Insert Molding and Encapsulation
Polyurethanes have low molding temperatures and pressures, making them ideal for encapsulating reinforcing inserts. The insert should not impede material flow.
If using a hollow insert, the ends must be sealed. Thermoplastic end caps have been successfully used to seal inserts. To promote good adhesion with the polyurethane, clean and roughen the inserts and treat them with an adhesion promoter, if necessary.
The type of RIM system used determines the recommended minimum distance between an insert and the mold wall. For solid materials, this distance is 1/8 inch. For foamed systems it is ¼ inch.
Encapsulated inserts have the following benefits:
|Reduce wall thickness|
|Absorb high stresses|
|Control Thermal Expansion|
Metal Stiffening Inserts
Molding metal inserts into RIM polyurethane materials will increase stiffness significantly. Inserts ofall types (flat plates, extrusions, tubes, and bars) can be encapsulated.Fully encapsulating inserts eliminates metal corrosion, while reducing thick cross sections, controlling deflection and thermal elongation,and absorbing high stresses.
The center of gravity for the insert should coincide with the center of gravity for the RIM material to reduce the potential for warping. This is due to the differences in the coefficients of linear thermal expansion in the materials. As the temperature increases, the polyurethane material will be in compression and the metal insert will be in tension.
As the temperature decreases from the ambient, the opposite happens. The relative cross-sectional areas of the two materials determine the ultimate elongation of the part.
Wood Stiffening Inserts
Typically cheaper than metal inserts, wood inserts can also be used to stiffen polyurethane parts. When a finished part is subjected to repeated loads, wood inserts may separate from molded polyurethane if the wood’s moisture content is higher than 6%. In cases where the wood cannot be dried to reduce the moisture, it must be sealed with a lacquer before molding.
Threaded inserts are particularly useful when components must be attached to RIM parts. Use appropriately sized, press-fit inserts with respect to boss-hole diameter. Use threaded inserts if your part is going to be frequently assembled and disassembled. With structural foam, molded-in inserts my offer greater pullout strength, because skin forms over the entire insert surface.
When using press-fit inserts with structural foam parts, mold the hole so that skin forms inside. Generally, press-fit inserts are preferred to molded-in ones, even though they are not as strong. Placing inserts on pins inside the mold can increase cycle time significantly.
Inserts have also been known to fall off pins during molding on rare occasions. The risk of potential tool damage must be weighed against the need for additional strength. The insert design, hole diameter, part density, and screw size determine the pullout force and stripping torque of threaded inserts.