Compression and Transfer Moulding of Thermosetting Polymers

Compression and transfer moulding are the primary manufacturing processes for crosslinked thermosetting resins (Phenol-Formaldehyde, Melamine-Formaldehyde, Epoxy, and Unsaturated Polyester BMC/SMC). Unlike thermoplastics, thermosets undergo an irreversible exothermic chemical crosslinking reaction (curing) inside the heated mold cavity ($150^\circ\text{C} - 180^\circ\text{C}$). These processes are vital for producing high-temperature electrical switchgear, automotive brake pads, circuit breakers, and structural composite panels.

By completing this lesson, you will be able to:

• Differentiate compression moulding from pot transfer moulding mechanisms.
• Calculate transfer pot ram pressure, clamping force ($F_c$), and cure time using the Gel-Time equation.
• Compare flash, positive, and semi-positive compression mold designs.
• Diagnose blistering, gas entrapment, and under-cure in thick-walled thermoset moldings.

3.1 Compression Moulding Sequence

Pre-formed or powdered charge is placed directly into an open, heated cavity. The upper mold half descends, compressing the resin, forcing it into cavity contours while activating thermal crosslinking.

3.2 Transfer Moulding Mechanics

Thermoset charge is pre-heated in a separate transfer pot and injected through sprue and runners into the closed mold cavity by a hydraulic ram, yielding superior dimensional accuracy and minimal parting line flash.

mermaid
graph TD
    A["Pre-heated Charge Placement in Pot / Cavity"] --> B["Hydraulic Ram Descent & Pressure Application"]
    B --> C["Resin Liquefaction & Cavity Flow"]
    C --> D["Exothermic Chemical Crosslinking (Cure Phase)"]
    D --> E["Mold Opening & Hot Ejection of Cured Component"]

Transfer Pot Pressure & Hydraulic Clamping Force

The required press clamping force ($F_c$) must exceed the total separation force exerted by transfer pot pressure ($P_{pot}$) across cavity projected areas:

Where $A_{total} = (N \times A_{single}) + A_{runner}$.

Worked Numerical Example:

Problem: A 4-cavity transfer mold produces phenolic resin electrical switch housings. Each housing has a projected area $A_{single} = 40.0\text{ cm}^2$. The runner system contributes an additional $A_{runner} = 20.0\text{ cm}^2$. The transfer pot ram exerts a hydraulic fluid pressure $P_{pot} = 35.0\text{ MPa}$ ($350\text{ bar}$). Using a safety factor $S_f = 1.20$, calculate the required hydraulic press clamping tonnage.

Solution:

1. Calculate total projected area ($A_{total}$):

1. Calculate Clamping Force ($F_c$):

1. Convert to Metric Tonnage ($1\text{ Tonne} = 9.80665\text{ kN}$):

• Electrical Switchgear: Phenolic (Bakelite) circuit breaker housings moulded on 100-Tonne transfer presses. (Illustrative Indian industry scenario based on electrical equipment manufacturing hubs in Mumbai/Pune).
• Automotive Brake Pads: Friction material compression moulding using phenolic binder resin under high tonnage.

• Crosslinking / Curing: Irreversible 3D covalent network formation driven by heat.
• Transfer Pot: Chamber where charge is plasticized before hydraulic injection into closed cavity.
• Breathe Cycle: Momentary mold opening during initial compression to release trapped moisture and volatiles.

1. Strong, A. B. (2005). Plastics: Materials and Processing, 3rd Ed., Pearson.
2. ISO 295:2004 — Plastics — Compression moulding of test specimens of thermosetting materials.