DESIGN AND MANUFACTURING OF THE INJECTION MOLD FOR METAL-INSERTED RUBBER PARTS USING CAD/CAM/CAE TECHNOLOGIES: A CASE STUDY OF THE ENGINE MOUNTING

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ASME International Mechanical Engineering Congress and Exposition ASME2008 October 31 - November 6, 2008, Boston, Massachusetts, USA

IMECE2008-66843

DESIGN AND MANUFACTURING OF THE INJECTION MOLD FOR METAL-INSERTED

RUBBER PARTS USING CAD/CAM/CAE TECHNOLOGIES:

A CASE STUDY OF THE ENGINE MOUNTING

Supasit Rodkwan

Center of Excellence in Rubber Mould, Research and Development Institute of Industrial Production

Technology and Department of Mechanical Engineering, Kasetsart University, Bangkok

Thailand.

Wacharapong Chookaew

Thailand. Rungtham Panyawipart

Thailand.

Chana Raksiri

Technology and Department of Industrial Engineering, Kasetsart University, Bangkok

Thailand.

ABSTRACT

In general, rubber parts used in automotive applications are designed and manufactured without any inserts; it is significant, however, that for certain parts, such as an engine mounting that, the metal core must be used to increase the compressive strength of products. With the lack of numerical tools used to predict the rubber flow characteristics and the interaction between rubber and metal insert during the molding process, mold designers must rely on their experience and the trial-and-error method to design molds. Therefore, in this research, an application of CAD/CAM/CAE on the design and manufacturing of an injection mold for the engine mounting rubber made of a blend of Natural Rubber (NR) and AcryloNitrile-Butadiene Rubber (NBR) is performed. A CAD model of the part is constructed and then a two-cavity mold as well as various runner patterns and gate positions are designed and further analyzed using CAE. Subsequently, with use of the CAM system, a metal-inserted mold and related components are manufactured and used to produce the rubber engine mounts on the vertical rubber injection machine. The empirical and numerical resulting parameters, including part geometry, injection pressure, and part temperature at various injection stages, correlate well. This information provides mold designers and manufacturers a better understanding of the rubber behavior during curing in the metal-inserted rubber molding process so that various mold components can properly designed and effectively used. Consequently, better mold and

product quality with less defects as well as reduced production time can be obtained.

INTRODUCTION

Recently, there is increasing global demand on rubber products from various industries such as electronics and automotive applications. One of the widely used automotive rubber products is the engine mounting, which can be a challenge to design and manufacture. It is used to absorb the vibration from the engines of vehicles [1]. In this certain product, rubber is injected through an injection molding process, and the metal core is used in order to increase the product compressive strength. The traditional design and manufacturing of injected molds like this engine mounting are based on trial-and-error method and the moldmaker’s experience. This can result in unsatisfactory molds and product quality. Issues can occur such as misplacement of the position of gates and runners, low strength part, under-filled mold cavity, excessive flash, air trap and peeling-off on rubber/metal interface. Consequently, the objective of this research is to design and to manufacture the injection mold, using Computer Aided Design/Manufacturing/Engineering (CAD/CAM/CAE) technology to minimize the trial-and-error process, for metal-inserted rubber parts with a case study of the engine mounting used in heavy trucks. Both numerical and empirical work is performed for verification. The research result can lead to better understanding of rubber flow characteristics, improved

Proceedings of IMECE2008 2008 ASME International Mechanical Engineering Congress and Exposition October 31-November 6, 2008, Boston, Massachusetts, USA

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lead times, and costs due to design and manufacturing.

As a better understanding of the rubber injection process and its modeling is developed, there are still several difficulties associated with both rheological and thermal characterization, such as the influence of fillers in rubber compound, the cross linking reaction, the viscoelastic behavior of the rubber compound during cure and uncured stages. More work needs to be performed in these modeling areas. Nevertheless, in general, modeling of rubber injection molding process can be simplified and considered as an unsteady, isothermal flow of a non-Newtonian medium with viscous heat generation. The model accounts for both rheological and vulcanization data of various rubber compound used. The process can be divided into the filling and post-filling stages. Generally, the governing equations of the injection process are expressed by the conservation of mass, momentum, and energy. The continuity equation for an incompressible flow (Equation 1), the momentum equations for an inelastic non-Newtonian fluid in the absence of body and inertial force (Equations 2 and 3), and the non-isothermal energy equation (Equation 4) can be written as follows [10]:

0 =

∂

+

∂

+

∂

z

w

y

v

x

u

(1)

0 =

∂

+

∂

⎟

⎠

⎞

⎜

⎝

⎛

∂

x

P

z

u

η

(2)

0 =

∂

+

∂

⎟

⎠

⎞

⎜

⎝

⎛

∂

y

P

z

v

η

(3) •

+

Φ

+

∂

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

∂

+

∂

+

∂

Q

z

T

k

y

T

x

T

t

T

C

_p _th ₂ 2

ρ

(4)

Where u, v, w are velocity components in the x, y, and z direction, respectively, P is pressure,

η

is viscosity, and

ρ

,

C

_p,

k

_th, T,

Φ

,

•

Q

are density, specific heat, thermal conductivity, temperature, viscous heating and the rate of heat generated by reaction of vulcanization, respectively.

MATERIALS AND METHODS

In this industrial research-based work, a case study of the engine mounting as shown in Figure 1. is chosen. The vertical injection machine used in this work, as shown in Figure 2, has a clamping force size of 200 tons and an injection capacity of

2000 cc. The part modeling and the injection simulation work is carried out using Unigraphics and 3D-SIGMA. Using a preprocessor module [11], the STL file of the part (see Figure 3.) is used to construct the various components of the mold (see Figure 4.), such as the cavity plate, the core plate, the heater plate, and the inlet. Subsequently, both the rubber part and mold are meshed into elements using the enmeshment step. The size of each set of element is varied upon the specific part of the product and the mold. Then, the following material definition tasks are assigned: part material, insert material, mold material, tempering material. In this work, Natural Rubber (NR) and Nitride Butadine Rubber (NBR) were used as rubber materials of the part. The initial injection temperature is empirically found, in addition to other material data, such as density, heat conductivity, heat capacity, curing rate and dynamic viscosity, which are measured at three different temperatures. In addition, the conventional steel grade X45NiCrMo4 is used as a mold material. Then, initial

temperatures of 100˚C for rubber material, 160˚C for mold and 160˚C for heater, are chosen. An injection pressure of 100 MPa is used. Later, the injection process parameters and definitions such as cycle time, result selection, insert placement, mold closing, filling, curing, mold opening, external cooling, and channel are selected. A mold is designed for the two cavity injection process with the mold size of 360x180x149 mm. The runner of 8 mm diameter is chosen for circular and semi-circular types. The gate with a rectangular cross section of 2x6 mm is used depending on location. The various gate and runner combinations can be classified to six cases as shown in Figure 5. The gate positions are depicted in Figure 6. The assembly drawing of the injection mold for engine mounting is also shown in Figure 7.

Figure 1. The original engine mounting part

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Figure 2. The vertical injection machine

Figure 3. Three dimensional model of the engine mounting

Figure 4. Mold construction in a preprocessor module

1. Semi-circular/Side entry 2. Circular/Side entry

3. Semi-circular/Middle entry 4. Circular/Middle entry

5. Semi-circular/Two entries 6. Circular/Two entries Figure 5. Six gate and runner combinations

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Side entry Middle entry

Two entries Gate location Area Figure 6. Various gate positions

Figure 7. The assembly drawing of the injection mold for engine mounting

RESULTS

Studying the injection percentage and simulated patterns of rubber flow through the designed runner with the six aforementioned combinations, the rubber flow pattern can be initially seen at the injection rate of 0-50%. When the injection percentage is increased to 80-100%, a defect in the part is noticeable, as shown in Figure 8., in gate and runner combination No. 5 at the 80% injection rate. Similar results are also found for combinations No. 4 and No. 6. The simulated results reveal that there is high possibility for air traps to occur in the engine mounting rubber part for gate and runner combinations No. 4, 5, and 6. It can affect the lower adhesion strength on the interface between metal insert and rubber. As a result, the possible combinations are reduced to No. 1, 2, and 3.

Figure 8. Possible defect for the gate and runner combination No. 5

Using the results found on the rubber flow pattern in previous section and the simulated resulting pressure found in Table 1., the gate and runner combinations No. 1 and 2 (having lower inlet pressure than No. 3) are further considered for mold manufacturing.

Table 1. Pressure in the runner

Pressure in the runner (bar)

No. 1 No. 2 No. 3

855.5 722.0 989.0 Table 2. shows various simulated parameters such as pressure, velocity, shear rate, and temperature on the gate and runner combinations No. 1 and 2. The gate and runner system No. 2 (Circular/Side entry type) is ultimately selected for the production due to its large cross sectional area, which is conducive to high flow rate.

Air trap location

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Table 2. The simulated parameter comparison Parameter

Combination No. 1 Combination No. 2

Pre ssure a t 100% Fi lling (b ar) V elo city at 10 0% F illing (cm /s) S h ear Rate at 100% Filling (1/ s ) T emperature at 100% F illing (˚ C)

After the mold design phase is completed, the mold production can be performed. The first step is to obtain the complete 3D mold drawing with the gate and runner system selected using Computer Aided Design (CAD). Then, Computer Aided Manufacturing (CAM) software is used in order to generate the programming and codes for each mold components on the Computer Numerical Control (CNC) machine. The machining process of the mold components consist of milling, turning, drilling, Electro Discharge Machining (EDM), and grinding. The metal inserts and the final assembly of mold components and inserts are shown in Figures 9 and 10, respectively. In addition, the final rubber-injected part of engine mounting manufactured is shown in Figure 11.

The simulated rubber flow pattern results are compared with the empirical data at the various injection stages as shown in Table 3, and similar flow directions have been found. The rubber flows to the area opposite the gate and then starts filling until it reaches the gate itself. The air is then released in the direction shown in Figure 12. This design prevents air from accumulating in the interface of the rubber and metal insert, which will maintain the adhesion strength on that area. The final design for gate and runner pattern is shown in Figure 13.

Additionally, it can also be seen that the results correlate well. This shows the great benefit of using Computer Aided Engineering (CAE) technology to accurately predict the rubber flow behavior in the rubber injection molding process.

Figure 9. Metal insert components

Figure 10. The final assembly of mold components and inserts

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Table 3. Simulated and empirical result comparison on the rubber flow pattern at various injection rates

Simulated result Empirical work

60% Injection rate

80% Injection rate

100% Injection rate

Figure 12. Air release direction and filling pattern

Figure 13. The final design selection for gate and runner of the engine mounting

CONCLUSIONS

In this research, CAD/CAM/CAE was applied to the design and manufacturing of the injection mold for the engine mounting rubber made of a blend of Natural Rubber (NR) and AcryloNitrile-Butadiene Rubber (NBR). A CAD model of the part was designed in addition to a two-cavity mold and various runner patterns and gate positions, and further analyzed using CAE. Subsequently, with a use of the CAM system, a metal-inserted mold and related components were manufactured and used to produce the rubber engine mounts on the vertical rubber injection machine. The empirical and numerical parameters, including part geometry at various injection stages, correlated well. It was determined that the significant mold design parameters for engine mounting are the runner pattern and gate position, the mechanism of part removal from the mold, as well as air release direction. Therefore, it can be seen that this information can provide mold designers and manufacturers with a better understanding of rubber behavior during curing in the metal-inserted rubber molding process so that various mold components can properly designed and effectively used. Consequently, better mold and product quality can be obtained with less defects as well as reduced production time.

ACKNOWLEDGEMENTS

The funding supports both from the Center of Excellence in Rubber Mould (CERM), as a part of the Mold and Die Industry Development Project under the supervision of Thai-German Institute (TGI), Royal Thai Ministry of Industry and the Industrial Division (Department V), Thailand Research Fund (TRF) are greatly appreciated.

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Circular/Side entry

First full filled part

Air vent direction

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