Wall thickness rules for moulded parts

The nominal wall decides everything downstream

A moulded part is a wall with features attached to it. The nominal wall sets the injection pressure, the cooling time, the shrink, the warp and the stiffness of the finished part. It also decides where sink marks appear and how much cycle time the tool will need for the life of the programme. A wall callout on a drawing is worth more scrutiny than a cosmetic tolerance on a face nobody touches.

Four requirements pull against each other. The cavity must fill before the gate freezes, the section must cool fast enough to be economic, the part must carry its load, and the visible face must look acceptable. Thicken the wall and fill becomes easier but cooling time rises roughly with the square of the thickness. Thin the wall and the cycle drops, but the press needs more pressure and the tool needs tighter alignment.

That is why wall thickness is not a styling decision that engineering fixes later. It is the first number agreed between the part designer, the moulder and the toolmaker, and it is expensive to change once steel exists.

Typical nominal walls by material

The usable range depends on melt viscosity, filler content and the distance the material must travel. Unfilled nylon will fill a wall that would stop a polycarbonate flow dead. Glass filled grades fill thin sections more easily but introduce orientation and warpage that a uniform wall alone will not solve.

The table below is a starting point for a first layout. Treat it as a range to discuss with the moulder, then confirm against the material supplier data sheet for the exact grade and the flow length in your part.

Change section gradually, not in a step

The most common wall defect is not a thin wall or a thick wall. It is the step between them. Where a 2 mm wall meets a 4 mm wall, the thick section cools last and pulls molten material from the thin section while it is still shrinking. The result is a sink on the thick side, a void inside it, or a warp that bows the whole part.

The fix is a taper. A rule that works well in practice is to spread the change over a length of at least three times the difference in thickness, which gives a taper of roughly 3:1 or gentler. A 2 mm step then needs about 6 mm of transition. Longer is better where the surface is visible or the part must sit flat.

Corners need the same treatment in two dimensions. An inside corner with a sharp radius concentrates stress and creates a local thick section in the corner itself. A fillet of at least half the wall thickness on the inside, and a matching outside radius, keeps the section closer to nominal and improves flow.

• Keep at least 80 percent of the part at the nominal wall, including features.
• Taper any change over three times the thickness difference or more.
• Use an inside corner fillet of at least 0.5 times the wall, with a matching outside radius.
• Avoid thick bosses and pads standing on a thin wall without a taper or a rib.

Sink marks and the rib base rule

A sink mark is a local depression on the surface opposite a thick section. It forms because the thick region shrinks more than the thin skin around it and drags the visible face inwards. The depth depends on the section, the material, the packing pressure and the cooling, so a moulder can reduce it but rarely remove it entirely on a badly proportioned part.

Ribs are the usual cause. A rib attached to a 2.5 mm wall behaves like a thick section at its base if the base is the same width as the wall. Keep the rib base at about half to two thirds of the nominal wall and the sink risk drops sharply. A thicker rib buys stiffness but shows a line on the opposite face, which matters on a visible housing cover and matters less on an internal bracket.

Bosses follow the same logic. A solid boss is a cylinder of thick material. Core it to a wall of roughly half the nominal section, support it with a rib or gusset, and it will cool at a rate closer to the surrounding wall.

Thin walls, thick walls and what each costs

A thin wall is cheaper per part in material and cycle time, but it demands more from the press, the tool and the material. Injection pressure rises as the wall narrows, and the flow length that can be filled before the gate freezes falls. The ratio of flow length to wall thickness is a useful early check: unfilled commodity plastics commonly fill ratios in the range of 100 to 250, glass filled engineering grades often less, and high flow grades more. The exact figure comes from the grade data, not from a table in a design guide.

A thick wall is more forgiving to fill and stiffer, but every extra millimetre adds cooling time and mass. It also increases the chance of voids in the core of the section, particularly in semi crystalline materials where shrink is high. Where a thick section is unavoidable, a cored pocket is usually better than a solid block, because it removes material from the slow cooling core rather than adding pack pressure.

For a part that must be both stiff and light, ribs are the answer. They move material away from the neutral axis, so a ribbed 2 mm wall can outperform a flat 4 mm wall at a fraction of the mass and cycle time.

Wall thickness in electrical housings

Electrical parts add a second set of constraints. The wall must provide dielectric creepage and clearance, it must survive a glow wire or flame test at the thickness used, and it must hold a terminal or a breaker in place without creeping under load. Those requirements can push the wall above what the structural load alone would need.

Flammability ratings are thickness dependent. A material listed as V-0 at 1.5 mm may only reach V-2 at 0.8 mm, and the rating may change with colour as well. If the drawing calls for V-0, the wall on the drawing has to be the wall that was tested. The same applies to glow wire ratings, where the result is reported at a specific thickness and often a specific colour.

Tracking resistance matters at the surface rather than through the section. A thin wall between two live parts can pass a dielectric test when new and fail after surface contamination, so the creepage path along the surface is often the governing dimension. Wall thickness and rib layout together set that path.

• State the flame rating and the thickness at which it applies.
• Check that ribs and partitions give the creepage distance the standard requires.
• Allow for insert or terminal pull-out loads in the boss wall.
• Confirm the rating at the production colour, not the natural sample.

Checks to run before the tool is cut

A section analysis takes an hour and can save a tool change. Cut the model at the worst three or four places: through the thickest boss, through the rib field, through the thinnest wall at the end of flow and through any step between sections. Measure the actual wall at each cut rather than trusting the model to be uniform.

Then run a mould flow study on the agreed gate position. It will show the fill pattern, the pressure at the end of fill, the temperature at the last point to fill, the weld lines and the predicted sink and warp. The study is only as good as its inputs: grade, melt temperature, tool temperature and gate size all need to be real.

Finally, agree the wall in writing with the moulder. A DFM response that says the wall is acceptable at 1.8 mm but not at 1.5 mm is useful. A response that says the design is fine is not.

1. 01

   Measure the sections

   Cut the model through the thickest and thinnest regions and record the real wall, not the nominal.

2. 02

   Check the flow ratio

   Compare the longest flow path to the wall thickness against the grade data for the material.

3. 03

   Review sink risk

   List every rib, boss and pad and check its base against the nominal wall.

4. 04

   Run the flow study

   Confirm fill pressure, weld line position and predicted warp at the intended gate.