INJECTION MOULDING

Description

Injection moulding is a manufacturing process where a hot molten polymer is injected into a mould cavity with the shape of the piece that will be produced. The molten polymer is pushed into the mould under very high pressure. The polymer starts to cool due the contact with the mould walls. The pressure in the extruder is maintained until stops the pushing of more material. The piece will cool to a temperature where it becomes sufficiently rigid so that it can be ejected without any deformation. Once the piece is ejected, the mould is closed and the piece cools slowly at room temperature.

Manufacturing Engineering and Technology, S. Kalpakjan, (1995)

Contacts

Dipl.-Ing. Oliver Pfannschmidt
Head of Injection Moulding Department of IKV - Institute of Plastics Processing at Aachen University of Technology
D-52062 Aachen, Pontstraße 49
GERMANY
Tel. +49 241-803827
Fax +49 241-8888262
email: pfannschmidt@ikv.rwth-aachen.de

Dr. Poul Kyvsgaard Hansen
Aalborg University - Department of ProductionAalborg University
Fibigerstræde 16, Room 2.225
DK-9100 Aalborg.
DENMARK
Phone: +45 96358935
Fax: +45 98153030
E-Mail: i9phkh@iprod.auc.dk

Alberto Fernando Silva

ISQ - Instituto de Soldadura e Qualidade
F1 - R&D / Production Technologies
TagusPark
Apartado 119
2781-951 Oeiras Codex
PORTUGAL
Phone: +351 214 228 100 Ext. 2609
Fax: +351 214 228 126
Email: AFSilva@isq.pt
Web: http://www.isq.pt

Characteristics

Costs (tooling / unit)	High initial tool and die costs. Low cost per unit.
Volume (units / day)	Some thousands of pieces a day (the number always depends of the geometry of the piece).
Possible Materials	Most of all thermoplastics (e.g. polystyrene, polyethylene, polyvinylchloride, polycarbonate, nylons); thermoset materials (e.g. phenolic resins, allyl resins); elastomers.
Size / Shape	Sizes may range from very small to very large. The limiting size factor is the moulding equipment. The design is flexible to produce relatively large and complex shapes which are determined by the mould. Could be produced a wide range of materials with repeatability within tolerances.
Mechanical Properties	Pressure: 10-150 MPa Typical cycle times: 10-100 seconds Minimum inside radius: 0,25-3,20 mm Minimum thickness: 0,4 mm Maximum thickness: 25 mm
Special Remarks	Injection moulding produces pieces in large volume at high production rates. Labour costs per piece are low and the process could be automated. The pieces require a little or no surface finishing with a minimum scrap losses. However, the initial equipment investment (moulds, machinery and auxiliary tools) is high. Accurate cost prediction for moulding job is difficult, so high start-up and running costs are possible especially with small runs. Pieces must be designed for effective moulding. Viscosity, temperature and pressure in the mould are continually changing and cannot be measured.

Application / Case Study

Injection moulding is the most widely used polymeric fabrication process. A number of variations of the basic injection moulding process have been developed for special products like air injection moulding, foam injection moulding, etc. One of them is sandwich injection moulding. This process is used for geometrically simple pieces to achieve specific product properties like low weight combined with high strength. The mould is first partly filled with a polymer, and then another polymer is injected into the first polymer through a separate nozzle. So it could be obtained a sandwich plastic pieces with different types of plastic inside and on surface. In this way pieces with high quality surfaces can be made from plastic types with low surface quality. So, the filler can be a recycled polymer. The materials that could be processed with this technique are: high density polyethylene (HDPE), polyamide nylon (PA), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyethylene (PE) and polypropylene (PP).

A sandwich moulding technique (co-injection) developed by Addmix Ltd. (London, UK) can be utilized by all injection moulders operating standard single-screw injection moulding machines. The company explains sequenced screw loading by reference to the production of a sandwich-injection moulded part. Here, the first material going into the mould forms the skin of the part. As the core material fills the mould, the skin material is stretched over the surface of the mould cavity. The gate of the mould is then closed off with more skin material. The plasticizing screw is designed for conveying and preparing material, yet many regard it as a mixing device. The sequenced screw loading technique utilizes this notion with a special two-component computerized dosing system that loads the screw with a sequence of two materials. It attaches directly to the feed throat of a single-barrel injection moulding machine and replaces the hopper. Different granules are fed into the feeder in a predetermined sequence. The materials travel through the machine screw and barrel, maintaining the feed sequence. Material A enters the cavity forming the skin of the component, and material B follows filling its core to produce a sandwich-moulded component. The materials travel through the moulder in separate zones and this produces parts that compare with those produced by a traditional twin injection unit machine, according to the company.

It is important that the materials should have overlapping temperature zones and be compatible. Not only do the base materials need to be compatible, but also the additives such as colouring and glass binders, consequently material suppliers need to be consulted. If material A is a compound material with colour and material B is virgin or recycled material, the skin will be fabricated of the compound material and the core will be the virgin or recycled material. As the two materials are conveyed through the barrel, a small degree of mixing occurs at their interface. In some cases this can improve bonding. The mixed zone also allows the production of parts with a high concentration of colour on the skin of the component and a low concentration at the core. Machine cycle times can be reduced by introducing blowing agent in parts where there are heavy sections. Improved cycle times are often the main objective for acquiring the system. The sequenced screw loading technique has benefits in key design areas claims the company. These include the ability to use recycled core materials to improve product cost; minimize the use of costly colouring; use a foam core to save weight and improve the look of a product.