Whereas a minority of machines actuate by means of crosshead plungers or puller plates,very large machines tend to employ cross-headed assemblies to distribute the larger forces as evenly as possible. The majority of moulding machines ejection actuate on the centre line of the platens in line with the injection unit. (ii) Is the intended moulding machine able to actuate the designed ejection system incorporated into the mould design? Machine designs may vary with respect to ejection actuation method. (i) Is the ejection force sufficient to eject the component within the working limitations of the moulding machine? Mould designers will often design a whole mould without giving ejection force requirement a thought,resulting in disastrous consequences. (ii) Will additional ribs be required to enable ejection to take place without distortion resulting? (c) Machine considerations. (i) What amount of ejection induced distortion can be tolerated dimensionally? Moulded components can easily be distorted during ejection if unsupported and especially in the case of thin-walled mouldings. (b) Component dimensional considerations. (ii) Does sufficient draft angle exist on the mould cavity walls to avoid component ‘scuffing’ or ‘dragging’ occurring during ejection? Adequate drafting of any moulded component is essential and should be considered as part of the component design before the designing of the mould. (i) Can ejector pin marks be tolerated on visual presentation faces? Reversing the cavity within the mould design is not an uncommon practice to overcome this problem. Such a list should include references to the following: (a) Component aesthetics. It may prove beneficial to construct a short list of requirements for the component to be moulded. Choice of ejection methodīefore selecting an ejection method,sufficient thought must be given to the various component requirements. It will be evident that careful consideration must be given to the design and choice of ejection method when designing both components and moulds for injection moulding purposes. Aesthetic considerations of the component.Choice of ejection method is usually dependent on some factors, for example: The function of an ejection system is to enable the removal of the moulded component from the mould once solidified.Įjection mechanisms vary enormously, both in function and design. Proper surface finishing takes up a considerable portion of the total mold production time. Wherever possible the surface should be polished, since even microscopic scratches and indentations are filled with plastics melt under the high injection pressure, thus preventing smooth ejection. Since self-acting slight lubrication of the mold surfaces that come in contact with the molded parts is possible only in rare cases,there is a danger that scratches and cracks will occur when the mold is opened or the parts ejected, particularly if the molding compound is very hard.Ī mirror finish, which is easily obtained by buffing a hard steel surface,is in no way adequate it is far more important to grind and polish the areas in the direction of ejection with an oilstone to eliminate any scratches and indentations. These measures include, first of all, careful polishing of all mold surfaces that come in contact with the plastics melt as well as a certain draft in the direction of draw. Shrinkage also causes the molded parts to sit tight on the cores since usually parts are ejected after they have cooled down.Īs a result, extraordinary measures are required to be able to eject finished parts. If parts with accurate dimensions are needed, allowances must be made for this shrinkage when establishing the dimensions of the cavities. Injection moulding compounds exhibit a certain amount of shrinkage,i.e., in a cooled condition, the volume is somewhat smaller than in a heated condition.
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