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In this paper the design of a plastic injection mould for producing warpage testign specimen and for performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould is presented.
2 methodology
2.1 Design of warpage testing specimen
This section illustrates the design of the warpage testing specimen to be used in plastic injection mould. It is clear that warpage is the main problem that exists in product with thin shell feature. Therefore, the main prupose of the product development is to design a plastic part for determining the effective factors in the warpage problem of an injection moulded part with a thin shell.
The warpage testing specimen is developed from thin shell plastics. The overall dimensions of the specimen were 120 mm in length, 50 mm in width and 1 mm in thickness. The material used for producing the warpage testing specimen was acrylonitrile butadiene stylene (ABS) and the injection temperature, time and pressure were 210℃, 3 s and 60 MPa, respectively.
2.2 Design of plastic injection mould for warpage testing specimen
This section describes the design aspects and other considerations involved in designing the mould to produce warpage testign specimen. The material used for producing the plastic injection mould for warpage testing specimen was AISI 1050 carbon steel.
Four design concepts had been considered in designing of the mould including:
i. Three-plate mould (Concept 1) having two parting line with single cavity. Not applicable due to high cost.
ii. Two-plate mould (Concept 2) having one parting line with single cavity without gating system. Not applicable due to low production quantity per injection.
iii. Two-plate mould (Concept 3) having one parting line with double cavities with gating and ejection system. Not applicable as ejector pins might damage the product as product is too thin.
iv. Two-plate mould (Concept 4) having one parting line with double cavities with gating system, only used sprue puller act as ejector to avoid product damage during ejection.
In designing of the mould for the warpage testing specimen, the fourth design concept had been applied. Various design considerations had been applied in the design.
Firstly, fthe mould was designed based on the platen dimension of the plastic injection machine used (BOY 22D). There is a limitation of the machine, which is the maximum area of machine platen is given by the distance between two tie bars. The distance between tie bars of the machine is 254 mm. Therefore, the maximum width of the mould plate should not exceed this distance. Futhermore, 4 mm space had been reserved between the two tie bars and the mould for mould setting-up and handing purposes. This gives the final maximum width of the mould as 250 mm. The standard mould base with 250 mm×250 mm is employed. The mould base is fitted to the machine using Matex clamp at the upper right and lower left corner of the mould base or mould platen.
The mould had been designed with clamping pressure having clamping force higher than the internal cavity force (reaction force) to avoid flashing from happening.
Based on the dimensions provded by standard mould set, the width and the height of the core plate are 200 and 250 mm, repectively. These dimensions enabled design of two cavities on core plate to be placed horizontally as there is enough space while the cavity plate is left empty and it is only fixed with sprue bushing for the purpose of feeding molten plastics. Therefore, it is only one standard parting line was designed at the sur4face of the product. The product and the runner were released in a plane through the parting line during mould opening.
Standard or side gate was designed for this mould. The gate is located between the runner and the product. The bottom land of the gate was designed to have 20° slanting and has only 0.5 mm thickness for easy degating prupose. The gate was also designed to have 4 mm width and 0.5 mm thickness for the entrance of molten plastic.