International Journal of Metallurgical & Materials Science and Engineering (IJMMSE) ISSN 2278-2516 Vol. 3, Issue 2, Aug 2013, 1-14 © TJPRC Pvt. Ltd.
DESIGN AND DEVELOPMENT OF INJECTION MOULD FOR O-GIVE COMPONENT K.SRINIVASULU REDDY Professor, Department of Mechanical Engineering, Sreenidhi Institute of Science & Technology (An autonomous Institution under JNTU Hyderabad), Hyderabad, Andhra Pradesh, India
ABSTRACT The high cost and long lead times of traditional tooling makes it difficult for manufacture of complicated contour components. This paper presents an approach to produce the mould for missile component O-Give having aerodynamic shape. The mould is designed and modeled in moldflow insight software and mould is prepared using conventional and CNC machining.
KEYWORDS: O-Give, Conventional and CNC Machining, Thermoplastic and Thermoset Polymers INTRODUCTION Injection molding is considered one of the most common plastic part manufacturing processes. It can be used for producing parts from both thermoplastic and thermoset polymers. The process usually begins with taking the polymers in the form of pellets or granules and heating them to the molten state. The melt is then injected into a chamber formed by a split-die mold. The melt remains in the mold and is either chilled down to solidify (thermoplastics) or heated up to cure (thermosets). The mold is then opened and the part is ejected. Injection molding is used to produce thin-walled plastic parts for wide variety of applications.
Process Cycle The process cycle for injection molding is very short, typically between 2 seconds and 2 minutes, and consists of the following four stages: Clamping Prior to the injection of the material into the mold, the two halves of the mold must first be securely closed by the clamping unit. Each half of the mold is attached to the injection molding machine and one half is allowed to slide. The hydraulically powered clamping unit pushes the mold halves together and exerts sufficient force to keep the mold securely closed while the material is injected. The time required to close and clamp the mold is dependent upon the machine - larger machines will require more time. This time can be estimated from the dry cycle time of the machine. Injection The raw plastic material, usually in the form of pellets, is fed into the injection molding machine, and advanced towards the mold by the injection unit. During this process, the material is melted by heat and pressure. The molten plastic is then injected into the mold very quickly and the buildup of pressure packs and holds the material. The amount of material that is injected is referred to as the shot. The injection time is difficult to calculate accurately due to the complex and changing flow of the molten plastic into the mold. However, the injection time can be estimated by the shot volume, injection pressure, and injection power.
2
K. Srinivasulu Reddy
Cooling The molten plastic that is inside the mold begins to cool as soon as it makes contact with the interior mold surfaces. As the plastic cools, it will solidify into the shape of the desired part. However, during cooling some shrinkage of the part may occur. The packing of material in the injection stage allows additional material to flow into the mold and reduce the amount of visible shrinkage. The mold cannot be opened until the required cooling time has elapsed. The cooling time can be estimated from several thermodynamic properties of the plastic and the maximum wall thickness of the part. Ejection After sufficient time has passed, the cooled part may be ejected from the mold by the ejection system, which is attached to the rear half of the mold. When the mold is opened, a mechanism is used to push the part out of the mold. Force must be applied to eject the part because during cooling the part shrinks and adheres to the mold. In order to facilitate the ejection of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to injection of the material. The time that is required to open the mold and eject the part can be estimated from the dry cycle time of the machine and should include time for the part to fall free of the mold. Once the part is ejected, the mold can be clamped shut for the next shot to be injected. The mould design of o-give component developed in moldflow is given in figure 1 After the injection molding cycle, some post processing is typically required. During cooling, the material in the channels of the mold will solidify attached to the part. This excess material, along with any flash that has occurred, must be trimmed from the part, typically by using cutters. For some types of material, such as thermoplastics, the scrap material that results from this trimming can be recycled by being placed into a plastic grinder, also called regrind machines or granulators, which regrinds the scrap material into pellets. Due to some degradation of the material properties, the regrind must be mixed with raw material in the proper regrind ratio to be reused in the injection molding process.
Figure 1: O-Give Injection Mould In order for the molten plastic to flow into the mold cavities, several channels are integrated into the mold design. First, the molten plastic enters the mold through the sprue. Additional channels, called runners, carry the molten plastic from the sprue to all of the cavities that must be filled. At the end of each runner, the molten plastic enters the cavity through a gate which directs the flow. The molten
Design and Development of Injection Mould for O-Give Component
3
plastic that solidifies inside these runners is attached to the part and must be separated after the part has been ejected from the mold. However, sometimes hot runner systems are used which independently heat the channels, allowing the contained material to be melted and detached from the part. Another type of channel that is built into the mold is cooling channels. These channels allow water to flow through the mold walls, adjacent to the cavity, and cool the molten plastic. The detailed view of moulding tool is shown in figure 2.
Figure 2: Detailed View of Moulding Tool O-give Milan Missile is a second generation anti tank guided weapon of high accuracy used by Indian army to destroy enemy battle tanks. It has a range of two km wit h high hit probability of 94% with an inch exactness of aimed target. O-give is part of warhead assembly and gets mounted on threaded ring to match the external profile of the rear charge casing at rear. The criticality of this component is the external profile which experiences the aerodynamic thrust and the wall thickness to match the required weight and sustain the burst pressure. This component is made of PC 1041 Polycarbonate material which is manufactured by injection moulding process. Because of the requirements of high strength to weight ratio and to withstand aerodynamic forces during its function, the PC 1041 polycarbonate material is used as the O-give material. This component is of 117mm in diameter, 245mm in length, 1.25-0.35 wall thickness and 120gms in weight. Realizing the component with these controls through Injection moulding is very critical. O-Give is to be subjected to a test with pressure exerted in axial direction which is greater than or equal to 8 kN as shown in figure 3.
Figure 3: Axial Load on O-give
INJECTION MOULD DESIGN Component Specifications Component Name
: O-give
Material
: PC 1041
4
K. Srinivasulu Reddy
Material Description
: 40% Glass fiber reinforced Polycarbonate.
Glass fiber content
: 40±4
Water absorption
: 0.15% (max)
(24 Hours immersion in water) Weight
: 110±6gr
Material Properties Mechanical Tensile strength at break
: 120-150 MPa
Elongation at break
: 2-4 %
Flexural strength
:160-190 MPa
Flexural modulus
: 8000 – 9500 MPa
Notched Izod impact strength
: 140-170 J/m
Specific gravity
: 1.50±0.03
Thermal Thermal capacity
: 80 cal/gr
Deflection temperature under load : 135-140°C (at 1.8 MPa/ 3.2 mm thickness) Vicat softening temperature
: 152 - 158°C
(10N/ rate B/ 3.2 mm thickness) Electrical Volume resistivity
: > 10 15 Ohm-cm
It is an injection moulding grade of Polycarbonate reinforced with 40% glass fiber. The product exhibits excellent strength, creep resistance, impact properties, dimensional stability and good flow because of following factors
Extremely low moisture absorption with no resultant dimensional changes.
Low overall shrinkage and only slight shrinkage differences.
Very slight, uniform thermal expansion.
It has excellent electric and dielectric properties.
Design Calculations Shot Capacity Shot capacity is the amount of material that a machine can inject in one stroke of the plunger. It is a machine related parameter; this parameter depends on the type of material to be injected. For a screw type of machine swept volume (Vs) of the injection cylinder is generally mentioned. Shot capacity can be calculated from the swept volume.
5
Design and Development of Injection Mould for O-Give Component
Shot capacity (g) = swept volume x ρ x C Where ρ = density of plastic at normal temperature C is a constant and 0.85 for crystalline materials and 0.93 for amorphous materials The machine used here is a screw type machine SP 130 whose swept volume is 130 cc. Shot capacity should be more than the amount of material that should be injected into the mould, which includes total weight of the components to be molded with runners, gates and sprue. Clamping Force As the material is injected into the mould, the force exerted by the material tends to open up the mould. Hence a clamping force is required to keep the mould in closed position. The machine supplies this clamping force by the help of hydraulic or mechanical actuation. Most of the machines use hydraulic arrangement for supplying locking force. This clamping force required to keep the mould closed during injection must exceed the force given by the product of the opening pressure in the cavity and the total projected area of all impressions and runners. Clamping force = Projected area of molding x (½ to
of injection pressure)
Clamping force = 202.42T Plasticizing Capacity It is the rate at which the machine can supply molten plastic per hour. The press to be selected, therefore, should be capable of plasticizing sufficient material to maintain the expected molding cycle. It is mentioned as Kg/hr in specifications of the machine. This parameter is material dependent and Polystyrene (PS) is used for specification. Plasticizing rate of material X = Plasticizing rate of PS x Thermal capacity of PS Thermal capacity of X
Cooling Period The principal factor that generally controls the cycle time is the cooling time, which is necessary to produce a warp free component. Uniform cooling of the component is necessary to produce a defect free component.
T=
t 2 Tx Tm ln 2 4 Tc Tm
Where T = min. molding set up time in sec t = max. thickness of the component α = thermal diffusivity of material ( mm2/ sec °C ) Tx = heat distortion temperature of material ( °C ) Tm = temperature of mould ( °C ) Tc = cylinder or barrel temperature ( °C )
6
K. Srinivasulu Reddy
Runner Size When deciding the size of the runner the designer must consider the following factors:
The wall section and Volume of the molding;
The distance of the impression from the main runner or sprue; Runner cooling consideration; The range of mould makers cutter available and
The plastic material to be used. √W x 4√L D=
3.7
Where D = Runner diameter (mm) W = weight of molding (gr) L = length of runner (mm) Deflection of Side Walls C P d4 y= Et3 Where y = deflection of side walls ( cm) C = constant (From table 1) P = maximum cavity pressure (kgf/cm2) d = total depth of cavity wall (cm) E = modulus of elasticity for steel (2.1 x 10 6 Kgf/ Cm2) t = thickness of cavity wall (cm) Table 1: L/d Ratio Ratio of the Length of Cavity Wall to the Depth of Cavity Wall 1:1 2:1 3:1 4:1 5:1
Value of C 0.044 0.111 0.134 0.140 0.142
Strength of Guide Pillars The guide pillar system is primarily concerned with alignment of the mould faces as they close during the moulding cycle. But in certain cases the pillars have also the subsidiary functions of protecting the core and acting as locating pins when the mould is being assembled.
Design and Development of Injection Mould for O-Give Component
7
The size of the guide pillars is of great importance in the design of the mould. The working diameter of the pillars and the number of guide pillars will depend on the size of the mould and whether or not any side force is likely to be exerted on it. So the guide pillars should withstand side forces to be exerted on it. The moulds with deep and heavy cross- sectional cores exert side thrusts and the guide pillars should be strong enough to absorb t hem without any damage. The surface of the pillar must be hard and wear resisting. This is achieved by machining he pillar from low carbon steel which is then case hardened. This process gives a surface which resists pick-up and scoring as the pillar continuously enters and leaves the guide bush. If the guide pillar is likely to be subjected to bending forces the use of carburizing nickel chrome steel is to be preferred. The normal size range of guide pillars obtainable as standard parts (to reduce mould cost) is in between 10 mm to 38 mm working diameter. Injection Mould Design in Autodesk Inventor An injection mould is designed for the component. Mould is designed in a 3D modeling software Autodesk inventor. Core and Cavity are extracted from a module of Autodesk moldflow insight. The basic procedure for the design of injection mould is: Parting Surface A "parting line" is the line of separation on the part where the two halves of the mold meet. The line actually indicates a parting "plane" that passes though the part. It should be such that the component sticks to the moving side and the ejection of the component is simple. Care should be taken that the mould parts are easy to manufacture, which is governed by parting surface. Feed System Feed system comprises of sprue, runners and gate. They will guide the material from exit of the nozzle to the cavity in the mould. As the mould to be designed is of single cavity direct sprue feeding is the best option. But the outer surface of the mould should not have a visible feed mark. Hence, sprue, runner and gate are used. The feed mark can be minimized and the cost is also normal. This calls for an inclined sprue as the mould center and the location where feeding is decided are at an offset. Sprue Design Considerations The dimensions of sprue depend primarily on the dimensions of the molded part and especially its wall thickness. Here are some general guidelines that should be considered;
The sprue must not freeze before any other cross section. This is necessary to permit sufficient transmission of holding pressure.
The sprue must de mold easily and reliably. The included angle in the sprue is generally taken as 2° to 5°. The maximum inclination angle of sprue is 38°. The
inclination angle of the sprue is 25°. Gate Design Considerations A gate is a small opening (or orifice) through which the polymer melt enters the cavity. Gate design for a particular application includes selection of the gate type, dimensions, and location. It is dictated by the part and mold design, the
8
K. Srinivasulu Reddy
part specifications (e.g., appearance, tolerance, concentricity), the type of material being molded, the fillers, the type of mold plates, and economic factors (e.g., tooling cost, cycle time, allowable scrap volume). Gate design is of great importance to part quality and productivity. The cross section of the gate is typically smaller than that of the part runner and the part, so that the part can easily be "de-gated" (separated from the runner) without leaving a visible scar on the part. The gate thickness is usually two-thirds of the part thickness. Since the end of packing can be identified as the time when the material in the gate drops below the freeze temperature, the gate thickness controls the packing time. A larger gate will reduce viscous (frictional) heating, permit lower velocities, and allow the application of higher packing pressure for a longer period of time. If the requirement is good surface finish, better dimensional stability and appearance then a larger gate is favorable. Selection of gate location should be such that it ensures rapid and uniform mold filling. Position weld lines and air/gas vents so they have the least effect on the appearance and strength of the part. Since gates are locations of high residual stress, position them away from areas that will experience high external stress during use. The gate location should be at the thickest area of the part, preferably at a spot where the function and appearance of the part are not impaired. This leads the material to flow from the thickest areas to thinner areas to the thinnest areas, and helps maintain the flow and packing paths. Gate location should be central so that flow lengths are equal to each extremity of the part.
Position the gate away from load-bearing areas. The high melt pressure and high velocity of flowing material at a gate cause the area near a gate to be highly stressed.
Position the gate away from the thin section areas, or regions of sudden thickness change. This will avoid Hesitation or Sink marks and voids. Avoiding common problems: Improperly positioned gates often cause the following problems; we have to keep them in mind when designing
the delivery system.
Gate symmetrically to avoid warpage Symmetrical parts should be gated symmetrically, to maintain that symmetry. Asymmetric flow paths will allow
some areas to be filled, packed, and frozen before other areas are filled. This will result in differential shrinkage and probable warpage of the parts.
Vent properly to prevent air traps The gate location should allow the air present in the cavity to escape during injection to prevent air traps. Failure
to vent the air will result in a short shot, a burn mark on the molding, or high filling and packing pressure near the gates.
Enlarge the gate to avoid jetting Gate location and size should prevent jetting. The string appearance or spaghetting size strands of melt in shots.
Jetting can be prevented by enlarging the gate or by locating the gate in such a way that the flow is directed against a cavity wall.
Position weld lines carefully The gate location should cause weld and meld lines, if any, to form at appropriate positions that are not objection-
Design and Development of Injection Mould for O-Give Component
9
able to the function or appearance of the part.
Gate length Gate length should be as short as possible to reduce an excessive pressure drop across the gate. A suitable gate
length ranges from 1 to 1.5 mm. Gate size Gates should be small at the beginning of the design process so they can be enlarged, if necessary. Reducing the gate size is not easy as enlarging it.
Gate thickness The gate is normally 50 to 80 percent of the gated wall section thickness. For manually trimmed gates, the gate
thickness can occasionally be the same as the gated wall section thickness. For automatically trimmed gates, the gate thickness is typically less than 80 percent of gated wall section thickness, to avoid part distortion during gate breaking. Typical diameters for gate and for pin and submarine gates range from 0.25 to 2.0 mm.
Freeze –off time The freeze –off time at the gate is the maximum effective cavity packing time. However, if the gate is too large,
freeze off might be in the part, rather than in the gate, or if the gate freezes after the packing pressure is released, flow could reverse from the part, back into the runner system .A well-designed gate freeze-off time will also prevent back flow of the injected material.
Fiber-filled materials Fiber-filled materials require larger gates to minimize breakage of the fibers when they pass through the gate. Us-
ing small gates such as submarine, tunnel, or pin gates can damage the fillers in filled materials. Gates that deliver a uniform filling pattern (such as an edge gate) and thus, a uniform fiber orientation distribution are preferable to point-type gate. Ejection System This arrangement helps to remove the component from the mould after the mould is cooled. All the components of the mould, which take care of ejection, are called as ejection system. A stripper mould is very similar to the two plate mould except for the ejection system. This design has a stripper plate for ejection, where as the standard one has pins or sleeve as the ejector. This is illustrated in figure 4. The advantage of a stripper plate is the increased surface area for ejection that it offers.
Figure 4: Stripper Plate in Ejection System
10
K. Srinivasulu Reddy
This mould is having a stripper plate ejection system, where the wall thickness at ejection area is less; we have to use stripper plate ejections which apply the uniform load throughout the surface. During moving half opening, initially core and stripper plates together open until the end of limit bolt or pull rod stop its position. After touching the limit bolts the stripper plate motion stops, but the core plate will continuously move backside to the end. Due to this action the components automatically ejected from the stripper plate. The ejection system consists of stripper insert (Figure 5), stripper plate (Figure 6), stripper rod, pull rods, guide pillars, core, core plate etc.
Figure 5: Stripper Insert
Figure 7: Cavity Insert
Figure 6: Stripper Plate
Figure 8: Core Insert
Mold Cooling Mold cooling accounts for more than two-thirds of the total cycle time in the production of injection molded thermoplastic parts. An efficient cooling circuit design reduces the cooling time, which, in turn, increases overall productivity. Moreover, uniform cooling improves part quality by reducing residual stresses and maintaining dimensional accuracy and stability. The mold itself can be considered as a heat exchanger, with heat from the hot polymer melt taken away by the circulating coolant. After the holding phase, the plastic continues to cool until it reaches a temperature at which it is rigid enough to be removed from the mold and remain adequately stable. Too short a cooling time results in a part with excessive shrinkage or warpage. Too long a cooling time results in excessive molded-in stresses (and possible breakage), as well as an uneconomical cycle time. The temperature of the plastic is not uniform when it is removed from the mold. Plastic is a poor conductor of heat. The temperature of the core of the plastic part when it is removed from the mold is higher than the surface temperature. The core takes longer to cool and shrink than the surface. There are always some molded-in stresses as a result of this differential cooling. The greater the part wall- thickness, the greater this differential cooling and stress. For very thick
Design and Development of Injection Mould for O-Give Component
11
walls, the core temperature can be so high that even though the part looks all right when it is removed from the mold, the heat from the core material can remelt the surface and cause all sorts of difficulties. For this reason it is sometimes appropriate to place thick-walled parts into a cooling fluid to keep the surfaces rigid until the core is fully cooled.In this design normal procedure of oil cooling for cavity insert (Figure 7) and baffle type oil cooling is introduced for core insert (Figure 8). Frame or Mold Base Mould base consists of top and bottom plate, locating ring, guide pillars, guide bush, stripper plate, stripper insert, support pillars, insulation plate etc. Mould base is to be designed by referring to machine mould space. Machine mould space is the space given in the machine for mounting the mould. This includes size of mould plates, shut height of the tool, clamping slots, distance between tie bars etc. Injection mould assembly, moving side assembly and fixed side assembly are shown in figures 9,10 & 11 respectively.
Figure 9: Injection Mould Assembly
Figure 10: Moving Side Assembly
Figure 11: Fixed Side Assembly
12
K. Srinivasulu Reddy
MOLD FLOW ANALYSIS Flow analysis is done in Mold flow Plastics Insight software. Mold flow Plastics Insight (MPI) software represents the most comprehensive suite of definitive tools for simulating, analyzing, optimizing, and validating pla stics part and mold designs. To avoid the high costs and time delays associated with problems discovered at the start of manufacturing, it is necessary to consider the combined effects of part geometry, mater ial selection, mold design and processing conditions on the manufacturability of a part. Using predictive analysis tools to simulate the injection molding process, one can evaluate and optimize interactions among these variables during the design phase itself. With MPI analyses, we can simulate the filling, packing and cooling phases of thermoplastics molding processes using materials with or without fillers and fiber reinforcements, as well as predict post -molding phenomena such as part warpage. We can also simulate material flow and cure of reactive molding processes. Fill, flow, warp and cooling analysis can be done in MPI. The filling phase of the injection molding process greatly affects the quality of the end product. By utilizing filling analysis the processing characteristics of an injection mold can be investigated and optimized at the design stage. Improvements in part quality are achieved by refining gate points, the position of weld lines, eliminating gas traps, balancing pressure drops and r educing stress levels. Flow analysis predicts the flow of molten plastics material during the filling cy cle and facilitates the understanding of filling characteristics of the part and thereby helps to validate the part design for producing good quality molded parts. Part design can be reviewed based on the results and suitably modified for improving the part quality. Flow analysis can be used in:
Polymer Selection: Flow analysis determines the behavior of polymer and its filling characteristics during the molding cycle and therefore helps to evaluate different polymers and select the most suitable one.
Optimization of Feed System Design: Feed system of the mold controls the distribution of material flow into the mold cavities. Flow analysis facilitates evaluation of alternate feed systems to achieve balanced filling.
Process Optimization: Flow analysis simulates the filling and packing phases of the molding cycle. Different process conditions can be evaluated and most appropriate process parameters can be chosen to produce the part. For a set of process inputs (injection time, melt and mold temperature, pack profile etc) the following results can be seen: injection pressure, Clamp force, temperature at flow front, weld line locations, air trap locations, volumetric shrinkage, etc. Based on these results the most optimum process cond ition can be chosen. The cooling circuit is often an after-thought, designed to fit in with other aspects of the mold tool, such as ejection
and the feed system. However uniform and efficient removal of the heat from the polymer is essential to the end quality of the molding and cycle time. The cooling system should be given a high priority at the tool design stage. Warpage analysis evaluates the dimensional characteristics of a molding once ejected from the mold. In add ition diagnostic capabilities allow the cause of warpage to be identified, and remedied during the early stages of design. Warpage analysis is also a very useful tool for investigating practical solutions for existing moldings that have disto rtion problems.
CONCLUSIONS Mold flow analysis of o-give missile component is carried and the pressure drop is studied. The pressure drop varies from 0 to 62.75 MPa for the component made and it is found that the designed component is safe as per the
13
Design and Development of Injection Mould for O-Give Component
pressure drop simulations calculated by moldflow analysis tool as shown in Figure 13.
Figure 12: O-Give Component
Figure 13: O-Give Pressure Drop
REFERENCES 1.
Martinez et al [2011], “characterization of viscous response of a polymer during fabric IMD injection process by means a spiral mold” Journal of Measurement, Elsevier,vol 44,Issue 10 Dec 2011, pp 1806-1818.
2.
S.H.Tang et al [2006], “Design and thermal analysis of plastic injection mould” Journal of materials processing Technology, Vol 171, Issue 2, Jan 2006, pp 259-267.
3.
T.Barriere et al [2002], “Improving mould design and injection parameters in metal injection moulding by accurate 3D finite element simulation” Journal of materials processing technology, vol -125-126, sep 2002, pp 518-54
4.
David kazmer [2011], “ 31-Design of plastic parts” Applied Engineering Plastics Hand book” pp 535 -551.
