What are the factors that affect the shrinkage of plastics? How to design modified formula?

01. Plastic Shrinkage Profile

The shrinkage rate of plastic refers to the percentage of the difference between the size of the plastic part at the molding temperature and the size after it is taken out of the mold and cooled to room temperature. It reflects the degree of size reduction of the plastic part after it is taken out of the mold and cooled. Factors affecting plastic shrinkage include plastic species, molding conditions, mold structure, etc. Different plastics have different shrinkage rates. In addition, the shrinkage rate of plastics has a lot to do with the shape of the plastic part, the complexity of the internal structure, and whether there are inserts.

Factors affecting plastic shrinkage are as follows:

1. Influence of raw materials

(1) Types of raw materials

Different types of plastic raw materials have different shrinkage rates, see Table 1 for details.

Table 1 Shrinkage of different plastic raw materials

It can be seen from Table 1 that the shrinkage rate of PPS reinforced with 40% for thermoplastics is the lowest, and the shrinkage rate of epoxy resin for thermosetting plastics is the lowest, and the shrinkage rate of both plastics is 0.2%. Fluoroplastics have the largest shrinkage rate, which can reach a maximum of about 6%; the second largest shrinkage rate is low-density polyethylene, with a maximum shrinkage rate of 5%.

 

(2) Crystallinity of raw materials

The same raw material has different crystallinity, and its shrinkage rate is different. The smaller the crystallinity, the smaller the shrinkage rate. Among the many influencing factors, the crystallinity of the resin has the greatest impact on shrinkage.

 

(3) Molecular weight of raw materials

The larger the molecular weight of the same raw material, the smaller its shrinkage. Resins with the same molecular weight have good fluidity and low molding shrinkage.

 

(4) Modification of raw materials

After the resin is modified by adding other resins, elastomers, and fillers, its shrinkage rate decreases to varying degrees.

 

The shrinkage rate of the resin is different, which greatly affects the molding accuracy of its products during the melt processing process. In order to process high-precision plastic products, resins with low shrinkage must be used. Taking PP resin as an example, its shrinkage rate is 1.8% to 2.5%, and the smaller the molecular weight, the greater the shrinkage rate. PP is difficult to make high-precision products. For this reason, major companies in the world are actively researching to reduce the resin's own shrinkage rate under the condition of ensuring the resin performance. For example, Basel has developed a low shrinkage rate PP named Hifax CA207A with a low shrinkage rate of only 1%.

 

2. The effect of molding process on the shrinkage rate of plastic products

(1) The molding temperature remains unchanged, the injection pressure increases, and the shrinkage rate decreases;

(2) Keeping the pressure increases, the shrinkage rate decreases;

(3) The melt temperature increases and the shrinkage rate decreases;

(4) The mold temperature is high and the shrinkage rate increases;

(5) The pressure holding time is long and the shrinkage rate is reduced, but the shrinkage rate is not affected after the gate is closed;

(6) The cooling time in the mold is long, and the shrinkage rate is reduced;

(7) The injection speed is high, the shrinkage rate tends to increase slightly, and the impact is small;

(8) The molding shrinkage is large and the post-shrinkage is small; the post-shrinkage is large in the first two days and stable in about a week.

 

3. Effect of structure on product shrinkage

(1) The shrinkage rate of thick-walled plastic parts is larger than that of thin-walled plastic parts;

(2) The shrinkage rate of plastic parts with inserts is smaller than that without inserts;

(3) The shrinkage rate of plastic parts with complex shapes is smaller than that of simple shapes;

(4) The shrinkage rate of the dimension in the length direction of the plastic part is smaller than that in the thickness direction;

(5) The shrinkage rate of the inner hole is large, and the shrinkage rate of the shape is small.

 

4. Effect of mold structure on shrinkage of plastic products

(1) The gate size is large and the shrinkage rate is reduced;

(2) The shrinkage rate in the direction perpendicular to the gate decreases, and the shrinkage rate in the direction parallel to the gate increases;

(3) The shrinkage rate away from the gate is smaller than that near the gate;

(4) The shrinkage rate of the plastic part with mold restrictions is small, and the shrinkage rate of the unrestricted plastic part is large.

02. Modification method to reduce plastic shrinkage

Of course, the most effective way to reduce the shrinkage of plastics is various modification technologies, as detailed 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, the shrinkage rate is shown in Table 2.

Table 2 Shrinkage rate of glass fiber reinforced PP with different contents

 

The long glass fiber reinforced plastics developed in recent years have certain advantages in shrinkage rate, and 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 types, properties, particle sizes and surface treatment degrees affect the modification effect on resin shrinkage.

(1) Shape of filler

Different types of inorganic fillers have different shapes and have different effects on shrinkage. The specific order of impact is flake > needle > particle > spherical. For example, flake montmorillonite and mica can significantly reduce the shrinkage of composite materials.

For example, the effect of adding the same content of various fillers on the shrinkage of PP in PP is shown in Table 3.

Table 3 Effect of different fillers on shrinkage of PP

 

For another example, 20% fillers of different shapes are added to PP (HHP10), and the shrinkage rate of the composite material is shown in Table 4.

Table 4 Shrinkage of different shapes of filler composite PP (20% filler + 8% POE)

(2) The particle size of the filler

Different particle sizes of the same filler have different effects on the shrinkage rate, and the smaller the particle size, the greater the effect on the shrinkage rate. Taking talcum powder filled PP (20% talc powder + 8% POE) as an example, the effects of different particle sizes of talc powder on shrinkage are shown in Table 5.

 

Table 5 Effect of talcum powder with different particle sizes on shrinkage of PP composites

(3) Treatment of filler

Whether the same filler is surface treated or not when it is compounded with the resin has different effects on the shrinkage rate. Fillers with surface treatment are more effective in reducing shrinkage. For example, taking ABS filled with 10% talcum powder as an example, the specific effects are shown in Table 6.

 

Table 6 Effect of surface treatment on shrinkage of talcum powder/ABS composites

(4) The amount of filler

Different additions of the same filler have different effects on shrinkage. The greater the addition, the greater the decrease in shrinkage. Taking PP filling as an example, the impact of different filling amounts of talcum powder on shrinkage is shown in Table 7.

 

Table 7 Effect of Different Contents of 1250 Mesh Talc Powder on Shrinkage Rate of PP Composites

 

 

3. Reduce crystallization modification

(1) Add crystallization reducing aid

Commonly used crystallization-reducing aids are small molecular compounds, which break the regularity of macromolecules and hinder the movement of macromolecules in the molten state after addition, thereby achieving the purpose of reducing crystallization. For example, an organic decrystallization agent A provided on the market has a yellow-white powder appearance and a melting point of about 60°C. Add 0.5% to 1% to PP. Another example is an inorganic decrystallization agent B available on the market. The appearance is white powder, the pH value is 8, and the whiteness is 95%. Add 0.5% to 1% to PP.

 

(2) Add other resins

If a small amount of LDPE and HDPE are blended in PP, the crystallization during PP processing can be destroyed, and non-crystalline resins such as PS, ABS, PMMA, PC, etc. can also be added.

 

A specific example is adding other resins to UP resin, which can greatly reduce the shrinkage of the product:

The first generation: PS and PE are added, and the shrinkage rate is reduced to 0.2% to 0.4%.

The second generation: PMMA is added, and the shrinkage rate is reduced to 0.05%.

The third generation: PVAC is added, and the shrinkage rate is reduced to 0.03%.

 

Taking the addition of PE resin in PP as an example, the effects of different contents of PE on the shrinkage of PP are shown in Table 8.

 

Table 8 Effect of different content of PE on shrinkage of PP

 

It can be seen from Table 8 that the effect of adding other resins on shrinkage is far less than that of fiber filling and inorganic filling.

 

4. Blending elastomers

Elastomers that can be added include POE, EPDM, SBS, etc. All kinds of elastomers have similar effects on plastic shrinkage, see Table 9 for details.

 

Table 9 Effect of different content of elastomer on shrinkage of PP

 

It can be seen from Table 9 that when the elastomer content is less than 5%, the effects of various elastomers on the shrinkage of PP are basically the same. Only after the elastomer content exceeds 5%, the degree of influence of different elastomer varieties diverges, and the order of impact on shrinkage is POE>EPDM>SBS, which is consistent with the toughening effect of elastomers on PP.

 

In PP, the inorganic filler is mixed with the elastomer, and the shrinkage rate is reduced better. When the talc content is 20%, the effect of different contents of POE on the shrinkage of PP is shown in Table 10.

 

Table 10 Effect of different POE contents on shrinkage of PP filled with 20% talc powder

 

 

5. Chemical graft modification

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

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