DFM Checklist for Injection Molded
Plastic Parts.

1. Check Wall Thickness

Wall thickness is one of the most important factors in plastic part design. Uneven wall thickness can create sink marks, voids, warpage, internal stress, longer cooling time, poor dimensional control, weak molded parts, and cosmetic defects.

Thick wall sections are harder to cool and pack. When thick and thin sections exist together, shrinkage-related stresses can develop in the transition region. Malloy recommends avoiding thick sections where possible and using coring, ribs, and edge stiffeners to maintain stiffness without unnecessary plastic mass.

2. Check Ribs and Gussets

Ribs are used to increase stiffness without making the full wall thicker. But ribs can also create sink marks, warpage, and ejection problems if designed badly.

Tres notes that rib thickness depends strongly on the type of thermoplastic. For example, crystalline materials with higher shrinkage usually need thinner ribs, while lower-shrinkage or filled materials may allow thicker ribs. He also notes that draft is required for mold ejection, and textured surfaces require more draft.

3. Check Bosses and Screw Posts

Bosses are common in plastic housings, but they are also common failure points. Poor boss design can cause sink marks, cracking, screw stripping, weak retention, stress concentration, mold filling problems, and visible marks on outer surfaces.

Malloy notes that bosses can create local wall thickness changes where cooling stresses develop, and excessive wall thickness around bosses can lead to sink marks or shrinkage voids.

4. Check Draft Angles

Plastic parts must release from the mold. Without draft, parts may stick, scratch, drag, deform, or require high ejection force.

Check draft on outer walls, inner walls, ribs, bosses, snap-fits, shutoff faces, button openings, connector openings, deep pockets, and textured surfaces.

• Add draft early in CAD.
• Increase draft for textured surfaces.
• Avoid zero-draft walls unless truly required.
• Confirm mold opening direction before detailing.
• Avoid late draft changes because they can disturb fit and appearance.

Draft is not just a tooling detail. It affects product dimensions, assembly fit, and surface quality.

5. Check Parting Line

The parting line is where the two mold halves meet. A poor parting line can create flash, mismatch, visible cosmetic defects, sealing issues, or assembly problems.

For premium products, parting line planning should happen during product architecture, not after the final CAD model is ready.

6. Check Gate Location

Gate location controls how plastic enters the cavity. It affects flow, weld lines, shrinkage, packing, fiber orientation, cosmetic marks, and part strength.

Check where the gate mark will appear, if it will be hidden, if flow will reach all areas properly, if weld lines will appear in high-stress or cosmetic areas, if thick areas can be packed, if flow length is too long, and if venting is possible at the end of flow.

A common direction is to gate into thicker regions when thick and thin sections exist, because the thicker section needs more packing and takes longer to solidify. Malloy notes that gating into the thick section is preferred in such cases because it allows packing of the thick area even after thinner sections have solidified.

7. Check Weld Lines

Weld lines form where two plastic flow fronts meet. They can be cosmetic defects, but they can also become weak areas.

Weld lines are especially risky near snap-fits, screw bosses, living hinges, pressure-loaded areas, sealing surfaces, impact zones, display windows, and visible cosmetic surfaces.

Do not ignore weld lines. In many plastic parts, the weakest location is not where the CAD looks thin, but where the flow front creates a weak knit line.

8. Check Sink Marks

Sink marks happen when thick plastic sections shrink more than surrounding areas. Common locations include behind ribs, behind bosses, around screw posts, near thick corners, near snap-fit bases, at thick logo areas, and around mounting pads.

Check if ribs are too thick, boss bases too heavy, or cosmetic surfaces backed by thick features. Consider if material can be cored out, if stiffness can be achieved with ribs instead of mass, or if gate and packing can improve the issue.

Sink marks are both cosmetic and engineering problems. A sink-prone area may also contain internal stress or voids.

9. Check Warpage Risk

Warpage is often caused by uneven shrinkage. Common causes include uneven wall thickness, unbalanced ribs, poor gate location, long flow length, uneven cooling, thick bosses, asymmetric geometry, glass fiber orientation, and poor stiffness balance.

Check if the part will stay flat, if covers will match correctly, if screw holes align, if gaps remain even, and if sealing surfaces remain flat.

Warpage should be reviewed before tooling. Mold-flow analysis can help, but design judgment is still required.

10. Check Ejection

The molded part must come out of the tool without damage.

Check if there is enough draft, if ejector pin locations are acceptable, if ejector marks will appear on cosmetic surfaces, if deep ribs are difficult to eject, if the part will stick to the wrong mold half, if undercuts are intentional, and if the part can deform during ejection.

Ejection problems can create scratches, stress marks, deformation, or slow cycle time.

11. Check Tolerances

Plastic parts cannot be toleranced like machined metal parts. Dimensions are affected by material shrinkage, mold temperature, packing pressure, cooling rate, part geometry, fiber orientation, moisture absorption, post-mold shrinkage, and warpage.

Check if tight tolerances are truly required, if critical dimensions are clearly identified, if PCB and gasket fits are checked, and if tolerance stack-up is reviewed.

Tight tolerances increase mold cost, inspection cost, rejection risk, and process sensitivity. Use tight tolerances only where function needs them.

12. Check Assembly Method

A plastic part should be designed for assembly from the beginning.

Check how the part will be assembled, if screws/snap-fits/inserts are used, if the operator can assemble it easily, if there is clear alignment, if wires or gaskets can be trapped, and if the assembly forces are acceptable.

DFMA thinking is important because assembly decisions affect total product cost, not only part cost. Boothroyd, Dewhurst, and Knight describe DFMA as a process where DFA first simplifies product structure, then DFM supports material/process selection, cost estimates, and detail design for manufacturing cost reduction.

13. Check Fasteners and Inserts

Plastic parts often use screws, heat-set inserts, molded-in inserts, or self-tapping screws.

Check if the screw type is correct for plastic, if the pilot hole is correct, if the boss is strong enough, if the insert is too close to the edge, and if heat staking or insert installation will damage the part.

Avoid using metal-style fastening rules directly in plastic. Plastic creeps, relaxes, cracks, and behaves differently under stress.

14. Check Surface Finish and Texture

Surface finish affects appearance, draft, ejection, scratches, flow marks, and tooling cost.

Check which surfaces are cosmetic vs functional, if texture will need extra draft, if gloss variation will be visible, if gate and ejector marks are hidden, and if ribs or bosses will read through the surface.

For visible plastic products, cosmetic review should be part of DFM, not a separate late-stage activity.

15. Check Material Selection

Material affects everything.

Check if the material is suitable for the load, impact strength, temperature, long-term creep, UV exposure, chemical compatibility, snap-fits, surface finish, and availability.

Do not select material only by price. A cheaper resin can become expensive if it causes warpage, cracking, poor finish, or field failure.

16. Check Tooling Complexity

Every design feature has tooling impact.

Check if undercuts are avoidable, if side cores or lifters are required, if shutoffs are too thin or weak, if the mold can be cooled properly, and if the part can be ejected safely.

Simple tooling usually means lower cost, shorter lead time, easier maintenance, and better production consistency.

17. Check Testing and Validation Plan

DFM does not end with CAD review. Before production approval, plan testing for first article inspection, assembly fit, screw boss strength, snap-fit retention, drop impact, heat aging, UV/chemical exposure, gasket compression, IP testing, vibration testing, and cosmetic inspection.

3D printed prototypes are useful for early learning, but they do not fully validate injection molded behavior. Molded samples must be tested before production approval.