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編號：
畢業(yè)設計(論文)外文翻譯
（原文）
學 院： 機電工程學院
專 業(yè)： 機械設計制造及其自動化
學生姓名： 韋良華
學 號： 1000110129
指導教師單位： 機電工程學院
姓 名： 陳虎城
職 稱： 助教
2014年 5 月 26 日
a r t i c l e i n f o
Article history:
Received 25 October 2010
Received in revised form
12 January 2011
Accepted 14 January 2011
Available online 21 January 2011
Keywords:
Microcellular injection molding
Plastic foaming
Swirl-free surface
a b s t r a c t
Microcellular injection molding is the manufacturing method used for producing foamed plastic parts.Microcellular injection molding has many advantages including material, energy, and cost savings as well as enhanced dimensional stability. In spite of these advantages, this technique has been limited by its propensity to create parts with surface defects such as a rough surface or gas flow marks. Methods for improving the surface quality of microcellular plastic parts have been investigated by several researchers. This paper describes a novel method for achieving swirl-free foamed plastic parts using the microcellular injection molding process. By controlling the cell nucleation rate of the polymer/gas solution through material formulation and gas concentration, microcellular injection molded parts free of surface defects were achieved. This paper presents the theoretical background of this approach as well as the experimental results in terms of surface roughness and profile, microstructures, mechanical properties, and dimensional stability.
l Introduction
The commercially available microcellular injection molding process (a.k.a. the MuCell Process) consists of four distinctive steps, namely, gas dissolution, nucleation, cell growth, and shaping [1]. In the gas dissolution stage, polymer in the injection barrel is mixed with supercritical fluid (SCF) nitrogen, carbon dioxide, or another type of gas using a special screw which is designed to maximize the mixing and dissolving of the gas in the polymer melt. During injection, a large number of nucleation sites (orders of magnitude higher than conventional foaming processes) are formed by a rapid and substantial pressure drop as the polymer/gas solution is injected into the mold cavity, thus causing the formation of cells (bubbles). During the rest of the injection molding cycle, cells continue to grow to fill and pack out the mold and subsequently compensate for the polymer shrinkage as the material cools inside the mold. The cell growth is driven by the amount and spatial distribution of the dissolved gas. The cell growth is also controlled by processing conditions such as melt pressure and temperature as well as material properties such as melt strength and gas solubility. Finally, the shaping of the part takes place inside the mold until the mold opens allowing the part to be ejected.
Since the microcellular injection molding process was invented, there have been numerous studies on process, material, and technical developments aimed at materializing the full process potential. According to previous studies [1-5], microcellular injection molding offers a number of advantages such as cost savings, weight reduction, ease in processing due to low viscosity, and outstanding dimensional accuracy. Due to these advantages, the microcellular injection molding process has been used in many industries such as automotive, electrical goods, and home appliances using a broad range of thermoplastics. Despite these advantages, however, the surface imperfections associated with microcellular injection molded partsdsuch as unique gas flow marks, referred to as swirl marks throughout this paper, and a lack of smoothnessdstill remain one of the main drawbacks surrounding microcellular injection molding. In order to eliminate or reduce these surface imperfections there have been several studies attempted, as reported in Refs. [6-14]. Some researchers have focused on temperature modification of the mold surface to improve the surface quality of microcellular injection molded parts [6-8]. With polymeric foam, it was found that bubbles forming at the advancing melt front are first stretched by the fountain flow behavior toward the mold surface and subsequently dragged against the mold wall causing swirl marks [9]. During the filling stage of polymer melts, keeping the mold wall temperature high enough for bubbles at the mold surface to beeliminated improves the surface quality of microcellular injection molded parts. By controlling the mold temperature rapidly and precisely using mold temperature control units or other kinds of thermal or surface heating devices, microcellular foamed plastics with glossy and swirl-free surfaces can be produced.
There have also been efforts to eliminate the swirl marks on microcellular injection molded parts without any mold temperature controller. In particular, it was proposed that inserting an insulator onto the mold wall might help keeping the interface temperature between the mold and the polymer melt high. This technique basically yields the same result as temperature modification of the mold [10]. Thermal analysis and experimental results prove that the addition of an insulator layer on the mold can improve the surface quality of microcellular injection parts [11].
Another method of producing parts with an improved surface quality leads to a microcellular co-injection molding process [12]. In this technique, a proper amount of solid skin material is injected prior to the injection of a foaming core material. This can yield a sandwiched (solid skinefoamed coreesolid skin) structure with a surface finish similar to a conventionally molded component while partially maintaining the advantages of microcellular injection molding.
Another approach for improving the surface quality of microcellular
injection molded parts is the gas counter pressure process [13,14]. In this process, a high-pressure gas is injected into the mold prior to the polymer/gas solution to suppress cell nucleation and bubble growth while the polymer/gas solution is being injected into the mold cavity. Toward the end of injection, counter gas pressure is released and bubbles begin to form within the cavity. Since a majority of the part surface is already solidified, gas flow marks are eliminated.
In spite of these efforts to improve the surface quality, there have been difficulties in applying the microcellular injection molding process in industries requiring parts with high surface qualities because these techniques entail additional equipment which results in high costs or maintenance. There have been no reported studies on improving the surface quality of microcellular injection molded parts without any additional equipment or modification to existing equipment.
This paper proposes a novel approach to improve the surface quality of microcellular injection molded parts by controlling the cell nucleation rate. In this study, the cell nucleation rate was dramatically lowered or delayed by controlling the degree of supersaturation so that cell nucleation was delayed during the filling stage. After the polymer/gas solution volumetrically filled the mold cavity, intentionally delayed nucleation occurred and bubbles formed in the polymer matrix, except on the surface where the material had already solidified upon touching the mold surface. Theoretical background and experimental results are described in this paper. Microstructure, surface profile, surface roughness,mechanical properties, and dimensional stability are also investigated in this study.
2. Theoretical
2.1. Nucleation theory for polymeric foams
In polymeric foams, nucleation refers to the initial stage of the formation of gas bubbles in the polymeregas solution. For nucleation,
gas bubbles must overcome the free energy barrier before they can survive and grow to macroscopic size [15]. According to classical nucleation theories [16-18], the nucleation rate is controlled by the macroscopic properties and states of the polymer and gas such as solubility, diffusivity, surface tension, gas concentration, temperature, and the degree of super saturation.
One representative equation for the nucleation rate of polymeric foams was reported by Colton and Suh [19,20]. In addition to the mathematical representation, they also verified their nucleation theory experimentally for a batch foaming process using a high pressure vessel. The nucleation equation for microcellular foams dominated by the classical nucleation theory [16e18] can be expressed as
N=fCex(-?G**/kT)
where N is the nucleation rate, f is the frequency of atomic molecular lattice vibration, C is the concentration of gas molecules, k is the Boltzmann’s constant, T is the absolute temperature, and ?G**is the activation energy barrier for nucleation.
According to previous studies [19,20], the nucleation rate of polymeric foams is composed of two components: a homogeneous term and a heterogeneous term. The activation energy for homogeneous nucleation is given by
?Ghom**?16πr33?P2
where g is the surface energy of the bubble interface and ?P.is
assumed to be the gas saturation pressure. More precisely,
?P=|Pr'-Pr| where Pr` is the pressure that is exerted in a high
pressure vessel and Pr is the pressure of the supersaturated vapor in
the sample [16]. That is, DP is the pressure difference between the
pressure that is applied to the sample and the pressure of the supersaturated vapor in the sample. When the pressure that saturates
the gas in a high pressure vessel is suddenly released to trigger the so-called thermodynamic instability by rendering the sample into the supersaturated state, Pr` becomes 1 bardso low compared to Pr that DP can be approximated as Pr.
On the other hand, the activation energy for heterogeneous nucleation is affected by a geometric factor that depends on the contact (wetting) angle between the polymer and the particle and can be expressed as
?Ghet**=?Ghom**×f(θ) (3a)
fθ=12-34cosθ+14cosθ3 (3b)
where f(q) is a geometric factor that is dependent upon the contact
angle, θ, of the interface between the polymer and a second phase,
and has values of less than or equal to 1. For a typical wetting angle
of around 200 on the interface between a solid particle and the polymer melt, the geometric factor is 2.7X10-3, suggesting that the energy barrier for heterogeneous nucleation can be reduced by three orders of magnitude with the presence of an interface [20,21].
l 2.2. Nucleation theory for microcellular injection molding
In the batch foaming process, the theory of Colton and Suh was verified by their experiments. Due to the large difference between the pressure exerted in a high pressure vessel and the pressure of the supersaturated vapor in the sample, the gas pressure dissolved in the polymer, the?P in the Gibbs free energy equation, can be approximately assumed to be the saturation gas pressure. The assumption that ?P is the gas saturation pressure is fairly reasonable in a batch foaming process although the ?Pcan still have an error of about 30-40% due to overestimation as reported in a previous study [15].
The nucleation theory by Colton and Suh is a simplified form derived and modified from classic nucleation theories [16-18] and is generally adequate for the batch foaming process. However, there is a need for this theory to be modified in cases of microcellular injection molding and extrusion systems because ?P cannot be directly controlled and measured. To predict nucleation in microcellular injection molding and extrusion processes more precisely, this paper proposes a cell nucleation theory of a different form, which includes a term for the degree of supersaturation because it is a directly controllable factor.
To avoid misestimating ?P, and to consider the degree of supersaturation, a more proper activation energy equation for nucleation can be derived from the following equation [16,17]
?P=|Pr'-Pr|=2rrc （4）
where rc is the radius of a characteristic droplet, and the W.
Thomson equation
RTlnPrP∞=2r?Mr?p （5）
where P∞ is the pressure of the saturated vapor (i.e., the equilibrium
pressure), R is the universal gas constant, M is the molar mass, and p is the density. These equations yield
?P=RTρlnPrP∞M （6）
which can be alternatively expressed as
?P=ktρ1lnS (7)
whereρ1is the molecular density of the bulk liquid, and S(=PrP∞)
is defined as the degree of supersaturation.
Thus, the activation energy equation (cf. Equation (2)) for nucleation in the microcellular injection molding process can be given by
?G**=16πr33(kTρ1lnS)2 （8）
Hence it can be stated that the activation energy for nucleation is inversely proportional to the square of the natural logarithm of the supersaturation degree.
In the microcellular injection molding process, the polymer/gas
solution becomes a metastable supersaturation solution when it is
injected into the mold cavity. This is because the amount of gas able to be dissolved in the polymer in the presence of a rapid pressure drop is less than the gas amount originally dissolved in polymer melts. In particular, assuming the air in the cavity is properly vented, the pressure at the advancing melt front is at the atmospheric pressure. The solubility of a gas in a polymer at atmospheric pressure and processing temperature can be obtained by an Arrhenius-type expression with regard to temperature [22]
S@1 atm; melt temperature=S@STPexp?(-?HsR(1Tmelt-1298)) (9)
where S@STP is the solubility of the gas in the polymer at standard
temperature and pressure conditions (298 K and 1 atm). The parameter DHs is the molar heat of sorption, and Tmelt is the polymer melt temperature.
Thus, the degree of supersaturation is given by
S=mgS@STPexp?(-?HsR(1Tmelt-1298)) (10)
where mg is the gas dosage which can be controlled by the supercritical
fluid (SCF) supply system.
The heat of sorption, ?HsRg, of various polymer/gas systems at standard temperature has been studied and summarized in many previously published studies. In order to obtain the degree of supersaturation for a polymer/gas solution in the microcellular injection molding process, one has to either measure the solubility of the gas in the polymer at standard temperature and pressure or consult published data on the solubility of the gas in the polymer. Then, the activation energy barrier for nucleation in Equation (8), ?G**, can be obtained based on the calculated degree of supersaturation and the surface energy of the bubble interface, γ. Given the activation energy barrier and the frequency factor, f, the nucleation rate (expressed in Equation (1)) can then be calculated.The estimate of the surface energy of the bubble interface and the frequency factor is discussed below.
In microcellular injection molding, the polymer/gas solution can
be treated as a liquid mixture. Thus, the surface energy of the
bubble interface, g, can be expressed as [23,24]
γmix=γpolymerρmixρpolymer4(1-wgas) （11）
where γpolymer is the surface energy of the polymer, P′S are the
densities, and wgas is the weight fraction of gas.
In addition, a frequency factor for a gas molecule, f, in Eq. (1) can
be expressed as [24-26]
f=Zβ(4πrc2) (12)
where z is the Zeldovich factor, which accounts for the many clusters that have reached the critical size, rc., but are still unable to grow to sustainable bubbles. The parameter b is the impingement rate at which gas molecules collide with the wall of a cluster. The parameter Zβcan be used as a correction factor and is determined experimentally.
Once the nucleation rate as a function of the degree of supersaturation
is obtained, one can control the gas (SCF) content in the polymer melt to control or delay the onset of cell nucleation so that no bubble will form at the advancing melt front during the injection filling stage, thus, allowing microcellular parts with solid, swirl-free surface to be injection molded.
3. Experimental
3.1. Materials
The material used in this study was an injection molding grade
low density polyethylene, LDPE (Chevron Phillips Chemical Company, Texas, USA). It has a melt index of 25 g/10 min and a density of 0.925 g/cm3.
To confirm the theory for improving surface quality by controlling
the degree of supersaturation, a random copolymer polypropylene (PP)was also used in this study. The PP used in this study was Titanpro SM668 (Titan Chemicals Corp., Malaysia), with a melt flow index of 20 g/10 min and a density of 0.9 g/cm3. Both materials were used as received without any colorant, fillers, or additives.
Commercial grade nitrogen was used as a physical blowing agent for the microcellular injection molding trials.
3.2. Microcellular injection molding
In this study, an Arburg 320S injection molding machine (Arburg,Germany) was used for both the solid conventional and microcellular injection molding experiments. The supercritical fluid (SCF) supply system used in this study was the S11-TR3 model (Trexel, Woburn,MA, USA). The total gas dosagewas controlled by adjusting the gas injection time, t, and the gas injection flowrate,m_ g. A tensile test mold, which produces tensile test specimens that meet the ASTM D638 Type I standards, was used for this experiment.
For injectionmolding of both LDPE and PP tensile test specimens,
nozzle and mold temperatures were set at 221 。C and 25 。C, respectively. The cycle time was 40 s. An injection speed of 80 cm3/s was employed. In this study, six different gas dosages (concentrations) were used for injection molding of LDPE as shown in Table 1. Also, four different gas dosages were employed for microcellular injection molding of PP. The supercritical fluid was injected into the injection barrel at 140 bar pressure to be mixed with the polymer melts in this experiment. The weight reduction of foamed versus solid plastic partswas targeted at 8 _ 0.5% for each specimen. For the conventional injectionmolding experiment, the shot size of 20.2 cm3 and a packing pressure of 800 bars were employed for 6 s. For the microcellular injection molding experiments, the shot size of the polymer melt was 19.2 cm3 and the packing stage was eliminated.
3.3. Analysis methods
To analyze the surface roughness of the molded tensile bar specimens, a Federal Surfanalyzer 4000 (Federal Product Corporation, RI, USA)was used. The surface roughnesses of conventional and microcellular injection molded parts were evaluated at three locations shown in Fig. 1 and the averaged surface roughness based on measurementsdone at all three locationswas recordedandreported. The cutoff, drive speed, and drive length for the test were 0.75 mm, 2.5 mm/s, and 25 mm, respectively. For each process condition, ten specimens and three points on each specimen were tested.
In addition to the surface roughness, swirl marks commonly observed in microcellular injection molded samples can also be clearly revealed by a 3-D surface profiler. Zygo NewView (Zygo Corporation, CT, USA), a non-contact 3-D surface profiler, was employed to examine the surface profile of injection molded parts in this study using a scan distance of ±10 mm.
A JEOL JSM-6100 scanning electron microscope with an accelerating
voltage of 15 kV was employed for observing the microstructures of the foamed parts. To observe the cross section of the microcellular injection molded parts, test specimens were frozen by liquid nitrogen and subsequently fractured. Representative images of each process condition were selected and cell sizes and densities were analyzed. A UTHSCSA Image Tool was employed as the ima