Design and thermal analysis of plastic injection mould
S.H. Tang , Y.M. Kong, S.M. Sapuan, R. Samin, S. Sulaiman
Abstract
窗外雨
This paper prents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould. The technique, theory, methods as well as consideration needed in designing of plastic injection mould are prented. Design of mould was carried out using commercial computer aided design software Unigraphics, Version 13.0. The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved using a commercial finite element analysis software called LUSAS Analyst, Version 13.5. The software provides contour plot of temperature distribution for the model and also temperature variation through the plastic injection molding cycle by plotting time respon curves. The results show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions. This uneven cooling effect at different regions of mould contributed to warpage.
Keywords: Plastic Injection mould; Design; Thermal analysis
1.Introduction
Plastic industry is one of the world’s fast est growing industries, ranked as one of the few billion-dollar industries. Almost every product that is ud in daily life involves the usage of plastic and most of the products can be produced by plastic injection molding method [1]. Plastic injection molding process is well known as the manufacturing process to create products with various shapes and complex geometry at low cost[2].
The plastic injection molding process is a cyclic process. There are four significant stages in the process. The stages are filling, packing, cooling and ejection. The plastic injection molding process begins with feeding the resin and the appropriate additives from the hopper to the heating/injection system of the injection plastic injection molding machine [3]. This is the “filling stage” in which the mould cavity is filled with hot polymer melt at injection temperature. After the cavity is filled, in the “packing stage”, additional polymer melt is packed into the cavity at a higher pressure to compensate the expected shrinkage as the polymer solidifies. This is followed by “cooling stage” where the mould is cooled until the part is sufficiently rigid to be ejected. The last step is the “ejection stage” in which the mould is opened and the part is ejected, after which the mould is clod again to begin the next cycle [4].
The design and manufacture of injection molded polymeric parts with desired properties is a costly process dominated by empiricism, including the repeated modification of actual tooling. Among the task of mould design, designing the mould specific supplementary geometry, usually on the core side, is quite complicated by the inclusion of projection and depression [5].
In order to design a mould, many important designing factors must be taken into consideration. The factors are mould size, number of cavity, cavity layouts, runner systems,
gating systems, shrinkage and ejection system [6].
In thermal analysis of the mould, the main objective is to analyze the effect of thermal residual stress or molded-in stress on product dimension. Thermally induced stress develop principally during the cooling stage of an injection molded part, mainly as a conquence of its low thermal conductivity and the difference in temperature between the molten resin and the mould. An uneven temperature field exists around product cavity during cooling [7].
During cooling, location near the cooling channel experiences more cooling than location far away from the cooling channel. This different temperature caus the material to experience differential shrinkage causing thermal stress. Significant thermal stress can cau warpage problem. Therefo
re, it is important to simulate the thermal residual stress field of the injection-molded part during the cooling stage [8]. By understanding the characteristics of thermal stress distribution, deformation caud by the thermal residual stress can be predicted.
In this paper the design of a plastic injection mould for producing warpage testing specimen and for performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould is prented.
2.Methodology
2.1. Design of warpage testing specimen
This ction illustrates the design of the warpage testing specimen to be ud in plastic injection mould. It is clear that warpage is the main problem that exists in product with thin shell feature. Therefore, the main purpo of the product development is to design a plastic part for determining the effective factors in the warpage problem of an injectionmoulded part with a thin shell.
The warpage testing specimen is developed from thin shell plastics. The overall dimensions of the specimen were 120mmin length, 50mmin width and 1mmin thickness. The material ud for produci
ng the warpage testing specimen was acrylonitrile butadiene stylene (ABS) and the injection temperature, time and pressure were 210 ◦C, 3 s and 60MPa, respectively.
2.2. Design of plastic injection mould for warpage testing specimen教师座右铭
This ction describes the design aspects and other considerations involved in designing the mould to produce warpage testing specimen. The material ud for producing the plastic injection mould for warpage testing specimen was AISI 1050 carbon steel.工作性质怎么填写
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 the product is too thin.
Iv. Two-plate mould (Concept 4) having one parting line with double cavities withgating system, only ud 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, the mouldwas designed bad on the platen dimension of the plastic injection machine ud (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. Furthermore, 4mm space had been rerved between the two tie bars and the mould for mould tting-up and handling purpos. This gives the final maximum width of the mould as 250 mm. The standard mould ba with 250mm×250mmis employed. The mould ba is fitted to the machine using Matex clamp at the upper right and lower left corner of the mould ba or mould platen.
成人不自在
The mould had been designed with clamping pressure having clamping force higher than the internal cavity force (reaction force) to avoid flashing from happening.
Bad on the dimensions provided by standard mould t, the width and the height of the core plate
are 200 and 250 mm, respectively. The 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 purpo of feeding molten plastics. Therefore, it is only one standard parting line was designed at the surface of the product. The product and the runner were relead 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 0.5mm thickness for easy de-gating purpo. The gate was also designed to have 4mm width and 0.5mm thickness for the entrance of molten plastic.
In the mould design, the parabolic cross ction type of runner was lected as it has the advantage of simpler machining in one mould half only, which is the core plate in this ca. However, this type of runner has disadvantages such as more heat loss and scrap compared with circular cross ction type. This might cau 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 6mm 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 symmetrical 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 cau air to trap in the cavity when the mould is clod and cau short shot or incomplete part. Sufficient air vent was designed to ensure that air trap can be relead 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. The cooling channels were drilled on both cavity and core plates. The cooling channels provided sufficient cooling of the mould in the ca of turbulent flow.
In this mould design, the ejection system only consists of the ejector retainer plate, sprue puller and also the ejector plate. 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 ud or located at product cavities becau the product produced is very thin, i.e. 1 mm. 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
怎么设置壁纸From the mould designed and fabricated, the warpage testing specimens produced have some defects during trial run. The defects are short shot, flashing and warpage. The short shot is subquently 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 the 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 ◦C 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 material ud for producing warpage testing specimen was ABS and the injection temperature, time and pressure were 210 ◦C, 3 s and 60MPa res pectively. The material lected for the mould was AISI 1050 carbon steel. Properties of the materials were important in determining temperature distribution in the mould carried out using finite element analysis.
Due to symmetry, the thermal analysis was performed by modeling only the top half of the vertical cross ction or side view of both the cavity and core plate that were clamped together during injection.
Modeling for the model also involves assigning properties and process or cycle time to the model. This allowed the finite element solver to analyze the mould modeled and plot time respon graphs to show temperature variation over a certain duration and at different regions.
For the product analysis, a two dimensional tensile stress analysis was carried using LUSAS Analyst, Version 13.5. Basically the product was loaded in tension on one end while the other end is
clamped. Load increments were applied until the model reaches plasticity.
抗美援朝故事4conclusion
In future, however, it is suggested that the product rvice condition should be determined so that further analysis may be carried out for other behaviors under various other loading. that affect warpage. The testing specimen was produced at low cost and involves only little finishing that is de-gating.
The thermal analysis of plastic injection mould has provided an understanding of the effect of thermal residual stress on deformed shape of the specimen and the tensile stress analysis of product managed to predict the tensile load that the warpage testing specimen can withstand before experiencing failure.
References
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(2002) 1.
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[4] A.T. Bozdana, O¨. Eyerc´ıog˘lu, Development of an Expert System for the Determination of Injection Moulding Parameters of Thermoplastic Materials: EX-PIMM, J. Mater. Process. Technol. 128 (2002) 113.
[5] M.R. Cutkosky, J.M. Tenenbaum, CAD/CAM Integration Through Concurrent Process and Product Design, Longman. Eng. Ltd., 1987, p. 83.
[6] G. Menges, P. Mohren, How to Make Injection Molds, cond ed., Hanr Publishers, New York, 1993, p 129.
[7] K.H. Huebner, E.A. Thornton, T.G. Byrom, The Finite Element Method for Engineers, fourth ed., Wisley, 2001, p. 1.
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