Everything about moulding gets harder with size, and it gets harder for reasons that are easy to compute in advance. The press has to hold the tool shut against pressure acting on the whole projected area. The melt has to travel from the gate to the furthest point without freezing. And the tool has to be big enough to contain all of it, which is where the money goes.
Clamp tonnage and projected area
Injection pressure acts on the part's projected area in the direction the tool opens, and the press must hold the tool shut against that force. So clamp requirement scales with area rather than with volume, and a large flat part can need an enormous machine while weighing very little. Large presses are scarcer and their hourly rate is much higher, which is often the binding constraint.
Flow length and fillability
Melt cools as it travels, so there is a maximum ratio of flow length to wall thickness for each resin. A large part therefore needs either a thicker wall, an easier flowing resin, or several gates fed sequentially. Multiple gates create weld lines wherever the flow fronts meet, so sequential valve gating is used to control where those land.
The tool and its cost
A large tool is heavy, needs a large mould base, needs far more cooling circuits to remove heat evenly, and needs handling equipment to install. It is also more expensive to modify and more expensive to store. On large parts the tooling decision is often the whole business case, which is why thermoforming and structural foam moulding compete so strongly at this size.
The alternatives at size
Thermoforming makes large covers and panels with a fraction of the tooling cost, and one good face. Structural foam moulding uses a blowing agent to fill large thick sections at much lower pressure, so a smaller press can make a bigger part, at the cost of a swirled surface. Rotational moulding makes large hollow parts with very cheap tooling and long cycles.
Questions people ask about large part injection molding
What limits how big a moulded part can be?
Clamp tonnage against the part's projected area, shot size, and the flow length the resin can fill at that wall thickness. All three are computable in advance.
Why does a thin flat part need such a large press?
Because clamp force scales with projected area, not with volume. A large flat part presents a lot of area for injection pressure to push against.
How is a large part filled?
Usually through several gates, often valve gated in sequence so the weld lines fall where they are acceptable rather than wherever the flows happen to meet.
What are the alternatives for large parts?
Thermoforming for covers and panels, structural foam moulding for thick large parts at lower pressure, and rotational moulding for large hollow parts.