Sheet Molding Compound | Compression Molding Process
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. It's a suitable material for this application, but let's try to understand what SMC really is.
Although we do not run Sheet Molding Compound parts, they certainly have their benefits and may be the way to go for certain parts. But, in many cases, Long Fiber Injection (LFI) can provide the same quality of product for a lower price and with less waste. In order for you to make an informed decision, it’s important to understand exactly what advantages each substance and process offers.
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 straightforward 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.
However, there is something that does what SMC and compression molding does even better – and it’s called long fiber injection.
What is LFI?
Long fiber injection (LFI) molding is an even shorter, more efficient and cost-effective process used to create highly complex and detailed parts. It shares many of SMC’s advantages, but outperforms SMC in a number of key aspects. Since the process was brought to America by Romeo RIM in 1999, it has gained immense popularity and been used for automotive parts, housing, furniture and more.
LFI consists of polyurethane mixed with chopped glass fibers – as the name suggests, longer fibers are used for greater strength and reinforcement. The fiber-polyurethane mix is sprayed directly into the mold using a machine; the molding process uses lower pressure than traditional compression molding and is completed within minutes.
The resultant product is significantly – in fact, approximately 40% – more lightweight than SMC, and over 60% more lightweight than metals such as steel and aluminum. Despite the decreased weight, it offers similar strength, durability, and quality as heavier polyesters.
Choosing LFI means that you will save even more money on the molding process with SMC! The entire process, from filling the mold to removing the finished product, is fully automated and carried out by Romeo RIM’s accurate, efficient robot. In addition, Romeo RIM operates the world’s largest LFI press, allowing parts up to 12 feet by 12 feet to be produced quickly and with ease. As with labor costs, the volume of waste or scrap material produced is practically zero.
Anything SMC can do, LFI can do better, or so we like to think…
– Short production cycle? Absolutely.
– Ability to create complex parts, including B-side geometry? Certainly.
– Impact resistance? Definitely.
– In-mold painting? You bet!
In fact, LFI is even more suited for in-mold painting than other materials. LFI produces Class-A finishes right out of the mold. Need a glossy or textured finish? No problem – LFI can even mimic fine textures such as stone and wood grain.
Whether you’re making car exteriors, spa technology, or siding for a brand new home, LFI is right for you.
Contact us here at Romeo RIM today to get the process started.
SMC is a good material because of its impact resistance. It can be found in many applications and industries: food service trays, swimming pools, electrical appliances, and the automotive industry can be found in body panels