Influencing factors of plastic shrinkage

Time:2022-08-28 11:49:47 / Popularity: / Source:

01. Overview of plastic shrinkage

Shrinkage rate of plastic refers to percentage of difference between size of plastic part at molding temperature and size of plastic part after it is taken out of mold and cooled to room temperature. It reflects extent to which plastic part is reduced in size after it is removed from mold and cooled. Factors that affect plastic shrinkage rate include plastic varieties, molding conditions, mold structure, etc., and shrinkage rates of different plastics are different. In addition, shrinkage rate of plastic is also closely related to shape of plastic part, complexity of internal structure, and whether there are inserts.
Factors that affect plastic shrinkage rate are as follows:

1 Influence of raw materials

(1) Variety of raw materials
Different types of plastic raw materials have different shrinkage rates, see Table 1 for details.
Table 1 Shrinkage rate of different plastic raw materials
plastic shrinkage 
plastic shrinkage 
It can be seen from Table 1 that shrinkage rate of thermoplastic energy plastics with 40% reinforced PPS is the lowest, and shrinkage rate of thermosetting plastics with epoxy resin is the lowest, and shrinkage rate of both plastics is 0.2%. The largest shrinkage rate is fluorine plastics, which can reach up to about 6%; the second largest shrinkage rate is low-density polyethylene, with a maximum shrinkage rate of 5%.
(2) Crystallinity of raw materials
Same raw material has different crystallinity, and its shrinkage rate is different. The smaller crystallinity, the smaller shrinkage rate. Among many influencing factors, crystallinity of resin has the greatest influence on shrinkage rate.
(3) Molecular weight of raw materials
The larger molecular weight of same raw material, the smaller shrinkage rate. Resins with same molecular weight have good fluidity and low molding shrinkage.
(4) Modification of raw materials
After adding other resins, elastomers and fillers to resin for modification, shrinkage rate decreases to varying degrees.
Shrinkage rate of resin is different, which greatly affects molding accuracy of its products during melting process. In order to process high-precision plastic products, it is necessary to use low-shrinkage resins. Taking PP resin as an example, its shrinkage rate is 1.8% to 2.5%, and the smaller molecular weight, the greater shrinkage rate, so it is difficult for PP to make high-precision products. For this reason, major companies in the world are actively researching to reduce shrinkage rate of resin itself under condition of ensuring resin properties.

2 Influence of molding process on shrinkage rate of plastic products

(1) Molding temperature remains unchanged, injection pressure increases, and shrinkage rate decreases;
(2) Keep pressure increasing and shrinkage rate decreasing;
(3) Melt temperature increases, and shrinkage rate decreases;
(4) Mold temperature is high, and shrinkage rate increases;
(5) Pressure holding time is long and shrinkage rate is reduced, but shrinkage rate is not affected after gate is closed;
(6) Cooling time in mold is long, and shrinkage rate is reduced;
(7) Injection speed is high, shrinkage rate tends to increase slightly, and impact is small;
(8) Molding shrinkage is large, and post-shrinkage is small; post-shrinkage is large in the first two days and stable in about a week.

3 Influence of structure on product shrinkage

(1) Thick-walled plastic parts have a larger shrinkage rate than thin-walled plastic parts;
(2) Shrinkage rate of plastic parts with inserts is smaller than that without inserts;
(3) Shrinkage rate of plastic parts with complex shapes is smaller than that of simple shapes;
(4) Shrinkage rate of dimension in length direction of plastic part is smaller than that in thickness direction;
(5) Inner hole shrinkage rate is large, and shape shrinkage rate is small.

4 Influence of mold structure on shrinkage rate of plastic products

(1) Gate size is large and shrinkage rate is reduced;
(2) Shrinkage rate in direction perpendicular to gate decreases, and shrinkage rate in direction parallel to gate increases;
(3) Shrinkage rate far from gate is smaller than that near gate;
(4) Shrinkage rate of plastic part with mold restrictions is small, and shrinkage rate of unrestricted plastic part is large.

02. Modification method to reduce plastic shrinkage

Of course, the most effective way to reduce plastic shrinkage is various modification technologies, which are described below.

1 Various types of fiber filling

Fibers include various types of inorganic fibers and organic fibers. Taking glass fiber as an example, adding different contents of glass fiber to PP resin, shrinkage rate is shown in Table 2.
Table 2 Shrinkage rate of glass fiber reinforced PP with different contents
Fiber content (%) 0 5 10 15 20 25 30 35
Shrinkage(%) 1.8 1.3 1.3 1.1 0.9 0.7 0.6 0.5
Long glass fiber reinforced plastics developed in recent years have certain advantages in shrinkage rate, their longitudinal and transverse shrinkage rates are small and consistent.

2. Inorganic filler filling

Inorganic raw materials such as talcum powder, calcium carbonate, barium sulfate, mica powder, wollastonite and montmorillonite, etc., different filler varieties, properties, particle size and surface treatment degree all affect modification effect of resin shrinkage.
(1) Shape of filler
Different types of inorganic fillers have different shapes and have different effects on shrinkage rate. Specific effect is in the order of flakes > needles > particles > spherical. For example, flakes of montmorillonite and mica reduce shrinkage rate of composite materials. .
For example, effect of adding same content of various fillers to PP on shrinkage of PP is shown in Table 3.
Table 3 Effects of different types of fillers on PP shrinkage
Filler content (%) 0 5 10 15 20 25 30 35 40
Calcium carbonate 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.0 0.8
Talcum powder 1.8 1.6 1.5 1.4 1.3 1.2 1.0 0.9 0.7
Mica powder 1.8 1.6 1.4 1.3 1.2 1.1 0.9 0.7 0.6
For another example, adding 20% fillers of different shapes to PP (HHP10), shrinkage rate of composite material is shown in Table 4.
Table 4 Shrinkage rate of different shapes of filler composite PP (20% filler + 8% POE)
Filler PP (HHP10) Ball barium sulfate Granulated calcium carbonate Needle wollastonite Flakes of talc Sheet mica
Shrinkage(%) 1.93 1.532 1.128 1.045 1.035 1.010
(2) Particle size of filler
Particle size of same filler is different, and effect on shrinkage rate is also different. The smaller particle size, the greater effect on shrinkage rate. Taking talc-filled PP (20% talc + 8% POE) as an example, effects of different particle sizes of talc on shrinkage are shown in Table 5.
Table 5 Effects of different particle sizes of talc on shrinkage of PP composites
Particle size of talc 1250 2000 3000 5000
PP shrinkage rate (%) 1.05 1.00 0.90 0.80
(3) Handling of fillers
For same filler, whether surface treatment is performed when compounding with resin has different effects on shrinkage rate. Surface-treated fillers have a greater effect on reducing shrinkage. For example, take 10% talc filling ABS as an example, specific effect is shown in Table 6.
Table 6 Effect of surface treatment on shrinkage of talc/ABS composites
Composition Shrinkage(%) Tensile strength (MPa) Elongation at break (%) Notched impact strength (kJ/m2)
ABS 0.56 48 3.55 19
ABS+talc powder 0.48 43 2.2 15
ABS + aluminate treated talc 0.42 47 2.95 21
(4) Amount of filler
Different amount of same filler has different effects on shrinkage rate. The greater amount of filler added, the greater decrease in shrinkage rate. Taking PP filling as an example, effect of different filling amounts of talc on shrinkage is shown in Table 7.
Table 7 Effects of different contents of 1250 mesh talc powder on shrinkage rate of PP composites
Filling amount (%) 0 10% 20% 30% 50%
Talcum powder 1.95 1.35 1.10 0.8 0.75

3. Reduce crystal modification

(1) Add crystallization reducing aids
Commonly used additives for reducing crystallization are small molecular compounds, which damage regularity of macromolecules and hinder movement of macromolecules in molten state after adding, so as to achieve purpose of reducing crystallization. For example, an organic anti-crystallizing agent A provided on the market, appearance is yellow-white powder, melting point is about 60 ℃, and 0.5% to 1% can be added to PP. Another example is a commercially available inorganic crystallizer B, which is white powder in appearance, pH 8, and whiteness 95%. It is sufficient to add 0.5% to 1% to PP.
(2) Add other resins
If a small amount of LDPE and HDPE is blended in PP, crystallization in PP processing can be destroyed, and amorphous resins such as PS, ABS, PMMA, PC can also be added.
A specific example is adding other resins to UP resin, which can greatly reduce shrinkage rate of product:
First generation: adding PS and PE, shrinkage rate is reduced to 0.2% to 0.4%.
Second generation: adding PMMA, shrinkage rate is reduced to 0.05%.
Third generation: adding PVAC, shrinkage rate is reduced to 0.03%.
Taking addition of PE resin into PP as an example, effects of different contents of PE on shrinkage rate of PP are shown in Table 8.
Table 8 Effects of different contents of PE on shrinkage of PP
PE content (%) 0 5 10 15 20 25 30 35
LDPE shrinkage (%) 1.8 1.6 1.55 1.50 1.45 1.40 1.35 1.30
HDPE shrinkage (%) 1.8 1.5 1.45 1.35 1.25 1.20 1.10 1.05
It can be seen from Table 8 that effect of adding other resins on shrinkage is much smaller than that of fiber filling and inorganic filling.

4 Blended elastomers

Elastomers that can be added include POE, EPDM, SBS, etc. Effects of various types of elastomers on plastic shrinkage are similar, see Table 9 for details.
Table 9 Effect of different content of elastomer on PP shrinkage
Elastomer (%) 0 5 10 15 20 25 30
POE 1.8 1.6 1.45 1.40 1.25 1.20 1.10
EPDM 1.8 1.6 1.50 1.45 1.30 1.25 1.20
SBS 1.8 1.6 1.55 1.50 1.40 1.35 1.30
As can be seen from Table 9, when elastomer content is less than 5%, effects of various types of elastomers on shrinkage rate of PP are basically same. Only after elastomer content exceeds 5%, degree of influence of different elastomer varieties is different. Order of influence on shrinkage rate is POE>EPDM>SBS, which is consistent with toughening effect of elastomers on PP.
In PP, inorganic fillers and elastomers are mixed and added, and shrinkage rate reduction effect is better. When talc content is 20%, effects of different contents of POE on shrinkage of PP are shown in Table 10.
Table 10 Effect of different POE content on shrinkage of PP filled with 20% talc
POE content (%) 0 2 4 6 8 10
Shrinkage(%) 1.20 1.18 1.14 1.10 1.05 1.00

5 Chemical graft modification

After PP grafting, crystallinity can be reduced, so as to achieve purpose of reducing shrinkage.

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