Some plastics can never be moulded thin
Flow length, and the limits of getting material into a cavity.
Some plastics can never be moulded thin because molten plastic begins cooling and stiffening the instant it touches a comparatively cold mould wall, and in a very thin cavity that stiffening front can meet itself and seize the material solid before it has travelled far enough to fill the far corners, regardless of how much pressure is used to push it in.
Two frozen skins closing on a hot core
As molten plastic enters an injection moulding cavity, the layer touching the cold wall solidifies almost immediately, while the material in the centre of the channel stays hot and mobile for longer, insulated by the layer that has already frozen. In a thick cavity that hot core has room to keep moving and can carry material a long way. In a thin one, the two frozen skins growing in from opposite walls start much closer together, and they meet, pinching off the flowing core, far sooner.
The time for a wall to freeze through grows with the square of its thickness, because heat has to cross the whole distance from the centre to the steel. Halving a wall therefore gives the plastic roughly a quarter of the time to flow before the channel closes, and the distance it can travel, its flow length, shrinks sharply with it. Once the core is pinched shut, extra injection pressure pushes nothing further, since there is no longer a channel left to push through.
The material decides how bad this gets. A plastic that flows like warm syrup when molten travels a long way through a thin channel before its skins meet, while a stiffer one, particularly a grade loaded with glass fibres or other filler to make the finished part stronger, moves sluggishly from the moment it enters and covers far less ground in the same time. The stiffest of them cannot fill a thin wall of any useful length at all, which is the sense in which some plastics can never be moulded thin.
Spreading butter straight from the fridge
Spreading cold butter across a large slice of bread shows the same limit. The butter tears and clumps before it reaches the far edges, while the same butter left out to soften spreads all the way across under the same knife pressure because it stays workable for longer. Plastic that has begun stiffening as it flows behaves like the cold butter, and pressing harder with the knife, or with the injection ram, does little for a material that has run out of workable time before it has run out of distance.
Fixing the part instead of the machine
A part with walls too thin for its size and material shows short shots, patches near the far edges of the cavity the plastic never reached, however the machine's pressure or temperature are adjusted afterwards. Pressure helps only while the flowing core is still open, and any pressure applied after it has pinched shut simply loads stress into the tool. An experienced process engineer diagnosing a short shot therefore checks first whether the fault lies in a setting that can still be adjusted, or in a flow-length limit built into the part's geometry that no setting will overcome.
When it is the geometry, the wall has to be thickened, split into shorter runs each fed from its own entry point, or moulded from a more freely flowing grade of plastic. Thin walls are entirely normal on small parts fed from a well-centred gate, where the distance to the far edge stays comfortably inside what the material can manage, and they only become a risk once a design asks a thin section to carry material a long way from its entry. Adding a second gate partway along a long, thin feature, so it fills from both ends, is a standard way to keep the weight saving that the thin wall was chosen for in the first place.