For previous reading, please refer to What are factors that affect plastic shrinkage?.
Modification methods to reduce plastic shrinkage
Of course, the most effective way to reduce plastic shrinkage is various modification technologies, which are detailed as follows.

 

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 of glass fiber reinforced PP with different contents

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.

Inorganic raw materials include talc powder, calcium carbonate, barium sulfate, mica powder, wollastonite and montmorillonite. Different filler types, properties, particle sizes and surface treatment levels all affect modification effect on resin shrinkage rate.
(1)Shape of filler
Different types of inorganic fillers have different shapes and have different effects on shrinkage. Specific order of influence is flake > needle > granular > spherical. For example, flaky montmorillonite and mica can significantly reduce shrinkage 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

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 filler composite PP (20% filler + 8% POE) with different shapes
(2) Particle size of filler
Different particle sizes of same filler will have different effects on shrinkage rate. The smaller particle size, the greater impact 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 Effect of talc powder with different particle sizes on shrinkage of PP composite materials
(3) Treatment of fillers
Whether same filler is surface treated when compounded with resin will have different effects on shrinkage rate. Surface-treated fillers have a greater effect on reducing shrinkage. For example, taking ABS filled with 10% talc powder as an example, specific effects are shown in Table 6.

Table 6 Effect of surface treatment on shrinkage of talc/ABS composite materials
(4)Amount of filler
Different amounts of same filler added will have different effects on shrinkage rate. The greater amount added, the greater decrease in shrinkage rate. Taking PP filling as an example, effects of different filling amounts of talc powder on shrinkage are shown in Table 7.

Table 7 Effect of different contents of 1250 mesh talc powder on shrinkage of PP composite materials

 

(1) Add crystallization reducing aids
Commonly used crystallization reducing aids are small molecule compounds. After being added, they destroy regularity of macromolecules and hinder movement of macromolecules in molten state, thereby achieving purpose of reducing crystallization. For example, an organic crystallization reducing agent A available on the market has a yellow-white powder appearance and a melting point of about 60℃. Just add 0.5% to 1% to PP. Another example is an inorganic crystallization reducing agent B available on the market. It appears as a white powder with a pH value of 8 and a whiteness of 95%. Just add 0.5% to 1% to PP.
(2)Add other resins
If a small amount of LDPE and HDPE are blended into PP, crystallization during PP processing can be destroyed, amorphous resins such as PS, ABS, PMMA, and PC can also be added.
Specific examples include 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 reduces shrinkage to 0.05%.
Third generation: adding PVAC, shrinkage rate is reduced to 0.03%.
Taking addition of PE resin to PP as an example, effects of different contents of PE on shrinkage of PP are shown in Table 8.

Table 8 Effect of different contents of PE on PP shrinkage
As can be seen from Table 8, impact of adding other resins on shrinkage is much smaller than impact of fiber-filled and inorganic fillers.

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 contents of elastomer on PP shrinkage
It can be seen from Table 9 that when elastomer content is less than 5%, effects of various types of elastomers on shrinkage of PP are basically same. However, after elastomer content exceeds 5%, influence of different elastomer varieties diverges. Order of influence on shrinkage is POE>EPDM>SBS, which is consistent with toughening effect of elastomers on PP.
Inorganic fillers and elastomers are mixed and added to PP to achieve a better shrinkage reduction effect. 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 contents on shrinkage of 20% talc filled PP

After PP grafting, crystallinity can be reduced, thereby achieving purpose of reducing shrinkage.