Thermoplastic Materials - Characteristics and Properties,Thermoplastic materials are divided into two major categories based on their heat resistance and characteristics: heat-resistant thermoplastics and general-purpose thermoplastics.
Types and properties of plastics:
△Thermodynamic changes of plastics
Heat-resistant thermoplastics can be molded into fixed-shape plastic parts at high temperatures, and maintain a fixed shape after cooling. If heated again, they can soften and flow, and be molded into fixed-shape plastic parts again through repeated processing - this is reversible. Since thermoplastic materials do not undergo fundamental chemical changes during the molding process, the scrap materials can be recycled and reused, known as "secondary material" or "regrind."
Thermoplastics undergo physical changes during molding, meaning they simply cannot change their original state when heated, but remain amorphous and unable to flow after cooling. Therefore, thermoplastic materials cannot be repeatedly heated and remolded, so the scrap from thermoplastic materials generally cannot be reused.
This book primarily discusses "plastics" - excluding rubber, which is also a thermoplastic material.
Compared with plastic molding processes, although injection molding has various unique capabilities and advantages, it also has some inherent defects and shortcomings. Understanding the main molding characteristics of plastic injection molding is the prerequisite and guarantee for correct mold design and quality improvement of molded parts.
(1) Shrinkage
Whether plastic parts can maintain dimensional stability under normal temperature conditions when molded from the mold cavity and cooled to room temperature, the size will be slightly smaller than the original mold cavity. This characteristic is called shrinkage, which can be compensated through molding temperature control.
This shrinkage is not only caused by the thermal expansion and contraction of the plastic itself, but also relates to various molding process conditions and mold design factors. After the plastic is cooled, the shrinkage rate of the part is the molding shrinkage, which can be reduced or improved through adjustment of process parameters or small changes in mold cavity size.
Plastic parts undergo secondary shrinkage for a period of time after molding, also known as post-molding shrinkage, while maintaining the same molding conditions:
① The shrinkage of plastic parts is not uniform. Due to the fact that the thermal shrinkage rate of plastics changes with physical and chemical conditions in different internal parts, the shrinkage rate of plastic parts after cooling to room temperature varies in size and is not completely uniform. Therefore, there should be certain limitations on the molding dimensional accuracy of plastic parts, and precision should be appropriately improved through mold design.
② The post-shrinkage of plastic parts. During the molding process, due to various internal stresses, chemical reactions, and various external forces - primarily molding pressure - the plastic part continues to exist after molding with residual stress. After molding, due to various residual stress effects, the plastic part size continues to change slightly after production. Generally, most molded parts stabilize within 10h after molding, and basically stabilize after 24h, but it takes more than 10 days for them to completely stabilize. Paying attention and taking corresponding molding measures is the key to controlling post-molding shrinkage.
To stabilize the final dimensions of plastic parts after molding, heat treatment is sometimes required after molding. Heat treatment allows the thermoplastic to be maintained at a certain temperature causing softening; removing external forces on the molding shrinkage allows proper internal compensation to occur after molding, which can reduce the dimensions of the molded part at higher molding temperatures.
③ Directional shrinkage of plastic parts. During the molding process, the orientation effect of polymers along the flow direction leads to anisotropy in the plastic part. The shrinkage of the part will inevitably differ depending on the direction of the material flow: generally, shrinkage is greater and strength is higher along the material flow direction, while shrinkage is smaller and strength is lower in the direction perpendicular to the material flow. Simultaneously, due to the uneven distribution and density of additives in different parts of the plastic part, shrinkage is also uneven, resulting in differential shrinkage and making the plastic part prone to warping, deformation, and even cracking.
(2) Fluidity
During the molding process, the ability of plastic to fill the mold cavity under certain temperature and pressure is called the fluidity of the plastic. This is a unique comprehensive technical indicator for injection molding. During the molding period, attention should be paid to the dimensions of the mold cavity and its related parameters. When the molding pressure is too large or too small, the influence of fluidity should also be considered.
The size of fluidity has a significant relationship with the molecular structure of plastics. Plastics with linear molecular structures or lower molecular weights have less obstruction to molecular flow, resulting in greater fluidity. When adding fillers to plastics, the fluidity increases with higher filler content. The influence of various design factors and molding process conditions on plastic fluidity is also applicable to the fluidity of polymer compounds.
The fluidity of plastics is not an invariant value, and mold design can adjust it to a great extent. If the fluidity of plastics is good, it does not necessarily mean that every aspect of the injection molding process is smooth and satisfactory. Conversely, if the fluidity is poor, it can be improved by increasing the injection molding temperature or pressure. However, if the fluidity is too great, it can easily cause defects such as flash in the production of plastic parts. Therefore, in the molding process, the use of plastic materials should consider all aspects of influencing factors and comprehensively choose appropriate plastics. Only then can quality be guaranteed and the molding process parameters and mold design be appropriately selected, ultimately achieving the purpose of controlling and improving quality.
According to the fluidity of common plastics in mold design requirements, the fluidity classification of thermoplastic injection molding can generally be divided into three categories:
① Good fluidity plastics: such as nylon, polyethylene, polypropylene, polystyrene, acrylic, cellulose acetate butyrate, and polyoxymethylene.
② Medium fluidity plastics: such as modified polystyrene, ABS, AS, polyformaldehyde, vinyl chloride homopolymer, and polytetrafluoroethylene.
③ Poor fluidity plastics: such as polycarbonate, hard polyvinyl chloride, polysulfone, polyimide, aromatic polyester, and fluoroplastics.
The main factors affecting the fluidity of injection molding plastics are:
① Plastic temperature. When the plastic temperature is high, the fluidity increases correspondingly with the temperature of different plastics. For example, polystyrene, polypropylene, polyamide, polyoxymethylene, modified polystyrene, cellulose acetate, and ABS have very sensitive temperature dependence on fluidity; for polyvinyl chloride, polyformaldehyde, and polymethyl methacrylate, the impact of temperature changes on fluidity is relatively small.
② Injection pressure. Increasing injection pressure can overcome the resistance generated by the melt flow, and correspondingly increase the melt filling speed, forming greater fluidity.
③ Mold structure. Such as the form of the gating system, gate location and size, cavity shape, exhaust system, mold temperature, wall thickness of plastic parts, and the presence of inserts, the number and location of inserts - these all directly affect the actual filling situation within the mold cavity and have a significant impact on plastic fluidity.
(3) Heat Sensitivity
The viscosity of some plastics changes with temperature during the molding process at lower temperatures, and the plastics remain relatively stable. However, when the temperature is maintained at a higher molding temperature for an extended period, or the cross-sectional area of the flow passage is too small, or the shear rate is too high, phenomena such as discoloration, degradation, and decomposition may occur due to the increased shear action. Plastics with this characteristic are called heat-sensitive plastics. Such as rigid PVC, polyvinyl chloride, polyformaldehyde, polyfluoroethylene, and fluoroplastics. Heat-sensitive plastics decompose and release gases during degradation, which corrode the mold and affect the appearance of plastic parts. Moreover, their physical and mechanical properties also deteriorate.
When heat-sensitive plastics undergo heat-induced decomposition or degradation during heating, various decomposition products will be generated, some of which are harmful to the human body. Molds and equipment must be kept clean. Impurities or dirt may cause localized overheating and lead to material degradation.
Agents and additives can prevent further decomposition. For example, adding heat stabilizers to rigid PVC can improve its decomposition effect.
When heat-sensitive plastics are molded under conditions where work is overheated or decomposition occurs, some measures must be taken during mold design. Heat stabilizers can be added to the materials, or suitable equipment (screw-type injection machines) can be used. Strict control must be maintained over molding temperature, barrel temperature, heating time, screw rotation speed and pressure; and measures such as preventing material retention and preventing equipment and molds from being contaminated should be taken.
(4) Moisture Sensitivity
The moisture sensitivity of plastics refers to the sensitivity to moisture decomposition at high temperature and high pressure, such as polycarbonate which is a typical moisture-sensitive plastic. Even if it contains a small amount of moisture, it will decompose at high temperature and high pressure. Therefore, moisture-sensitive plastics must be strictly controlled for moisture content before molding and must undergo drying treatment.
(5) Hygroscopicity
Hygroscopicity refers to the affinity of plastics for moisture. Based on this, plastics can be broadly divided into two categories: one is plastics with water absorption or adhesion properties, such as polyamide, polycarbonate, polyester, ABS, etc.; the other is plastics that neither absorb water nor adhere to moisture, such as polystyrene, polypropylene, and polyethylene.
For plastics with water absorption tendencies, if the moisture content before molding is not removed and exceeds a certain limit, then during the molding process, moisture will change into gas and cause the plastic to decompose, resulting in reduced flowability of the molded plastic, difficulty in molding, and deterioration of the surface quality and mechanical properties of the plastic parts. Therefore, to ensure the smooth progress and quality of molding, for plastics with large hygroscopicity and adhesion to moisture, moisture must be removed before molding and drying treatment must be performed. Attention must also be paid to appropriate settings of barrel temperature and external heating of the injection molding machine.
(6) Compatibility
Compatibility refers to the ability of two or more different types of plastics to not undergo phase separation in the molten state.
If two types of plastics are incompatible, phase separation will occur during the melting and molding process, resulting in delamination, peeling, and surface defects. The incompatibility of plastics is related to their molecular structure. Molecular structures that are similar or easily compatible with each other are compatible, such as high-pressure polyethylene and low-pressure polyethylene, polypropylene mixed with each other; molecular structures that are dissimilar are difficult to be compatible, such as the mixture of polyethylene and polystyrene. The compatibility of plastics is also commonly called miscibility. Understanding this characteristic of plastics can help determine the compatibility of similar or common raw materials, which is one of the important ways to improve plastic performance.