SMC Flowability And Its Relationship With Molds

SMC (Sheet Molding Compound) is a glass fiber-reinforced thermosetting molding compound, inherently high in viscosity and long in fiber length.,Flowability is entirely driven by high-temperature and high-pressure molding, and the mold structure is the key external factor determining the quality of material flow and product defects,The two mutually restrict each other.

 

I. The temperature zoning of the mold directly alters the flow capacity of SMC.

 

The temperature of the mold affects the viscosity of the resin.SMC The matrix unsaturated polyester becomes soft when heated and thus acquires fluidity:

 

Low mold temperature: The resin viscosity is high, and the resistance of the material flow increases sharply, resulting in insufficient filling, material shortage, and poor strength of the weld line;

 

High mold temperature: The surface resin rapidly undergoes premature curing (pre-gelling), forming a hard shell that blocks the flow of the internal material, leading to bubbles, delamination, and flow patterns. The constant temperature range for conventional SMC compression molding molds: 130～155℃. For large and thin-walled products, temperature control needs to be divided into zones, and the material flow end should be appropriately heated.

 

The temperature difference of the mold causes a difference in flow If the temperature difference between the mold core and the mold cavity is too large, the material will preferentially flow to the area with higher temperature and lower resistance, resulting in uneven distribution of glass fibers and local material shortage.

 

 

II. The design of the cavity structure determines the material flow path and the flow resistance.

 

1. Product wall thickness

· Thick-walled cavity: The material flow channel is wide, with low flow resistance and smooth filling, but it is prone to gas bubbles accumulation;

· Thin-walled cavity: The gap is narrow, and the glass fibers are very likely to bridge and block the material, resulting in a significant decrease in fluidity. It is necessary to increase the press machine pressure and optimize the placement of the material.

 

2.Round corners, draft angle, reinforcing ribs

Sharp corners, small R angles: The resistance to material flow change is high, causing fibers to be tangled and accumulated, and the flow to be interrupted; when the mold's rounded corner R is ≥ 2mm, the flow can be significantly improved;

Deep ribs, narrow grooves: They belong to narrow flow channels, and SMC glass fibers are prone to getting stuck, making the filling of the mold difficult. It is necessary to widen the root of the ribs and add overflow grooves;

Insufficient draft angle: The friction resistance on the side walls of the mold cavity increases, hindering the material from advancing forward.

 

3.Cavity area and process length

The longer the process, the greater the cumulative resistance along the line of SMC, and the more significant the decline in terminal fluidity; for long-process products, it is necessary to reasonably distribute the raw materials and increase the overflow edge to assist in discharging.

 

 

III. The molding / overflow system is the key mold structure for controlling the flow.

 

Overflow tank (overflow edge) Creating overflow areas around the mold is the most common method to improve the flow of SMC: During the molding process, excess resin, air, and low-viscosity materials flow into the overflow channel first, continuously providing a flow driving force to the interior of the mold cavity, removing air bubbles and preventing weld lines. Mold without an overflow channel is prone to insufficient filling.

Blank positioning cavity, flow guiding slope A guiding slope is made at the material feeding area of the mold to guide the SMC to spread evenly in all directions, preventing the material blockage from causing the material to flow unidirectionally or causing local fiber aggregation.

Exhaust structure The depth of the mold vent groove is generally 0.03 to 0.08 mm: If the gas cannot be discharged, it will cause an air blockage, directly blocking the material flow; poor air venting will manifest as seemingly poor fluidity, but in fact it is due to the obstruction of gas.

 

Surface finish of the mold, friction at the parting surface, and its impact on flow

·Rough cavities, scratches, corrosion pits: Increase the friction resistance between SMC and the mold wall, slow down the flow rate of the material, and cause flow marks on the surface of the product;

 

·Excessive gap between the parting surface: After applying pressure, the resin flows out in large quantities from the parting surface, resulting in insufficient material inside the cavity, seemingly indicating insufficient flowability; too small a gap leads to poor exhaust and high resistance.

 

VI. The stiffness of the mold and the mold closing gap indirectly affect the effective fluidity.

 

After the press machine applies pressure, if the mold rigidity is insufficient, there will be slight mold expansion, the thickness of the local cavity will increase, and the material flow distribution will become disorderly. Uneven mold closing gap will result in inconsistent material flow speed at different locations, uneven thickness of the product, and disordered fiber orientation.

 

V. Correspondence between Common Flow Defects in Molds and Their Causes

 

unhealthy phenomenon	The core cause of the mold
Insufficient materials, insufficient filling of the mold	Long process, thin walls, narrow ribs, low mold temperature, absence of overflow channel, blocked exhaust port
Surface swirls and ripples	Poor cavity finish, large temperature difference, excessive sharp corners, and uneven material flow speed
Layering, bubbles	Insufficient overflow, too small exhaust groove, and excessively high local mold temperature causing premature gelation.
Local glass fiber accumulation	Flow channel abrupt change, right-angle corner, deep and narrow reinforcing rib
Fusion weld marks are fragile.	There is no overflow drainage or exhaust system at the junction where the two materials meet.

 

 

VII. Optimization Plan for Improving the Fluidity of SMC in Mold End Elevation

 

• Molding cavity: All corners are rounded with larger radii, the narrow ribs are thickened at the root, and a guiding structure is added at the long process section;
• Temperature control: The mold has independent heating circuits. The temperature in the feeding area is slightly lower, and the temperature at the end of the material flow is appropriately increased;
• Overflow and exhaust: Continuous overflow edges are set along the product contour. Exhaust grooves are added separately at the corners and the material convergence points;
• Surface treatment: The mold cavity is polished to a mirror finish to reduce the friction resistance of the material wall;
• Mold closing control: Increase the rigidity of the mold template, uniformly control the mold closing gap, and avoid excessive overflow at certain local areas.

 

sum up

 

The SMC's own formulation (resin, glass fiber, fillers) determines the basic flow performance, but the mold is the carrier for the full exertion of the flow: temperature, cavity geometry, venting, surface friction, and mold rigidity jointly control the flow resistance, flow direction, and filling speed. For the same batch of SMC raw materials, changing the mold or modifying the cavity structure will result in completely different molding effects with significant differences in flowability.