SMC/BMC Mold Design

You have to know that a good matched metal chrome plated steel mold is required for optimum production conditions when one is molding bulk molding compound (BMC) or sheet molding compound (SMC) parts. When the mold maker is expected to design the mold, the molder must furnish the following minimum information: 
1. Part print
2. Master model
3. Size of press to be used with the mold, including mounting hole patterns, etc.
4. Expected molding pressure
5. Type of molding compound to be used
6. Molding material shrinkage
7. Ejector cylinder line pressure
8. Operating temperature ranges for both core and cavity
9. Type of steel to be used. 

Mold Steel

Preliminary mold drawings should have been completed by the responsible design group and approved by the molder before ordering steel.
The anticipated production quantities expected from the mold or the product end use or both should dictate the choice of mold steel
When steel forgings are used, a certification of the steel composition, vacuum degassing, and hardness should be supplied by the steel company to the moldmaker.
Stress relieving of both the core and the cavity are necessary after rough duplicating. The steel company should provide the time and temperature requirements for the proper stress relieving.
The anticipated production quantities expected from the mold or the product end use or both should dictate the choice of mold steel
When steel forgings are used, a certification of the steel composition, vacuum degassing, and hardness should be supplied by the steel company to the moldmaker.
Stress relieving of both the core and the cavity are necessary after rough duplicating. The steel company should provide the time and temperature requirements for the proper stress relieving.

The anticipated production quantities expected from the mold or the product end use or both should dictate the choice of mold steel
When steel forgings are used, a certification of the steel composition, vacuum degassing, and hardness should be supplied by the steel company to the moldmaker.
Stress relieving of both the core and the cavity are necessary after rough duplicating. The steel company should provide the time and temperature requirements for the proper stress relieving.

 

Shear Edge Design and Specifications

The shear edges on an FRP mold differentiate it from other compression type molds. For SMC the design of the shear edges is particularly important. The purpose of the shear edges on an SMC mold is to provide a thin section at the extremities of the material flow that will cure quickly and then provide a dam to prevent further material escape, thereby establishing a back pressure on the curing material in the mold. As such, the shear edges no longer actually shear the glass reinforcement, as was the case peviously when oversized preforms had to be cut to size in the mold. Figure 3.2 consists of a sketch of both a horizontal and a vertical-type shear edge design. The following should be specified for SMC shear edges on molds:
1. Clearances should be from metal-to-metal fit to a maximum opening of 0.005 inches at the mold operating temperature of 300°F(149°C).
2. Shear edges on both the core and the cavity should be flame hardened to a Rockwell C of 55-60.
sticking or tearing of the flash. Shear edges should be polished well into the rind area.
4. Minimum rind clearances should be:
a. horizontal shears-1/2 inch
b. vertical shears-1 inch.

3. Final polishing should be in the direction of draw to prevent

References
1. Raymond W. Meyer,  “Handbook of Polyester Molding compounds and Molding Technology”, Chapman & Hall, London, 1987

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