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In th emould design, the parabolic cross section type of runner was selected as it has the advantage of simpler machining in one mould half only, which is the core plate in this case. However, this type of runner has disadvatages such as more heat loss and scrap compared with circular cross section type. This might cause the molten plastic to solidify faster. This problem was reduced by designing in such a way that the runner is short and has larger diameter, which is 6 mm in diameter.
It is important that the runner designed distributes material or molten plastic into cavities at the same time under the same pressure and with the same temperature. Due to this, the cavity layout had been designed in synnetrical form.
Another design aspect that is taken into consideration was air vent design. The mating surface between the core plate and the cavity plate has very fine finishing in order to prevent flashing from taking place. However,this can cause air to trap in the cavity when the mould is closed and cause short shot or incomplete part. Sufficient air vent was designed to ensure that air trap can be released to avoid incomplete part from occurring.
The cooling system was drilled along the length of the cavities and was located horizontally to the mould to allow even cooling. These cooling channels were drilled on both cavity and core plates. The cooling channels provided sufficient cooling of the mould in the case of turbulent flow.
In this mould design,the ejection system only consists of the ejector retainer plate,sprue puller and also the ejector plat. The sprue puller located at the center of core plate not only functions as the puller to hold the product in position when the mould is opened but it also acts as ejector to push the product out of the mould during ejection stage. No additional ejector is used or located at product cavities because the product produced is very thin,i.e,1mm. Additional ejector in the product cavity area might create hole and damage to the product during ejection.
Finally, enough tolerance of dimensions is given consideration to compensate for shrinkage of materials.
3 Results and discussion
3.1 Results of product production and modification
From the mould designed and fabricated, the warpage testing spectimens produced have some defects during trial run. The defects are short shot, flashing and warpage. The short is subsequently eliminated by milling of additional air vents at corners of the cavities to allow air trapped to escape. Meanwhile, flashing was reduced by reducing the packing pressure of the machine. Warpage can be controlled by controlling various parameters such as the injection time, injection temperature and melting temperature.
After these modifications, the mould produced high quality warpage testing specimen with low cost and required little finishing by de-gating.
3.2 Detail analysis of mould and product
After the mould and products were developed, the analysis of mould and the product was carried out. In the plastic injection moulding process, molten ABS at 210℃ is injected into the mould through the sprue bushing on the cavity plate and directed into the product cavity. After cooling takes place, the product is formed. One cycle of the product takes about 35 s including 20 s of cooling time.
The matrial used for producing warpage testing specimen was ABS and the injection temperature,time and pressure were 210℃, 3 s and 60MPa respectively. The material selected for the mould was AISI 1050 carbon steel. Properties of these materials were important in determining temperature distribution in the mould carried out using finite element analysis.
The critical part of analysis for mould is on the cavity and core plate because these are the place where the product is formed. Therefore, thermal analysis to study the temperature distribution and temperature at through different times are performed using commercial finite element analysis software called LUSAS Analyst, Version 13.5. A two-dimensional thermal analysis is carried out for to study the effect of thermal residual stress on the mould at different regions.