Hu Guanghong and Wang Yue

*National Engineering Research Center of Die & Mold CAD, Shanghai Jiao Tong University, Shanghai, China* 

#### **1. Introduction**

In recent years, the polymer resin price is rising due to the petroleum shortage. How to save plastics on the premise to ensure the plastics part quality is one of the research hotspots. Microcellular foam injection molding process is developed in this background. Microcellular foam technology was invented by MIT in the early 1980's [1]. The traditional foaming processes, which produce bubbles larger than 0.25mm, are not feasible due to excessive loss of strength. Thus, the idea was born to create microcellular foam to both save plastics and have reasonable strength.

Generally, microcellular foam process takes advantage of supercritical fluid (SCF) as physical blowing agent. CO2 and N2 are usually used as agent. The microcellular foam parts have uniform cell diameters of 1 to100 microns and cell density of 109 to 1015 cells per cubic centimeters. Figure 1-1 shows the scanning electron micrographs of microcellular polystyrene sample [2].

Fig. 1-1. Electron micrographs of microcellular polystyrene scanning sample [2].

Microcellular Foam Injection Molding Process 177

growing can be possible. Thus, the mixer temperature, MPP and SCF concentration affect

When millions of nuclei are generated and the nucleus is stable, the bubbles growth start. SCF concentration of mixer is higher than the SCF concentration inside bubbles. Due to the concentration difference SCF in the mixer enters the bubbles. And the gas bubbles grow up. Until the SCF concentration inside bubbles equals to the outside one or the melt is frozen, the gas bubbles will keep growing up. Thus, the final bubble morphology is determined by

Along with mold cooling, the melt temperature is decreased and the melt freezes up. The

From above microcellular foam injection molding process, the properties of microcellular foam injection molding parts are determined by nucleation process and final bubble morphology besides tradition injection process condition such as part shape, the kind of polymer, mold structure, process parameters. Thus microcellular foam injection molding

Due to SCF injected into the polymer melt, it is great affect the polymer melt viscosity, injection molding process cycle, part weight, mechanical properties and surface quality *etc*.

Due to the SCF dissolved in the polymer melt, the glass transition temperature of polymer melt becomes lower. So the polymer viscosity is decreased and the melt fluidity becomes better. Thus, the required injection pressure is lower than tradition injection and the requirement of injection machine properties is less. Figure 1-3 shows the effect of SCF on the PA, PBT melt viscosity [4]. The results indicate that the viscosity is decreased after the SCF is

It should be pointed out that the effect of SCF on the polymer viscosity is determined by the polymer kind and filler. Because SCF can't be dissolved into the filler, it will not affect the filler viscosity. Thus comparing to the pure polymer, the effect of SCF on the viscosity of

Microcellular foam injection molding technology can reduce the cycle time. The reasons mainly include: (1). because the gas in the bubbles can provide the packing pressure, the packing and holding phase can be eliminated. (2). When the millions of nucleus are generated and bubbles grow up, they are all endothermic reaction. So the cooling time is saved. (3). Due to the bubbles in the part, the part weight is reduced. The cooling time is also saved. (4). The lower viscosity means higher filling speed. The filling time becomes short. Generally 20%~50% cycle time can be saved by microcellular foam injection molding

process has distinct characters comparing to the traditional plastics injection.

**1.2 Microcellular foam injection molding process characters** 

the nuclei process and the final nucleus density.

the SCF concentration and injection process parameters.

bubbles stop growing up. And the shape of part is fixed.

**Bubbles growth** 

**Product typing** 

**1.2.1 Melt viscosity** 

polymer with filler is less.

**1.2.2 Injection cycle time** 

added.

Now microcellular foam technology is extended into many other plastics forming process such as extrusion, injection, blowing process. And microcellular foam technology is widely used in the homework appliance, aerospace and auto industry *etc*. In this book, microcellular foam injection molding process is mainly discussed.

#### **1.1 Microcellular foam injection process principle**

During microcellular foam injection molding process, SCF is injected into the polymer melt. And the single phase of polymer- SCF mixed solution is obtained under certain temperature and pressure. When the mixer is injected into the mold, the pressure of the single-phase solution is dropped from microcellular process pressure (MPP) to atmospheric pressure. The nucleation phenomena occur due to the gas separated out of the mixer. Then these nuclei finally grow up to stable bubbles.

Figure 1-2 shows the microcellular foam injection molding process. And generally the microcellular foam injection molding process is described as following four steps.

Fig. 1-2. Illustration of microcellular polymer foaming process [3].

#### **Polymer-SCF single phase generation**

During microcellular foam injection molding process, the supercritical nitrogen (N2) or carbon dioxide (CO2) is injected into plastics injection machine barrel and dissolved into polymer melt. Then a single phase polymer-SCF solution is generated under the definite temperature and pressure. In this stage, the concentration of SCF is determined by saturation, microcellular process pressure (MPP) and the mixer temperature. These parameters also significantly affect the final bubbles size.

#### **Homogeneous nucleation**

Theoretically, only when the polymer-SCF mixer is in the thermodynamics equilibrium and millions of nuclei are generated at the same time, homogeneous nucleation will be possible. When the polymer-SCF single phase mixer is injected into mold cavity, the mixer pressure is changed from MPP to the atmospheric pressure. Thus a rapid pressures unloading occurs. Then, SCF separates from the single phase mixer, and a large number of nuclei are generated. With the nucleus growing, free energy of the mixer is also increasing. Only when the nucleus size is bigger than the critical one, the nucleus will be stable. And the bubble growing can be possible. Thus, the mixer temperature, MPP and SCF concentration affect the nuclei process and the final nucleus density.

#### **Bubbles growth**

176 Some Critical Issues for Injection Molding

Now microcellular foam technology is extended into many other plastics forming process such as extrusion, injection, blowing process. And microcellular foam technology is widely used in the homework appliance, aerospace and auto industry *etc*. In this book,

During microcellular foam injection molding process, SCF is injected into the polymer melt. And the single phase of polymer- SCF mixed solution is obtained under certain temperature and pressure. When the mixer is injected into the mold, the pressure of the single-phase solution is dropped from microcellular process pressure (MPP) to atmospheric pressure. The nucleation phenomena occur due to the gas separated out of the mixer. Then these nuclei

Figure 1-2 shows the microcellular foam injection molding process. And generally the

During microcellular foam injection molding process, the supercritical nitrogen (N2) or carbon dioxide (CO2) is injected into plastics injection machine barrel and dissolved into polymer melt. Then a single phase polymer-SCF solution is generated under the definite temperature and pressure. In this stage, the concentration of SCF is determined by saturation, microcellular process pressure (MPP) and the mixer temperature. These

Theoretically, only when the polymer-SCF mixer is in the thermodynamics equilibrium and millions of nuclei are generated at the same time, homogeneous nucleation will be possible. When the polymer-SCF single phase mixer is injected into mold cavity, the mixer pressure is changed from MPP to the atmospheric pressure. Thus a rapid pressures unloading occurs. Then, SCF separates from the single phase mixer, and a large number of nuclei are generated. With the nucleus growing, free energy of the mixer is also increasing. Only when the nucleus size is bigger than the critical one, the nucleus will be stable. And the bubble

microcellular foam injection molding process is described as following four steps.

microcellular foam injection molding process is mainly discussed.

Fig. 1-2. Illustration of microcellular polymer foaming process [3].

parameters also significantly affect the final bubbles size.

**1.1 Microcellular foam injection process principle** 

finally grow up to stable bubbles.

**Polymer-SCF single phase generation** 

**Homogeneous nucleation** 

When millions of nuclei are generated and the nucleus is stable, the bubbles growth start. SCF concentration of mixer is higher than the SCF concentration inside bubbles. Due to the concentration difference SCF in the mixer enters the bubbles. And the gas bubbles grow up. Until the SCF concentration inside bubbles equals to the outside one or the melt is frozen, the gas bubbles will keep growing up. Thus, the final bubble morphology is determined by the SCF concentration and injection process parameters.

#### **Product typing**

Along with mold cooling, the melt temperature is decreased and the melt freezes up. The bubbles stop growing up. And the shape of part is fixed.

From above microcellular foam injection molding process, the properties of microcellular foam injection molding parts are determined by nucleation process and final bubble morphology besides tradition injection process condition such as part shape, the kind of polymer, mold structure, process parameters. Thus microcellular foam injection molding process has distinct characters comparing to the traditional plastics injection.

#### **1.2 Microcellular foam injection molding process characters**

Due to SCF injected into the polymer melt, it is great affect the polymer melt viscosity, injection molding process cycle, part weight, mechanical properties and surface quality *etc*.

#### **1.2.1 Melt viscosity**

Due to the SCF dissolved in the polymer melt, the glass transition temperature of polymer melt becomes lower. So the polymer viscosity is decreased and the melt fluidity becomes better. Thus, the required injection pressure is lower than tradition injection and the requirement of injection machine properties is less. Figure 1-3 shows the effect of SCF on the PA, PBT melt viscosity [4]. The results indicate that the viscosity is decreased after the SCF is added.

It should be pointed out that the effect of SCF on the polymer viscosity is determined by the polymer kind and filler. Because SCF can't be dissolved into the filler, it will not affect the filler viscosity. Thus comparing to the pure polymer, the effect of SCF on the viscosity of polymer with filler is less.

#### **1.2.2 Injection cycle time**

Microcellular foam injection molding technology can reduce the cycle time. The reasons mainly include: (1). because the gas in the bubbles can provide the packing pressure, the packing and holding phase can be eliminated. (2). When the millions of nucleus are generated and bubbles grow up, they are all endothermic reaction. So the cooling time is saved. (3). Due to the bubbles in the part, the part weight is reduced. The cooling time is also saved. (4). The lower viscosity means higher filling speed. The filling time becomes short. Generally 20%~50% cycle time can be saved by microcellular foam injection molding

Microcellular Foam Injection Molding Process 179

PS 1.5 30 Acetal 1.5 15 PET 5 30 TPE 1.5 20 PP (30%Talc) 2.1 25 HDPE 5 60 PC/ABS 2.1 23 PA 1.2 9 PA(40% Filler) 2 15 PC 7.2 45

Also, the parts mechanical properties are changed due to the bubbles. The former researches indicate that the part bend strength of microcellular foam polymer is almost same as the solid polymer. Thus microcellular foam technology can be used to produce the inner structure part. However it is quite different situation for the part tensile strength. The tensile property data shows that the tensile strength of microcellular foams decreases in proportion to the foam density. It means that a 50% relative density foam can be expected to have 50% of the strength of the solid polymer. To the part impact strength, it is more sensitive to variation from polymer to polymer. And the results cannot be generalized. However the Gardner impact strength of PVC foam experiment results show that the impact strength decreases linearly with foam density. It should be pointed out that the impact strength of

Polyphenylsulfone 5 50

Table 1-1. Effect of microcellular foam process on weight reduction [5].

Fig. 1-5. PBT mechanical properties on the different weight reduction ratio.

Lzod Impact

(KJ/m2x10-1)

Solid

10% Weight reduction 17% Weight reduction 27% Weight reduction

Flexural Strength (MPa)

**1.2.4 Part mechanical properites** 

0

Tensile Strength (MPa)

20

40

60

80

100

Polymer Part thickness(mm) Weight reduction (%)

Fig. 1-3. Effect of SCF on melt viscosity [4].

process. Figure 1-4 shows the comparison between microcellular foam injection molding process cycle and traditional injection one.

Fig. 1-4. Comparison between microcellular foam injection molding process cycle and traditional injection process.

#### **1.2.3 Part weight**

Due to the bubbles in the part, the polymer obviously can be saved. Generally the part weight can be reduced as 0.5mm thickness weight by microcellular foam injection molding process. At the same time, all kinds of polymer, even including the high temperature polyphenylsulfone, can be formed by this technology. The effect of microcellular foam injection molding process on the weight reduction is shown in the Table 1-1.

Plastics Melt

Melt with Nitrogen

Melt with Carbon Dioxide

process. Figure 1-4 shows the comparison between microcellular foam injection molding

PBT PA

Material Type

Fig. 1-4. Comparison between microcellular foam injection molding process cycle and

injection molding process on the weight reduction is shown in the Table 1-1.

Due to the bubbles in the part, the polymer obviously can be saved. Generally the part weight can be reduced as 0.5mm thickness weight by microcellular foam injection molding process. At the same time, all kinds of polymer, even including the high temperature polyphenylsulfone, can be formed by this technology. The effect of microcellular foam

Fig. 1-3. Effect of SCF on melt viscosity [4].

0

0.2

0.4

Normalize Viscosity

0.6

0.8

1.2

1

process cycle and traditional injection one.

traditional injection process.

**1.2.3 Part weight** 


Table 1-1. Effect of microcellular foam process on weight reduction [5].

#### **1.2.4 Part mechanical properites**

Also, the parts mechanical properties are changed due to the bubbles. The former researches indicate that the part bend strength of microcellular foam polymer is almost same as the solid polymer. Thus microcellular foam technology can be used to produce the inner structure part. However it is quite different situation for the part tensile strength. The tensile property data shows that the tensile strength of microcellular foams decreases in proportion to the foam density. It means that a 50% relative density foam can be expected to have 50% of the strength of the solid polymer. To the part impact strength, it is more sensitive to variation from polymer to polymer. And the results cannot be generalized. However the Gardner impact strength of PVC foam experiment results show that the impact strength decreases linearly with foam density. It should be pointed out that the impact strength of

Fig. 1-5. PBT mechanical properties on the different weight reduction ratio.

Microcellular Foam Injection Molding Process 181

parallels the flow direction. The difference between them is that there are no broken bubbles

Michaeli and Cramer point out that the silver streaks are flow marks of the polymer-SCF mixer on the mold cavity surface. It's the shear deformation of the bubbles that are close to the surface. Because of different bubble sizes, the depth of silver threads is different and then the parts surface roughness is different. Compared with silver trips, silver threads will

When many tiny bubbles converge at the part thin wall place, it makes a thin polymer layer separate from the main part body. This phenomenon is called surface blistering. (see Figure 1-5c). Surface blistering most likely appears in the parts that are made by crystalline polymer without filler such as POM. Surface blistering can be eliminated by adjusting the

Post-blow is similar to the internal blistering and always appears at the place of hot spots (see Figure 1-5d). The post-blow defect is caused by following two factors. One is that the cooling is not enough at the hotspots; the other is that too much gas enters the some certain bubbles due to the high SCF concentration and form some large size bubbles. When the pressure inside the bubbles is higher than the outside one, the post-blow will happen. So the method to eliminate

In addition to the above serious defects, surface roughness is another problem that limits the application scope of microcellular foam injection molding process. During bubbles growing up, some small bubbles break up near the surface, and the gas is trapped on the mold surface when the polymer-SCF mixer begins to solidify. So the surface roughness of

According to above chapters, all the advantages and disadvantages are all caused by the SCF injected into the polymer melt. Before introduction microcellular foam injection

Supercritical fluid is any substance at certain temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like gas, and dissolve materials like liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, and allowing many properties of supercritical fluid to be "fine-tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and nitrogen are the most commonly used supercritical fluids for microcellular foam injection

molding. Figure 2-1 shows the Carbon dioxide pressure-temperature phase diagram.

microcellular foam injection process parameters and improving the mold design.

this defect is to enhancing cooling at the hot spots and adjusting SCF concentration.

microcellular foam injection parts is higher than that of traditional injection parts.

**2. Microcellular foam injection molding theories** 

molding theories, supercritical fluid is firstly discussed.

at the surface to the latter.

**Surface blistering** 

**Post-blow** 

**Surface roughness** 

**2.1 Supercritical fluid** 

cause larger surface roughness [18].

polymer added filler is decreased less than one without any filler. The main reason is that the filler properties and content percent great affect the part impact strength. And SCF has no effect on the fillers[6-13]. Figure 1-5 shows the bend strength, tensile strength and impact strength of PBT (30% GF) on the solid polymer and different weight reduction ratio. The results present the almost same rules as above.

### **1.2.5 Surface quality**

As said above, microcellular foam injection molding process presents nice formability and lots of advantages. But still due to SCF, the part surface quality is worse than tradition process. Typical surface defects are swirl marks, silver streak, surface blistering, post-blow and large surface roughness. These defects limit the application scope of microcellular foam injection process seriously. Figure 1-6 shows main surface defects of microcellular foam injection molding parts.

Fig. 1-6. Surface defects of microcellular foam injection molding parts (a) swirl mark[14]; (b) silver streak[14]; (c) surface blistering[15]; (d) post-blow[15].

#### **Swirl marks**

Grooves on the part surface are caused by the trapped gas on the mold surface when the polymer-SCF mixer begins to solidify. And the area of grooves surface shows positive correlation. The shape of these grooves is slender along the flow direction, and the aspect ratio of grooves indicates the size of shear strength which is caused by the polymer-SCF mixer filling behavior in the mold cavity. Swirl marks are these grooves whose shapes are curled (see Figure 1-5a).

Yoon propose that the glass transition temperature (for the amorphous polymer) or the melt temperature (for the crystalline polymer) is one of the important effect factors on the swirl mark forming [16]. Zhang YT points out that swirl marks always appear near the gate [17]. While the polymer-SCF mixer is injected into mold cavity, many parameters in different mold cavity area are varied. Generally near the gate, the temperature is higher, viscosity of the polymer-SCF is smaller, and melt strength is lower. So the gas near the gate is easy to diffuse to the mixer surface, and the bubbles near the surface break up easily.

#### **Silver streak**

Silver streak is a defect that shows silver gloss in the sunlight (see Figure 1-5b). Silver streak of microcellular foam injection parts shows two different appearances. One is called silver thread because its boundary looks like a thread. This defect is caused by the broken bubbles at the surface of melt. The other is called silver strip because it looks like a strip which parallels the flow direction. The difference between them is that there are no broken bubbles at the surface to the latter.

Michaeli and Cramer point out that the silver streaks are flow marks of the polymer-SCF mixer on the mold cavity surface. It's the shear deformation of the bubbles that are close to the surface. Because of different bubble sizes, the depth of silver threads is different and then the parts surface roughness is different. Compared with silver trips, silver threads will cause larger surface roughness [18].

#### **Surface blistering**

180 Some Critical Issues for Injection Molding

polymer added filler is decreased less than one without any filler. The main reason is that the filler properties and content percent great affect the part impact strength. And SCF has no effect on the fillers[6-13]. Figure 1-5 shows the bend strength, tensile strength and impact strength of PBT (30% GF) on the solid polymer and different weight reduction ratio. The

As said above, microcellular foam injection molding process presents nice formability and lots of advantages. But still due to SCF, the part surface quality is worse than tradition process. Typical surface defects are swirl marks, silver streak, surface blistering, post-blow and large surface roughness. These defects limit the application scope of microcellular foam injection process seriously. Figure 1-6 shows main surface defects of microcellular foam

(a) (b) (c) (d)

Grooves on the part surface are caused by the trapped gas on the mold surface when the polymer-SCF mixer begins to solidify. And the area of grooves surface shows positive correlation. The shape of these grooves is slender along the flow direction, and the aspect ratio of grooves indicates the size of shear strength which is caused by the polymer-SCF mixer filling behavior in the mold cavity. Swirl marks are these grooves whose shapes are

Yoon propose that the glass transition temperature (for the amorphous polymer) or the melt temperature (for the crystalline polymer) is one of the important effect factors on the swirl mark forming [16]. Zhang YT points out that swirl marks always appear near the gate [17]. While the polymer-SCF mixer is injected into mold cavity, many parameters in different mold cavity area are varied. Generally near the gate, the temperature is higher, viscosity of the polymer-SCF is smaller, and melt strength is lower. So the gas near the gate is easy to

Silver streak is a defect that shows silver gloss in the sunlight (see Figure 1-5b). Silver streak of microcellular foam injection parts shows two different appearances. One is called silver thread because its boundary looks like a thread. This defect is caused by the broken bubbles at the surface of melt. The other is called silver strip because it looks like a strip which

diffuse to the mixer surface, and the bubbles near the surface break up easily.

Fig. 1-6. Surface defects of microcellular foam injection molding parts (a) swirl mark[14];

(b) silver streak[14]; (c) surface blistering[15]; (d) post-blow[15].

results present the almost same rules as above.

**1.2.5 Surface quality** 

injection molding parts.

**Swirl marks** 

**Silver streak** 

curled (see Figure 1-5a).

When many tiny bubbles converge at the part thin wall place, it makes a thin polymer layer separate from the main part body. This phenomenon is called surface blistering. (see Figure 1-5c). Surface blistering most likely appears in the parts that are made by crystalline polymer without filler such as POM. Surface blistering can be eliminated by adjusting the microcellular foam injection process parameters and improving the mold design.
