Wall thickness is the single most influential number in a moulded part. It sets the cooling time and therefore the cycle and therefore the price; it decides whether the part sinks or warps; and it determines whether the cavity can be filled at all. Almost every other design rule in moulding exists to protect it.
Why uniformity matters more than the number
Thick sections cool last and shrink after their neighbours have solidified, pulling the surface in as a sink mark and locking stress into the part as warp. A part of uniform wall thickness avoids both without any process intervention. Where thickness must change, blend it gradually rather than stepping it, and core out heavy sections rather than accepting them.
Typical ranges and what sets them
Each resin has a practical range that reflects its flow behaviour and shrinkage. Stiff, high viscosity engineering resins need more wall to fill a given flow length; easy flowing commodity resins need less. The real limit is flow length divided by wall thickness: a long thin part will not fill however good the machine is, which is why gate position and wall thickness are decided together.
Thin wall moulding
Below the normal range for a resin, moulding becomes a specialist discipline: very high injection speeds and pressures, tooling built to withstand them, cooling designed to remove heat from a much thinner section, and machines specified for the job. It is done routinely in packaging because the material saving at volume is enormous, and it is not something a general moulder can simply attempt.
Ribs, bosses and everything attached to the wall
A rib thicker than about half to two thirds of the wall creates a thick section behind it and sinks on the visible face. A boss made solid does the same. The standard solution is to keep ribs thin and add more of them, and to core bosses out and support them with ribs or gussets. This one rule removes most of the sink marks a first design carries.
Questions people ask about injection molding wall thickness
What wall thickness should I use?
Whatever is uniform and within the practical range for your resin. Uniformity matters more than the specific number, and the resin supplier or moulder will give the range.
Why does my part have sink marks?
Almost always a thick section: a heavy wall, a rib too thick relative to the wall, or a solid boss. It is a design problem rather than a process one.
What is thin wall moulding?
Moulding below the normal thickness range for a resin, requiring very high injection speed and pressure, specialised tooling and machines. It is used at volume where material saving justifies it.
How thick can a rib be?
Roughly half to two thirds of the adjoining wall. Thicker and it creates a heavy section that sinks on the opposite face.