22-09-2014, 10:37 AM
RESOURCE EFFICIENT INJECTION MOULDING WITH LOW ENVIRONMENTAL IMPACTS
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INTRODUCTION
Injection moulding ranks as one of the most widely used processes for producing plastics products. As in other forming processes, the characteristics of injection moulding procedures and products are significantly affected by the quality of moulds used i.e. tools that are mounted into injection moulding machine to produce repeatable products .Once produced, the moulds are used in production for many years. Since raw materials are becoming scarcer and more expensive, and the costs of energy is also increasing the strategy of mould design should not aim only at cost reduction but also at reducing resource consumption and emissions throughout its entire life cycle. This paper presents an approach to compare and optimize mould design and production process parameters from technical, economic and also environmental point of view.
OVERVIEW OF CASE STUDY
The case study is presented with three alternative mould designs put side by side. They enable an injection of the same plastic product, shown in Fig. 1, but differ in technical solutions, which contribute to the productivity and resource efficiency during their use phase (injection moulding of plastic products). It is expected that one million products will be produced throughout the mould’s life cycle.
Mould Production
For smaller moulds, such as the one investigated in the case study, the main standard metal elements are sawed to rough shape from rolled slabs and bars which are then milled, turned and grinded to final tolerance geometry. Mould components like plates, injectors, ejectors, guiding elements and others are commercially available as standard elements. The patterns which form the final product geometry within standard plates (sometimes also called active mould surfaces) are usually produced by CNC milling and/or Electric Discharge Machining (EDM). In our case only the CNC milling process was used.
INJECTION MOULDING
Injection molding is a manufacturing process for producing parts by injecting material into a mould. Injection moulding can be performed with a host of materials, including metals, glasses , confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, moulds are made by a mouldmaker (or toolmaker) from metal, usually either steel or aluminum, and precision machine to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panel of cars.
MOLD DESIGN
The mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). Plastic resin enters the mold through a sprue or gate in the injection mold; the sprue bushing is to seal tightly against the nozzle of the injection barrel of the molding machine and to allow molten plastic to flow from the barrel into the mold, also known as the cavity. The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the A and B plates. These channels allow plastic to run along them, so they are referred to as runners. The molten plastic flows through the runner and enters one or more specialized gates and into the cavity geometry to form the desired part.
Conclusion
In conclusion this project was useful in exploring and understanding how different control factors have different effects on a desired characteristic for a part. Through the use of design expert it was shown how some control variables have a strong effect on a specified characteristic, while others have little to no effect. Through design expert it was found that the variables affecting the final weight of the plastic mold injected part the most, were pressure and flow weight. Each of these factors had a larger impact than the nozzle temperature, barrel temperature and dwell time combined. Even though this experiment was a half factorial, a lot of useful Data was collected about the different factors and how they interact with each other.