Plastic shrinkage refers to percentage difference between size of a plastic part at molding temperature and size after it is removed from mold, cooled to room temperature. It reflects degree of size reduction of a plastic part after it is removed from mold and cooled. Factors affecting plastic shrinkage include plastic type, molding conditions, mold structure, etc. Shrinkage rates of different plastics are different. In addition, shrinkage rate of plastics is also closely related to shape of plastic part, complexity of internal structure, whether there are inserts, etc.
Factors affecting plastic shrinkage are as follows:

(1) Raw material type
Plastic raw materials of different types have different shrinkage rates. See Table 1 for details.

 

Table 1 Shrinkage rates of different plastic raw materials
As can be seen from Table 1, shrinkage rate of 40% reinforced PPS is the lowest among thermoplastic performance plastics, and shrinkage rate of epoxy resin is the lowest among thermosetting plastics. Shrinkage rates of both plastics are 0.2%. Fluorine plastics have the largest shrinkage rate, which can reach up to about 6%; low-density polyethylene has the second largest shrinkage rate, which can reach up to 5%.

(2) Crystallinity of raw materials

Shrinkage rate varies with crystallinity of same raw material. The smaller crystallinity, the smaller shrinkage rate. Among many influencing factors, crystallinity of resin has the greatest impact on shrinkage rate.
(3) Molecular weight of raw materials
The larger molecular weight of same raw material, the smaller its shrinkage rate. Resins with same molecular weight have a smaller molding shrinkage rate with good fluidity.
(4) Modification of raw materials
After resin is modified by adding other resins, elastomers, and fillers, its shrinkage rate decreases to varying degrees. Different shrinkage rates of resins greatly affect molding accuracy of their products during melt processing process. In order to process high-precision plastic products, resins with low shrinkage rates must be used. Taking PP resin as an example, its shrinkage rate is 1.8% to 2.5%, and the smaller molecular weight, the greater shrinkage rate. It is difficult to make high-precision products with PP. To this end, major companies around the world are actively researching how to reduce shrinkage of resin while ensuring performance of resin. For example, Basel has developed a low-shrinkage PP with a brand name Hifax CA207A, which has a shrinkage of only 1%.

(1) When molding temperature remains unchanged, injection pressure increases and shrinkage decreases;
(2) When holding pressure increases, shrinkage decreases;
(3) When melt temperature increases, shrinkage decreases;
(4) When mold temperature is high, shrinkage increases;
(5) When holding time is long, shrinkage decreases, but shrinkage is not affected after gate is closed;
(6) When cooling time in mold is long, shrinkage decreases;
(7) When injection speed is high, shrinkage tends to increase slightly, but impact is small;
(8) Molding shrinkage is large, and post-shrinkage is small; post-shrinkage is large in the first two days and stabilizes after about a week.

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

(1) The larger gate size, the smaller shrinkage rate;
(2) Shrinkage rate in direction perpendicular to gate decreases, and shrinkage rate in direction parallel to gate increases;
(3) Shrinkage rate far away from gate is smaller than that near gate;
(4) Shrinkage rate of part with mold restrictions is small, and shrinkage rate of part without restrictions is large.

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

Fibers include various inorganic fibers and organic fibers. Taking glass fiber as an example, shrinkage of PP resin with different glass fiber contents is shown in Table 2.

 

Table 2 Shrinkage of PP reinforced with different glass fiber contents
Long glass fiber reinforced plastics developed in recent years have certain advantages in shrinkage, their longitudinal and transverse shrinkage are small and consistent.

Specific inorganic raw materials include talcum powder, calcium carbonate, barium sulfate, mica powder, wollastonite and montmorillonite. Different filler types, properties, particle sizes and surface treatment degrees all affect modification effect on resin shrinkage.
(1) Filler shape
Different types of inorganic fillers have different shapes and different effects on shrinkage. Specific order of influence is flake > needle > particle > spherical. For example, flake montmorillonite and mica have a significant effect on reduction of composite shrinkage.
For example, effect of adding same amount of various fillers to PP on shrinkage of PP is shown in Table 3.

 

Table 3 Effect of different types of fillers on PP shrinkage
For example, when 20% fillers of different shapes are added to PP (HHP10), shrinkage of composite material is shown in Table 4.

Table 4 Shrinkage of PP (20% filler + 8% POE) composited with fillers of different shapes
(2) Filler particle size
Particle size of same filler has different effects on shrinkage. The smaller particle size, the greater effect on shrinkage. Taking talc-filled PP (20% talc + 8% POE) as an example, effect of talc of different particle sizes on shrinkage is shown in Table 5.

 

Table 5 Effect of talc of different particle sizes on shrinkage of PP composite materials
(3) Filler treatment
For same filler, whether or not it is surface treated when compounded with resin will have different effects on shrinkage. Fillers with surface treatment have a greater effect on reducing shrinkage. For example, taking ABS filled with 10% talc as an example, specific effect is shown in Table 6.

 

Table 6 Effect of surface treatment on shrinkage of talc/ABS composites
(4) Filler dosage
Same filler dosage has different effects on shrinkage. The greater dosage, the greater reduction in shrinkage. Taking PP filling as an example, effects of different talc dosages on shrinkage are shown in Table 7.

 

Table 7 Effect of different contents of 1250 mesh talc on shrinkage of PP composites

(1) Adding crystallization reduction aids
Commonly used crystallization reduction aids are small molecular compounds. After addition, they destroy regularity of macromolecules in molten state and hinder movement of macromolecules, thereby achieving purpose of reducing crystallization. For example, an organic crystallization reduction agent A available on the market has a yellow-white powder appearance and a melting point of about 60℃. It can be added to PP at 0.5% to 1%. Another example is an inorganic crystallization reduction agent B available on the market. It has a white powder appearance, a pH value of 8, and a whiteness of 95%. It can be added to PP at 0.5% to 1%.
(2) Adding other resins
If a small amount of LDPE or HDPE is blended into PP, crystallization during PP processing can be destroyed. Non-crystalline resins such as PS, ABS, PMMA, and PC can also be added.
For example, adding other resins to UP resin can greatly reduce shrinkage of product:
First generation: adding PS and PE, shrinkage is reduced to 0.2% to 0.4%.
Second generation: adding PMMA, shrinkage is reduced to 0.05%.
Third generation: adding PVAC, shrinkage is reduced to 0.03%.
Taking addition of PE resin to PP as an example, effect of different PE contents on shrinkage of PP is shown in Table 8.

 

Table 8 Effect of different PE contents on shrinkage of PP
It can be seen from Table 8 that effect of adding other resins on shrinkage is far less than that of fiber filling and inorganic fillers.

Elastomers that can be added include POE, EPDM, SBS, etc. Effects of various elastomers on shrinkage of plastics are similar. See Table 9 for details.

 

Table 9: Effects of Different Elastomer Contents on PP Shrinkage
Table 9 shows that when elastomer content is below 5%, effects of various elastomers on PP shrinkage are essentially same. However, after elastomer content exceeds 5%, degree of influence of different elastomer types diverges. Order of influence on shrinkage is POE > EPDM > SBS, consistent with toughening effect of elastomers on PP.
Adding inorganic fillers to PP mixed with elastomers achieves a greater shrinkage reduction effect. Effects of different POE contents on PP shrinkage at a talc content of 20% are shown in Table 10.

 

Table 10: Effects of Different POE Contents on Shrinkage of 20% Talc-Filled PP

Grafting PP can reduce crystallinity, thereby reducing shrinkage.