Is ʼDraftʼ necessary for parts designed for plastic molding in SolidWorks?

ByCADbrief Staff

Short Answer

Yes—draft is usually necessary for plastic-molded parts in SolidWorks because it helps the part release from the mold without scuffing, sticking, or damaging shutoff surfaces. The most common professional method is applying the Draft feature using the pull direction from the mold opening direction. Limitation: some cosmetic or non-molded faces may not require draft.

What You Need to Know Before

Warning: A part can look correct in the model and still fail in tooling if vertical faces have zero or negative draft relative to the pull direction. A very common hidden issue is assigning draft from the wrong neutral plane or pull direction, which creates undercuts and ejection problems.

How to Add Draft in SolidWorks

  • Command: Draft

  • Shortcut: No default keyboard shortcut

  • Quick Steps:

    1. In the CommandManager, go to Features > Draft, then choose the most common method: Neutral Plane Draft.
    2. Select the neutral plane or planar face that should remain fixed, then select the faces to draft based on the mold pull direction.
    3. Enter the draft angle, enable Flip direction if needed, and confirm the feature after previewing the taper direction.

Variables & Settings

  • Key Setting: Draft Angle

  • Expert Setting: The draft angle directly controls manufacturability and ejection behavior. In most plastic part workflows, designers start with approximately 1° to 3° per side depending on material, surface texture, depth, and tooling requirements.

Why it Fails

  • Cause 1 (Geometry): Fillets, ribs, or drafted faces may intersect or collapse when the angle is too large for the available wall height.
  • Cause 2 (layers/Locks): Imported geometry or locked body relationships can prevent clean face selection, especially in multi-body or dumb-solid models.
  • Cause 3 (Command/Logic): The draft is applied from the wrong neutral plane or incorrect pull direction, causing negative draft or hidden undercuts.

Quick Fix & Best Practice

  • Quick Fix: Use Mold Tools > Draft Analysis before and after applying Draft to verify positive draft relative to the tooling direction.
  • Manager’s Verdict: Add draft early in the model whenever the part is intended for injection molding; avoid leaving it until final detailing because later edits around ribs, bosses, and fillets often become unstable.

FAQ

Do all plastic faces need draft?
No, but most faces parallel to the mold pull direction should have draft.

How much draft is typical in SolidWorks plastic parts?
Usually 1° to 3°, but textured surfaces often need more.

Can fillets be added before draft?
Yes, but in many professional workflows draft is added before some final fillets to reduce rebuild issues.

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