Injection moulding is the most important of all the commercial methods of plastics processing. Many variations have been developed and one of the rapidly expanding fields is multi-material injection moulding. This is particularly important where processors are looking to gain technological advantages over rivals by adding value to products. Whilst tooling costs can be very high, cost savings can be made by eliminating assembly steps. This review looks at the many techniques being used, from the terminology to case studies.
There are many issues involved in moulding different types of materials together. Advantages are gained in the product by combining different properties. Recyclate can be used as a core material with virgin resin skin. However, there are potential problems. Compatibility is important for interfacial adhesion. Different materials have varying rheological properties and optimal moulding conditions, which can limit material choice. This is a big area for research as there have been few studies on co-moulding incompatible polymers.
The three primary types of multi-material injection moulding examined are: multi-component, multi-shot and over-moulding.
Multi-component moulding can be further subdivided. Co-injection moulding involves making sequential injections into the same mould with one material as the core and one as the skin. It is also known as sandwich moulding because the core is fully encapsulated. Bi-injection moulding is the simultaneous injection of different materials through different gates. Interval injection moulding, also known as marbling, is the simultaneous injection of different materials through different gates giving limited mixing.
Multi-shot moulding describes any process where distinct material shots are applied to produce the final component. This includes transfer moulding, core back moulding and rotating tool moulding.
Over-moulding includes both insert moulding and lost core moulding, the latter produces hollow parts.
This review describes the basic types of multi-material injection moulding, the issues surrounding combining different types of polymers and examples of practical uses of this technology. It is clearly written and difficult concepts are explained with illustrations.
The abstracts from the Polymer Library include many more examples of the use of this technology, giving names of companies and organisations involved in this field.
1.1 Multi-Component Moulding
1.1.1 Co-Injection Moulding
1.1.2 Bi-Injection Moulding
1.1.3 Interval Injection Moulding
1.2 Multi-Shot Moulding
1.2.2 Core Back
1.2.3 Rotating Tool
1.4 Business Trends
2. Injection Moulding Basics
2.1 Stages of Injection Moulding
2.1.2 Mould Filling
2.1.3 Packing and Solidification
2.2 Differential Shrinkage and Cooling Effects
2.3 Microstructure of Injection Mouldings
3. Material Selection
3.1 Material Bonding Properties
3.2 General Material Properties
4. Multi-Component Injection Moulding
4.1 Co-Injection Moulding
4.1.1 Material Selection for Co-Injection Moulding
4.1.2 Co-Injection Moulding: Different Techniques
4.1.3 Sequential Injection: Single Channel Technique
4.1.4 Sequential Injection: Mono-Sandwich Technique
4.1.5 Simultaneous Injection: Two Channel Technique
4.1.6 Simultaneous Injection: Three Channel Technique
4.1.7 Part Design and Tooling Requirements for Co-Injection Moulding
4.1.8 Rheology and Mould Filling: Why and How Co-Injection Moulding Works
4.1.9 Immiscible Materials Research in Co-Injection Moulding
4.1.10 Co-Injection Moulding Applications - Case Studies
4.1.11 Recycling and Legislation
4.1.12 Discussion and Conclusions
4.2 Bi-Injection Moulding
4.3 Interval Injection Moulding
5. Multi-Shot Moulding
5.1 Machine Technology
5.1.1 Injection Unit Configurations
5.1.2 Plastication Design
5.1.3 Machine Type
5.2 Core Back Moulding
5.3 Rotating Tool Moulding
5.4 Transfer Moulding
5.5 Multi-Shot with a Single Injection Unit
5.6 Material Selection for Multi-Shot
5.6.1 Material Properties
5.6.2 Material Process Order
5.6.3 Using Thermoset Materials
5.6.4 Liquid Silicone Rubber (LSR)
5.6.5 Thermoplastic Elastomers (TPEs)
5.7 Multi-Shot Moulding Applications - Case Studies
5.7.1 Trio Knob
5.7.2 Stanley Screwdriver
5.8 Limitations to Multi-Shot Moulding
6.1 Insert Moulding
6.2 Lost Core Moulding
7. The Future?
Abbreviations and Acronyms
Abstracts from the Polymer Library Database
Dr Goodship is a Senior Research Fellow with 14 years experience in industry, expertise in coinjection moulding technology and a particular interest in recycling. Jo Love is an experienced materials engineer currently working on in-mould decoration. The authors are based at the Warwick Manufacturing Group in the Advanced Technology Centre at the University of Warwick.