Meaning
Physical void created at the boundary between the polymer and the mould surface caused by the volumetric contraction of the plastic during the cooling phase. An interfacial air gap forms as the density of the polymer increases and the part pulls away from the rigid walls of the tool. This gap creates a significant barrier to heat transfer because the thermal conductivity of air is much lower than that of the polymer or the metal.
Its presence slows down the cooling process, which can lead to longer cycle times and potential defects like sink marks. Managing the formation of this gap is a key challenge in high precision injection moulding.
Separation Mechanism
Physical separation occurs because polymers have a high coefficient of thermal expansion and shrink as they lose heat. During the initial filling and packing stages, the high pressure of the melt keeps the plastic in tight contact with the mould, but once the pressure is released and the material solidifies, the interfacial air gap begins to grow. The width of this gap is influenced by the part geometry, the wall thickness and the specific shrinkage characteristics of the resin.
In areas with complex features or ribs, the part may be held against the wall longer, leading to non uniform gap formation. This variation causes some parts of the component to cool faster than others, which can induce internal stresses. Proper part design with uniform wall thicknesses helps to minimize the negative effects of this separation.
Cooling Impedance
Presence of air at the interface acts as a thermal insulator that limits the efficiency of the cooling system. The interfacial air gap becomes the dominant resistance in the thermal path, meaning that even very cold water in the cooling channels cannot pull heat out of the part effectively. This insulation effect keeps the core of the part at a higher temperature for a longer period, which can cause the part to warp after it is ejected from the tool.
Moulders often use a longer packing time or higher pressures to delay the formation of the gap as much as possible. By keeping the polymer in contact with the tool for a longer duration, the heat removal is maximized during the critical early stages of the cooling cycle. This is especially important for thick walled parts where the total heat content is high.
Dimensional Accuracy
Control over the final size of the part depends on how the material shrinks within the tool and after ejection. The interfacial air gap directly impacts the final dimensions because it changes the rate at which the part reaches its final state. If the gap forms too early, the part may not cool enough to become rigid, leading to excessive shrinkage once it is out of the mould.
When sourcing materials, engineers look for resins with predictable and consistent shrinkage rates to ensure that the gap formation is repeatable. Using regrind can complicate this, as different batches may have different molecular weights and shrinkage behaviors. Monitoring the gap through pressure sensors or thermal probes allows for adjustments to the process to maintain the parts within the required tolerance limits.
