**3. Microcellular foam injection molding products surface defects and solutions**

As said in above chapter, microcellular foam injection molding parts have many advantages such as saving material and energy, reducing cycle time, and parts excellent dimensional stability. Despite these advantages, the low parts' surface quality limits its application scope seriously. Typical defects are swirl marks, silver streak, surface blistering, post-blow, large surface roughness. The details are introduced on above chapter.

### **3.1 Technologies to improve surface quality**

Until now, many technologies for improving surface quality have been studied. The typical technologies include Gas Counter Pressure (GCP), Rapid Heating Cycle Molding (RHCM) and Film Insulation which is derived from RHCM.

#### **Gas Counter Pressure (GCP)**

When polymer-SCF mixer is injected into the cavity, counter pressure can prevent bubbles growth due to the high cavity pressure. When the injection is completed, the high cavity pressure is released, and then the bubbles begin to grow up. However, the surface melt has solidified at that time. So the parts surface quality can be as satisfied as traditional injection parts'.

GCP method can control the bubbles growth and remove the swirl marks. But it is not suitable for mass production due to the complex mold structure and high cost.

#### **Rapid Heating Cycle Molding (RHCM)**

Compared with conventional injection molding process, RHCM process is that the mold is rapidly heated before filling stage. The heated mold temperature is higher than the polymer thermal deformation temperature. Then the filling and packing process are going. Afterward, the mold is rapidly cooled. Finally, the products are ejected from the mold. So RHCM process circle is finished [18].

RHCM technology is widely used to improve the surface quality of injection molding parts. For example, to improve optical transparence and decrease birefringence of polystyrene, radiation heating on injection mold is proposed to directly control the temperature during the filling stage. A polycarbonate lens with a variation thickness from 1.5 mm to 7 mm can be successfully produced by electric heaters combined with chilly water cooling method.

Previous discussions about microcellular foam injection parts surface defects show that the melt temperature on the cavity surface affects the parts surface quality obviously. RHCM can meet the temperature requirement. On Oct., 2010, Trexel Inc., the supplier of the MuCell microcellular foaming technology, announced to promote MuCell for injection molding parts with Class-A/high-gloss surface finish at a global licensing agreement with Ono Sangyo Co. Ltd.. Chen SC and Li HM has successfully demonstrated the usefulness of a variable mold temperature in improving parts surface quality during microcellular foam injection molding process [14]. Figure 3-1 shows their experimental results.

Figure 3-2 shows that the effect of mold temperature on the surface roughness is very insignificant when the mold surface temperature is below 100Ԩ. The surface roughness

Microcellular Foam Injection Molding Process 199

SCF mixer is higher than its glass transition temperature or the melting point (140Ԩ for the PC resin), gas bubbles flow marks do not form on the surface of the microcellular foam injection parts. So to improve the microcellular foam injection surface quality, RHMC

RHCM technology can evidently improve the surface quality, but the heating equipment is necessary and complicated and the mold should be surface finish, good corrosion resistance and excellent hot strength. These will lead to more cost. Based on the theory of RHCM, the insulated films is stick on the surface of mold core to control the melt temperature on the cavity surface. This method is called Film Insulation. At present, the reported materials that

Polymer film (82%PET+18%PC) is used as insulated film to improve surface quality. Table

Film thickness [mm] Surface roughness [μm] Improved efficiency [%] 0 26 0 0.125 5.6 78 0.188 1.8 93 Table 3-1. Surface toughness and improved efficiency under different thicknesses of films

Table 3-1 shows that the surface roughness decreases obviously from 5.6μm to 1.8μm when the film layer thickness increases from 0.125mm to 0.188 mm. Compared with parts molded at mold temperature of 60 ℃ without film layer, the surface quality can be greatly improved

PTFE insulated film is also used [29]. And the experiment results in terms of surface roughness, surface profile of conventional and microcellular injection molded parts with and without the insulated film are discussed. Table 3-2 shows the thermal analysis of the

Thickness of PTFE [μm] Interfacial Temperature [°C] Heat Fluxes [kW/m2] 75 59 113 125 76 102 175 90 93.5 225 104 85.8

Table 3-2. Predicted interfacial temperatures and heat fluxes with different thickness of

The experiment results show that the swirl marks are eliminated under the condition of the film thickness bigger than 175μm. Because of the excellent properties about low thermal conductivity (*k*=0.25W/( mK)), low coefficient of friction(<0.1) and high melting point

corresponding microcellular foam injection molding experiments.

(327Ԩ), PTFE is very suitable to be used as insulated film.

can be used as insulation film include PEEK, PTFE, PET/PC, and so on[27-28].

process is one of useful methods.

3-1 is the experiment results [14].

without a significant cycle time increase.

used for molding [14].

PTFE [29].

**Film insulation** 

Fig. 3-1. Effect of mold temperature on the surface roughness of microcellular foam injection molded parts [26].

Fig. 3-2. Surface visual quality molded under different mold temperatures [26].

decreases from 25μm to 6.5μm when the mold surface temperature increases from 100Ԩ to 160Ԩ. When the mold temperature reached a critical value of approximately 180Ԩ, the surface roughness begins to level off at 5μm.

Figure 3-2 reveals that visible surface flow marks were eliminated with the mold temperatures higher than 160Ԩ. The reason is that when the temperature of the polymerSCF mixer is higher than its glass transition temperature or the melting point (140Ԩ for the PC resin), gas bubbles flow marks do not form on the surface of the microcellular foam injection parts. So to improve the microcellular foam injection surface quality, RHMC process is one of useful methods.

#### **Film insulation**

198 Some Critical Issues for Injection Molding

19.3

14.8

6.5

Melt temperature=300Ԩ Injection flow rate=90cm3/sec

5.1 5.2 5.4

26.0 25.0

Fig. 3-1. Effect of mold temperature on the surface roughness of microcellular foam injection

0 50 100 150 200 250

**Mold Tempreture(**Ԩ**)**

60Ԩ 100Ԩ 120Ԩ 140Ԩ

160Ԩ 180Ԩ 200Ԩ 220Ԩ

decreases from 25μm to 6.5μm when the mold surface temperature increases from 100Ԩ to 160Ԩ. When the mold temperature reached a critical value of approximately 180Ԩ, the

Figure 3-2 reveals that visible surface flow marks were eliminated with the mold temperatures higher than 160Ԩ. The reason is that when the temperature of the polymer-

Fig. 3-2. Surface visual quality molded under different mold temperatures [26].

surface roughness begins to level off at 5μm.

molded parts [26].

0

5

10

15

**Surface Roughness(**

**μ**

**m**)

20

25

30

RHCM technology can evidently improve the surface quality, but the heating equipment is necessary and complicated and the mold should be surface finish, good corrosion resistance and excellent hot strength. These will lead to more cost. Based on the theory of RHCM, the insulated films is stick on the surface of mold core to control the melt temperature on the cavity surface. This method is called Film Insulation. At present, the reported materials that can be used as insulation film include PEEK, PTFE, PET/PC, and so on[27-28].

Polymer film (82%PET+18%PC) is used as insulated film to improve surface quality. Table 3-1 is the experiment results [14].


Table 3-1. Surface toughness and improved efficiency under different thicknesses of films used for molding [14].

Table 3-1 shows that the surface roughness decreases obviously from 5.6μm to 1.8μm when the film layer thickness increases from 0.125mm to 0.188 mm. Compared with parts molded at mold temperature of 60 ℃ without film layer, the surface quality can be greatly improved without a significant cycle time increase.

PTFE insulated film is also used [29]. And the experiment results in terms of surface roughness, surface profile of conventional and microcellular injection molded parts with and without the insulated film are discussed. Table 3-2 shows the thermal analysis of the corresponding microcellular foam injection molding experiments.


Table 3-2. Predicted interfacial temperatures and heat fluxes with different thickness of PTFE [29].

The experiment results show that the swirl marks are eliminated under the condition of the film thickness bigger than 175μm. Because of the excellent properties about low thermal conductivity (*k*=0.25W/( mK)), low coefficient of friction(<0.1) and high melting point (327Ԩ), PTFE is very suitable to be used as insulated film.

Microcellular Foam Injection Molding Process 201

[13] Yin Z, Heath RJ, Hourston DJ. Morphology, mechanical properties, and thermal

[14] Chen Shiachung, Li Haimei, Hwang Shyhshin, *et al*. Passive mold temperature control

[15] Hu Guanghong: Research on key technologies of microcellular foam injection molding

[16] Yoon JD, Hong SK, Kim JH, et al. A mold surface treatment for improving surface

[17] Zhang Yatao, Li Haimei, Hwang Stanley, et al.Surface Defects and Morphology of

[18] Wang Yuea, Hu Guanghong.Research progress of improving surface quality of

[19] Goel SK, Beckman EJ. Generation of microcellular polymeric foams using supercritical

[20] Fang Kaitai, Orthogonal and Symmetrical Experiment Design [M]. Beijing: Scientific

[21] Barlow EJ, Langlois WE. Diffusion of Gas from a Liquid into an Expanding Bubble [J].

[22] Han S J, Kennedy P, Zheng R, *et al*, Numerical analysis of microcellular injection

[23] Gramann P, Turng L S, Chandra A *et al*, Modeling cell nucleation during microcellular

[24] Moldflow Pty. Ltd. Moldflow Plastics Insight Training Manual [M]. Moldflow Pty. Ltd ,

[25] Osorio A, Turng L S, Mathematical modeling and numerical simulation of cell growth

[26] Chun Shiachuang, Lin Yuwan, Chien Reander, *et al*. Variable Mold Temperature to

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Film Insulation method makes the interfacial temperature with a thin layer of insulated film higher than that of the conventional injection mold. These results show that Film Insulation is as acceptable as RHCM.

#### **4. Summary**

In this chapter, microcellular foam injection molding process is introduced. Based on the analysis of the characters of microcellular foam injection molding process, the nucleation theory and bubble growth model are described. Then the effect of process parameters on the cell morphology is detailed studied. At last, the part surface defects of microcellular foam injection molding process are introduced. At the same time, the methods to overcome such defects are referred.

#### **5. References**


Film Insulation method makes the interfacial temperature with a thin layer of insulated film higher than that of the conventional injection mold. These results show that Film Insulation

In this chapter, microcellular foam injection molding process is introduced. Based on the analysis of the characters of microcellular foam injection molding process, the nucleation theory and bubble growth model are described. Then the effect of process parameters on the cell morphology is detailed studied. At last, the part surface defects of microcellular foam injection molding process are introduced. At the same time, the methods to overcome such

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[3] Zhai Wentao,Yu Jian,He Jiasong. Research progresses in preparation of microcellular

[4] Hyde LJ, Kishbaugh LA. The MuCell® Injection Molding Process: A Strategic Cost

[6] Williams JM, Wrobleski DA. Microstructures and properties of some microcellular foams

[7] Matuana LM, Park CB, Balatinecz JJ. Structures and mechanical properties of

[8] Kumar V, Juntunen RP, Barlow C. Impact strength of high relative density solid state

[9] Ozkul MR, Mark JE, Aubert JH. Elastic and plastic mechanical responses of microcellular

[10] Ozkan E. Thermal and mechanical properties of cellular polystyrene and polystyrene

[11] Nimmer RP, Stokes VK, Ysseldyke DA. Mechanical properties of rigid thermoplastic

[12] Progelhof RC, Kumar S, Throne JL. High speed puncture impact studies of three low

foams [J]. Journal of applied polymer science. 1993, 48(5): 767-774

[5] Bill N, Mark B. The Supercritical Fluid (SCF) Delivery System[J/OL]. Trexel. Inc

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polymers by supercritical fluid technique [J]. Chinese polymer bulletin. 2009, 3: 1-

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pressure styrene thermoplastic structural foam plaques [J]. Advances in polymer

is as acceptable as RHCM.

**4. Summary** 

defects are referred.

10

1-16

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symposium, USA, 2003

technology. 1983, 3(l): 15-22

washington.edu/ vkumar/microcel/

**5. References** 


**Part 5** 

**Other Topics** 


**Part 5** 

**Other Topics** 

202 Some Critical Issues for Injection Molding

[31] Colton J.S,The nucleation of microcellular thermoplastic foam[D]. Massachusetts: MIT,

[30] Wikipedia, the free encyclopedia. Supercritical fluid. [online] Available: http://en.wikipedia. org/wiki/Supercritical\_fluid

1985.

**9** 

**Insert Molding Process** 

*1National Taiwan Ocean University, 2National Taiwan Normal University,* 

*3National Defense University,* 

*Taiwan, R.O.C.* 

 **Employing Vapour Chamber** 

Jung-Chang Wang1, Tien-Li Chang2 andYa-Wei Lee3

Insert molding process is a simplified injection molding method that eliminates secondary processing and assembly. In this process, the metal inserts are firstly formed, and placed in the mold during injection molding (the metal inserts can be designed into a grooved pattern, allowing them to be connected closely with the plastics), and then the mold is closed for injection molding. Although insert molding process can greatly improve the assembly and manufacturing procedure, the joining of two materials is the main problem yet to be solved. Because the molten plastic drives the air out of the mold cavity from the vent resulting in welding lines of a plastic part during the filling step in an injection molding process, then there will be a V-notch formed between the plastic and the mold wall if the air cannot exhaust before the melted plastic fronts meet. Thus, this chapter concerns about a V-notch found on the exterior surfaces of welding lines, which will form between the plastic and the mold wall. Not only are they appearance defects, but they also decrease the mechanical strength of the parts. Once the plastics are filled, the temperature of plastics bypassing one side of the inserts may decline more quickly than that of plastics contacting the temperature side of the mold wall, so a weld line may be formed when meeting again after bypassing the inserts. The strength at the position of weld line is generally lower than that at the region of plastics; moreover, the metal inserts are generally located at the stress region when the product is utilized. Hence, the rupture of plastics often occurs from the weld line at the rear of the metal inserts, leading to damage of products (Wang & Tsai 2011). The key approach is to rapidly and uniformly

increase the temperature of inserts before the plastics enter into the mold cavity.

Some studies indicated that, during the injection molding process, the defect of weld line could be resolved by adjusting the mold temperature. The locations of the welding lines are usually decided by the part shapes and the gate locations. Welding lines can be eliminated by the following three methods. The first method is increasing the molten plastic temperature. The viscosity of the molten plastic fluid decreases with the increasing temperature, which improves the flow pattern of the plastic, and reduces the depth of the Vnotch of the welding lines. However, degradation of the material strength sometimes occurred if the melting temperature is too high. The second method is increasing the number of the vents. Increasing the number of the vents (eg. ejecting pin or inserts) at the

**1. Introduction** 
