Twelve adjustable design elements that determine whether a molded part builds cleanly, cycles fast, and looks right. Click any card to compare good vs. bad practice.
Taper on vertical walls so the part releases from the steel
Every surface parallel to the direction of mold pull needs a slight outward taper. Without it, the part wedges against the steel and refuses to eject cleanly — you get scratches, stress whitening, or fractured walls. Draft also extends mold life by reducing scraping load on polished surfaces.
Round every inside corner to relieve stress and guide flow
Sharp internal corners are the fastest route to part failure. Stress concentrates at the apex during cooling and again under load, initiating cracks. They also choke flow and create cooling differentials. Outside corners matter too — a small round prevents steel chipping and flash-prone feather edges.
Stiffen parts without thickening walls
Ribs add rigidity without bulking up the main wall — critical because thick walls sink, warp, and lengthen cycle time. But ribs have their own rules. Too thick and they sink themselves; too tall and they won't fill or they buckle; too close together and the steel between them can't cool.
Mounting features for screws, pins, and inserts
A boss is any projecting cylinder used for fastening or alignment. The common mistake is modeling it as a solid lump, which creates a thick section that sinks, warps, and cools slowly. Design a boss as a tube whose wall matches the rest of the part, supported by gussets where it joins the main wall.
Features that block straight ejection
An undercut is anything that mechanically locks the part to one half of the mold — a snap feature, a side hole, an external groove. Resolving them requires side actions, lifters, or collapsible cores, which dramatically raise tooling cost and cycle time. The first question in DFM review: can this undercut be eliminated?
Where molten plastic enters the cavity
The gate controls flow direction, pressure distribution, weld-line location, and how visible the gate vestige will be. Get it wrong and you see sink, voids, jetting, or knit lines straight across the cosmetic face. Get it right and the part fills uniformly with the scar hidden.
Where the two mold halves meet — and always show
The parting line is where the two mold halves come together, and it will almost always show as a faint seam or flash on the finished part. Every part has one — the question is where you put it. A bad parting line runs across a polished A-surface; a good one disappears into an edge, a radius, or a recessed feature.
Where the part gets pushed out of the mold
Ejector pins leave small circular witness marks — raised or depressed — on the surface they contact. Place them on cosmetic faces and every part ships with blemishes. Place them on the B-side and you never see them. Pin size and count also matter: too few or too small and you deform thin walls or leave drag marks.
Hollow out thick sections to keep walls uniform
Thick sections are the enemy of injection molding. They cool slowly, creating sink marks outside and voids inside as the skin solidifies before the core. The fix is counterintuitive: carve the thickness out from the back, turning a solid block into a shell with uniform walls. The part keeps its envelope shape but molds cleanly and cycles faster.
Through-holes vs blind holes and core-pin limits
Every hole in an injection-molded part is formed by a steel pin in the mold called a core pin. Long, thin core pins are cantilevers — they deflect under flow pressure and drift out of position. That's why blind holes have strict aspect ratios and why through-holes (supported on both ends) are always preferred when geometry permits.
Part numbers, warnings, and branding molded into steel
Text on a molded part is inverted into the mold steel. If letters stand proud on your part, they're engraved into the mold — a single durable cavity. If letters are recessed into your part, they're raised on the mold — thin fragile steel that chips off. That's why raised text is the industry default.
SPI finishes and their draft-angle demands
Textured surfaces look premium and hide minor defects, but every texture cuts into the mold steel — and the part has to climb over that texture during ejection. Without enough draft, the texture drags the part like sandpaper. Texture depth and draft angle are tightly coupled; you cannot specify one without considering the other.