Parameters simply | What exactly is melt viscosity

Time：2022-11-03 08:41:22 / Popularity： / Source：

Melt viscosity

Melt viscosity is a characteristic that reflects difficulty of plastic melt flow and is a measure of melt flow resistance. The higher viscosity, the greater flow resistance and the more difficult flow. Molecular shape of polyethylene and its molecular weight distribution are different, its melt viscosity will have different performances.
When melting temperature or applied pressure causes speed to change, it will affect melt viscosity of addition polymer or polycondensation polymer. Figure 1-1 and Figure 1-2 show relationship between melt viscosity, temperature and pressure of various resins.

As can be seen from Figure 1-1, resins such as cellulose acetate, polystyrene, polymethyl methacrylate, nylon and polycarbonate have a sharp decrease in viscosity with increase of temperature, so slope of curve is large, while polyethylene and polyoxymethylene resins are not sensitive to temperature.

Some plastic viscosity is affected by temperature

No	Plastic	Sensitivity to temperature
1	CA	Highest
2	PS	High
3	PP	Slightly lower
4	PE	Normal
5	POM	Bad

Figure 1-2 and Figure 1-3 show change of melt viscosity of each resin when speed (called shear rate) changes due to pressure during measurement.
It can be seen from Figure 1-2 that melt viscosity of polystyrene and various polyethylene resins shows a sharp decline trend with increase of speed, while melt viscosity of nylon, polyoxymethylene, polymethyl methacrylate and polycarbonate is not sensitive to speed.
Thermoplastic polyester and polypropylene are compared in Figure 1-3. For example, in low speed region of 100 cm/s, viscosity of thermoplastic polyester is almost unchanged when shear speed is changed, while viscosity of polypropylene resin decreases sharply with increase of shear speed.

Effect of Pressure on Plastic Melt Viscosity (Example)

No	Name	Melting temperature (℃)	Pressure change range/Mpa	Viscosity increase factor
1	PS	131-165	0-126.6	134
2	ABS	130-160	14-175.8	100
3	PE	105-136	0-126.6	14
4	HDPE	105-137	14-175.8	4.1
5	LDPE	105-125	14-175.8	5.6
6	MDPE	110-120	14-175.8	6.8
7	PP	160-176	14-175.8	7.3

Thermoplastic resins have such a tendency. If its melt viscosity is sensitive to temperature, it will be insensitive to shear rate; conversely, what is sensitive to shear rate is insensitive to temperature. The only exception is polystyrene, whose melt viscosity is sensitive not only to temperature but also to shear rate.

Sensitivity of plastic melt viscosity to shear rate

No	Plastic	Sensitivity
1	ABS (most sensitive)	Sensitivity to shear decreases sequentially
2	PC
3	PMMA
4	PVC
5	PA
6	PP
7	PS
8	LDPE (least sensitive)

Polystyrene (PS) is the easiest resin to mold and process because its resin viscosity can be reduced simply by increasing melting temperature, or by increasing speed at which molten resin is injected into mold (injection speed).
For resins containing functional groups like nylon, optimal molding temperature (actual injection temperature) is near melting temperature, and its temperature adjustable range is small; due to addition polymer composed of active atomic groups, its optimal molding temperature is much higher, temperature can be adjusted in a wide range, and melt viscosity can be reduced by increasing injection speed. Properties of addition polymer resin can be better exerted by injection molding machine with multi-stage injection speed.

How common plastics improve flow properties

No	Plastic	Common name	How to improve
1	PE	Polyethylene	Increase screw speed
2	PP	Polypropylene	Increase screw speed
3	PA	Nylon (polyamide)	Increase temperature
4	POM	POM	Increase screw speed
5	PC	Polycarbonate	Increase temperature
6	PS	Polystyrene	Both will work
7	ABS	-	Increase temperature
8	PVC	PVC	Increase temperature
9	PMMA	PMMA	Increase temperature

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