Discussion on the internal stress of plastic products
In the production field of plastic products, internal stress is a factor that cannot be ignored. So, is there actually a relationship between the internal stress in plastic products and the production process and raw materials? Meanwhile, how should the internal stress be eliminated, and what are the testing methods?
From a scientific perspective, the internal stress in plastic products is closely related to the production process. Parameters such as injection molding temperature, pressure, and cooling rate during the production process will all have an impact on the generation and distribution of internal stress. For example, if the injection molding temperature is too high, the unevenness of cooling when the plastic melt flows in the mold will increase, thus leading to the generation of internal stress. And if the cooling rate is too fast, the plastic surface cools and solidifies rapidly while the interior is still in a hot state, and this temperature difference will trigger internal stress. However, internal stress has no direct relation to the raw materials themselves. The raw materials mainly determine the basic properties of plastic products, such as strength and toughness, but will not directly cause the generation of internal stress.
Regarding the method of eliminating internal stress, an effective means is to heat the plastic product above its glass transition temperature for heat treatment. When the temperature reaches above the glass transition temperature, the mobility of the plastic molecular chains increases, allowing them to rearrange, thereby releasing the internally accumulated stress. This heat treatment method is like giving the plastic product a "relaxing massage", making its internal structure more stable.
As for the testing method of internal stress, although few people mention it, it can be judged by observing the changes in the mechanical strength of plastic products. If there is internal stress in a plastic product, its mechanical strength will decrease significantly. This is because internal stress will form tiny defects and cracks inside the product, which will weaken the product's load-bearing capacity and make it more prone to damage when subjected to force.
Concept analysis of melt index and stress index
In the plastics industry, the melt index is a very important concept. It refers to the mass of plastic flowing through a certain orifice per unit time under certain conditions. Simply put, the melt index reflects the fluidity of plastics under specific conditions.
The measurement method is to measure the rate at which the molten plastic is extruded through a fine - hole die of a specified length and diameter by means of timing measurement under conditions such as the specified temperature, load, and the position of the piston in the cylinder. This rate can be used to determine the uniformity of the polymer flow rate during the manufacturing process. Taking the melt index of PPR pipes as an example, MFI 230/2.16 0.35g/10min means that at a temperature of 230°C and with a load of 2.16 kg applied, 0.35 g of plastic will flow out every ten minutes. This is like observing the number of vehicles passing through in a specific passage within a certain period of time to understand the traffic efficiency of the vehicles.
In - depth analysis of linear low - density polyethylene (LLDPE)
Introduction
Linear low-density polyethylene (LLDPE) differs significantly in structure from general low-density polyethylene, and the key difference lies in the absence of long branches. The linearity of LLDPE is determined by its production and processing process, which is different from that of LDPE. Usually, LLDPE is produced by the copolymerization of ethylene and higher α - olefins, such as butene, hexene or octene, at lower temperatures and pressures. The LLDPE polymer produced by this copolymerization process has a narrower molecular weight distribution than general LDPE, and its linear structure endows it with unique rheological properties.
The melt flow characteristics of LLDPE are very suitable for the requirements of new processes. Especially in the film extrusion process, it can produce high-quality products. It has been widely used in all traditional markets of polyethylene. Its enhanced stretch, penetration, impact and tear resistance make it very suitable for making films. In addition, its excellent environmental stress crack resistance, low-temperature impact resistance and warpage resistance also make it very attractive in pipe and sheet extrusion and all molding applications. The latest application area of LLDPE is as a geomembrane, used as a liner for waste landfills and liquid waste ponds, playing an important role in preventing leakage.
Production and characteristics
The production of LLDPE usually starts with transition metal catalysts, especially those of the Ziegler or Phillips types. A new process based on cycloolefin metal derivative catalysts is also an option for LLDPE production. The actual polymerization reaction can be carried out in solution and gas-phase reactors. Generally, octene and ethylene are copolymerized in a solution-phase reactor, while butene and hexene are polymerized with ethylene in a gas-phase reactor. The LLDPE resin produced in the gas-phase reactor is in the form of particles, which can be sold as powder or further processed into pellets.
In recent years, a new generation of ultra-LLDPE based on hexene and octene has been launched by companies such as Mobil, Union Carbide, Novacor, and Dow Plastics. These materials have a large toughness limit and have shown new potential in the application of automatic bag removal. At the same time, very low density PE resins (with a density lower than 0.910 g/cc) have also emerged in recent years. The flexibility and softness of VLDPEs cannot be achieved by LLDPE.
The characteristics of resin are mainly reflected in the melt index and density. The melt index can reflect the average molecular weight of the resin and is mainly controlled by the reaction temperature. It should be noted that the average molecular weight has no relation to the molecular weight distribution (MWD), and the choice of catalyst will affect the MWD. The density is determined by the concentration of the comonomer in the polyethylene chain. The comonomer concentration controls the number of short branches (the length of which depends on the type of comonomer), thereby controlling the density of the resin. The higher the comonomer concentration, the lower the density of the resin.
Structurally, LLDPE differs from LDPE in the number and type of branches. High-pressure LDPE has long branches, while linear LDPE only has short branches. The structural difference between LLDPE and HDPE lies in the number of short branches. HDPE has fewer short branches and is therefore a material with a higher density. The physical properties of LLDPE are controlled by its molecular weight, MWD, and density.
The reason why LLDPE is superior to LDPE ultimately depends on its uses. Generally, in all applications, products made from LLDPE have stronger rigidity. Although according to the ATSM standard for low-density materials, the densities of both LLDPE and LDPE are between 0.91 - 0.925, LLDPE forms a higher crystalline structure because it has no long branches. This higher crystallinity enables LLDPE to produce products with higher rigidity and also raises its melting point by 10 - 15℃ compared to LDPE.
LLDPE has higher tensile strength, puncture resistance, tear resistance and elongation, and these properties make it particularly suitable for making films. If hexene or octene is used instead of butene as the comonomer, its impact resistance and tear resistance can even be further improved. For a given resin with the same melt index and density, the impact and tear properties of hexene and octene LLDPE resins can be increased by up to 300%. The longer side chains of hexene and octene resins act like "knot" molecules between the chains, improving the toughness of the compound.
Resins produced with cycloolefin metal derivative catalysts have unique properties. A narrower MWD improves the comonomer distribution, giving them better film transparency, sealability, and impact strength. These properties are similar to those of LLDPE produced with Ziegler catalysts. However, in terms of transparency, LLDPE has similar drawbacks to LDPE. The film has poor turbidity and gloss, mainly because its higher crystallinity results in surface roughness of the film. Nevertheless, the transparency of LLDPE resin can be improved by blending it with a small amount of LDPE.
Processing
Both LDPE and LLDPE have excellent rheological properties or melt flowability. However, LLDPE has lower shear sensitivity because it has a narrow molecular weight distribution and short branches. During the shearing process (such as extrusion), LLDPE can maintain a higher viscosity and is therefore more difficult to process than LDPE with the same melt index. In extrusion, the lower shear sensitivity of LLDPE allows the stress relaxation of polymer molecular chains to occur more quickly, and thus reduces the sensitivity of physical properties to changes in the blow-up ratio. In melt stretching, LLDPE generally has a lower viscosity at various strain rates, which means it does not exhibit strain hardening during stretching like LDPE. This is because the entanglement of molecular chains in LDPE leads to a surprisingly large increase in viscosity as the deformation rate increases, while the lack of long branches in LLDPE prevents the polymer from becoming entangled. This property is very important for film applications because LLDPE films can be more easily made into thinner films while maintaining high strength and toughness. The rheological properties of LLDPE can be summarized as "rigid under shear" and "soft under extension".
When LLDPE is used to replace LDPE, the film extrusion equipment and conditions must be modified. The high viscosity of LLDPE requires the extruder to have greater power and provide higher melt temperature and pressure. The die gap must be widened to avoid reducing the output due to high back pressure and melt fracture. The general die gap sizes of LDPE and LLDPE are 0.024 - 0.040 in. and 0.060 - 0.10 in. respectively.
The "soft when stretched" characteristic of LLDPE is a drawback during the film blowing process. The film bubble of LLDPE blown film is less stable than that of LDPE. A general single-lip air ring is sufficient for stabilizing LDPE, but the unique film bubble of LLDPE requires a more sophisticated double-lip air ring for stabilization. Cooling the inner film bubble with a double-lip air ring can increase the stability of the film bubble and improve the film production capacity at high productivity. In addition to better cooling of the film bubble, many film production plants use the method of blending with LDPE to enhance the performance of LLDPE. Theoretically, the extrusion of LLDPE can be completed on existing LDPE film equipment when the concentration of LLDPE in the LDPE blend reaches 50%. However, when processing 100% LLDPE or LDPE blends rich in LLDPE, the equipment of a general LDPE extruder must be improved. Depending on the lifespan of the extruder, it may be necessary to widen the die gap, improve the air ring, modify the screw design for better extrusion, and increase the motor power and torque if necessary. For injection molding applications, generally, there is no need to improve the equipment, but the processing conditions need to be optimized. Rotational molding requires LLDPE to be ground into uniform particles (35 mesh). The processing process includes filling the mold with powdered LLDPE, heating and rotating the mold biaxially to distribute the LLDPE evenly, and removing the product from the mold after cooling.
Application
LLDPE has penetrated most of the traditional polyethylene markets, including films, molding, pipes, and wires and cables. Anti - seepage geomembranes are a newly developed LLDPE market. As a large - scale extruded sheet, they are used for landfill liners and waste pond liners to prevent leakage or contamination of the surrounding areas.
In the film market, LLDPE has already occupied a large share. For example, it is used to produce bags, garbage bags, stretch wraps, industrial liners, towel liners, shopping bags, etc. These applications fully utilize the improved strength and toughness of LLDPE. Although the transparent film market, such as bread bags, has always been dominated by LDPE because of its better haze, the blend of LLDPE and LDPE can improve the strength, puncture resistance and stiffness of the film without significantly affecting its transparency.
Injection molding and rotational molding are the two largest molding applications of LLDPE. Due to its superior toughness, low temperature and impact strength, it is theoretically suitable for manufacturing products such as waste bins, toys and refrigeration appliances. In addition, the high environmental stress crack resistance of LLDPE makes it suitable for injection molding of molded lids in contact with oily foods, rotationally molded waste containers, fuel tanks and chemical tanks. In the coating layer of pipes, wires and cables, the application market of LLDPE is relatively small, but its high burst strength and environmental stress crack resistance can meet the requirements of these fields. Currently, 65% - 70% of LLDPE is used for making films.
A Comprehensive Analysis of Commonly Used Plastic Materials for Toys
Introduction: The Inseparable Bond between Toy Manufacturing and Plastic Materials
Hello, everyone! I'm engaged in toy procurement and fully understand the importance of plastic materials in the toy manufacturing field. Different plastic materials endow toys with diverse characteristics. There are huge differences from appearance to performance, from molding processes to usage safety. Next, I'll introduce in detail several plastic materials commonly used in toys at present, as well as relevant knowledge such as their uses and molding processes.
ABS: The All-rounder in the Toy Industry
Component characteristics
ABS is an acrylonitrile - butadiene - styrene polymer. Acrylonitrile (A) is like the "staunch guardian" of toys, endowing the products with relatively high hardness and significantly improving their wear resistance and heat resistance. Imagine a toy made of ABS that can maintain its shape in various environments and withstand a certain degree of friction and high - temperature tests. Butadiene (B) is like the "flexible envoy" of toys. It enhances the flexibility of the material, enabling the toy to maintain good toughness, elasticity, and impact resistance. Even if a child accidentally drops the toy, it is not easily damaged. Styrene (S) is like the "shape - changing magician" of toys. It enables the material to maintain good formability, including excellent fluidity and colorability, and can also maintain the rigidity of the material. Moreover, depending on different components, ABS has derived various specifications and grades to meet the needs of different toys.
Material advantages
ABS has a unique advantage, which is its excellent electroplating property. It has the best electroplating performance among all plastics. This enables toys to present a bright metallic appearance, enhancing the toys' aesthetics and attractiveness. Compared with GPPS, the impact strength of ABS is significantly improved, which means that toys made of ABS are more durable. The ABS raw material is light yellow and opaque, but the surface finish of the products is good, giving people a high - quality visual experience. At the same time, it has a small shrinkage rate and stable dimensions, which can ensure the precise size and shape of toys.
Chemical characteristics
However, ABS also has its "little quirks". It is not resistant to organic solvents. For example, when it is dissolved in ketones, aldehydes, esters and oxidized hydrocarbons, it will form an emulsion flow (i.e., ABS cement). This requires special attention during the use and maintenance of toys to avoid contact with these organic solvents. In addition, ABS also has good blending properties. Blending with PVC can improve toughness, flame resistance and anti-aging performance; blending with PC can improve impact strength and heat resistance.
Molding process
Before the molding process, ABS needs to be fully dried to make the moisture content <0.1%. the drying conditions are a temperature of 85℃ and a time of more than 3 hours. due to its good fluidity, it is prone to produce injection molding burrs. therefore, the injection pressure should be controlled between 70 - 100mpa and should not be too high. the barrel temperature should not exceed 250℃ either. the temperature of the front barrel is 160 - 210℃, the middle barrel is 170 - 190℃, and the rear barrel is 160 - 180℃. excessively high temperature will cause the decomposition of plastic components, resulting in a decrease in fluidity. the mold temperature is generally between 40 - 80℃. if high requirements are placed on the appearance of toys, the mold temperature also needs to be increased accordingly. the injection speed is mainly medium and low, and the injection force is 80 - 130mpa. finally, the internal stress of abs products also needs to be inspected, with the standard that the products do not crack after being immersed in kerosene for 2 minutes.
MBS: The Transparent and Flexible Sprite
MBS is also known as transparent ABS, which is a polymethyl methacrylate - butadiene - styrene copolymer. It is like a transparent elf, with advantages such as transparency, good toughness, resistance to acids and alkalis, good fluidity, easy molding and coloring, and dimensional stability. These characteristics make MBS very suitable for making toys that require a transparent appearance and a certain degree of toughness, such as transparent toy shells and display boxes. This allows children to see the internal structure of the toys while ensuring the durability of the toys.
SBS (K - resin): A small but powerful transparent and elastic material
SBS (K-resin) is a polymer of butadiene and styrene, such as KR01, KR03, etc. It is known for its transparency and good elasticity, just like a transparent elastic balloon, which is easy to form. In toy manufacturing, SBS is often used to make some parts that require elasticity and a transparent appearance, such as elastic balls, transparent elastic connectors, etc., adding more fun and functionality to toys.
PS material: A combination of hard plastic and unbreakable plastic
Type characteristics
PS materials include polystyrene (GPPS rigid plastic, HIPS modified polystyrene). GPPS has high transparency but is relatively brittle. When an appropriate amount (5 - 20%) of butadiene rubber is added to GPPS for modification, HIPS is obtained. It is opaque, milky white or slightly yellowish, and has good impact resistance, and is called "unbreakable plastic". HIPS and GPPS can also be mixed for injection molding as needed. The more the GPPS component, the better the surface gloss and fluidity of the product. For example, when HIPS:GPPS = 7:3 or 8:2, sufficient strength and surface quality can be maintained.
Molding process
GPPS has a wide molding temperature range, fast heating flow and curing speed, and a short molding cycle. On the premise of filling the mold cavity, the barrel temperature should be slightly lower, with the front barrel temperature at 200°C and the rear barrel at 160°C. Due to the good fluidity of GPPS, high injection molding pressure (70 - 130 PMa) is not required during molding. Too high pressure will instead increase the residual internal stress in the product, especially the plastic parts are prone to cracking after spraying. The fluidity of modified PS, namely HIPS, is slightly worse than that of GPPS. A relatively high injection speed is advisable to reduce the weld lines (water lines), but an excessively high speed may cause flash (burrs) or cracking during demolding. The mold temperature is generally between 30°C and 50°C. Polystyrene (GPPS) has low hygroscopicity, so it generally does not need to be dried before molding, while modified polystyrene (HIPS) needs to be dried at a temperature of 60°C - 80°C for 2 hours.
Polypropylene (PP): Characteristics and Processes of Crease-resistant Rubber
Material properties
Polypropylene (PP), also known as folding glue, belongs to crystalline plastics. It is translucent and lightweight (with a density of 0.91), and can even float on water, which makes toys made of PP feel very light in the hand. It has good fluidity and moldability, and its surface is shiny. However, there will be marks at the colored parts compared with PE. Its high molecular weight results in high tensile strength, high yield strength (fatigue resistance), and high chemical stability. It is insoluble in organic solvents, but this also makes it difficult for spraying, ironing, and bonding. In addition, PP has excellent wear resistance and good impact resistance at room temperature. However, it has a large molding shrinkage rate (1.6%), the dimensions are relatively unstable, and plastic parts are prone to deformation and shrinkage.
Molding process
Polypropylene has good fluidity. A relatively low injection pressure can fill the mold cavity. However, if the pressure is too high, flash is likely to occur, and if it is too low, the shrinkage will be severe. The injection pressure is generally between 80 - 90 MPa. The holding pressure is about 80% of the injection pressure, and a relatively long holding time is advisable for shrinkage compensation. It is suitable for rapid injection. To improve the poor exhaust, the exhaust groove should be slightly deeper, about 0.3 mm. Due to the high crystallinity of polypropylene, the temperature of the front barrel is between 200 - 240 °C, the middle barrel is between 170 - 220 °C, and the rear barrel is between 160 - 190 °C. In fact, lower temperatures are used to reduce flash and shrinkage. Because the material has a large shrinkage rate, the cooling time should be appropriately extended to accurately control the dimensions of the plastic parts. The mold temperature should be low (20 - 40 °C). If the mold temperature is too high, the crystallinity will be high, the intermolecular interaction will be strong, and the product will have good rigidity and gloss, but poor flexibility and transparency, and the shrinkage will also be obvious. The back pressure is preferably 0.1 MPa. In the dry powder coloring process, the back pressure should be appropriately increased to improve the mixing effect.
Polyvinyl chloride (PVC): A toy material with adjustable softness and hardness
Material characteristics
Polyvinyl chloride (PVC) is an amorphous plastic, and its raw material is transparent. By adding plasticizers, its softness and hardness can be adjusted within a wide range, which makes it widely used in toy manufacturing. PVC is flame-retardant and self-extinguishing, but it has poor thermal stability. It is soluble in cyclohexanone, tetrahydrofuran, dichloroethane, etc. Soft rubber thinner (containing cyclohexanone) is used for spray painting. PVC is mainly used for slush molding (and making dolls), and can be used to create various soft and cute toy images.
Molding process
Soft PVC has a relatively large shrinkage rate (1.0 - 2.5%), and the polar molecules of PVC are prone to absorb moisture. It needs to be dried before molding. The drying temperature is 85 - 95°C, and the drying time is 2 hours. During the molding process, long - term and repeated heating in the barrel will cause the decomposition of vinyl chloride monomers and HCL (i.e., degradation). Therefore, the dead corners in the mold cavity and the machine head should be cleaned regularly. Adding ABS to soft PVC can improve its toughness, hardness and mechanical strength. Since the molding temperature of PVC is close to its decomposition temperature, the barrel temperature should be strictly controlled. As low a molding temperature as possible should be used, and at the same time, the molding cycle should be shortened to reduce the residence time of the melt in the barrel. The temperature of the front barrel is 160 - 170°C, the middle barrel is 160 - 165°C, and the rear barrel is 140 - 150°C. The mold runner and gate should be as short, thick and thick - walled as possible to reduce pressure loss and fill the cavity as soon as possible. High - pressure and low - temperature injection is suitable for the injection pressure, and the back pressure is 0.5 - 1.5MPa. The wall thickness of PVC products should not be too thin and should be above 1.5mm; otherwise, it is difficult for the material flow to fill the cavity. The injection speed should not be too fast, so as to avoid severe friction when the molten material passes through the gate, which will increase the temperature and easily cause shrinkage marks. The mold temperature should be as low as possible (about 30 - 45°C) to shorten the molding cycle and prevent the plastic parts from deforming when demolding. If necessary, the plastic parts need to be shaped by a sizing die. To prevent the cold slug from blocking the gate or flowing into the mold cavity, a larger cold slug well should be designed to accumulate the cold slug.
In conclusion, different plastic materials each have their own advantages and disadvantages in toy manufacturing. Understanding their characteristics and molding processes can help us better select suitable materials and produce high-quality, safe and interesting toys.