Development of a heat-generable mold insert and its application to the injection molding of microstructures

Highlights

• A micro-heater-embedding mold insert for micro injection molding is proposed.

• Silicon-based micro heater exhibits stable properties and heating performance.

• We reduce the de-molding force and destruction by using dynamic partial heating.

Abstract

This work presents a heat-generable mold insert for micro injection molding that solves the problem of de-molding destruction. This mold insert is constructed from silicon wafer by silicon micro-fabrication. Micro electrical heating lines were formed in the wall of the micro mold cavity to control the temperature distribution and the sequence of local solidification of the filled plastic during injection molding. This design reduces the shrinking stress of the plastic filled in the mold. The micro electrical heating lines embedded in the cavity wall are silicon-based with specified resistance, and were fabricated by doping phosphorus ions precisely into the surface of the silicon cavity wall. Ion-implantation was adopted to dope phosphorus ions. The performance of the novel mold insert was studied. Then, the developed mold insert was applied for the injection molding of micro-structures with high aspect ratios. Experimental results reveal that electrical heating lines formed within the novel mold insert can supply stable heating power. These electrical heating lines are used to heat the cavity wall of the silicon mold insert and the nearby plastic with appropriate timing at sufficient power in the cooling stage, such that the de-molding force associated with contraction of the patterned plastic grips to the micro-structured mold insert, is reduced. Furthermore the de-molding destruction of the injection molded micro-structures can be eliminated. Optical micro-structures with aspect ratios of up to eight were successfully injection-molded.

Introduction

Micro-structured components have many potential applications. Plastics are very suitable for their fabrication. Therefore, technology for manufacturing plastic micro-structures is very important. Plastic micro-structures of high quality must be fabricated more economically as micro-structured components are becoming more extensively utilized. Injection molding is anticipated strongly to favor the mass production of plastic micro-structures of stable and good quality. However, the common characteristic of designed micro-structured components is that column- or wall-shaped micro-structures with high aspect ratios densely stand erect on a base plate, and these micro-structures cannot be given a draft or a taper angle because of limitations on the functional requirements of the product and the fabrication of mold insert. Accordingly, de-molding interference or the gripping of the mold insert by cooled molded plastic is liable to occur, causing de-molding fracture during the injection molding of plastic components with surface micro-structures [1], [2], [3]. This problem is considered to be caused by the stress field that is established by the difference between the shrinkage ratios of the plastic and the mold insert material. Although the shrinkage ratio of plastic can be regulated by controlling the specific volume of plastic inside the mold cavity with pressure applied during cooling, uniformly distributing the pressure to ensure uniform shrinkage of the molded plastic is difficult [4], [5]. Additionally, the pressure on the plastic during cooling may cause not only residual stress in the molded product but also damage to the mold insert. Hence, the de-molding problem associated with the injection molding of micro-structures described above cannot be expected to be solved by applying a pressure to the plastic during cooling [6].

In recent years, variotherm mold technique [7], [8], [9] and surface modification such as PVD coating [10] have been applied to help the filling and de-molding in the injection molding of microstructures. Nevertheless, the issue due to thermal shrinkage described above does not completely be solved yet especially when the diameter (or the thickness) of the microstructure as small as 10 μm [10]. The dynamical mold temperaturing process leads to an increase in the cycle time as used in conventional process [11], [12]. The high temperature range variation can also decrease the lifetime of the mold [13], [14]. Moreover, high temperature inhomogeneities may occurs on the variothermally tempered mold wall [15].

This study proposes a novel mold insert and a new strategy for controlling temperature in a mold. The mold insert has an independent heating function that can be employed to increase rapidly the temperature of the walls of micro-cavities during cooling in an injection molding process, actively controlling the temperature distribution and the sequence of solidification of the plastic inside the mold cavity. This mold insert can greatly reduce the shrinking stress in the cooled plastic, solving the de-molding problem described above without having to use other auxiliary design such as the thermal stress barrier [16].