FLOW FRONT ANALYSIS OF TIR LENS OF LEDS

WITH INJECTION MOLDING

Chao-Chang A. Chen, Feng-Chi Lee

Department of Mechanical Engineering, National Taiwan University of Science and Technology,

Taipei, Taiwan

Abstract

This research is to investigate the TIR (Total

Internal Reflection) lens as secondary optical

element mounted on LED for landscape and indoor

illumination. The TIR lens is like a cone bell with

the ratio of gate thickness to cavity thickness is

about 1: 10. The simulation software, Moldex 3D is

used to simulate the melt front behavior for 3D

filling. Experimental results show that injection

speed, mold temperature and melt temperature

significantly affect the final quality of TIR lens.

Molding parameters are obtained in this research to

improve the optical performance and that can be

applied on LED market in the future.

Keywords: Injection Molding, Secondary Optical

Elements, TIR Lens, LED

Introduction

Injection molding has become widespread used

for mold precision plastic parts such as optical

lenses optical disc, and diffraction gratings since the

dimensional accuracy and stability can be strictly

satisfied for high precision parts [1, 2]. This research

is to investigate the TIR (Total Internal Reflection)

lens that is mounted on LED for landscape and

indoor illumination. Fig. 1 shows the function of

TIR lens and LED. The function of TIR lens is

designed with a totally internal reflecting face to

collimate radiant light from LED. The TIR lens has

quite large volume of mold filling and the ratio of

gate thickness to cavity thickness is about 1: 10.

Fig. 2 shows the part and gate geometry of TIR lens.

The gate dimension, gate type and gate location are

limited. A commercial mold flow software,

Moldex3D R9.0 was used for simulating the melt

front behavior for investigating the fountain flow

effect and roll up phenomenon with compared with

the experimental results.

The fountain flow effect discussed by Rose [3],

as the phenomenon of deceleration and outward

motion of fluid particles when they approach a

slower moving interface. A "reverse fountain flow"

occurs in the retreating fluid. Li [4] presented that

the viscous flow in the filling stage of injection

molding can be described in terms of an one-

dimensional fully developed main flow and a

complex two-dimensional flow near the advancing

front to often termed as the "fountain flow".

In short shot experimental, that melt into the

cavity melt front can be observed with roll up, weld

line and air trap. Therefore, this research is to

investigate the flow front for 3D volumetric filling

with roll up behavior by injection molding and also

provides with a suggestion of controlled parameters.

Fig. 1 Schematic of TIR lens and LED

Fig. 2 Schematic of TIR lens

Experimental set up and methods

The polymer material and injection molding

machine used in this research were optical grade

PMMA (Asahi Delpet 80N) and a full electrical

injection molding machine (FANUC -15ia), respectively. The mold temperature was controlled

by a water circulation controller and measured by a

thermal couple. The mold of experiment and

injected TIR lens are shows in Fig. 3.

The short shot experiments were controlled by

the melt volume of polymer with screw position of

injection molding machine. Experiment observed the

behavior of the melt font when melt plastic is shot

into the cavity. Additionally, it can also be adjusted

to the multi injection speed stage by short shot

experiments.

When the short shot experiments were

completed, the molding window experiment is

obtained with the injection velocity and mold

temperature in the first stage of molding window

experiment. The second stage of molding window

experiment was proceeded by the injection velocity

and melt temperature. The air trap and weld line

defects were selected to evaluate the molding

window as the melt front into the gate stage.

Fig. 3 Photo of injection mold and TIR lens

Results and discussion

Fig.4. shows the schematic of fountain flow. In

filling stage of injection molding, the skin of the

melt contacted with the cold mold cavity freezes

rapidly, while the central core of melt remains

molten. When additional melt is injected, it flows

into this central core, displacing the melt already

there, which forms a new flow front. The outward

flow contacts the wall, freezes and forms the next

section of skin while the forward flow forms the

new molten core. Therefore, the flow pattern is often

called fountain flow or bubble flow.

Fig. 5 shows the result of short shot

experiments by injection molding. Fig. 6 shows the

schematic of flow effect. Most fountain flow effects

of conventional injection molding were discussed in

2D instead of 3D. In this research, the TIR lens was

molded by a cavity of 3D filling. The resin filling

into the gate are not restrained as the top of resin

was continued filling, but the lower molten contact

with cold cavity wall freeze rapid. With continuous

3D filling and solidification, the melt polymer was

covered by surface tension force and viscosity of

melt polymer effect. It can lead to packing the

solidify layer of lower polymer. Thus the weld line

and air trap are easily formed. After filling, packing

and cooling stages, the weld line is condensed by

pressure effect and most appears on lens surface

In the first experiment, Fig. 7 shows the results

of molding window were performed by changing

injection speed and mold temperature. Mold

temperature increased from 60 to 100 and the injection speed increased from 1mm/s to 7mm/s

when melt flow was entered from gate location.

In the secondly experiment, Fig. 8 shows the

results of molding window were performed by

changing injection speed and melt temperature. Melt

temperature increased from 240 to 270 and setting the injection speed increased from 1 mm/s to

6 mm/s when the melt flow was entered from gate

location. The molding window was accepted since

there is no air trap and weld line phenomenon.

Table 1 shows the setting of final parameters of

injection molding by experimental molding window.

The first injection speed was 40 mm/s that filling the

runner system rapidly. The second injection speed

was set 4 mm/s for preventing the roll up

phenomenon of melt front. When a melt front was

stable, the third injection speed was increased from

20 mm/s to 40 mm/s until all of cavity was filled.

Finally, the packing and cooling parameters were set.

Fig. 9 shows the result of short shot by molding

flow analysis software Moldex3D R9.0 and

experiments. The top view of short shot results

shows that are similar from this direction. But there

are some of air trap and weld line phenomenon in

experimental results.

Fig.4 Schematic of fountain flow

Fig. 5 Short shot experiment of roll up phenomenon.

Fig. 6 Schematic of TIR lens of flow effect

Fig. 7 Molding window of TIR lens by injection

speed and mold temperature.

Fig. 8 Molding window of TIR lens by injection

speed and melt temperature.

Fig. 9 Compare with short shot results by Moldex3D

and experiments.

Tab. 1 Final injection molding parameters of TIR lens

Parameters Stage Value Unit

Injection Speed 1 40 mm/s

2 4

3 20

4 40

Packing Pressure/

Packing Time 1 100/7 MPa/s

2 80/4

3 40/2

Cooling Time

30 s

Conclusions

This study has investigated the significant

injection parameters to the TIR lens of 3D filling

and also found the setting of injection parameters

through short shot experiments.

Some conclusions can be drawn as following:

1. Increasing the mold temperature and melt

temperature can reduce defect and solidified

layer.

2. Decreasing the injection speed from gate location

can reduce the weld line and air trap.

3. Using the short shot experiment to adjust the

multi stage, the injection speed needs to control

the gate of injection rate.

Results of this study can be further applied to

develop related injection compression molding of

thick optical elements with 3D filling of multi-

scaled structures.

References

1. Yang, S.Y.; Ke, M.Z. Experimental Study on the Effects of Adding Compression to Injection

Molding Process, Advances in Polymer Technology, v 14, n 1, p 15-24, 1995.

2. Yoshii, Masaki; Kuramoto, Hiroki; Kawana,

Takeshi; Kato, Kazunori. The Observation and Origin of Micro Flow Marks in the Precision

Injection Molding of Polycarbonate, Polymer Engineering and Science, v 36, n 6, p 819-826,

1996.

3. W. Rose, Fluid-Fluid interface Steady Motion, Nature, 1961, 191,242.

4. C. S. Li; C. F. Hung; and Y. K. Shen, Computer Simulation and Analysis of Fountain Flow in

Filling Process of Injection Molding, Journal of Polymer Research, v1, n2,p 163-173, 1994.

Moldex3D analysis Part (By screw position)

0.47s

11mm

0.67s

15mm

0.76s

17mm

0.87s

19mm

0.98s

21mm