Most injection moulding problems are decided before the tool is cut. Wall thickness, draft, ribs, bosses and radii are fixed in the CAD file, and every one of them either helps the part fill, pack and eject cleanly or guarantees a fight at the production meeting. These plastic part design guidelines collect the DFM rules that matter most for moulded parts, with the numbers buyers should expect to see on a drawing. If you are sourcing moulded parts rather than designing them, this is still your checklist: the same rules tell you whether a supplier's DFM feedback is real or boilerplate. The custom molding services catalogue lists suppliers who work to these rules daily.
Wall Thickness: Uniform Beats Thin
Aim for one wall thickness across the part wherever the design allows. Thick sections cool last, sink visibly on the surface and trap stress that shows up later as warp or cracking. Where the design forces two thicknesses, transition with a generous taper rather than a step. Nominal walls between 1.5 and 3 mm cover most moulded parts; going thinner is possible but pushes pressure and tooling requirements up quickly.
Draft Angles and Ejection
Every surface that pulls off the core or out of the cavity needs draft. Without it, parts scuff, stick or deform during ejection, and polished tools make the problem worse, not better. Reasonable expectations:
- 1 degree per side as an absolute minimum on lightly textured surfaces.
- 1.5 to 2 degrees per side for general production parts.
- 3 degrees or more on deep draws and on surfaces with heavy texture, where each 0.03 mm of etch depth demands roughly another degree of draft.
- Zero draft only where the design genuinely permits, and then only with a toolmaker's written agreement.
Ribs, Bosses and Gussets
Ribs stiffen without adding mass, but a rib is a junction: make its base too thick and you sink the surface it stands on. Keep rib thickness at roughly half to two-thirds of the adjoining wall, and give the root a fillet rather than a sharp corner. Bosses for self-tapping screws need controlled wall thickness around the hole and enough draft to release; gussets should support the boss from a direction that does not fight the flow path. When a part is stiffened by material choice instead of geometry, material guides such as this comparison of supplier screening and verification practice help keep expectations realistic.
Radii, Corners and Stress Concentration
Sharp internal corners concentrate stress and slow the flow front. Fillet internal corners generously - radii of half the wall thickness are a good starting point - and keep external corners rounded to protect both the part and the tool. Snap-fit hooks need a generous root radius and a defined flex direction; hooks loaded across the mould flow direction crack long before the design cycle predicts.
Shrinkage and Tolerances
Shrinkage is material-specific and direction-sensitive, and it is the reason "standard tolerance" questions have no single answer:
| Design feature | Practical guideline |
|---|---|
| Wall thickness | Nominal plus or minus 0.1-0.15 mm on a 2 mm wall |
| Flatness on large thin parts | Allow 0.3-0.5 mm over 100 mm unless a fixture is quoted |
| Hole to hole distance | Plus or minus 0.1 mm on dimensions below 50 mm |
| Shrinkage allowance | Set per material family and confirmed by the toolmaker on the tool drawing |
Tight tolerances are achievable, but they are priced: tighter process control, longer cycles and more sampling rounds. Decide which dimensions actually need the tight band, and mark the rest as reference.
A DFM Checklist Before You Send Drawings
- Is the wall thickness uniform, or are transitions tapered?
- Does every pulling surface carry draft, with texture depth accounted for?
- Are rib bases, boss walls and snap hooks dimensioned against the wall, not guessed?
- Are internal corners filleted and load paths defined for snap features?
- Are tolerances tight only where function demands, with the rest marked reference?
- Has the toolmaker confirmed shrinkage and gate strategy against your material?
Run the list before the RFQ, not after. Suppliers who answer DFM feedback with specific numbers are designing the tool; those who only quote a price are hoping the design is already sound. For a broader view of when moulding is the right process at all, this comparison of CNC machining vs injection molding is the natural next read.
