By Andy from Baiyear factory
Upated November 2, 2022
Here is the news center of Baiyear’s injection molding industry. Next, Baiyear will divide the injection molding process into several articles to introduce the analysis of the raw materials of the injection molding process, because there are too many content. Next is the third article.
(5). BS (K material)
1. Performance of BS
BS is a butadiene-styrene copolymer, which has certain toughness and elasticity, low hardness (softer) and good transparency. The specific gravity of BS material is 1.01f\cm3 (similar to water). The material is easy to color, has good fluidity, and is easy to shape and process.
2.The process characteristics of BS
The processing temperature range of BS is generally 190-225 °C, and the mold temperature is preferably 30-50 °C. The material should be dry before processing, because of its better fluidity, the injection pressure and injection speed can be lower.
(6). PMMA (Acrylic)
1. Performance of PMMA
PMMA is an amorphous polymer, commonly known as plexiglass. Excellent transparency, good heat resistance (heat deformation temperature of 98 ° C), and good impact resistance. Its products have medium mechanical strength, low surface hardness, and are easily scratched by hard objects and leave traces, which are similar to PS. It is not easy to be brittle and cracked, and the specific gravity is 1.18g/cm3.
PMMA has excellent optical properties and weather resistance properties. The penetration of white light is as high as 92%. PMMA products have very low birefringence and are especially suitable for making video discs. PMMA has room temperature creep properties. Stress cracking can occur with increasing load and time.
2. Process characteristics of PMMA
The processing requirements of PMMA are strict, and it is very sensitive to moisture and temperature. It should be fully dried before processing (recommended drying conditions are 90 ° C, 2~4 hours). °C) and molding under pressure, the mold temperature is preferably 65-80 °C.
The stability of PMMA is not very good, and it will be degraded by high temperature or prolonged residence time at higher temperature. The screw speed should not be too large (about 60%), and the thicker PMMA parts are prone to “voids”, which need to be processed by using a large gate, “low material temperature, high mold temperature, slow speed” injection method.
3.Typical application range: automotive industry (signal equipment, instrument panels, etc.), pharmaceutical industry (blood storage containers, etc.), industrial applications (video discs, light diffusers), consumer goods (drink cups, stationery, etc.).
(7) PE (polyethylene)
1. The performance of PE
PE is the plastic with the largest output among plastics. It is characterized by soft quality, non-toxicity, low price, convenient processing, good chemical resistance, not easy to corrode, and difficult to print. PE is a typical crystalline polymer.
It has many types, commonly used are LDPE (low density polyethylene) and HDPE (high density polyethylene), which are translucent plastics with low strength and a specific gravity of 0.94g/cm3 (smaller than water); very low density LLDPE resins (The density is lower than 0.910g/cc, and the density of LLDPE and LDPE is between 0.91-0.925).
LDPE is softer, (commonly known as soft rubber) HDPE is commonly known as hard soft rubber. It is harder than LDPE and is a semi-crystalline material. Environmental stress cracking occurs. The internal stress can be reduced by using materials with very low flow characteristics, thereby reducing the cracking phenomenon. It is easy to dissolve in hydrocarbon solvents when the temperature is higher than 60 °C, but its resistance to dissolution is better than that of LDPE.
The high crystallinity of HDPE results in its high density, tensile strength, high temperature distortion temperature, viscosity and chemical stability. Stronger penetration resistance than LDPE. PE-HD has lower impact strength. Properties are mainly controlled by density and molecular weight distribution.
HDPE suitable for injection molding has a narrow molecular weight distribution. For the density of 0.91~0.925g/cm3, we call it the first type of PE-HD; for the density of 0.926~0.94g/cm3, it is called the second type of HDPE; for the density of 0.94~0.965g/cm3, it is called the second type of HDPE It is the third type HDPE.
The flow characteristics of this material are very good, with an MFR between 0.1 and 28. The higher the molecular weight, the poorer the flow characteristics of LDPE, but the better the impact strength. HDPE is prone to environmental stress cracking. Cracking can be mitigated by using materials with very low flow properties to reduce internal stress. HDPE dissolves easily in hydrocarbon solvents when the temperature is higher than 60C, but its resistance to dissolution is better than that of LDPE.
LDPE is a semi-crystalline material with high shrinkage after molding, between 1.5% and 4%.
LLDPE (Linear Low Density Polyethylene) has higher tensile, penetration, impact and tear resistance properties that make LLDPE suitable for films. Its excellent resistance to environmental stress cracking, low temperature impact resistance and warpage resistance make LLDPE attractive for pipe, sheet extrusion and all molding applications. The latest application of LLDPE is as a mulch for landfills and linings for waste ponds.
2. Process characteristics of PE
The most notable feature of PE parts is that the molding shrinkage rate is large, which is prone to shrinkage and deformation. PE material has low water absorption, so it does not need to be dried. PE has a wide processing temperature range and is not easy to decompose (decomposition temperature is 320°C). If the pressure is large, the density of the part will be high and the shrinkage rate will be small.
The fluidity of PE is medium, the processing conditions should be strictly controlled, and the mold temperature should be kept constant (40-60℃). The degree of crystallization of PE is related to the molding process conditions. It has a higher freezing temperature and a lower mold temperature, and the crystallinity is lower. During the crystallization process, due to the anisotropy of shrinkage, the internal stress is concentrated, and the PE parts are prone to deformation and cracking.
The product is placed in a water bath in hot water at 80°C, which can relax the pressure to a certain extent. During the molding process, the material temperature and mold temperature should be higher, and the injection pressure should be lower under the premise of ensuring the quality of the parts. The cooling of the mold is particularly required to be rapid and uniform, and the product will be hot when demoulding.
HDPE drying: drying is not required if stored properly. Melting temperature 220~260C. For materials with larger molecules, the recommended melting temperature range is between 200 and 250C.
Mold temperature: 50~95C. Plastic parts with wall thickness below 6mm should use higher mold temperature, and plastic parts with wall thickness above 6mm should use lower mold temperature. The cooling temperature of the plastic part should be uniform to reduce the difference in shrinkage. For optimal machining cycle time, the cooling channel diameter should be no less than 8mm and the distance from the mold surface should be within 1.3d (where “d” is the diameter of the cooling channel).
Injection pressure: 700~1050bar. Injection Speed: High-speed injection is recommended. Runners and gates: The runner diameter is between 4 and 7.5mm, and the runner length should be as short as possible. Various types of gates can be used, and the gate length should not exceed 0.75mm. Especially suitable for the use of hot runner molds.
The “soft-on-stretch” property of LLDPE is a disadvantage in the blown film process, and the blown film bubble of LLDPE is not as stable as that of LDPE. The die gap must be widened to avoid reduced throughput due to high back pressure and melt fracture. The general die gap dimensions of LDPE and LLDPE are 0.024-0.040 in and 0.060-0.10 in, respectively.
3. Typical application range:
LLDPE has penetrated most traditional markets for polyethylene, including film, molding, pipe, and wire and cable. Anti-leakage mulch is a newly developed LLDPE market. Mulch, a large extruded sheet used as landfill and waste pool liners to prevent seepage or contamination of surrounding areas.
Examples include the production of bags, garbage bags, elastic packaging, industrial liners, towel liners and shopping bags, all of which take advantage of this resin’s improved strength and toughness. Clear films, such as bread bags, have been dominated by LDPE because of its better haze.
However, blends of LLDPE and LDPE will improve strength. Penetration resistance and stiffness of LDPE films without significantly affecting film clarity.
HDPE application range: refrigerator containers, storage containers, household kitchenware, sealing covers, etc.
To be continued, if you want to know more, please feel free to contact us. Baiyear is a large-scale comprehensive factory integrating plastic mold manufacturing, injection molding and sheet metal processing. Or you can continue to pay attention to the news center of our official website: www.baidasy.com , we will continue to update the knowledge news related to the injection molding processing industry.
Contact:Andy Yang
What’s app : +86 13968705428
Email: Andy@baidasy.com
Post time: Nov-29-2022