Polyethylene injection molding process conditions and performance analysis of polyoxymethylene

Time：2023-01-05 08:47:59 / Popularity： / Source：

Polyethylene injection molding process conditions

01 Barrel temperature

Barrel temperature during PE injection molding is mainly based on density of PE and value of melt flow rate, secondly it is also related to type of injection machine and shape of product. Since PE is a crystalline polymer, crystal grains will absorb a certain amount of heat during melting, so barrel temperature should be about 10℃ higher than its melting point. Generally, the barrel temperature of LDPE can be controlled at 140-180℃, while that of HDPE can be controlled at 180-220℃. Minimum value is taken at rear end of barrel, and maximum value is taken at the front end.

02 Mold temperature

Mold temperature has a great influence on crystallization status of product. High mold temperature will slow cooling of melt, and crystallinity of product will be high, so strength will be high, but shrinkage rate will increase; mold temperature will be low, and melt cooling rate will be fast. Transparency and toughness are improved, but internal stress also increases, product is easy to warp and deform. Choice of mold temperature is related to density of PE. Normally, mold temperature of LDPE is 30-60℃, and that of HDPE is 50-80℃. Products with a wall thickness below 6mm should use a higher mold temperature, and products with a wall thickness above 6mm should use a lower mold temperature.

03 Injection pressure

Selection of injection pressure is mainly based on fluidity of melt, wall thickness and shape of product. Due to good fluidity of PE in molten state, a lower injection pressure can be used during processing, generally 60-80MP; for some products and molds with thin walls, long processes, complex shapes, and narrow gates, injection pressure should be properly increased to about 120MPa. Although increasing injection pressure can reduce shrinkage of the product, if it is too high, overflow will appear and internal stress of product will increase.

04 Holding pressure

About 30% to 60% of injection pressure.

05 Plasticizing pressure

Control it at 5-20MPa, if back pressure is too low, it will easily cause uneven product quality and dispersion.

06 Injection speed

Choose medium speed or slow speed, and high-speed injection is not suitable, because PE has a tendency to melt fracture during high-speed injection, but high injection speed is required for thin-walled packaging containers.

07 Screw speed

Torque of screw is low, allowing high screw speeds. As long as plasticizing process can be completed before end of cooling time.

08 Molding cycle

In addition to proper mold filling time and cooling time, there should also be sufficient pressure holding time to compensate for defects such as lack of material, bubbles, and dents caused by melt shrinkage. Holding time is determined by thickness of product and cross-sectional area of runner, generally between 10 and 40s. Injection molding process conditions of PE are shown in the table below.

PE injection molding process conditions

Process conditions		LDPE	HDPE
Cylinder temperature/℃	Conveyor section	140-160	150-160
	Compressed section	150-170	170-180
	Homogenization section	160-180	180-200
Nozzle temperature/℃		150-170	160-180
Mold temperature/℃		30-50	50-70
Injection pressure/MPa		60-100	70-100
Holding pressure/MPa		40-50	50-60
Screw speed/(r/min)		60-100	40-80
Molding cycle/s		40-140	40-120

Performance analysis of polyoxymethylene material

1. Physical properties of POM material

Polyoxymethylene is a high polymer with relatively high crystallinity. Its comprehensive mechanical properties are much better than other plastics. For example, it has high strength and high hardness, its elastic modulus is larger than that of ordinary plastics. Many, it has specific strength and specific stiffness very close to metal materials; POM material has relatively high impact strength and fatigue strength, has good wear resistance.

2. POM material structure

According to its synthesis method, polyoxymethylene resin has two types: homopolyoxymethylene and copolymerized polyoxymethylene. Homopolyoxymethylene is prepared by homopolymerization method, and copolymerization method is used to make polyoxymethylene. Although their molecular structures are all linear structures, they are slightly different. Due to different molecular structure, performance is different. Compared with the two materials, homopolyoxymethylene has higher density, higher melting point, and better strength, but its thermal stability, acid and alkali resistance are worse than that of copolymerized polyoxymethylene; other than the first time, copolymerized polyoxymethylene has much better thermal stability. And it is easy to shape, so development of polyoxymethylene copolymer is faster than that of homopolyoxymethylene.

3. Thermal and chemical properties of polyoxymethylene materials

Deformation of polyoxymethylene needs to reach a higher temperature, and its maximum allowable continuous use temperature is about 100℃. Thermal stability of polyoxymethylene material is relatively poor, it is easy to decompose under influence of its temperature when heated, it will also age under conditions of light and oxygen. Temperature range that can melt polyoxymethylene material is relatively small. Once melts, its speed will be very fast and solidification will be very fast. Its products are prone to surface defects such as hair spots and wrinkles during casting, so strict control must be taken in terms of equipment parameters and mold temperature control during molding to avoid them. Polyoxymethylene is easily soluble in organic solvents, and it is resistant to dilute acids, but not to strong acids.

Main performance indicators of polyoxymethylene

Density/kg*dm-3	1.41	Impact strength an/kJ*m-2	202
Shrinkage rate/s	1.5~3.0	Tensile modulus of elasticity Et/MPa	2.5*1000
Water absorption (24h)	0.12~0.15	Flexural modulus Qf/MPa	104
Melting pointt/℃	180~~200	Tensile strength Qt/MPa	69
Calculate Shrinkage (%)	1.5~3.0	Specific volume v/dm3*kg-1	0.71
Hardness/HBW	11.2 M78	Breakdown strength E/kv*mm-1	18.6

Required wall thicknesses for acetal materials are listed in Table 2 below.

Minimum wall thickness and recommended wall thickness of POM

Minimum wall thickness	Recommended wall thickness for small plastic parts	Recommended wall thickness for medium-sized plastic parts	Recommended wall thickness for large plastic parts
0.8	1.4	2.3	3-4.5

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