Compression moulding, transfer moulding and melt fracture: 22-Mec-A4 Design and Manufacture of Machine Elements, May 2018, Q2

Question 2: Compression moulding, transfer moulding and melt fracture

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Given. The two thermoset moulding processes, compression moulding and transfer moulding, and the extrusion defect known as melt fracture.

Find. Annotated principle sketches with the essential die elements (a); the process with the higher material utilisation (b); why the other one is nevertheless used (c); the closest metal processing analogues (d); and a sketch explaining melt fracture (e).

(a) Principle sketches and essential die elements

Figure 2.1 - Compression moulding (left) and transfer moulding (right). Compression moulding: a measured charge is placed directly in the open cavity and the upper force closes on it. Transfer moulding: the cavity is closed first, and a plunger drives the softened charge out of a heated pot, through a sprue and gate, into the cavity.

The essential elements of a compression mould are the upper force (punch), the heated cavity block, a flash land or pinch-off around the parting line that meters the escaping excess, guide pins, cartridge or platen heaters holding the tool at 150-200 °C, and ejector pins. A measured charge of preform, granule or bulk moulding compound is loaded cold into the open cavity; closing the press squeezes it out to the shape while the heat cross-links it, and the press is held closed for the cure time.

The essential elements of a transfer mould are the transfer pot, the transfer plunger (ram), the sprue, runner and gate that connect the pot to the cavity, the closed cavity and core, and the same heating and ejection provisions. The critical difference in principle is the sequence: the mould is fully closed before any polymer enters, so the charge arrives as a flowing melt through a restricted gate rather than as a lump that is squashed.

(b) Which process gives higher material utilisation

Compression moulding gives the higher material utilisation. Its only waste is the thin flash squeezed onto the flash land, typically a few per cent of the charge. Transfer moulding additionally loses the sprue, the runners and the cull - the residual disc of cured material left in the pot - which together commonly amount to 15-25 % of the shot. Taking representative figures of 4 % flash for compression moulding, and 4 % flash plus 18 % sprue, runner and cull for transfer moulding:

$$\eta_{\text{compression}} = \frac{100-4}{100} = 0.96, \qquad \eta_{\text{transfer}} = \frac{100-4-18}{100} = 0.78$$

The penalty is permanent, not a set-up loss, because these are thermosets: once cured, the sprue and cull cannot be reground and re-moulded. That single fact is what makes the comparison worth asking about, and it is why the answer would be different for an equivalent thermoplastic process.

(c) Why transfer moulding is used despite the waste

Because the cavity is closed before the material enters, transfer moulding can mould around delicate metal inserts, fine pins and thin sections without displacing or crushing them - the insert sees a controlled melt flow through a gate rather than the full closing force of a press acting on a solid charge. This is decisive for encapsulated electrical and electronic parts (semiconductor packages, coil bobbins, connectors), for parts with deep or slender cores, and for close-tolerance parts where the flash and the variable cavity height of compression moulding would be unacceptable. Transfer moulding also gives more uniform cure and shorter cycles on thick sections, because the material is pre-heated in the pot and enters the cavity hot.

(d) The closest metal processing techniques

Compression moulding is the polymer analogue of closed-die (impression-die) forging: a discrete billet is placed in a heated cavity and deformed to shape by the closing die, and a flash land controls the excess in exactly the same way. Transfer moulding is the analogue of die casting (and, in the cold-chamber form, of squeeze casting): a metered charge is held in a chamber and forced by a plunger through a sprue and gate into a closed die, leaving a biscuit or cull behind. The parallel extends to the defects - flash and underfill in the first, gate erosion, air entrapment and short shots in the second.

(e) Melt fracture

Figure 2.2 - Melt fracture: converging flow at the die entry stores elastic strain and generates stagnant corner vortices; above a critical wall shear stress the extrudate emerges grossly distorted.

A polymer melt is viscoelastic. As it converges from the wide barrel into the narrow die it is stretched along the flow direction, and the elastic component of that deformation is stored rather than dissipated. At low throughput the stored strain relaxes within the die land and the extrudate is smooth. Above a critical wall shear stress of roughly 0.14 MPa for most commodity thermoplastics, the residence time in the land is too short for relaxation, the flow in the entry region becomes unstable and oscillates between the stagnant corner vortices and the die throat, and the extrudate emerges with a gross helical, bamboo or chaotic distortion. This is melt fracture proper - a bulk instability, to be distinguished from sharkskin, which is a surface-layer tearing at the die exit (Q3). The cures are the same family: reduce output rate, raise melt temperature, streamline and lengthen the die entry with a gradual taper, or narrow the molecular weight distribution.

Question 2 - summary
Item	Answer
(b) Higher material utilisation	Compression moulding (about 96 % vs. 78 %) - no sprue, runner or cull
(c) Reason for using transfer moulding	Cavity closes before filling, so delicate inserts, thin sections and fine detail survive; better dimensional control and cure uniformity
(d) Metal analogue of compression moulding	Closed-die (impression-die) forging
(d) Metal analogue of transfer moulding	Die casting (cold-chamber) / squeeze casting
(e) Melt fracture	Elastic-instability at the die entry above about 0.14 MPa wall shear stress; gross helical or bamboo distortion of the extrudate

Topic. Thermoset moulding routes and the viscoelastic limit on extrusion rate.

Key relations. Material utilisation $\eta = (m_{\text{charge}} - m_{\text{waste}})/m_{\text{charge}}$; apparent wall shear rate for a round die $\dot{\gamma} = 4Q/(\pi R^{3})$ and wall shear stress $\tau_{w} = \eta_{\text{app}}\dot{\gamma}$, with melt fracture above $\tau_{w} \approx 0.14$ MPa.

Why this works. Both moulding processes cure the same chemistry; they differ only in whether the cavity is closed before or after the charge is introduced, and every consequence - waste, insert survival, tolerance, cycle time - follows from that one choice.

Common pitfall. Calling transfer moulding wasteful and stopping there. Thermoset scrap cannot be reground, so the waste is real, but the process is selected precisely when compression moulding would destroy the part; and confusing melt fracture (bulk, at the entry) with sharkskin (surface, at the exit).

Source. Kalpakjian and Schmid, 8th ed., Sections 19.7-19.8 (compression and transfer moulding) and 19.2 (extrusion defects); Groover, 7th ed., Sections 13.7-13.8.