How to Design for Injection Molding: Product Development
Getting Started: Design for Injection Molding
Design for injection molding starts with one goal: to produce plastic parts that look good, function well, and don’t slow down production.
Smart design cuts costs, avoids defects, and gets your product out faster. It’s about thinking ahead—about the mold, the machine, and the material—before you hit “go” on manufacturing.JDI Plastics has been serving clients across all industries for over 20 years. We constantly strive for industry-leading innovations and technology. Experience how a trusted and capable injection molding company can make your vision come to life.
What Does Design for Injection Molding Involve?
| Design Element | What It Affects | Why It Matters |
|---|---|---|
| Draft Angles | Part ejection from the mold | Reduces drag and prevents damage during release |
| Wall Thickness | Mold filling, cooling time, structural strength | Keeps the part balanced and avoids warping, sink, or voids |
| Radii and Corners | Stress distribution and material flow | Supports smooth resin flow and lowers risk of cracking |
| Undercuts | Tooling complexity and cost | May require lifters or side-actions—avoid if possible |
| Gate Location | Flow pattern and cosmetic quality | Impacts fill consistency and visible gate vestiges |
| Ejector Pin Placement | Surface finish and part stability | Prevents distortion or surface damage during ejection |
| Material Selection | Shrinkage, strength, chemical resistance | Affects every step from molding to end-use performance |
| Surface Finish & Texture | Appearance and release | Influences required draft and tool wear |
How Wall Thickness Affects Moldability and Quality
Wall thickness makes or breaks a molded part. Go too thick, and you’ll deal with sink marks, internal voids, and uneven cooling. Go too thin, and the part may not fill properly or survive long in use.
Stick to recommended ranges for your resin—polycarbonate, for example, works best between 0.040″ and 0.150″. Keep wall thickness as uniform as possible across the part. Sudden changes force the plastic to cool at different rates, which leads to warping and dimensional errors.
To add strength without adding bulk, use ribs or gussets. Coring out thick sections helps reduce material usage and speeds up cycle times without sacrificing integrity.
Why Draft Angles Matter for Injection Molding
No draft, no part—at least not without a fight. Plastic shrinks slightly when it cools and grips the mold. Parts drag during ejection without the right draft angle, leaving scuff marks or getting stuck entirely.
Start with 1 degree of draft per vertical inch. If your part has textured surfaces or deep pockets, bump that number up. The more aggressive the texture, the more draft you’ll need.
Don’t wait until tooling to think about this. Fixing a draft after the fact usually means returning to the CAD and re-cutting steel.
The Role of Radii and Corners in Stress Reduction
Plastic hates sharp corners. They block flow, trap stress, and become weak points under load. That’s why you need radii—curved transitions that keep the material moving smoothly and reduce internal pressure during molding.
A rounded corner flows like a river bend. Plastic follows the curve without bunching or hesitation. It also cools more evenly to reduce the risk of cracking, sink, or deformation.
Keep in mind that inside corners should always be larger than outside ones when possible. It’s a small change that helps flow and strengthens the part without complicating the mold.
Core-Cavity Design and Parting Line Planning
The core-cavity setup is the mold designer’s best friend. Instead of overcomplicating geometry with deep ribs or multiple actions, split the part logically between two clean mold halves. It speeds up machining, simplifies ejection, and cuts down on costly slide mechanisms.
Don’t wait until tooling to think about your parting line. It controls where the two halves of the mold meet, and it decides everything from surface finish to ejection behavior. If it’s poorly placed, you risk flash, sink, or tricky ejection paths.
A smart parting line gives you clean separation, minimal tooling wear, and fewer surprises during production. (1)
Managing Undercuts Without Killing Your Budget
In injection molding, undercuts are part features that prevent a molded part from being ejected straight out of a two-part mold. They block the part from releasing cleanly in the direction the mold opens (known as the “line of draw”).
Undercuts aren’t always avoidable, but they always cost extra. They require sliders, lifters, or inserts—each one adding time, complexity, and maintenance to the mold.
Before defaulting to side-actions, take a step back. Could a snap-fit be redesigned? Could a slot replace a window? Could you split the part into an assembly? Those tiny changes can shave thousands off your tooling cost.
When undercuts are necessary, go with clean solutions like pin-actuated side actions or lifter systems that work with the mold’s natural motion. Stay away from overly complex designs unless you absolutely need them.
Gating and Ejection: What to Know Before Tooling
Gates control how the plastic enters the mold, and ejector pins help push the finished part out. Both impact how your part looks, how it performs, and how easily it molds.
Place gates where the wall thickness is thickest—that’s where flow resistance is lowest. Avoid placing them near cosmetic surfaces or tight tolerances. A badly placed gate can leave a visible vestige or create uneven fill that leads to warping.
As for ejector pins, they need flat, strong contact points. Skip tight corners or cosmetic surfaces. Use rib backs, mounting bosses, or other low-visibility areas instead. The goal is to eject the part without distortion—or drama.
Material Selection Based on Application and Process
Choose a material that fits your part’s function and molds well under your process.
Use polycarbonate or acrylic for clarity, TPE or LDPE for flexibility, and PEEK or PPS for high heat resistance. Every resin flows, shrinks, and cools differently.
Modifiers like glass-fill or UV additives can improve performance but may impact flow, surface finish, or tool wear. If you’re unsure, test early or talk to your molder.
Surface Finish and Texture
Finish affects more than appearance. It changes how the part releases, how much draft you need, and how the tool wears over time.
Glossy surfaces need a polished mold and clean design. Textured surfaces can improve grip and hide flaws but require more draft to release cleanly.
Plan textures during design, not after. If the part uses moving mold components, apply the texture while closed to avoid mismatched seams.
Clip Actions, Living Hinges, and Flexible Design Features
Clips and living hinges allow complex functions in a single molded part, but they require smart geometry and the right material.
Use polypropylene for hinges. Keep the hinge thin and place the gate close to reduce stress.
Clips should flex without cracking. Use long arms and avoid thick or sharp edges. Test repeated use if the clip will be opened and closed often.
Stay steel safe on key features so you can fine-tune the mold later without major changes.
JDI Plastics: Helping Clients Design for Injection Molding
Good injection molding starts at the design stage. Keep your wall thickness consistent, add proper draft, round off sharp corners, and avoid features that complicate mold actions unless they’re absolutely needed.
Design with the mold in mind, and you’ll spend less time fixing problems and more time shipping parts.
JDI Plastics is proud to work closely with our clients to collaborate on important design elements aimed at increasing quality and efficiency of production.
Reference:
- UC Sandiago, Plastic Part Design for Injection Molding, https://extendedstudies.ucsd.edu/courses/plastic-part-design-for-injection-molding-mae-40033
