Feature article: PIM International, Vol.2 No. 1 March 2008, pages 21-28, 4048 words

Author: Dr. Georg Schlieper, Ingenieurbüro Gammatec, Germany

Ingenieurbüro Gammatec, Alleestr. 101, D-42853 Remscheid, Germany

Introduction

Dr. Georg Schlieper presents an overview of the injection moulding process, and of the basics of mould design for powder injection moulding. Special emphasis is given to the specific characteristics of feedstocks used in the manufacture of PIM parts and innovations that may bring future benefits are highlighted.

In the early days of powder injection moulding, about 20 years ago, when we discussed the many problems to be solved on the way to a reliable MIM process, my colleague and co-worker Jürgen Paal used to say 'Injection moulding is no problem!' Indeed, we had so many questions to be answered with respect to feedstock composition and preparation, binder removal and sintering that the injection moulding process in comparison appeared clearly defined and straightforward. Here we could rely on the injection moulding technology for thermoplastics, a well-established industry offering sophisticated machinery and a vast choice of experienced mould makers and manufacturing engineers. Therefore we did not care much about the injection moulding process and concentrated our activities on problems like binder development, the binder removal process, carbon control after sintering, and other such questions.

Today the basic questions of PIM have been answered and stable manufacturing processes are mostly established throughout the industry. Now it may be of interest for production engineers to again look at some of the basics of injection moulding such as mould making and the injection moulding process in more detail and to investigate where the specific characteristics of the materials processed require special care and possible refinement or further development. Then we can answer the question: 'Is injection moulding really no problem?' for ourselves. The reward could be better part-to-part consistency, fewer rejects and lower tooling cost. This review simply attempts to give the reader a basic understanding of the injection moulding stage of the overall PIM process and to serve as a stimulus for discussion rather than as a definitive guide.

Injection moulding of thermoplastic feedstock

When we look at the injection moulding process we will only consider high pressure injection moulding of thermoplastic feedstock. There are other methods to shape PIM feedstock such as low pressure injection moulding or extrusion. These less widely used moulding processes are not discussed here.

The main elements of an injection moulding machine are the hopper where the feed-stock granules are fed into the machine, the heated barrel with a screw for heating, compressing and injecting the feedstock into the mould with a nozzle at the hot end, and the clamping unit which contains the mould (Fig. 1). The controls of the machine allows for adjustment of the barrel temperature in several independent zones. The first heated zone serves to melt the feedstock quickly since the abrasiveness of the molten feedstock is much less than the solid granules. Then the temperature is homogenised inside the barrel......

Figures and Tables:

Fig. 1 Diagram of a typical injection moulding machine for PIM

Fig. 2 Schematic screw profile with feed zone, compression zone and metering zone (Courtesy Arburg)

Fig. 3 Various screw geometries (Courtesy Arburg)

Fig. 4 Boride coated surface (Courtesy Arburg)

Fig. 5 Screw tip with a non-return valve (Courtesy Arburg)

Fig. 6 Hot runner single nozzle (Courtesy Günther Heißkanaltechnik)

Fig. 7 Tool set for powder injection moulding (Reprinted with permission of ASM International®. All rights reserved. www.asminternational.org [1])

Fig. 8 Mould design with a single hot runner nozzle (Courtesy Günther Heißkanaltechnik)

Fig. 9 Computer model of mould filling [3]. As published in the proceedings of the Euro PM2007 Conference, October 2007 (Courtesy European Powder Metallurgy Association)

Fig. 10 Shrinkage during sintering (a) experimental, (b) computer model [3]. As published in the proceedings of the Euro PM2007 Conference, Toulouse, October 2007 (Courtesy European Powder Metallurgy Association)

Fig. 11 Gate options: tap gate, sub gate, and three plate tooling (Courtesy Dynamic Engineering) [5]

Fig. 12 Mould with laterally moving elements (Courtesy Dynamic Engineering) [5]

Fig. 13 Mould with an angle pin attached to the ejector plate (Courtesy Dynamic Engineering) [5]

Fig. 14 Part with an undercut (left) and solution with an angle lifter, right. (Courtesy Dynamic Engineering) [5]

Fig. 15 Four PIM parts in a mould with threaded cores (Courtesy Dynamic Engineering) [5]

Fig. 16 Collapsible cores (Courtesy Roher)

Fig. 17 Micro-gears made by PIM (Courtesy Kugele/wbk/Arburg)