Application guide

DMC and SMC Compression Moulding

Compression moulding turns a pre-made compound into a finished part under heat and pressure. Where the resin sits in that chain, and why it is bought differently from a lay-up resin.

The resin goes into a compound, not into a mould.

Compression moulding is the odd one out among the processes in this section, because the fabricator who presses the part usually does not handle liquid resin at all. The resin goes into a compound first — a dough moulding compound or a sheet moulding compound, made up with reinforcement, filler and additives — and it is that compound, not the resin, which is loaded into a heated mould and pressed. Understanding where the resin sits in that chain is the difference between buying it sensibly and buying it as though it were a lay-up resin.

What manufacturers build

The DMC/SMC Resin Technical Data Sheet issued by Samrat Poly Resins, India lists DMC and SMC moulding compounds, electrical housings, manhole covers, automotive panels, transportation components, water tanks and sanitaryware. These are parts that suit a pressed process: repeatable geometry, moderate to high volumes, and a finished surface straight out of the tool rather than a laminate that is trimmed and finished by hand.

Why this material suits the work

DMC/SMC Resin is published by Samrat Poly Resins, India as an orthophthalic unsaturated polyester resin with a non-accelerated cure system, a viscosity of 600 cPs at 25°C, a gel time of 12 to 18 minutes at 25°C, a styrene content of 35% and a specific gravity of 1.12 g/cm³. In a compounding operation the relevant questions are how the resin wets and carries a filler and reinforcement load, and how consistently it behaves from batch to batch, because a compound that varies produces parts that vary. Those are the manufacturer’s published figures for the supplied resin and describe the liquid, not the pressed part.

Required resin characteristics

Predictable behaviour when carrying a filler and reinforcement load rather than wetting out mat by hand
Batch-to-batch consistency, since a compound is made ahead of moulding and variation shows up later as scrap
A non-accelerated system, so the compounder controls the cure rather than inheriting it
Compatibility with the thickener, filler and additive package the compound formulation uses
Documentation a downstream moulder can be shown, because the part maker is often not the resin buyer

Process context

In general industry practice the resin is first made up into a compound: resin, filler, reinforcement, and the additive and thickening package the formulation calls for are mixed to a dough for DMC, or spread and impregnated into a sheet for SMC, which is then left to mature. The compound is cut or weighed into charges, placed in a heated matched-metal tool, and pressed. Heat and pressure flow the charge to fill the cavity and cure it, and the part is removed and trimmed. Compound formulation, maturation, charge weight, tool temperature, pressure and cure time are all decisions for the compounder and the moulder, governed by their own approved procedure and by the current supplier documentation. None of them is published here.

Advantages

A published non-accelerated system leaves cure control with the compounder rather than fixing it at supply
Published viscosity and gel-time figures give a compounder a documented starting point for trials
The supplier Technical Data Sheet and Safety Data Sheet are both on file and linked from the product page
One documented grade across a compounding run keeps the compound behaving the same way from batch to batch

Common products

Electrical housings, enclosures and switchgear components
Manhole covers and utility access products
Automotive and transportation panels
Moulded water tank sections and sanitaryware

Common questions

What is the difference between DMC and SMC?

Both are compression moulding compounds and both are made from the same broad ingredients. The difference is the form: a dough moulding compound is a bulk dough, while a sheet moulding compound is produced and matured as a sheet. There is a dedicated comparison guide covering how the two differ and where each is normally used.

Can I use this resin for hand lay-up instead?

That is a question for the supplier documentation and your own process, not something a general guide should extend. The grade is published for compounding contexts. If the job is hand lay-up, the general-purpose grades are the documented route and there is a selection guide covering that choice.

Do you supply the finished compound, or the resin?

We supply the resin. Samrat FRP Traders is a trader, supplier, importer and reseller, and the compound is made by whoever formulates it. If your requirement is actually for a ready-made compound rather than a resin, say so at the enquiry stage, because those are different purchases.

Will this resin meet the electrical or fire requirement in my specification?

A resin grade on its own does not answer a finished-part requirement. Electrical and fire performance are properties of the moulded part and the full formulation, and they need the specific standard named and the supporting test evidence. For work where fire performance is specified, ask about the fire-retardant grades rather than assuming a general grade will carry it.

Compounding for DMC or SMC production?

Tell us the compound you are producing, the parts it feeds, the quantity you expect to take and the documentation your customers ask for, and we will review the grade and the current supplier documentation with you.

Contact Samrat FRP Traders