Study of Application of Ultrasonic Waveto Injection Molding

Atsushi Sato,1 Hiroshi Ito,2 Kiyohito Koyama21 Advanced Technology Research Laboratories, Idemitsu Kosan Co., Ltd., Ichihara, Chiba 299-093, Japan

2 Faculty of Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan

We have developed an ultrasonic injection molding(UIM) system, which applies ultrasonic waves to injec-tion molding, as a precision injection molding tech-nology. Molding of optical lenses showed that thelens weight increased when the ultrasonic wave wasapplied immediately after the resin was filled into acavity. Results showed that, by applying ultrasonicwaves, oscillatory flow was generated inside the cavityand consequently the weight of the lens wasincreased. The surface finish of the molded lens wasalso significantly improved in UIM when compared withthat in conventional molding. The part of shrinkageduring the packing and holding stages was suppressedby the oscillatory flow provided by the ultrasonic vibra-tion. Because of ultrasonic energy absorption, localheating was generated inside the resin, resulting in theformation of oscillatory flow during packing andholding stages. Local heating, occurring especiallybetween molten and skin layers, reduces the skinlayer’s deformation resistance. Consequently, replica-tion during the packing and holding stages is facili-tated by the UIM. Moreover, evaluation of the residualoptical strain of concave lens revealed that the strainwas much lower in UIM than in conventional molding.The decreased strain was attributed to the local heatgeneration by the ultrasonic waves. POLYM. ENG. SCI.,49:768–773, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

Injection molded precision products have consistently

improved in performance and function over the past

years. However, it is difficult to mold precision products

using conventional injection molding techniques. For that

reason, we investigated the application of ultrasonic

waves, which have the action of reducing apparent fric-

tion and causing local heating, to injection molding. Con-

sequently, we developed the ultrasonic injection molding

(UIM) system. Numerous studies have been investigated

polymer processing using ultrasonic waves [1–17]. How-

ever, except for the application of such vibration at a low

frequency range [18, 19], few such reports describe the

application of ultrasonic vibration to injection molding. In

our previous studies [20–22], the apparent fluidity of resin

was improved by the action of sound pressure and by the

reduction of apparent friction between the wall surface in

the cavity and the resin using the UIM system. Further-

more, the reaction force (inertia force) of vibration and

local heating improved the replication of microstructure of

the molded surface. In an earlier version of the UIM sys-

tem, the entire mold was designed to resonate longitudi-

nally at one wavelength by means of exposure to ultrasonic

waves. However, this system is difficult to apply when a

complex mold is in use. As a result, we improved the UIM

system and the current version is designed to focus ultra-

sonic waves on a part of resin flow channel in the mold.

In this study, the replication properties of an optical

lens’ surface microstructure, along with the residual

optical strain, were evaluated using the improved UIM

method.

EXPERIMENTAL

Material and UIM System

In our experiments, we used a commercial polycarbon-

ate (PC: Mv ¼ 22,000, MI ¼ 10 g/10 min, Idemitsu

Kosan Co. Ltd.). Figure 1 shows the improved UIM sys-

tem mounted on an injection machine. The system con-

sists of an ultrasonic oscillator, an oscillation controller

and a mold connected to a transducer, and a directional

converter of ultrasonic waves. The frequency of the ultra-

sonic oscillator was set at 19 kHz and its maximum

power was 1.2 kW. A 40-mm diameter rod was connected

to the directional converter of ultrasonic waves to focus

the ultrasonic waves on the molding product. The rod

vibrates at 19 kHz with the maximum amplitude of 11

lm (0-Peak). An injection machine with clamping force

of 3430 kN was used.

Correspondence to: Atsushi Sato; e-mail: atsushi.sato01@si.idemitsu.co.jp

DOI 10.1002/pen.21268

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

Optical Design of the Concave Lens

This study used a concave lens like that depicted in

Fig. 2. The focal distance of the concave lens was

designed according to Eq. 1.

1=F ¼ ðN � 1Þð1=R1 þ 1=R2Þ (1)

Therein, F denotes the focal distance, N represents the

refractive index of material, R1 is the radius of front cur-

vature, and R2 is the radius of rear curvature. In this

experiment, the design value of the focal distance is

20.25 m for PC using N ¼ 1.585.

As shown in Table 1, the lens was molded by injection

compression molding using a shut off nozzle under the

following conditions. The ultrasonic waves were applied

immediately after the cavity was filled with resin. All

lenses were molded with a constant thickness in the cen-

ter of 1.3 mm.

RESULTS AND DISCUSSION

Evaluation of Lens Weight

The results of lens weight measurements at various os-

cillation times with an amplitude of 11 lm are shown in

Fig. 3. Results show the lens weights produced by UIM

with increasing oscillation time up to 20 s were higher

than those produced by conventional molding. Lens

weight measurements at various amplitudes for the oscil-

lation time of 10 s are shown in Fig. 4. Lens weights

increased with increasing amplitude.

The results described earlier indicate that the resin flow

occurred in the mold because ultrasonic waves were

applied after the cavity was filled with resin. Conse-

quently, the oscillation part of the ultrasonic waves gener-

ated a sink mark, as shown in Fig. 5. Apparently, the

oscillatory flow stuffed the resin into the cavity from the

oscillation part.

Next, the oscillatory flow in the mold was evaluated

for injection molding in comparison with the results

obtained from injection compression molding. In this

evaluation, the lens shape depicted in Fig. 6 was used.

The molding conditions are presented in Table 2. Lens

weights were measured at various oscillation times, hold-

ing pressures, and amplitudes, as shown in Fig. 7. The

results reveal that lens weights increased with increasing

oscillation time in a manner similar to the results depicted

in Fig. 3. When the holding pressures were changed, it

was found that the oscillatory flow was generated easily

at low holding pressure because the lens weight increase

was greater at such pressures. The measurement results of

lens weights at various action timings, holding pressures,

and amplitudes are shown in Fig. 8. When the action tim-

ings were changed, lens weights increased as well. How-

ever, no further increases in lens weight for action timing

FIG. 1. Schematic drawing of UIM system.

FIG. 2. Shape of concave lens.

TABLE 1. Conditions of injection compression molding.

Injection machine: clamping force 3430 kN (AZ7000;

NISSEI PLASTIC INDUSTRIAL CO., LTD.)

Material : polycarbonate (TARFLON A2200;

IDEMITSU KOSAN CO., LTD.)

Molding temperature: 2808CMold temperature: 1208CAction of shut off nozzle: after injection finished

Compression force: 980 kN

Cooling time: 120 s

Ultrasonic oscillator: frequency 19 kHz (SONOPET1204B;

SEIDENSHA ELECTRONICS CO., LTD.)

Amplitude (0-peak): 0–11 lmAction timing: after the cavity was filled with resin

Oscillation time: 0–60 s

FIG. 3. Dependence of the lens weights on the oscillation time.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 769

were observed after 6 s from the injection start because

the gate had sealed, thus preventing further oscillatory

flow in the cavity. In general, the final shape of the injec-

tion molding product will be molded by the packing pres-

sure of the resin during the holding pressure stage. The

packing pressure in UIM would be higher than that of

conventional injection molding. Consequently, our results

showed that oscillatory flow in the cavity also occurred

during injection molding, independently of the shape of

the molding product.

Mechanism of Oscillatory Flow Generation

The results of our previous studies [20–22] indicate

that oscillatory flow is generated by local heating through

absorption of the ultrasonic energy. The temperature

increase of the oscillation part of the ultrasonic wave

(DT) is shown as Eq. 2.

DT ¼ I tð1� e�2aXÞ=ðXrHÞ (2)

Here, I denotes the sound intensity, t is the oscillation

time, a is the coefficient of ultrasonic energy absorption,

X is the distance, q is the density, and H is the heat

capacity. The sound intensity I is represented as Eq. 3.

FIG. 4. Dependence of the lens weights on the amplitude.

FIG. 5. The sink mark of oscillation part.

FIG. 6. Shape of plano-concave lens.

TABLE 2. Conditions of injection molding.

Injection machine: clamping force 980 kN (SG100M-HP; SUMITOMO

HEAVY INDUSTRIES, LTD.)

Material: polycarbonate (TARFLON A1500; IDEMITSU KOSAN

CO., LTD.)

Molding temperature: 2808CMold temperature: 1208CCooling time: 20 s

Ultrasonic oscillator: frequency 19 kHz (SONOPET1204B;

SEIDENSHA ELECTRONICS CO., LTD.)

Amplitude (0-peak): 0–11 lmAction timing: after 0–6 s from injection start

Oscillation time: 0–1 s

FIG. 7. Dependence of the lens weights on the oscillation time. l:

holding pressure 60 MPa, action timing 5 s after injection start, ampli-

tude 9 lm; ~: holding pressure 80 MPa, action timing 2 s after injection

start, amplitude 9 lm; n: holding pressure 80 MPa, action timing 5 s

after injection start, amplitude 9 lm; &: holding pressure 80 MPa,

action timing 5 s after injection start, amplitude 11 lm.

FIG. 8. Dependence of the lens weights on the action timing of ultra-

sonic waves from injection start. l: holding pressure 60 MPa, oscillation

time 0.5 s, amplitude 9 lm; ~: holding pressure 80 MPa, oscillation

time 0.5 s, amplitude 9 lm; ~: holding pressure 80 MPa, oscillation

time 0.5 s, amplitude 11 lm; n: holding pressure 80 MPa, oscillation

time 0.7 s, amplitude 11 lm.

770 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

I ¼ ð1=2Þrc ðoAÞ2 (3)

In that equation, c is the sound velocity, x is the angu-

lar frequency (¼2pf; where f is the frequency), and A rep-

resents the amplitude. For the material, the shape of the

product and frequency are constant, DT can be expressed

as Eq. 4 from Eqs. 2 and 3.

DT ¼ k1A2t (4)

Here, k1 is constant. The degree of increase in the

product weight (DV) can be represented as Eq. 5 if the os-

cillatory flow is generated by the increase of the tempera-

ture on the oscillation part.

DV ¼ k2A2t (5)

Here, k2 is constant. The plots of DV against A2t arepresented in Fig. 9, which shows that Eq. 5 is concluded

in the early region of A2t. Therefore, the generation of

oscillatory flow is one reason why the oscillation part is

heated by ultrasonic energy absorption. Calculation of DTat the oscillation part at a depth of 10 mm with 20 s using

the characteristics of PC at room temperature yields a

value of about 300 K. Furthermore, the pressure must

flow the resin at the oscillation part. The oscillation part

is compressed periodically at the high frequency (19 kHz)

as a result of heating. The compression pressure that

results from ultra-multistep compression at the oscillation

part and resin flow is considered to be generated by com-

pression pressure, which is similar to conventional injec-

tion compression molding process.

Evaluation of Surface Replication With SphericalMicrostructure of the Lens

Surface replication with the spherical microstructure of

the lens depicted in Fig. 2 was evaluated. Both sides of

the lens curvature were measured. The results of the focal

distance calculated with measuring curvatures by Eq. 1are shown in Fig. 10. The focal distance by UIM was

close to the design value with increasing oscillation time

when compared to conventional molding. The surface rep-

lication with the spherical microstructure of the lens was

improved. Furthermore, the relation between the focal dis-

tance and the lens weight is shown in Fig. 11. The focal

distance produced by UIM approached the design value

with increasing lens weight. Finally, the focal distance

would equal the design value at the theoretical lens

weight.

Figure 12 shows an evaluation of the lens surface

reproduction accuracy. The results of this experiment

show that the accuracy of the lens surface improved about

25% when using UIM compared to the surfaces produced

using conventional molding.

FIG. 9. Relationship between the increase in lens weights (DV) and

A2t.

FIG. 10. Dependence of the focal distance on the oscillation time.

FIG. 11. Dependence of the focal distance on the lens weights.

FIG. 12. Dependence of the peak-value on the oscillation time.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 771

Mechanism of Surface Replication Improvement

In general, concave lens surface replication can be

improved by suppressing the shrinkage of the molded

product during the packing and holding stages. When

using UIM, it is considered that the part of shrinkage dur-

ing the packing and holding stages was suppressed by the

oscillatory flow provided by the ultrasonic vibration.

Because of ultrasonic energy absorption, local heating

was generated inside the resin, resulting in the formation

of oscillatory flow during packing and holding stages.

Local heating, which often occurs between the molten

and skin layers, reduces the deformation resistance of the

skin layer. Consequently, replication during the packing

and holding stages would be improved using the UIM, as

shown in Fig. 13.

Evaluation of Lens Residual Optical Strain

Figure 14 shows the residual optical strain of a con-

cave lens observed using a polarizer. The residual optical

strain found to have decreased much more near the lens’

center when using UIM than when conventional molding

is used. A schematic illustration of observations on the re-

sidual strain of the lens, which was molded with the

application of 20 s of ultrasonic wave that had an off and

on ratio of 0.4 s/0.4 s, is depicted in Fig. 15. Several

black circles were observed with the polarizer, radiating

from the center. This result shows that the ultrasonic

wave was transmitted to the lens. The lens was vibrated

for the radial vibration mode in the mold. Consequently,

local heating, due to ultrasonic energy absorption,

occurred in the lens. Probably, the decrease in strain is

attributed to the local heat generation by the ultrasonic

waves. Namely, nonuniform cooling the lens by uneven

distribution of thickness was relaxed by local heating in

the mold during the cooling process.

CONCLUSIONS

By application of UIM to lens molding, local heating

and ultra-multistep compression pressure generated the

oscillatory flow. Replication properties were improved

because the oscillatory flow prevented the shrinkage of

the molded product during the packing and holding

stages. And also local heating by the ultrasonic energy

absorption decreased the residual optical strain.

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