Journal of Materials Processing Technology 155–156 (2004) 1201–1206

A new process of gas-assisted injection molding for faster cooling

Seong-Yeol Hana,∗, Jin-Kwan Kwagb, Cheol-Ju Kimb, Tae-Won Parkc, Yeong-Deug Jeongda Graduate School of Precision Mechanical Engineering, Pukyong National University, Busan, South Korea

b NARA M&D Co., Ltd, 50-1, Changwon, Gyeong Nam, South Koreac Department of Computer Aided Die and Mold, Changwon Polytechnic College, Changwon, Gyeong Nam, South Korea

d School of Mechanical Engineering, Pukyong National University, Busan, South Korea

Abstract

Gas-assisted injection molding (GAIM) process, that can be used to provide a hollow shape in a molding, is a variant of the conventionalinjection molding process. GAIM has many advantages such as reduction of material, sink mark, warpage, and lower injection pressure.Thus, GAIM has been widely applied in the industry to make moldings with a hollow channel such as handles, TV frames and so on. Onthe other hand, GAIM has some disadvantages such as slow cooling time and flow marks. In the disadvantages, hot gas core causes slowcooling of a molding and post-warpage.

To solve these problems, we devised a new GAIM process that has been called the reverse gas injection molding (RGIM). The RGIMhas two special units; one is the overflow buffer, which is used for reduction of a material, and the other the air unit, which is used for fastercooling of a molding. Through experiments verifying the efficiency of the cooling in the RGIM process, it was found that the efficiency ofthe RGIM process was approximately 50% better than the conventional GAIM process. Also, this experimental result was confirmed inthe numerical calculations and CAE simulations.© 2004 Elsevier B.V. All rights reserved.

Keywords: Gas-assisted injection molding (GAIM); Reverse gas injection molding (RGIM); Air unit; Overflow buffer

1. Background on gas-assisted injection molding

Injection molding represents the most important processfor manufacturing plastic parts. It is suitable for mass pro-ducing articles, since raw material can be converted into amolding by a single procedure[1]. According to Rosato[2],1986, approximately 32% by weight of all plastic parts aremanufactured by the injection molding process. Injectionmolding is widely used in the manufacture of a variety ofplastic components in consumer and commercial products.

Recently, an innovative injection molding process, calledgas-assisted injection molding (GAIM)[3,4], was developedfor producing parts with hollow shapes. The GAIM pro-cess has been known for more than 25 years. The originalidea of GAIM came from the so-called “injection blowing”method, which is widely used, particularly for the fabrica-tion of bottles and other relatively small hollow bodies. Theuse of pressurized gas for a conventional plastic injection

∗ Corresponding author.E-mail addresses: cozyhan@mail1.pknu.ac.kr (S.-Y. Han),yeky@nara-mnd.co.kr (J.-K. Kwag), ryh4851@nara-mnd.co.kr(C.-J. Kim), twpark@kopo.or.kr (T.-W. Park), ydjung@pknu.ac.kr(Y.-D. Jeong).

molding process is believed to have been first made com-mercially available by the invention of Friederich, which isdisclosed in US Patent No. 4,101,167 issued July 18, 1978.The Friendrich patent solved the problem of molding hollowshape bodies in a single injection molding operation[5].

Specifically, during those early years, the industry paidmost of its attention on the use of structural foam as a spe-cial process used for molding relatively thick-sectioned ar-ticles. These parts are light in weight and have acceptablesurface finish, i.e., without sink marks that are associatedwith conventional plastic injection molding. In recent years,attention has been concentrated on the use of gas assistancewith conventional plastic injection molding to achieve highproduct quality and productivity. Good surface quality, shortcycle times, lower clamp tonnage, material saving, weightreduction and minimization of part distortion or warpagecan all be achieved with proper utilization of gas assistanceinto a conventional plastic injection molding process.

There are two methods in conventional GAIM. The oneis “short shot”. The short shot is sequentially done by fol-lowing a simple three-step process. In the short shot pro-cessing, a molten polymer is initially filled in cavity about75–98% by ram speed control of the injection molding ma-chine. After a short delay period, compressed nitrogen gas

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cores out the molten polymer, filling the remainder of themold. The next step is the gas packing stage that compen-sates for the volumetric shrinkage of the polymer melt. Asthe plastic solidifies, the gas expands into volume created byshrinkage, locally packing out the part[6]. The short shotmethod is used for thick section moldings, typically handlesand tubular components. The advantage of the short shot isreduction in molded plastic weights. However, surface de-fects such as hesitation mark[7] may be visible when thegas is injected too late or the initial gas pressure is too low.The other is “full shot”. The full shot is injected to fill ornearly fill the mold cavity, but the plastic is not packed byan injection molding machine. After a selected time delay,first phase gas is injected. Second phase gas penetration oc-curs to compensate for volumetric shrinkage of the plasticas it cools. A uniform gas pressure is applied throughout theplastic. Gas is exhausted to atmosphere or for recovery be-fore mold opens. Plastic refill commences after the nozzlevalve is closed or after the plastic feed gate has solidified.The ‘full shot’ method is normally applicable for compo-nents in which there are thick and thin sections. The gasflows into the path of least resistance in the thicker sectionswhere the plastic interior is still in a molten state[5]. Thepushed melt needs to expel from the cavity to another place.The place is called overflow and wholly wastes material.

In the above two GAIM methods, there are been still somedisadvantages such as exploding at molding by high pres-sure gas, surface dimmed by mixed gas and melt, limitationof gas nozzle design and slow cooling of molding. In thesedisadvantages, slow cooling of the molding is one of themost important things connecting with a cycle time of man-ufacturing. The interior temperature of a hollow shape is in-creased by the injected gas, and rises almost to the meltingtemperature. The heated gas in the hollow shape acts likea hot core that causes the molding to cool slowly. Conse-quently, the cycle of GAIM process would be lengthened.

2. Reverse gas injection molding (RGIM)

In conventional GAIM process, hesitation marks havebeen needed to remove. The material for injection moldinghas been needed to reduce more and more. The gas in ahollow shape acting as hot core has been needed to removefor faster cooling of molding. For satisfying these require-ments, we devised a new GAIM process, the reverse gas in-jection molding, which was registered in Korea Patent No.0286015. There are some special units that include an airunit that is used to vent and remove the hot nitrogen gas inhollow shape and the overflow buffer that is used to reducea material.Fig. 1shows the schematic of the RGIM system.

The RGIM process is divided into four steps (Fig. 2). Thefirst step is filling. A melt injected from a hot runner fillsthe mold cavity about 98–100% such as full shot method.The second step is hollowing out the melt after a constantdelay time. A pressured gas is injected into the mold cavity.

Fig. 1. Schematic of the RGIM system.

The pressured gas makes a hollow shape. Synchronously,the pushed melt that is still in a molten state in the middle ofcavity reversely re-enters into an overflow buffer. The thirdstep is holding. The pressured gas acts as holding pressure.The last step is air blowing. Completing holding phase for afew seconds, air is blown for a few seconds by the air unit tocool the molding and to remove the hot gas in hollow shape.We applied a cutter that automatically acts to vent hot gasby the air unit.

In the RGIM process, there are two differences from thetraditional GAIM process. The first is that there isn’t thephysical overflow for the expelling of a melt. Instead ofexpelling, the melt re-enters into the overflow buffer by theinjected gas while hollowing out the melt. This step savesmaterial. So, this new GAIM process was named as theRGIM. The second is that there is the air unit in this system.This air unit vents and removes the hot gas for faster coolingin the hollow shape by air blowing.

3. The injection molding experiments in the RGIMprocess

The experimental equipments consisted of a moldthat included the function of the RGIM process, a gaspressure-generating unit and an air unit. The mold hasone cavity and was used to make an electric microwave’shandle. The mold temperature was 50◦C. The gaspressure-generating unit was made by GAIN Technologies[8]. The air unit is for cooling a molding (Fig. 3). The airtemperature was 15◦C. We used the LG injection moldingmachine (LGH 140N) (Fig. 4).

3.1. DOE for the RGIM process

The experiments were conducted to verify the main cool-ing efficiency of the RGIM with two kinds of polymersthat are general purpose polystyrene (GPPS, LG Chemi-cal 25SPI) and polypropylene (PP, LG Chemical M580). Toincrease efficiency and confidence of the experiment, weplanned the experiments with the Taguchi method[9]. The

S.-Y. Han et al. / Journal of Materials Processing Technology 155–156 (2004) 1201–1206 1203

Fig. 2. Four steps of the RGIM process.

Fig. 3. Gas pressure-generating unit and air unit.

ANOVA in statistical software MINITABTM was used forthe analysis of the experimental results[10]. Three injec-tion variables that would have most effective on the moldingquality were selected. These were also used as variables inthe design of experiment (DOE).Table 1shows the variablesand levels for the experiment.

Table 2shows the DOE schedules for this experiment.The experimental samples were molded five times at eachprocess condition. After dismissing the first and last samples,the averaged value of the other three samples was taken asinput data.

Fig. 4. Injection molding machine.

3.2. Experiment and analysis for GPPS

The GPPS polymer was used first in the experiment. Wemeasured the temperature, length (Fig. 5) and weight of theejected moldings.Table 3shows the measured values.

Table 1Experimental variables and levels

Melt temperature(◦C)

Delay time forair blowing (s)

Duration time forair blowing (s)

Level 1 210 21 50Level 2 220 31 60Level 3 230 41 70

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Table 2Design of experimental for the RGIM process

Processcondition

Melttemperature(◦C)

Delay timefor airblowing (s)

Duration timefor airblowing (s)

1 210 21 502 210 31 603 210 41 704 220 21 605 220 31 706 220 41 507 230 21 708 230 31 509 230 41 60

Fig. 5. Molding produced by the RGIM.

The molding was first molded without air blowing at melttemperature 220◦C with the GPPS. When the molding wasejected, after cooling for 30 s in the mold, the tempera-ture of the molding was 175◦C. In the main experiment,the seventh process condition showed the lowest moldingtemperature in the ejected moldings. The highest moldingtemperature was measured at the eighth. In between sev-

Table 3Measured temperature, length and weight under each process condition

Processcondition

Moldingtemperature(◦C)

Length 1(mm)

Length 2(mm)

Weight (g)

1 118.0 213.3 250.0 1062 105.0 213.7 250.7 1063 107.7 213.7 251.0 1064 107.3 211.7 249.3 1065 97.4 213.3 251.0 1066 113.1 209.7 248.0 1057 94.0 213.3 251.0 1068 120.0 208.7 246.0 1069 111.0 210.7 247.3 106

Fig. 6. S/N ratio plot of molding temperature for GPPS.

enth and eighth, the difference of the molding temperaturewas 26◦C. Comparing the molding temperature measuredin the seventh process condition with the molding tempera-ture measured without air blowing, the efficiency of coolingin molding increased approximately 50%. In the length ofmolding, the longest molding occurred in the third processcondition. The eighth process condition molded the short-est molding. Although the process conditions changed, theweight of moldings rarely changed.

The S/N ratio for the “smaller is better” characteristic wascalculated based on the measured molding temperature inTable 3. The Fig. 6 shows that the duration of air blowingwas the main variable affecting cooling efficiency, amongthe three variables. The predicted optimum process was melttemperature 220◦C, delay time for air injection 21 s andduration of air blowing 70 s by the ANOVA.

3.3. Experiment and analysis for PP

In the experiment of PP polymer, the values of tempera-ture, length (Fig. 5), height and gloss of the ejected moldingswere measured.Table 4shows the measured values.

Comparing the temperature of molding measured inthe third process condition with the temperature measuredwithout air blowing, the efficiency of cooling in moldingincreased about 45%. The length, heights and gloss ofmoldings hardly changed.

The S/N ratio is calculated based on the measured mold-ing temperature inTable 4. The Fig. 7 shows that the du-ration of air blowing was a main variable affecting on the

Table 4Measured temperature, length and weight under each process condition

Processcondition

Moldingtemperature(◦C)

Length 1(mm)

Height(mm)

Gloss(GU)

1 126.6 210.1 47.21 38.692 122.6 210.3 47.24 36.663 117.0 210.5 47.03 38.024 126.0 209.6 47.39 33.835 120.0 210.5 47.10 33.636 131.6 209.9 47.51 31.857 123.0 210.3 47.18 31.658 129.6 210.2 47.53 34.509 128.0 210.6 47.39 27.70

S.-Y. Han et al. / Journal of Materials Processing Technology 155–156 (2004) 1201–1206 1205

Fig. 7. S/N ratio plot of molding temperature for PP.

molding temperature. The predicted optimum process wasmelt temperature 210◦C, delay time for air blowing 31 s andduration of air blowing 70 s.

4. Numerical calculation of cooling time for the RGIM

The efficiency of air blowing was calculated with a nu-merical cooling equation. It needed to assume two casesfor the calculation of molding cooling time. The first casewas that a molding was cooled without air blowing. Thesecond case was that a molding was cooled by air blowingand heat transfer between mold and molding. Polycarbonate(PC) polymer was used for calculating. The reason of us-ing of PC that needs to be melt with high temperature wasclearly to show the efficiency of this system.

PC’s density (ρ) is 1.17 g/cm3, the diffusivity (R)is 0.0004547 cal/s cm◦C and the specific heat (Cp) is0.2319 cal/g◦C. The melt temperature (TM) is 300◦C, themold temperature (TW) was 60◦C and the ejecting temper-ature (TE) was 70◦C.

The following Fourier cooling equation was used for cal-culating cooling time[11]

tc = s2

π2αln

(8

π2

TM − TW

TE − TW

)(1)

α = R

ρCp

(2)

Fig. 9. Temperature distribution without air blowing during 30 s.

Fig. 8. Model for numerical calculation of heat transfer.

Table 5Calculation equations for heat exchange[12]

Equations Values

Input heat Q = WCp �T 6.981 kcal

Output heat Q1 = haA1(TM − Ta) 0.047 kcal/sQ2 = hRA2(TM − TW) 0.0638 kcal/s

Where Q: total input calories through molding (kcal);W: weight ofmodel (g);Cp: specific heat of model (kcal/g◦C); �T: difference betweenmelt temperature and ejecting temperature of molding (◦C); Q1: caloriesby heat transfer per second (kcal/s);ha: heat transfer coefficient of airblowing (kcal/mm2 h◦C) 1.72E+8; A1: surface area of hollow shape(mm2) 4.71E+5; TM: melt temperature (◦C); Ta: air temperature (◦C);Q2: calories removed by air blowing per second (kcal/s);hR: heat transfercoefficient of mold (kcal/mm2 h◦C) 5.0E+7; A2: surface area of betweenmolding and mold (mm2) 1.41E+6; Tw: mold temperature (◦C).

wheretc is the cooling time (s),s the wall thickness (mm),αthe thermal diffusivity (mm2/s)In the first result, the coolingtime without air blowing was 733 s as calculated with theFourier coolingEq. (1).

For the second case, the calculated total input calories ofthe molding wasQ, the calories of heat transfer betweenmold and molding per second wasQ1 and the removed calo-ries by air blowing in molding per second wasQ2. Fig. 8shows the model for numerical calculation.

It was assumed that the amount of the removed calorieswould be the average of the sum for the calories removedby air blowing (Q1) and heat transfer (Q2). So, the coolingtime of molding was calculated 126 s. This cooling time byair blowing was faster 5.5 times than the case 1 (Table 5).

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Fig. 10. Temperature distribution with air blowing during 20 s.

5. Temperature distribution simulation by marcsoftware

The heat transfer was simulated in order to verify the effi-ciency of the RGIM about the mentioned two cases with theMARC software. The first case is that molding was cooledwithout air blowing in the mold. The second case is that themolding was cooled by air blowing and heat transfer be-tween mold and molding in the mold. The material of themodel was PC. The mechanical properties and all conditionsfor input data in the MARC were the same as the conditionsin the numerical calculation. When the air was not blowninto the hollow shape, the temperature of nitrogen gas andair normally increased similar to the melting temperature.So, it was assumed that the interior temperature of the hol-low shape would be 200◦C.

Fig. 9shows temperature distribution of molding that wascooled for 30 s without air place at the boundary betweenmold wall and molding. But there was not a transfer of heatat the hollow shape.

Fig. 10shows a temperature distribution of molding withair blowing for 20 s at the hollow shape. There was a heattransfer by air blowing at the hollow and the temperatureat the hollow shape decreased to 70◦C which is ideal forejecting from the mold.

6. Conclusion

The reverse gas injection molding was devised to solvethe problems of slow cooling time and to improve surfacequality on a molding produced in the conventional GAIM.For verifying the feasibility of the RGIM process, the exper-iment was conducted on the mold that was used to make mi-crowave handles. In the experimental results, the efficiencyof cooling on the RGIM process was better about 50% than

the conventional GAIM process. The results of the numer-ical calculation and CAE simulation for the heat transfershowed the feasibility of this system.

Acknowledgements

This work was supported by the BRAIN Korea 21 Projectand the NARA M&D Co. Ltd.

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