Rapid prototyping and vacuum casing technology in the production of injection moulding tools--case study.

Weiss, Edmund ; Weiss, Zenobia

1. INTRODUCTION

Market competition requires enterprises to manufacture high quality products with a good design and to bring them on to the market as quickly as possible. All products will be judged by the investors, by the distributors, by the service support providers and finally by the end user and it is therefore necessary to limit imperfections in the new product. This can be achieved using RP, RT and VC techniques which allow for the production of the finished product, as well as prototypes and tools, quicker than using more traditional and more expensive technology such as machining, EDM etc.

2. AIM OF THE WORK

Plastics products are generally produced on moulding machines which often require complicated and expensive tools. A method is required to make the tools quickly and inexpensively and the tools should be able to produce a few dozen different models or prototypes of different shapes, colours etc. Making the moulding tool from a vacuum-caste resin-composite allows for such a possibility. These composites have distinctly different mechanical and thermal conductivity properties in comparison to steel and aluminium tools which require adjustments for individual pressing conditions. This article considers the approaches connected with the creation of mould inserts and presents a simulation of the injection moulding process. The results of experiments into the selection of special injection moulding conditions are detailed and the threats to customer acceptance and quality control. are analyzed.

3. THE MANUFACTURING OF THE MOULD INSERTS

The experiments conducted were based on a knob used for common household equipment (Fig. 1). The manufacturing process for such an element consists of a number of stages. At first a model of the product (using any technology) should be prepared and then the equipment is prepared including a ring insert, the dimensions of which are compatible with the dimensions of moulding machine's form. The following main stages of the mould insert manufacturing process can be distinguished:

* preparation of the prototype model (original product or its model using any technology), including plastic thermal contract,

* preparation of the equipment, especially ring insert, the dimensions of which are compatible with the dimensions of the moulding machine's form (Fig. 2 and 3),

* the determination of the model parting planes and their fixing in the tool,

* tool resin preparation, casting of the form and its vacuum degassing (EP 310 composite by MCP-HEK was used),

* solidification of composite insert at 25[degrees]C estimated time 18-24 h.

[FIGURE 1 OMITTED]

A similar process should be applied to the second part of the form (Fig. 3). In order to strengthen the material both halves of the mould were heat treated at 40[degrees]C for about 4 hours. To further strengthen the properties of the material and thermal properties, the mould was stored at the following temperatures:

* 2h at 70[degrees]C,

* 1h at 100[degrees]C,

* 1h at 120[degrees]C,

* all night (12h) in 150[degrees]C.

The storage at the above temperatures should ensure an increase in the lifetime of the tools. The next operation involves removing the inserts and cleaning them with a machining processes. The aim of machining is to level off the contact surfaces of the moulds and the inserts. The drilling for the pusher follows.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

4. INJECTION PROCESS SIMULATION

The simulations of the flowing and cooling processes by injection enable for the early detection and assessment of behaviour patterns, and, as a consequence, can eliminate many future possible errors. It should be added that existing software supporting this area, does not deal with inserts produced by resin composites.

During the experiment the following simulation was carried out:

* injection point location analysis,

* time and probability of fulfilling of mould form,

* pressure analysis,

* changes in plastic temperatures after the injection,

* plastic bounding lines and existence of air bubbles in product,

* cooling quality and deformation possibility analysis,

* cooling quality of moulded piece using moulds produced by different materials (steel, aluminium, resin).

5. EXPERIMENTS WITH MOULDING OF KNOB IN RESIN COMPOSITE TYPE FORM

The inserts, produced using the described method (point 3), were assembled into a mould, a plate with pushers was added, the mould closing surface was checked and then the whole mould was mounted in an injection moulding machine. The next step was to carry out injection testing. The material used to made the moulded parts was PBT (Polybutylene terephthalate).The first attempt was made in similar conditions to those that would exist when using metal moulds. The experiments were conducted using the following parameters: injection pressure 70 MPa, cooling time 25 s, injection temperature 240[degrees]C, mould temperature 50[degrees]C, injection time 1.6 s.

Only 18 cycles of injections were carried out as the mandrel of the knob broke off and the plastic remained in one part of the mould. The reason for the break was an incomplete solidification process--the plastic did not solidify as the insert received too little heat from the part and the cause of this was the low thermal conductivity of the composite (1.45 W/mK). During the experiment, as a consequence of the compression of the insert's resin material under the high injection pressure (70 MPa) the material fashes at the borders of the die parting were observed, It was decided to carry out a set of tests with a reduction in the pressure. At a pressure of 50 Mpa, the moulded parts did not have such big fashes as in the first test. Further tests were carried out under the pressures of 40, 35 and 30 MPa. The moulded parts made at 50-35 MPa pressure had small fashes at the border of the die parting surfaces (about 0,05 mm). The making of moulded parts without fashes was possible at 30 MPa.

After the analysis of the results of the first experiment and after carrying out tests at a reduced pressure, the technological conditions for the second set of tests were planned. The following conditions were established: pressure: 30MPa, cooling time: 25 sec, injection temperature: 240[degrees]C, mould temperature: 40[degrees]C, injection time: 1,6 sec. 50 cycles of knob moulding were carried out and all the parts produced were of good quality. No problems of cooling were observed.

6. VALIDATION OF THE SIMULATION PROCESSES

The insert pressure established by technologists, based on experience, was, in-fact, 15 MPa higher then that estimated by the CAD/CAE system. Further experiments have shown that the best results can be achieved at a smaller pressure. The temperature of the plastic was selected in accordance with calculations in simulation and, as in the simulation, 100% of form fulfilment was achieved.

The simulation of the cooling process for the moulded parts did show that some difficulties with the cooling of the knob mandrel are possible. The time for the cooling of the material in the form (25s) was calculated by the system. In the first set of experiments it was determined that this time was too little as the composite insert was too hot and this is why, after 17 cycles, the moulded parts solidified and remained in the form. The analysis of the solidification shrinkage did demonstrate that there is the possibility to get a small deflection in form on the exterior of the product and this was observed in all parts.

7. CONCLUSIONS

The technology of Epoxy Tooling using Vacuum Casting for the making of the mould insert for the moulding machine from a resin composite is very useful for the production of a short-term product batch such as a prototype or information series in plastic (50-1000 pieces according to the composite used to made insert and according to the injected materials).

The strength and thermal conduction of the forms made from the resin composites are worse than metal moulding forms. That is why, in order to achieve a good quality product, there is the need to select special (lower) injections and cooling parameters.

Simulation programs are helpful in determining and eliminating problems connected with the forming of plastic materials.

8. REFERENCES

Chlebus E. (2003). Innowacyjne technologie Rapid Prototyping--Rapid Tooling w rozwoju produktu (Innovation production technology Rapid Prototyping-Rapid Tooling in product development), Oficyna Wydawnicza Politechniki Wroclawskiej, ISBN 8370857477, Wroclaw

Lokiec A. (2006). Symulacje procesu wtrysku w oparciu o program Cadmould (Simulation of the injection process on the basis of Cadmould programe), Available from: http://www.MESco.com.pl, 2006-04-20

MCP-HEK Tooling GmbH (April 2006). Firm information, Paderborn

Ratajczak P. (2006). Wytwarzanie wkladek do form wtryskowych z materialow kompozytowych w oparciu o technikq Vacuum Casting (Making of the inserts from resin composite with use of Vacuum Casting technique), MSc work, PUT Poznan

Weiss E. & Warczynski B. (2007). Using of Vacuum Casting Technique for Producing Functional Prototype (in Polish). In: Zeszyty Naukowe Politechniki Poznanskiej, Budowa Maszyn i Zarzqdzanie Produkcjq No. 5, Wyd. Pol. Poznanskiej, pp. 89-96, ISSN 1733-1919