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Rapid Prototyping & Manufacturing TechnologiesMixing Ratio is a term used to state the amount of each material to be mixed ina multi-component material. The mixing ratios for two-part RTV products aredescribed on the individual product data sheets and are given as a ratio byweight of each material.Pot life is the length of time that a catalyzed resin system retains a viscosity lowenough to be used in processing. This is also known as WORKING LIFE orUSABLE LIFE.Curing condition is the condition which can provide the optimum properties fora silicone rubber. This typically depends on time and temperature.4.2.3 Considerations of making RTV mouldTo duplicate a good product by soft tooling, planning is very important as thepart may be blocked inside the mould if the parting surface is not well defined.All holes should be filled by tapes to separate individual mould half beforepouring of silicone rubber.For master pattern with flat surface, the parting surface should be located on theflat surface and use plastic type as a marking. For master pattern with free-formgeometry, parting line should be located on the position which gives lowesteffect to the appearance.It is important to evaluate the pattern if there are any undercuts that would lockthe casting in the mold. Small undercuts can be ignored since the silicone rubberis flexible material. To deal with some deep holes feature such as boss, metalinserts can be used to replace the silicon material by attaching the inserts to thepart before pouring of moulding material.4.3 PU CastingEven RTV mould can be used for different material, PU is the most popularmaterial accompany with RTV mould for producing plastic components. Thereare two different approaches for PU casting, Top-down approach (direct gate)and Bottom-up approach (by gravity).For Top-down approach, PU is poured into the mould directly through a gateconnected on top of the part. Firstly, weighted material and RTV mould are putinto a vacuum chamber for degassing. PU resins are mixed together inside avacuum chamber, it is then poured into the mould via a funnel and air isreintroduced simultaneously to force the PU material into the mould. Thismethod is suitable for casting small components such as jewelry that has finedetail. Figure 4.3 shows the processes of PU casing by this top-down approach.Page 28IC Professional Training
Rapid Prototyping & Manufacturing TechnologiesFig. 4.3 Steps of PU castingFor Bottom-up approach, PU material is poured into the RTV mould and filledinto the moulding cavity by gravity. Vent channels are need to be added at thetop areas of part to allow air out. This method is most suitable for large castingpart in which the mould is unable to put inside a vacuum chamber. Figure 4.4shows the different set up of top-down and bottom-up approachesMaterial inputRTV MouldMaterial inputVent channelsCavityFig. 4.4 Left; top-down approach Right; bottom-up approachPage 29IC Professional Training
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