally not required, typically ranging from 80 to 140 MPa. Thin-walled products require higher pressure and speed.
Injection Speed: High-speed injection is recommended for better appearance and filling effect, especially when producing thin-walled products.
Multi-stage speed control helps avoid jetting marks and air entrapment.
Holding Pressure: PP has a high shrinkage rate, requiring sufficient holding pressure to compensate.
Holding pressure is typically set to 40%-60% of injection pressure. Holding time needs to be determined based on product wall thickness and gate size to ensure gate solidification.
Back Pressure: Set a lower back pressure, typically 5-15 bar, to avoid excessive shear heat.

 

PA (such as PA6, PA66) is a crystalline engineering plastic with high strength, abrasion resistance, and heat resistance, but its most prominent characteristic is its strong hygroscopicity and temperature sensitivity.
Material Properties and Pretreatment
PA is highly hygroscopic. Excessive moisture content will cause hydrolysis at high temperatures, leading to a sharp decline in mechanical properties and appearance defects such as silver streaks.
Therefore, thorough drying is primary prerequisite for successful PA injection molding.
It must be dried in a vacuum or hot air drying oven at 80℃ - 100℃ for 4-8 hours to ensure a moisture content below 0.1%.
PA has a high melting point and a narrow melting range, requiring strict temperature control.
Temperature Settings:
Bark Temperature: Due to PA's high melting point, processing temperature is correspondingly high.
Processing temperature for PA66 is typically 260℃ - 290℃, and for PA6, it's 230℃ - 260℃.
Temperature settings must be very precise. Too high a temperature can cause degradation and discoloration, while too low a temperature may cause crystallization and nozzle clogging.
Nozzle Temperature: A temperature control device must be installed. Temperature setting should be slightly lower than front section of barrel to prevent cold material from entering.
Mold Temperature: Mold temperature has a decisive impact on crystallinity, mechanical properties, and dimensional stability of PA.
High mold temperatures (80℃ - 120℃) result in higher crystallinity, thereby improving product's hardness, rigidity, and wear resistance, but shrinkage will increase.
For products requiring dimensional accuracy and toughness, lower mold temperatures (60℃ - 80℃) can be used.
Pressure and Speed Settings:
Injection Pressure: PA melt has low viscosity and good fluidity, therefore medium to high injection pressures (75 - 125 MPa) can be used, depending on product structure.
Injection Speed: High-speed injection is recommended to prevent ripples and incomplete filling caused by excessive cooling, resulting in better surface quality. For glass fiber reinforced PA, excessively high speeds can exacerbate glass fiber orientation effect, leading to anisotropic warping, requiring appropriate adjustments.
Holding Pressure and Time: PA has a high shrinkage rate, requiring longer holding times and sufficient holding pressure.
Back pressure: Set at 10-20 bar to ensure uniform plasticization.

 

PC is a non-crystalline engineering plastic known for its excellent impact strength, high transparency, and dimensional stability. However, it has high melt viscosity and is very sensitive to moisture and temperature.
Material Properties and Pretreatment
PC is extremely sensitive to moisture; even trace amounts can cause severe hydrolysis at high temperatures, leading to molecular chain breakage, brittleness, and appearance of silver streaks.
Strict drying is essential before injection molding, typically at 120℃ for 4-6 hours, ensuring a moisture content below 0.02%.
PC has high melt viscosity and is not sensitive to shear rate, but it is sensitive to temperature changes.
Temperature Settings:
Barrel Temperature: PC has a narrow processing temperature window, typically between 250℃ and 320℃. If temperature is too low, melt will not plasticize properly, resulting in internal stress in product and even short shots; if temperature is too high (above 320℃), material may degrade.
A typical setting is: 250℃ for rear section, 270℃ for middle section, and 290℃ for front section. Nozzle temperature: Slightly lower than front section temperature.
Mold temperature: PC has high requirements for mold temperature, usually set between 80℃ and 120℃. High mold temperature helps reduce internal stress in product, improves filling capacity, and improves surface quality. Too low a mold temperature will lead to high internal stress, easy cracking, and surface defects in product.
Pressure and speed settings:
Injection pressure: Due to high viscosity of PC melt, a very high injection pressure is required to completely fill mold cavity, typically 100-150 MPa, sometimes even higher.
Injection speed: Generally, a medium injection speed is used. Too slow an injection speed can lead to cold spots and weld lines; too fast an injection speed can cause scorching and silver streaks due to excessive shear heat and poor venting. Multi-stage injection speed is key to controlling PC product quality. Typically, a slow speed is used near gate, a fast speed is used when filling most of cavity, then a slow speed is used again at the end of filling.
Holding Pressure and Time: To prevent sink marks and ensure dimensional accuracy, a high holding pressure (approximately 60%-80% of injection pressure) and a long holding time are required. Due to non-crystalline nature of PC, its shrinkage mainly occurs during cooling phase, making holding pressure particularly important.
Back Pressure: A setting of 10-20 bar is sufficient. Excessive back pressure can cause PC degradation due to shear heat.
Screw Speed: Should not be too fast to avoid excessive shearing that could lead to PC degradation. A setting of 40-60 RPM is typically recommended.