What are differences between LDPE and HDPE injection molding processes? ?

Time:2023-03-21 08:56:36 / Popularity: / Source:

LDP low density polyethylene

LDPE plastic applications: bowls, cabinets, pipe connectors
Injection molding process conditions:
1. Drying: generally not required
2. Cylinder temperature: nozzle 140~190℃, front section 140~200℃, middle section 135~195℃, rear section 130~180℃
3. Mold temperature: 20~40C. In order to achieve uniform cooling and economical heat removal, it is recommended that diameter of cooling cavity be at least 8mm, and distance from cooling cavity to surface of mold should not exceed 1.5 times diameter of cooling cavity.
4. Injection pressure: up to 1500bar.
5. Holding pressure: up to 750bar.
6. Injection speed: It is recommended to use fast injection speed.
7. Runners and gates: Various types of runners and gates can be used. PE-LD is especially suitable for using hot runner moulds.
8. Chemical and physical properties: Density of commercial PE-LD materials is 0.91~0.94 g/cm3. PE-LD is permeable to gases and water vapor.
9. High coefficient of thermal expansion of PE-LD is not suitable for processing long-term products. If density of PE-LD is between 0.91~0.925 g/cm3, then its shrinkage rate is between 2%~5%; if density is between 0.926~0.94 g/cm3, then its shrinkage rate is between 1.5%~4 %. Current actual shrinkage rate also depends on injection molding process parameters. PE-LD is resistant to many solvents at room temperature, but aromatic and chlorinated hydrocarbon solvents can make it swell. Similar to PE-HD, PE-LD is prone to environmental stress cracking.
HDPE injection molding processes 

HDPE high density polyethylene

Typical application range: Refrigerator containers, storage containers, household kitchen utensils, sealing lids, etc.
Injection molding process conditions:
1. Drying: No drying is required if stored properly.
2. Melting temperature: 220~260C. For materials with larger molecules, recommended melting temperature range is between 200 and 250C.
3. Mold temperature: 50~95C. Plastic parts with a wall thickness below 6mm should use a higher mold temperature, and plastic parts with a wall thickness above 6mm should use a lower mold temperature. Cooling temperature of plastic parts should be uniform to reduce difference in shrinkage. For optimal cycle times, cooling channel diameter should be no less than 8mm and should be within 1.3d of mold surface (where "d" is cooling channel diameter).
4. Injection pressure: 700~1050bar.
5. Injection speed: It is recommended to use high-speed injection.
6. Runner and gate: Diameter of runner is between 4 and 7.5mm, and length of runner should be as short as possible. Various types of gates can be used, and gate length should not exceed 0.75mm. Especially suitable for use with hot runner moulds.
Chemical and Physical Properties:
High crystallinity of PE-HD leads to its high density, tensile strength, high temperature distortion temperature, viscosity and chemical stability.
PE-HD has stronger penetration resistance than PE-LD. PE-HD has lower impact strength. Properties of PH-HD are mainly controlled by density and molecular weight distribution. Molecular weight distribution of PE-HD suitable for injection molding is very narrow. For density of 0.91~0.925g/cm3, we call it the first type of PE-HD; for density of 0.926~0.94g/cm3, we call it second type of PE-HD; for density of 0.94~0.965g/cm3 , called third type PE-HD.
Flow characteristics of this material are very good with an MFR between 0.1 and 28. The higher molecular weight, the worse flow characteristics of PH-LD, but better impact strength.
PE-LD is a semi-crystalline material with high shrinkage after molding, between 1.5% and 4%.
PE-HD is prone to environmental stress cracking. Cracking can be mitigated by using materials with very low flow characteristics to reduce internal stresses. PE-HD is easy to dissolve in hydrocarbon solvents when temperature is higher than 60C, but its resistance to dissolution is better than that of PE-LD.
HDPE injection molding processes 

Low-density polyethylene (LDPE) performance, processing, advantages and disadvantages.

1. Introduction to LDPE

Low-density polyethylene (LDPE), also known as high-pressure polyethylene, is a plastic material that is suitable for various molding processes of thermoplastic molding and has good molding processability.

2. Performance of LDPE

After LDPE is heated, as temperature rises, crystallization gradually decreases, crystallization part disappears completely, polyethylene melts, and temperature at this time is melting point. Density of polyethylene increases, and melting point increases with increase of crystallinity, so melting point of polyethylene with different density is different. Melting point of LLDPE is 120~125C, which is between HP-LDPE and HDPE.
Melting point of comonomer LLDPE changes with increase or decrease of carbon atoms of comonomer, and melting point increases as number of carbon atoms increases. Since melting point of LLDPE is higher than that of HP-LDPE, molded product can be demolded at a higher temperature, which is fast and clean. LDPE film, high heat sealing strength. Fluidity of LDPE when temperature rises and change when load increases are mainly affected by molecular weight.
Since it is easier to measure melt flow rate of polyethylene than molecular weight, molecular weight characteristics of polyethylene are usually expressed by melt index (MI) or melt flow index (MFI). In molten state, melt viscosity of LDPE is a function of molecular weight, which increases with increase of molecular weight. Molecular weight is same, melt viscosity decreases as temperature rises. LDPE density has different toughness at room temperature.
Low-temperature LDPE naturally has good toughness and low embrittlement temperature, which is related to molecular weight. Molecular weight of LDPE increases and temperature decreases, value is -140C. Under same molecular weight, melt viscosity of LLDPE and HDPE with linear structure is higher than that of LDPE with nonlinear structure. When melt index is same, melt viscosity of LDPE is obviously lower than that of LLDPE and HDPE. Melt fluidity of former is obviously better than that of latter two, screw load is small, and calorific value is also small.

3. Processing method

LDPE grades can meet requirements of most thermoplastic forming processing technologies.
Including: film blowing, film casting, extrusion lamination, wire and cable lamination, injection molding, blow molding.

4. Main application

LDPE is mainly used for film products, such as agricultural film, ground covering film, agricultural film and vegetable greenhouse film, etc.; packaging film such as packaging for candy, vegetables, and frozen foods; blown film for liquid packaging (milk, soy sauce, juice, tofu, soy milk); heavy packaging bags, shrink packaging films, elastic films, lining films; construction films, general industrial packaging films and food bags, etc.
LDPE is also used in injection molding products, such as small containers, lids, daily products, plastic flowers, injection-stretching-blow molding containers, medical appliances, pharmaceuticals and food packaging materials. Extruded pipes, plates, wire and cable coating, profiled materials, thermoforming and other products; blow-molded hollow-formed products, such as food containers such as dairy products and jams, medicines, cosmetics, chemical product containers, tanks and other calcium plastic plates, foam plastics, etc. Rotationally formed rotomolded products are mainly used for large containers and storage tanks.

5. Advantages and disadvantages of LDPE

(1) Advantages
Low-density polyethylene is the lightest variety among polyethylene resins. Compared with high-density polyethylene, its crystallinity (55%~65%) and softening point (90~100C) are lower; it has good flexibility, extensibility, transparency, cold resistance and processability; its chemical stability Better, it can resist acid, alkali and saline solution; it has good electrical insulation and gas permeability; it has low water absorption; it is easy to burn. It is soft in nature, has good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance (can withstand -70C).
(2) Disadvantages
Disadvantage is that its mechanical strength, moisture barrier, gas barrier and solvent resistance are poor. Structure is not regular enough, crystallinity (55%-65%) is low, and crystallization melting point (108-126C) is also low. British mechanical strength is lower than that of high-density polyethylene, anti-seepage coefficient, heat resistance and sunlight aging resistance are poor. It is easy to age and decompose under sunlight or high temperature and change color, resulting in performance degradation. Therefore, low-density polyethylene is added when making plastic coils. Antioxidants and UV absorbers are added to improve its deficiencies.

6. Identification method

Sensory identification: soft to touch: white and transparent, but transparency is average.
Burning identification: upper part of burning flame is yellowish and the lower part is blue; when burning, it is smokeless, has smell of paraffin, melts and drips, and is easy to draw.

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