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The material used for producing the plastic injectionmould for warpage testing specimen was AISI 1050carbon steel.Four design concepts had been considered in designing ofthe mould including:i. Three-plate mould (Concept 1) having two parting linewith single cavity. Not applicable due to high cost.ii. Two-platemould (Concept 2) having one parting linewithsingle cavity without gating system. Not applicable dueto low production quantity per injection.iii. Two-plate mould (Concept 3) having one parting linewith double cavities with gating and ejection system. Notapplicable as ejector pins might damage the product asthe product is too thin.iv. Two-platemould (Concept 4) having one parting linewithdouble cavitieswith gating system, only used sprue pulleract as ejector to avoid product damage during ejection.In designing of the mould for the warpage testing spec-imen, the fourth design concept had been applied. Variousdesign considerations had been applied in the design.Firstly, themouldwas designed based on the platen dimen-sion of the plastic injection machine used (BOY 22D). Thereis a limitation of the machine, which is the maximum area ofmachine platen is given by the distance between two tie bars.The distance between tie bars of the machine is 254mm.Therefore, the maximum width of the mould plate shouldnot exceed this distance. Furthermore, 4mm space had beenreserved between the two tie bars and the mould for mouldsetting-up and handling purposes. This gives the final max-imum width of the mould as 250mm. The standard mouldbase with 250mm×250mmis employed. Themould base isfitted to themachine usingMatex clamp at the upper right andlower left corner of the mould base or mould platen. Dimen-sions of other related mould plates are shown in Table 1.The mould had been designed with clamping pressurehaving clamping force higher than the internal cavity force(reaction force) to avoid flashing from happening.Based on the dimensions provided by standard mould set,thewidth and the height of the core plate are 200 and 250mm,respectively.These dimensions enabled design of two cavitieson core plate to be placed horizontally as there is enoughspace while the cavity plate is left empty and it is only fixedwith sprue bushing for the purpose of feedingmolten plastics.Therefore, it is only one standard parting line was designed at the surface of the product. The product and the runner werereleased in a plane through the parting line during mouldopening.Standard or side gatewas designed for thismould.The gateis located between the runner and the product. The bottomland of the gate was designed to have 20◦ slanting and hasonly 0.5mm thickness for easy de-gating purpose.
The gatewas also designed to have 4mm width and 0.5mm thicknessfor the entrance of molten plastic.In the mould design, the parabolic cross section type ofrunnerwas selected as it has the advantage of simplermachin-ing in one mould half only, which is the core plate in thiscase. However, this type of runner has disadvantages such asmore heat loss and scrap comparedwith circular cross sectiontype. This might cause the molten plastic to solidify faster.This problem was reduced by designing in such a way thatthe runner is short and has larger diameter, which is 6mm indiameter.It is important that the runner designed distributesmaterialor molten plastic into cavities at the same time under thesame pressure and with the same temperature. Due to this,the cavity layout had been designed in symmetrical form.Another design aspect that is taken into consideration wasair vent design. The mating surface between the core plateand the cavity plate has very fine finishing in order to preventflashing fromtaking place.However, this can cause air to trapin the cavity when the mould is closed and cause short shotor incomplete part. Sufficient air vent was designed to ensurethat air trap can be released to avoid incomplete part fromoccurring.The cooling system was drilled along the length of thecavities and was located horizontally to the mould to alloweven cooling. These cooling channels were drilled on bothcavity and core plates. The cooling channels provided suffi-cient cooling of themould in the case of turbulent flow. Fig. 2shows cavity layout with air vents and cooling channels oncore plate.In this mould design, the ejection system only consists ofthe ejector retainer plate, sprue puller and also the ejector plate. The sprue puller located at the center of core plate notonly functions as the puller to hold the product in positionwhen the mould is opened but it also acts as ejector to pushthe product out of the mould during ejection stage. No addi-tional ejector is used or located at product cavities becausethe product produced is very thin, i.e. 1mm. Additional ejec-tor in the product cavity area might create hole and damageto the product during ejection.Finally, enough tolerance of dimensions is given consid-eration to compensate for shrinkage of materials.Fig. 3 shows 3D solid modeling as well as the wireframemodeling of the mould developed using Unigraphics.3. Results and discussion3.1. Results of product production and modificationFrom the mould designed and fabricated, the warpagetesting specimens produced have some defects during trialrun. The defects are short shot, flashing and warpage. Theshort shot is subsequently eliminated bymilling of additionalair vents at corners of the cavities to allow air trapped to escape. Meanwhile, flashing was reduced by reducing thepacking pressure of the machine. Warpage can be controlledby controlling various parameters such as the injection time,injection temperature and melting temperature.After these modifications, the mould produced high qual-ity warpage testing specimen with low cost and requiredlittle finishing by de-gating. Fig. 4 shows modifications ofthe mould, which is machining of extra air vents that caneliminate short shot.3.2. Detail analysis of mould and productAfter themould and productswere developed, the analysisofmould and the product was carried out. In the plastic injec-tion moulding process, molten ABS at 210 ◦C is injected intothe mould through the sprue bushing on the cavity plate anddirected into the product cavity. After cooling takes place,the product is formed. One cycle of the product takes about35 s including 20 s of cooling time.The material used for producing warpage testing speci-men was ABS and the injection temperature, time and pres-sure were 210 ◦C, 3 s and 60MPa respectively. The materialselected for the mould was AISI 1050 carbon steel.Properties of these materials were important in determin-ing temperature distribution in the mould carried out usingfinite element analysis. Table 2 shows the properties for ABSand AISI 1050 carbon steel.The critical part of analysis for mould is on the cavity andcore plate because these are the place where the product isformed.
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