Compression molding is a process used to make stock shape materials that are both thermoplastic and thermoset. It is achieved by placing a plastic material in a mold cavity that is formed through adding heat and pressure. These two in combination force the materials into the mold. The cycle of heat and pressure hardens the material and it can then be removed from the mold.
Achieving Stiffness for Compression Molding
Predominantly, if the metal part used in compression molding is tension or compression loaded, a ratio of metal part thickness to its extensional modulus, multiplied by the Bulk Molding Compound (BMC) material modulus (tension, compression, or flexural), is a successful formula to follow to achieve stiffness.
If the part is loaded in bending, it could be possible to make the metal part thicker, however, this is not necessarily needed. Ribs are easily added to a compression molded part to add additional stiffness with negligible material weight increase. Many companies, like Toray, give easy to navigate graphs for a variety of metals compared to BMC.
Image Credit: Toray Advanced Composites
Tools for Compression Molding
Matched-metal tooling is required for compression molding because it’s long-fiber BMC, with high fiber content, requires very high pressures up to 138 bar (2000 psi) to fill complex features. Additionally, tolerances on the core and cavity halves of the tool need to be very well controlled due to entrapped air that can escape, while fiber and resin cannot. This combination of requirements during compression molding can increase the cost of tooling beyond the basic cost of lamination tooling made from composite materials.
Nevertheless, this cost is lower than an injection molding tool. An unwritten rule for sufficient tooling return on investment is that the tool’s cost should be covered with 1000 production units. Indeed, this may be higher if the part is highly complex, or lower if it is not. For example, aluminum molds are lower priced compared to steel, however, they are less suitable for higher volume production runs. Multiple cavity tools can also help to reduce the mold costs because a higher amount of parts can be produced per mold cycle.
Billet Stock and 3D Behavior of Compression Molds
Billet stock compression molded blocks are a brilliant alternative to obtaining an expensive part-specific mold. Billet can be an important time and cost saver, especially when many geometry iterations on a part are anticipated in the prototype phase. In addition, it can save the cost of molds altogether if only a few parts are needed. Put simply; it is possible to machine the customized part from a billet, and hog-out a metal block to produce a part.
However, one essential limitation to machining from billet stock is that the billet block or plate is compression molded with BMC chips. These chips will layer into the compression molding tool with a near quasi-isotropic fiber dominated lay-up in the plane of the plate, but through the thickness it will be resin dominated. Therefore, if the machined part requires a feature that needs strength in an out of plane direction, this cannot be achieved.
Image Credit: Toray Advanced Composites
Figure 1: Billet Machined part; one leg of the “L” will have strong and stiff fiber properties and the other vertical leg will be weak because it is resin dominated in the direction along the leg. In contrast, the compression molded part has the same fiber-dominated properties in both legs.
Are Compression Molded Parts Viable?
Toray ‘Compression Molding Design Guide, Design Guidance for Long Chopper Fiber Compression Molding’ is a useful tool to begin to learn if compression molded parts are viable. The guide helpfully defines the process, lists material properties, gives recommendations on how to structurally design a part, shows the resulting tolerances, describes difficulties of molding part features and also comments on those that are straightforward, and details subtle differences when designing a part for compression molding.
After using the Toray guide, it is possible to contact a team member of Toray Advanced Composites Expert Services team about the part and the requirements. A 3D model of the current part will be required so a list of recommended changes can be provided in order to make the part more compression mold friendly.
Analyzing a Discontinuous Fiber Compression Molded Part
BMC parts made from discontinuous fibers can take on fiber alignments, other than those that are quasi-isotropic, and are geometry dependent. When fiber is placed into the mold cavity, it generally layers like a continuous fiber composite lay-up. Fibers that are distant from the sides of the mold will usually take on an in-plane quasi-isotropic lay-up orientation. At the edges of the mold, fibers tend to orient themselves parallel to the edge, making the properties of these fibers more orthotropic.
The definitive orientations are hard to determine, thus making analysis difficult. However, fiber orientation assumptions can be made, and an orthotropic FEA model can be created. Ultimately, it is possible to better approximate orientations using X-rays and destructive methods on prototype parts.
These properties above apply to thermoset BMC that is placed directly into mold features and does not move a lot in the tool during molding. For thermoplastics that are placed into the tool as a charge, there are flow models that can predict fiber angles as the BMC is pushed into different regions of the tool during molding. These flow models can be interfaced with FEA models that determine the fiber directions and predict resulting part performance.
This information has been sourced, reviewed and adapted from materials provided by Toray Advanced Composites.
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