Good injection molding design usually comes down to a few practical decisions: keep wall thickness reasonably consistent, add enough draft for ejection, avoid unnecessarily heavy ribs and bosses, and define critical tolerances and material requirements before the mold is built.
These details may look minor in a CAD model, but they can make a noticeable difference during mold trials. Sink marks, warpage, drag marks, difficult ejection, and expensive tool changes often trace back to decisions made early in the design stage.
Here are eight areas we recommend checking before moving a custom plastic part into tooling.
Why Review DFM Before Building the Mold?
A plastic part can look perfectly reasonable on screen and still be difficult to mold.
One common example is a housing with a thick mounting area surrounded by much thinner walls. The thicker section cools more slowly, and the problem may only become obvious during the first mold trial when sink marks or distortion appear.
The same applies to deep vertical walls without enough draft, oversized bosses, unnecessary undercuts, or tolerances that are tighter than the application actually requires.
This is why we prefer to review the part together with the material and tooling requirements before mold manufacturing starts. For custom parts moving toward volume production, an early injection molding DFM review can help identify potential tooling and molding issues before steel is cut.
A typical DFM review looks at:
- Wall thickness and transitions
- Draft and part ejection
- Ribs and bosses
- Undercuts
- Critical dimensions
- Material and expected shrinkage
- Gate and parting-line locations
- Cosmetic surfaces
Changes are much easier to discuss at this stage than after steel has already been cut.
1. Start with Consistent Wall Thickness
Wall thickness is one of the first things worth checking.
If one area is much thicker than the surrounding wall, it will usually cool more slowly. Depending on the material and geometry, that can show up as sink marks, distortion, or dimensional variation after molding.
For areas that need more stiffness, adding a rib is often better than simply adding more material.
Typical Wall Thickness Reference
| Material | Typical Wall Thickness |
|---|---|
| ABS | 1.2–3.5 mm |
| PC | 1.0–4.0 mm |
| PP | 0.8–3.8 mm |
| PA (Nylon) | 0.8–3.0 mm |
| POM | 0.8–3.0 mm |
These ranges are useful as an early design reference, not as fixed specifications. Part size, flow length, material grade, strength requirements, and mold design all affect the final choice.
If a thickness change cannot be avoided, a gradual transition is generally preferable to an abrupt step.
2. Don’t Leave Draft Until Tooling
Draft is easy to overlook when the design team is focused on function and appearance.
For many molded parts, 1°–2° is a practical starting point, but there is no single draft angle that works for every feature.
Deep walls often need more consideration. Textured surfaces can also require additional draft because they create more resistance during ejection.
Too little draft may not stop the mold from working, but it can create problems such as drag marks, scratching, difficult ejection, or cosmetic damage.
It is better to review draft while the CAD model is still easy to change.
3. Use Ribs to Add Stiffness, Not Extra-Heavy Walls
Ribs are useful when a housing, cover, or structural plastic part needs more rigidity.
But a rib should not become another thick section.
As an initial guideline, rib thickness is often around 40–60% of the adjacent nominal wall thickness. The exact value depends on the resin, geometry, cosmetic requirements, and structural needs.
A very thick rib can leave a sink mark on the opposite surface—particularly noticeable on a cosmetic housing.
The rib base should also transition smoothly into the main wall rather than creating a sharp internal corner.
4. Pay Attention to Bosses Around Screws and Inserts
Bosses are another area where we often see too much material concentrated in one location.
They are needed for screws, locating pins, threaded inserts, and assembly features, but making the boss thicker does not automatically make the part better.
A practical boss design should consider:
- Wall thickness around the boss
- Support ribs
- Radius at the base
- Draft
- Distance from nearby walls
- Screw or insert installation method
For parts that use heat-set or ultrasonic inserts, it helps to confirm the insert specification early. Changing insert size late in the project can mean changing both the molded feature and the tool.
5. Ask Whether Every Undercut Is Really Necessary
Some parts genuinely need undercuts. Snap fits, side holes, connectors, and other functional features may make them unavoidable.
The question is whether every undercut in the design provides enough value to justify the tooling required to create it.
Depending on the geometry, an undercut may need a slider, lifter, side action, or another mold mechanism. That adds components to the mold and can increase tooling cost and maintenance.
Sometimes moving a hole, opening one side of a feature, or slightly changing the geometry can remove the need for a side action altogether.
For high-volume parts, that seemingly small change can matter over the life of the tool.
6. Tighten the Tolerances That Matter—Not Every Dimension
A drawing covered in very tight tolerances does not necessarily produce a better molded part.
It often produces a more difficult part to manufacture.
We normally recommend identifying which dimensions actually control:
- Fit
- Alignment
- Sealing
- Movement
- Assembly
- Functional performance
Those dimensions deserve closer attention.
Other non-critical dimensions can often use more practical molding tolerances.
Plastic also behaves differently from machined metal. Shrinkage, temperature, moisture absorption, part geometry, and processing conditions can all affect the final dimensions.
The goal is not loose tolerances. It is appropriate tolerances in the right places.
7. Confirm the Material Before the Mold Is Finalized
Material selection should not be treated as a decision that can always be made after tooling begins. For custom injection molded parts, the resin grade should ideally be confirmed while the part geometry and tooling concept are still being reviewed.
Changing from ABS to PP, for example, changes the shrinkage behavior. Switching to a glass-filled nylon introduces different flow, wear, and dimensional considerations.
Those differences can influence the mold itself.
Common Materials from a DFM Perspective
| Material | Moldability | Shrinkage Tendency | What to Watch |
| ABS | Good | Moderate | Sink marks in thick areas |
| PC | Moderate | Low–Moderate | Processing temperature and flow |
| PP | Good | High | Shrinkage and warpage |
| PA | Good | Moderate–High | Moisture and dimensional change |
| POM | Good | High | Shrinkage and dimensional control |
This table is only a starting point. Filled grades, flame-retardant materials, and other modified polymers may behave quite differently from standard grades.
For abrasive glass-filled materials, tooling wear should also be considered when selecting mold steel and planning long-term production.
8. Decide What the Customer Will See
For visible plastic housings, appearance needs to be discussed before mold design—not after the first samples arrive.
The engineering team should know which surfaces are cosmetic and which will be hidden after assembly.
This helps when deciding where to place:
- Gates
- Ejector marks
- Parting lines
- Textures
- Weld lines where possible
For example, placing a gate mark on the front face of an electronic housing may be technically acceptable but commercially unacceptable.
Surface texture also matters. A deeper texture usually needs additional draft, so appearance and moldability are connected. If the project also requires painting, coating, polishing, or other surface finishing operations, these requirements should be identified before production planning is finalized.
Quick DFM Check Before Tooling
Before approving the design for mold manufacturing, we recommend checking a few basic questions:
- Is the wall thickness reasonably consistent?
- Are thickness changes gradual?
- Is there enough draft for ejection?
- Are the ribs and bosses properly proportioned?
- Are all undercuts necessary?
- Which dimensions are actually critical?
- Has the final material grade been confirmed?
- Are cosmetic surfaces clearly identified?
- Have gate and parting-line requirements been discussed?
- Is the expected production volume known?
If several of these questions are still unanswered, it is usually worth resolving them before tooling starts.
What to Send with an Injection Molding RFQ
A good RFQ does not need to be complicated, but the right information makes the initial review much more useful.
For most custom injection molding projects, we recommend providing:
- 3D CAD model
- 2D drawing if available
- Material or performance requirements
- Critical tolerances
- Surface texture or cosmetic requirements
- Expected order quantity or annual volume
- Assembly information
If the material has not been finalized, tell the manufacturer what the part needs to do instead—for example, operating temperature, chemical exposure, strength, flame rating, or appearance requirements.
That gives the engineering team enough information to discuss realistic options rather than simply guessing a resin.
At FSD Precision, custom projects can be supported from early design and engineering review and mold manufacturing through injection molding and related secondary operations, allowing design and production requirements to be considered together before tooling begins.
Internal Links: Injection Molding Service · Design & Engineering
Frequently Asked Questions
What is DFM in injection molding?
DFM means reviewing the part with the manufacturing process in mind before the mold is built. The goal is to identify features that could create molding problems, unnecessary tooling complexity, inconsistent quality, or avoidable cost.
How much draft should an injection molded part have?
For many parts, 1°–2° is a useful starting point. Deep features and textured surfaces often need more. The final draft should be reviewed according to the material, surface finish, geometry, and mold-opening direction.
Why does wall thickness matter so much?
Thick and thin areas cool at different rates. Large differences can lead to sink marks, warpage, internal stress, and dimensional variation. Keeping the wall reasonably uniform makes the molding process easier to control.
Can ribs make a plastic part stronger?
Yes. Properly designed ribs can increase stiffness without making the entire wall thicker. The rib still needs to be sized carefully because an oversized rib may create sink marks on the opposite surface.
Do all undercuts make the mold expensive?
Not necessarily. It depends on how the undercut is produced. However, features requiring sliders, lifters, or other moving mold components generally add complexity, so it is worth checking whether the undercut is really necessary.
When should the plastic material be selected?
Preferably before the mold design is finalized. Different polymers have different shrinkage, flow, temperature, wear, and dimensional behavior, all of which can influence tooling decisions.
What should I send for an injection molding quotation?
A 3D CAD model is the best starting point. If available, also send a 2D drawing, material requirements, critical tolerances, surface requirements, estimated quantity, and assembly information.
Can FSD review the part before we commit to tooling?
Yes. A DFM review can be used to discuss moldability, potential design changes, material considerations, tooling structure, and production requirements before mold manufacturing begins.
Have a Plastic Part Ready for DFM Review?
If you already have a CAD model or drawing, send it together with your material requirements, expected quantity, and any critical dimensions.
FSD Precision can review the project before tooling and help determine a practical route from mold manufacturing to injection molding and production.
Need help sourcing custom OEM parts?
Send drawings, samples, part numbers, material requirements, quantity, and application details for engineering review.