Deliverables
Industrial competitiveness is increasingly determined by the ability to reduce time to market, lower costs, improve production flexibility and to minimize environmental impacts on an ongoing basis. With regard to the traditional sector of plastic processing, gas (assisted) injection moulding [GAIM] is a technology with a high added value. GAIM products are characterized by one or more parts with a regular solid outer wall and a hollow core. Depending on the application, various advantages can be achieved by the application of GAIM: (1) greater freedom in design phase of moulded products, (2) savings of weight/material, (3) increased stiffness, (4) higher productivity, (5) improved dimensional stability especially of large products, (6) lower machine processing cost (use of less expensive machinery or less wear due to lower forces) (7) better surface quality, flatness and tolerances. Typical GAIM products are: hand grips, bumpers, dashboards (automotive sector), crates, beverage boxes, television cabinets, house hold and furniture applications.
GAIM is an existing technique, which has been implemented in the European plastic moulding industry only to a very limited scale. The main cause is the lack of knowledge in the sector concerning design and manufacture of GAIM products, the (expensive) quality control and missing process control. Therefore the major goal of the project was to develop practical design and processing tools.
The main objectives of the GAIM project were:
- to improve the knowledge of design and manufacturing of GAIM products, especially when using fibre rein-forced polymers;
- to determine design rules and processing conditions/criteria for GAIM products;
- to develop practical CAD and CAE tools for GAIM products and to improve these products, i.e. the simulation programmes GAIM-Worx (TNO) and Cadmould-3D (IKV);
- to develop an in-mould ultrasonic measurement system [US-IMM)] which acts as closed loop process monitoring (quality) tool for GAIM.
The work programme was arranged in such a way that it follows more or less the life cycle of the introduction of a new product (line). The project basically consists of 3 phases: the research and development phase, the implementation phase and the test phase. Each phase consisted of a number of subtasks.
A new US-IMM system has been developed capable of measuring important properties of the GAIM process and of a GAIM product. The US-IMM system can be used as a quality monitoring system. The US-IMM system, developed by Krautkrämer en tested by IKV, employs only one newly developed US sensor, operating in pulse-echo mode. The sensor, mounted behind the mould platen, can measure through the steel mould wall. Different research institutes working in the ultrasonic measurement area have not been able to accomplish this. The new sensor set-up does not leave surface marks on the product. So far this was un-avoidable and is still state-of-the-art in ultrasonic inspection of conventional injection moulding.
The "time of flight" curve (TOF curve), the measuring signal of the US-IMM system, contains characteristic GAIM product and processing properties. The TOF curve appeared to change with changing wall-thickness of the part resulting from temperature changes during cooling (wall shrinkage) and the imposed gas pressure profile (gas pressure - time). In other words, the TOF curve depends on the internal conditions within the part. Therefore, the direct continuous measurement of the wall-thickness turned out to be very difficult and was not pursued further. Krautkrämer developed software, which allows monitoring and recording a TOF-curve over the whole cycle time. The gas bubble formation, the removal of the gas pressure, the formation of an air gap in the mould, fingering, gas breakthrough and foaming can be deduced from the TOF curves. The residual wall thickness of a gas channel can be measured indirectly from the TOF curve.
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