Crystalline plastics have a distinct melting point, and molecules are arranged regularly when solid. Regularly arranged area is called crystalline area, and disordered area is called amorphous area (or "amorphous area"). Percentage of crystalline area is called crystallinity. Usually, polymers with a crystallinity of more than 80% are called crystalline plastics. Common crystalline plastics include: polyethylene PE, polypropylene PP, polyoxymethylene POM, polyamide PA6, polyamide PA66, polyethylene terephthalate PET, polyethylene terephthalate PBT, etc.

 

(1) Mechanical properties
Crystallization makes plastics brittle (impact resistance decreases), with poor toughness and ductility, especially when spherulite size is too large.
(2) Optical properties
Crystallization makes plastics opaque because light scattering occurs at interface between crystalline and amorphous regions. However, reducing size of spherulites to a certain extent not only improves strength of plastic (reduces intercrystalline defects) but also improves transparency (no scattering occurs when size of spherulites is smaller than wavelength of light). This explains why crystallization ≠ opacity!
(3) Thermal properties
Crystallized plastics do not appear in a highly elastic state when temperature rises. When temperature rises to melting temperature Tm, they appear in a viscous flow state. Therefore, use temperature of crystalline plastics is increased from Tg (glass transition temperature) to below Tm (melting temperature).
(4) Solvent resistance, permeability, etc. are improved because crystallization makes polymer molecules more closely arranged.

(1) Polymer chain structure. Polymers with good symmetry, no branches or few branches, small side groups, and large intermolecular forces tend to be close to each other and crystallize easily.
(2) Temperature: polymers move from disordered rolls to the surface of growing crystals. When mold temperature is high, activity of polymers is increased, thus accelerating crystallization.
(3) Pressure: if there is an external force during cooling process, it can also promote crystallization of polymers. Therefore, injection pressure and holding pressure can be adjusted to control the crystallinity of crystalline plastics in production.
(4) Nucleating agent: since low temperature is conducive to rapid nucleation, but slows down growth of grains, in order to eliminate this contradiction, nucleating agents are added to molding material, so that plastic can crystallize quickly at high mold temperature, and addition of nucleating agents usually makes crystals refined. This explanation can understand real role of nucleating agents.

(1) Crystalline plastics require more energy to destroy lattice when melting, so more heat needs to be input when converting from solid to molten melt, so plasticizing capacity of injection molding machine should be large and maximum injection volume should be increased accordingly.
(2) Crystalline plastics have a narrow melting point range. To prevent crystallization of rubber material from clogging nozzle when nozzle temperature is lowered, nozzle aperture should be appropriately increased, and a heating coil that can independently control nozzle temperature should be installed. Especially for materials with relatively high melting points, such as nylon double 6 with glass fiber, if nozzle (nozzle) does not have a separate heating coil, it is easy to freeze nozzle hole.
(3) Since mold temperature has an important influence on crystallinity, mold water channel should be as many as possible to ensure uniform mold temperature during molding.
(4) Crystallinity undergoes a large volume shrinkage during crystallization process, causing a large molding shrinkage rate. Therefore, its molding shrinkage rate should be carefully considered in mold design.
(5) Due to significant anisotropy and large internal stress, attention should be paid to location and size of gate, location and size of reinforcing ribs in mold design, otherwise warping and deformation are likely to occur, and it is quite difficult to improve it by molding process.
(6) Crystallinity is related to wall thickness of plastic part. If wall thickness is slow to cool, crystallinity is high, shrinkage is large, shrinkage holes and pores are prone to occur. Therefore, attention should be paid to controlling wall thickness of plastic part in mold design.

 

(1) Heat released during cooling is large, so it is necessary to cool fully. Pay attention to control of cooling time during high mold temperature molding;
(2) Difference in specific gravity between molten state and solid state is large, molding shrinkage is large, shrinkage holes and pores are prone to occur. Pay attention to setting of holding pressure;
(3) When mold temperature is low, cooling is fast, crystallinity is low, shrinkage is small, and transparency is high. Crystallinity is related to wall thickness of plastic part. If wall thickness of plastic part is large, cooling is slow, crystallinity is high, shrinkage is large, and physical properties are good. Therefore, mold temperature of crystalline plastics must be controlled as required;
(4) Anisotropy is significant, internal stress is large, and uncrystallized molecules tend to continue to crystallize after demolding. They are in an energy imbalance state and are prone to deformation and warping. Material temperature and mold temperature should be appropriately increased, injection pressure and injection speed should be moderate.
Note:
Injection molding parameters:

Definition: Temperature required to dry polymer in advance to ensure molding quality
Function:
1. Remove moisture from raw material.
2. Ensure quality of finished product.
Setting principle:
1. Polymer will not decompose or agglomerate (polymerize).
2. Drying time should be as short as possible and drying temperature should be as low as possible so as not to affect drying effect.
3. Drying temperature and time vary depending on raw material.

Definition: Set temperature added to material pipe to ensure smooth molding.
Function: Ensure good plasticization (melt) of polymer, smooth mold filling and molding.
Setting principle:
(1) It will not cause plastic decomposition and carbonization.
(2) It will rise from feeding section to nozzle.
(3) Nozzle temperature should be slightly lower than barrel front break temperature.
(4) Required temperature varies depending on type of material.
(5) It will not have a bad impact on quality of product.

Definition: Surface temperature of mold cavity that product contacts.
Function: Control cooling rate of product in mold cavity and apparent quality of product.
Setting principles:
(1) Consider properties of polymer.
(2) Consider size and shape of product.
(3) Consider structure of mold and runner system.

Definition: Speed at which molten plastic is injected from nozzle into mold under a certain pressure.
Function:
(1) Increasing injection speed will increase filling pressure.
(2) Increasing injection speed can increase flow length and make product quality uniform.
(3) High-speed injection is suitable for products with high viscosity, fast cooling rate, and long process flow.
(4) Flow is stable at low speed and product size is stable.
Setting principles:
(1) Prevent mold support and avoid overflow.
(2) Prevent excessive speed from causing air entrapment and burning. Generally, multi-stage injection is used.
(3) Under premise of ensuring quality of product, try to choose high-speed filling to shorten molding cycle.

Definition: Speed of screw during plasticization process.
Function: An important parameter that affects plasticizing ability and plasticizing quality. The higher speed, the higher melt temperature and the stronger plasticizing ability.
Setting principles:
(1) When adjusting melting speed, it is generally adjusted from low to high.
(2) Machine speed of screw with a diameter greater than 50MM should be controlled below 50RPM, and machine speed of screw with a diameter less than 50MM should be controlled below 100RPM. At the same time, refer to maximum surface speed allowed by different materials.

Definition: Maximum pressure generated at injection port at the front end of screw. Its size is closely related to oil pressure generated in injection cylinder.
Function: Used to overcome pressure loss of melt from nozzle--runner--gate--cavity to ensure that cavity is filled and desired product is obtained.
Setting principles:
(1) It must be within rated pressure range of injection molding machine.
(2) When setting, injection pressure should be about 20% higher than actual injection pressure.
(3) Try to avoid using high pressure at high speed to avoid abnormal conditions.

 

Definition: Pressure established in melt cavity during plasticization process of plastic.
Function:
(1) Increase specific gravity of melt.
(2) Make melt plasticized evenly.
(3) Reduce gas content in melt and improve plasticization quality.
Setting principles:
(1) Adjustment of back pressure should take into account properties of plastic raw materials.
(2) Adjustment of back pressure should refer to apparent quality and dimensional accuracy of product.

Definition: Clamping force applied to mold by clamping system to overcome expansion force that separates mold during injection and holding stages.
Function:
(1) Ensure that mold does not expand during injection and holding processes
(2) Ensure surface quality of product.
(3) Ensure dimensional accuracy of product.
Setting principles:
(1) Size of clamping force depends on size of product and size of machine.
(2) Generally speaking, the smaller clamping force is, the better, provided that product does not have burrs.
(3) Clamping force setting should not exceed rated pressure of machine.

Definition: From the time mold cavity is filled with plastic, injection pressure applied to mold cavity plastic continues until gate is completely cooled and closed. This period of time requires a relatively high pressure support, which is called holding pressure.
Function:

(1) Supplement amount of material near gate position, stop unhardened plastic in mold cavity from flowing back under residual pressure before gate is cooled and closed, to prevent shrinkage of product, avoid shrinkage, and reduce vacuum bubbles.

(2) Reduce sticking, bursting or bending of product due to excessive injection pressure.
Setting principles:
(1) Holding pressure and speed are usually set to 50~60% of maximum pressure and speed when plastic fills mold cavity.
(2) Length of holding time is related to material temperature. The higher temperature, the longer gate closure time and the longer holding time.
(3) Holding time is related to projected area and wall thickness of product. Thick and large products require longer time.
(4) Holding pressure is related to size, shape and size of gate. Holding pressure switching position, metering length and release stroke setting

Definition: After plasticization begins, screw begins to retreat from injection end position under force of plastic melt until retreat limit switch is reached during rotation. This process is called metering stroke.
Function: To ensure that there is enough plastic to fill mold cavity to obtain desired appearance and size of product.
Setting principles:
(1) Metering stroke should be set according to size of product and size of machine.
(2) Metering stroke should not be too large to prevent excess plastic from staying in material tube for too long and causing carbonization.
(3) Metering stroke should not be too small to ensure that there is enough filling and to avoid mechanical damage to screw and nozzle. There should be a buffer of 3~5mm.

Definition: Change in position of screw during injection process
Function: Combine speed and pressure to control flow state of plastic
Setting principles:
(1) Metering position is determined by filling amount of finished product. Usually, a 3~5mm impulse is added to this value to determine final setting.
(2) Switching point to second speed is usually switched to hot runner and material head position.
(3) Switching point to third speed is set with 90% filling degree of molded product.
(4) Pressure holding switching point is generally set at 90% filling degree of finished product. (Note: Above four sections are used as an example)

 

Definition: After screw is pre-plasticized (metered) in place, it retreats a certain distance in a straight line. This retreat action is called back loosening, and distance of loosening is called loosening amount or anti-delay amount.
Function: Function of back release is to increase specific volume of melt in metering chamber, reduce internal pressure, and prevent melt from flowing out of metering chamber.
Setting principles:
(1) It can be set according to viscosity, relative density and actual conditions of plastic raw materials. A larger release amount will cause melt to be mixed with bubbles, affecting quality of product.
(2) Setting of release amount should be adapted to screw speed and back pressure.
(3) For raw materials with higher viscosity such as PC materials, release amount can be set.

Definition: After screw injection is completed, it is not desirable to eject all molten material in screw head, but to leave some to form a buffer volume.
Function:
(1) Prevent mechanical damage accidents caused by contact between screw head and nozzle.
(2) Control repetitive accuracy of injection volume
Setting principles:
(1) Buffer volume should not be too large or too small. If it is too large, there will be too much residual material, resulting in pressure loss and raw material degradation. If it is too small, purpose of buffering cannot be achieved.
(2) Determination of buffer volume is generally 3~5mm.

Definition: Time from end of mold opening to end of mold opening after next injection cooling is completed
Function: Ensure that product is formed, completely cooled and fixed
Setting principles:
(1) Cycle should be as short as possible.
(2) Shortening cycle must be carried out under premise of ensuring product quality

Definition: Time required for product to cool and solidify without deformation after demolding
Function:
(1) Allow product to solidify
(2) Prevent product from deforming
Setting principle:
(1) Cooling time is an important part of cycle time and should be kept as short as possible while ensuring product quality.
(2) Cooling time depends on temperature of melt, mold temperature, product size and thickness.

Definition: Pressure applied after injection molding to prevent plastic backflow after injection and cooling shrinkage compensation
Function:
(1) Prevent melt backflow after injection molding.
(2) Cooling shrinkage compensation
Setting principle:
(1) Holding time varies with product thickness.
(2) Holding time varies with melt temperature. Higher temperatures require longer time and lower temperatures require shorter time.
(3) To improve production efficiency, holding time should be kept as short as possible while ensuring product quality.

Definition: Time it takes for melt to fill the entire cavity.
Function: Injection time is determined by factors such as injection pressure, injection speed and product size.
Setting principles:
(1) Keep injection time as short as possible while ensuring product molding.
(2) Injection time is affected by factors such as material temperature and mold temperature.

Definition: Time required for screw to reach metering end position after injection is terminated
Function: Ensure sufficient melting
Setting principles:
(1) Screw speed and back pressure are mutually controlled.
(2) Do not let molten plastic stay in screw for too long to avoid decomposition and carbonization of plastic under long-term high temperature conditions.

Definition: Time required to use drying equipment to dry raw materials in advance.
Function:
(1) Improve surface gloss, improve bending resistance and tensile strength, avoid internal cracks and bubbles.
(2) Improve plasticizing ability and shorten molding cycle.
(3) Reduce water and moisture in raw materials.
Setting principles:
(1) Drying time varies depending on raw material.
(2) Drying time should be set appropriately. If it is too long, drying efficiency will be reduced and raw material may even clump. If it is too short, drying effect will be poor.