SMC Compression Molding

Sheet Molding Compound: A Guide

You’ve probably heard the words “sheet molding compound” – aka “sheet molding composite” in connection with the manufacture of high-quality automotive parts. While sheet molding compound (SMC) certainly has its benefits, and may be the way to go for certain parts.

What is SMC?

Sheet molding compound is a type of reinforced polyester containing glass or carbon fibers. The fibers, which are typically 1” or greater in length, are suspended in a bath of resin – usually epoxy, vinyl ester, or polyester. The long fibers and sturdy resin combined produce a strong, cost-efficient material.

The method by which SMC is produced ensures complete integration of fibers and resin. The resin is applied in the form of a paste to a film; next, the fibers are cut and added to the paste. The substance is then squeezed between two films; it is compacted until it reaches the desired thickness and texture. Proper sheet molding compound must be stored for several days to “cure” until it can be sold and used.  It’s important to note that the material is not fully cured when stored, the final “cure” takes place in the mold when heat and pressure are applied to it.

SMC is usually sold in rolls, which are cut into smaller pieces called “charges”. Heat and pressure are applied to the charges in a process known as compression molding, creating anything from simple designs to complex parts.

The Compression Molding Process

Compression molding takes place in a device called a “compression molding press,” which is usually hydraulically powered. Most compression molding presses require that both the loading of SMC into the mold and the unloading of the finished product must be manually carried out by a skilled operator.

The press usually consists of the upper and lower halves of the mold placed between two heated plates. Pre-heated SMC is placed into the lower half of the mold. The upper plate is then lowered, applying up to 2000 psi of pressure to the mold.

Consistent application of heat and pressure causes the SMC to spread and properly fill every part of the mold. Compression molding can therefore be used to create even complex, detailed parts with great accuracy.

Advantages of SMC

The most important and most frequently talked about advantage of sheet molding compound is its light weight when compared to other substances, including metals and even other polyesters such as bulk molding compound (BMC). For this reason, SMC has replaced metal components as the primary material used for a number of automotive parts. It has also seen use in the manufacture of baths, spas, and arena, cinema and stadium seating.

In addition to its light weight, SMC is easy to produce, and can be manufactured in high volume. Combined with the straight forward nature and short production cycle of the compression molding process, parts made using SMC can be created in similarly high volume in a small amount of time.

However, SMC’s lightweight nature does not necessitate any sacrifice in strength. It is sturdy and durable, capable of resisting impact even at high speeds. The compound is even approved for use in the outer shells of passenger vehicles, where it regularly receives high scores in crash tests.

Lastly, sheet molding compound is an extremely economical choice. Manufacturers can save money on all aspects of the production process, from minimal labor costs to reduced waste. As a further bonus, SMC is time-efficient as well – it can be primed in-mold rather than needing to wait until after the molding process has finished.  However, to achieve class A finishes, a post mold painting process is required.

The introduction of SMC was justifiably seen as a revolution in parts manufacturing, heralding the shift away from metals and towards polymers. Even today, SMC is a solid choice with many tempting advantages.

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