Plastic Micro Features Produced By Microwelding Machine

Advanced Materials Research Vols. 328-330 (2011) pp 1453-1456

Online available since 2011/Sep/02 at www.scientific.net

© (2011) Trans Tech Publications, Switzerland

doi:10.4028/www.scientific.net/AMR.328-330.1453

Plastic Micro Features Produced by Microwelding Machine

H. Liu1,2,a, Y. Xu1,b, K.L. Yung1,c, C.L. Kang1,d, W. Tian1,e

1Department of Industrial & Systems Engineering, The Hong Kong Polytechnic University, Hung

Hom, Kowloon, Hong Kong SAR

2Shenzhen Graduate School, Harbin Institute of Technology, University Town, Xili, Shenzhen, Guangdong province, China

amfphilip@inet.polyu.edu.hk, bmfxuyan@inet.polyu.edu.hk, cmfklyung@inet.polyu.edu.hk,

dmfclkang@inet.polyu.edu.hk, emftianwei@inet.polyu.edu.hk

Keywords: micro feature; microwelding; micro molding

Abstract. The method of using microwelding machine to fabricate micro features on a stainless steel micro mould has been studied. The micro features obtained from molding using an ultrasonic microwelding machine have shown distinctive characteristics. The microflow behavior of polymer melt during the microwelding process is also studied. The micro molds of 0.2mm thick and through holes of 0.05mm and 0.2mm in diameters were used. Results show micro features can be produced using the microwelding machine with molding speed comparable to that of micro injection molding.

Introduction

The principal for the microwelding process is to generate heat using ultrasonic vibration. It operates when electrical energy of ultrasonic frequency (20–40 kHz) is transformed into mechanical energy. The vibration energy amplifier in the machine is called a horn. In the welding process the horn usually moves up and down on high frequency. By doing so it transmits the energy to the joint area where frictional heat is produced to melt the plastic material immediately and fuse them together. Unlike hot embossing process, where the mould is heated and cooled in the molding process, molding with micro welding machine only heat the polymer that close to the micro mould. This causes a tremendous reduction in molding time.

Lin and Chen had tried to apply ultrasonic forming to nanoimprint lithography (U-NIL) [1]. Ultrasonic oscillations have also been used to improve the weld line of different kind of polymers by promoting the molecular diffusion across the weld line [7]. These applications all involved new designs of the vibration systems and used extremely high frequency signal. According to our literature review, using commercial ultrasonic welding machine to produce micro products have never been reported yet.

In this paper, we will discuss the research on using microwelding machine to produce micro features on the plastic material. The characteristics of micro features produced under different conditions have been studied.

Theoretical Analysis

The ultrasonic vibration in the microwelding process will change kinetic energy into heat due to the high viscosity of polymer materials. The complex sinusoidal strain of the flat viscoelastic material sheet is [8]:

   

it

0

  0

cos t  i 0

sin t

(1)

The  0

is the viscoelastic material’s strain amplitude.  is the frequency of the vibration

force. i 

 1 , so the material’s complex modulus can be divided into different two parts:

E   E '  iE ''

(2)

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Where E’ is the storage modulus, which means the in phase component stay in the vibration state. And E’’ is the loss modulus, which means the out of phase component transfer the energy in to other state. Only consider the linearly material, the strain of the viscoelastic material can be derived as [9]:

  Re(  E  )  

E ' cos t  

E '' sin t

(3)

So the energy that dissipated will be:

Q  

E '' sin td Re( ' ) 

 2 E '' sin td cos t  2 2 E '' (t  sin t)

(4)

T 0

T 0 0

The large part of dissipated energy will change into heat, which will apply to the plastic material.

The heat that the material gets is ( Qavg : energy dissipated per unit time and per unit volume) [8]:

Q  1 E '' 2

(5)

avg 2 0

Experiments

(a) (b)

Figure 1. (a) stainless steel micromold1 (thickness is 0.2mm, with 0.2mm diameter 4×4 holes), (b) stainless steel micromold2 (thickness is 0.2mm, with 0.05mm diameter 4×4 holes)

(a) (b)

Figure 2. (a) Ultrasonic welding machine

(b) Hot-press machine

The micro stainless molds (in fig.1a and fig.1b) for the micromolding were made by

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