Sheet Molding Compound | Compression Molding Process

September 10, 2017

Sheet Moulding Compound: A Guide

You’ve probably heard the words “sheet moulding compound” – aka “sheet moulding composite” in connection with the manufacture of high-quality automotive parts. While sheet moulding compound (SMC) certainly has its benefits, and may be the way to go for certain parts, in many cases long fiber injection (LFI) can provide the same quality of product for a lower price and with less waste.

But 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 moulding 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 moulding compound must be stored for several days to “cure” until it can be sold and used.

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 moulding, creating anything from simple designs to complex parts.

The Compression Moulding Process

Compression moulding takes place in a device called a “compression moulding press,” which is usually hydraulically powered. Most compression moulding presses require that both the loading of SMC into the mould 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 mould placed between two heated plates. Pre-heated SMC is placed into the lower half of the mould. The upper plate is then lowered, applying up to 2000 psi of pressure to the mould.

Consistent application of heat and pressure causes the SMC to spread and properly fill every part of the mould. Compression moulding 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 moulding compound is its light weight when compared to other substances, including metals and even other polyesters such as bulk moulding 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 moulding 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 moulding 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 painted in-mould rather than needing to wait until after the moulding process has finished.

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 moulding 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 RomeoRIM 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 mould using a machine; the moulding process uses lower pressure than traditional compression moulding 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 the same strength, durability, and quality as heavier polyesters.

Choosing LFI means that you will save even more money on the moulding process with SMC! The entire process, from filling the mould to removing the finished product, is fully automated and carried out by RomeoRIM’s accurate, efficient robot. In addition, RomeoRIM operates the world’s largest LFI press, allowing any size part 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. 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 RomeoRIM today to get the process started!

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