Plastic parts fail more often than they should. Failed production runs lead to wasted materials, project delays, and budget overruns. The cause usually isn’t complicated—it’s just overlooked basics of injection molding design.
Few engineering programs teach the practical side of plastic manufacturing. New designers learn on the job, sometimes after costly mistakes have already been made. Even simple choices about thickness and corners determine if a product succeeds.
What looks good in CAD software doesn’t always work in the molding machine. Designers focus on how things look, while molders worry about how plastic flows into the mold. This gap causes most production headaches. Here are injection molding design tips that help parts work right the first time:
Top Injection Molding Design Tips
Plastic parts fail more often than they should. Failed production runs lead to wasted materials, project delays, and budget overruns. The cause usually isn’t complicated—it’s just overlooked basics of injection molding design.
Few engineering programs teach the practical side of plastic manufacturing. New designers learn on the job, sometimes after costly mistakes have already been made. Even simple choices about thickness and corners determine if a product succeeds.
What looks good in CAD software doesn’t always work in the molding machine. Designers focus on how things look, while molders worry about how plastic flows into the mold. This gap causes most production headaches. Here are injection molding design tips that help parts work right the first time:
1. Understand the manufacturing process
Know how injection molding works before you design your part. Plastic melts, gets pushed into a mold under pressure, cools down, and then gets pushed out. What seems simple involves many steps that can affect your design.
Every manufacturing process has limits. Plastic can’t flow into extremely thin spaces. Thick areas cool slowly and may create sink marks. Sharp corners cause stress points. Learning these basics helps you avoid problems before they happen.
Many parts fail because designers don’t consider how they’ll be made. A good design needs proper ejection features. Understanding the injection molding process produces better parts and reduces costly modifications.
2. Work with appropriate materials
Not all plastics work the same way. Different injection molding materials have different properties. In fact, some polymers are strong but brittle; others are flexible but weak. Pick your material based on what your part needs to do. Don’t use a rigid plastic if it needs to bend without breaking. If it needs to hold tight tolerances, avoid materials with high shrinkage. For instance, Polyethylene has chemical resistance but might be too flexible for structural parts. Match your material to what the part needs to do.
Different materials need different molding settings. Some thermoplastics need very hot molds; others need cold ones. Some flow easily through thin walls, others don’t. The material choice affects processing temperature, pressure, and cooling time. Talk with your manufacturer to know the right materials for your design and machines. However, here are some materials you might want to consider:
PET resin (like Eastman TritanTM)
Nylon, or ABS resin (like Eastman ABSTM).
3. Keep parts simple
Complex shapes cost more money. Every curve, hole, and feature in your injection molding design adds complexity to the mold. While a CNC machine can cut almost any shape into steel, each complex feature increases machining time and tool wear. Simple designs mean faster mold making, fewer errors, and lower tooling costs.
Side-actions also increase mold complexity. These sliding parts enable features that a simple open and close mold cannot create. Each side-action extends the cycle time, increases wear on the tool, and creates potential failure points. When possible, redesign features to eliminate these mechanisms, even if it means splitting a complex part into two simpler ones.
The fastest injection molding process comes from parts with smooth, continuous flow paths and easy ejection. Thin, uniform walls cool quickly and evenly. Simple shapes with proper draft angles release properly from the mold without sticking.
When you design with these principles, you create parts that cost less to tool. These parts will also run faster in production, making them cost-effective for high-volume productions.
4. Avoid sharp corners and oversized holes
Sharp corners create major problems in injection molded parts. They block the proper material flow, trap air, and create weak spots where cracks start. Adding small radii to corners (even 0.5mm helps) allows the plastic to flow smoothly during molding. It also distributes stress evenly during use.
Holes in plastic parts should follow basic sizing rules. For round holes, keep the diameter less than 3 times the wall thickness to avoid problems. Larger holes need support structures or should be designed as cutouts with rounded corners.
In addition, every sharp internal corner acts as a stress concentrator, focusing force onto a tiny area. When designing functional parts, these become the first points of failure. For critical load-bearing areas, use generous radii to distribute stress.
This simple change can determine if a part breaks during its first use or if it lasts for years. Good injection moulding design balances aesthetics with these practical requirements.
5. Size your ribs and bosses correctly
Ribs and bosses strengthen plastic parts without making them too thick. Keep ribs at 60% of the main wall thickness to avoid sink marks on visible surfaces. If your main wall is 2mm thick, make ribs 1.2mm thick. For bosses (the round mounting posts), keep the outer diameter 2-2.5 times wider than the screw hole.
Don’t cluster ribs too close together. Space them at least twice the wall thickness apart for proper cooling. Connect bosses to nearby walls with small supporting ribs rather than leaving them as standalone posts. This prevents them from breaking during assembly and helps plastic flow better during molding.
Don’t forget to add draft angles to ribs and bosses – at least 1° per side, more for textured surfaces. Many parts get stuck in the mold because designers forget this simple detail. Also, add small radii at the base of ribs and bosses to prevent sharp corners that create weak spots and slow down cooling.
6. Put gates in the right spot for better parts
Where you place the gate (where the plastic enters your part) matters a lot. Put gates in the thickest section so plastic flows from thick to thin areas. Bad gate location causes warping, weak spots, and visible marks. For parts where looks matter, hide gates on non-visible surfaces or tuck them into corners and mounting holes.
Different types of gates work better for different parts. Pin gating works for small parts but leaves a mark. Edge gates fill well and are less visible. Film gates spread plastic evenly for flat parts. Submarine gates break off when the mold opens, but only work for specific sizes and materials.
In mold cavities, the runner system needs to be balanced so all parts fill at the same rate. If some cavities fill before others, you’ll get inconsistent parts. The best setup uses identical path lengths to each cavity, but this isn’t always possible. When it isn’t, mold makers adjust runner diameters to control flow. Hot runner systems eliminate waste plastic but cost more upfront.
7. Place ejector pins carefully to protect parts
Ejector pins push your finished injection molding design out of the mold. First, place these pins in stronger areas like ribs, bosses, or thicker sections that can handle the pressure without breaking. Next, spread pins evenly across the part so no section gets pushed too hard. Without enough pins in the right spots, your parts can bend or break when ejected. For wide, flat areas, use ejector sleeves instead of pins since they spread the force better.
Your part needs to release easily from the mold. Therefore, add small draft angles (1-2°) on all vertical walls. Remember that textured surfaces need more draft, at least 3° for light texture and 5° for heavy texture. Features like deep ribs or tall bosses may likely stick when the mold opens, so they need extra attention during design.
Finally, keep ejector pins away from visible part surfaces because each pin leaves a small circular mark. When pins must touch visible areas, place them in spots that will be hidden. It can be placed in areas such as inside corners, under edges, or in areas covered later. For parts where appearance matters most, try to put all ejection on the non-visible side of the part.
8. Design parts that snap together easily
Good snap fits, and living hinges can eliminate the need for screws completely. First, design snap fits with a hook angle of 30-45° for parts that need to come apart or 90° for permanent connections. Living hinges should be thin (0.3-0.5mm) and made with materials like polypropylene.
Remember that these part features need selected material. And that’s because not all plastics can bend repeatedly without breaking. These part features usually create undercuts in your mold, which makes tooling more complex and expensive.
Self-locating features help parts fit together correctly the first time. For instance, add chamfers to guide parts into position. Then, use pins and sockets that only fit one way to prevent assembly mistakes. Similarly, different boss heights or sized alignment posts can serve as “mistake-proof” features. Although these design considerations seem small, they reduce assembly time and mistakes during production.
Finally, reduce the need for extra work after molding to keep costs down. For example, design text and logos into the mold rather than adding them later with painting. Include mounting points in your original design instead of drilling holes afterward. Also, think about packaging – adding stacking features can save packaging costs. Good design for manufacturing (DFM) looks at the entire product journey, not just the molding step.
9. Use simulation software to spot problems
Flow analysis software shows you how plastic will fill your mold before making any tooling. First, this virtual testing spots potential molding defects. It identifies issues like short shots (unfilled areas), weld lines, and air traps that could turn into voids. Plus, it helps you find the best gate location and size. Testing different designs on the computer helps you fix problems early. This way, you avoid costly mold fixes later.
Cooling analysis examines how heat leaves your part during injection molding design. Specifically, uneven cooling causes stress, warpage, and inconsistent dimensions. Through simulation, you can identify hot spots where thick sections stay warm longer.
As a result, you can properly position cooling channels for more even temperatures. This leads to faster production because cooling takes up 70 to 80% of the molding cycle. Even small cooling improvements can significantly increase production rates over the tool’s life.
Finally, warpage and shrinkage prediction help ensure parts have the right dimensions. Different materials shrink by different amounts. For example, elastomers might shrink 3-5%, while engineering plastics shrink 0.5-2%.
Simulation shows how this shrinkage will affect your specific part shape. It also predicts warping from uneven cooling or internal stress. Without this testing, you might need many expensive tool adjustments to get parts right. But with good simulation, you can fix these issues before cutting any steel, saving time and money.
10. Place parting lines strategically to improve part quality
The parting line is where the two halves of the mold meet, and it always leaves a visible line on your finished part. First, place this line along natural edges or corners where it will be less noticeable.
For parts where appearance matters, keep the parting lines away from visible surfaces. Instead, put it on the bottom, inside, or along edges that won’t be seen. Don’t place parting lines on flat, visible surfaces where any mismatch between mold halves will stand out. A well-placed parting line disappears, while a poorly placed one ruins the look of a good part.
Flash is the thin excess plastic that squeezes between mold halves. To reduce flash, ensure the shut-off surfaces where mold halves meet are wide enough and properly designed. Also, keep parting lines on a single plane when possible. Complex, 3D parting lines make proper shut-off harder to achieve.
The mold’s clamping force also affects flash. Complicated parting lines need higher clamping forces, which means larger machines and higher costs. Therefore, a simple, well-designed parting line saves money throughout production.
Finally, undercuts are features that prevent the part from being ejected from a simple two-part mold. You’ll need side actions (slides) or other special mold components that increase costs to handle undercuts.
However, clever parting line placement can sometimes eliminate the need for these extras. For example, placing the parting line along the widest part of a tapered feature can allow both sides to release naturally.
When side actions are necessary, design your part so that multiple undercuts can use the same side action. This way, you won’t need separate mechanisms. Remember, each additional side action increases mold cost and maintenance needs.
11. Advanced techniques for complex parts
Gas-assisted injection molding design creates hollow sections inside thick sections of plastic parts. First, plastic is injected. Then, nitrogen gas is pushed into the still-soft material, creating hollow channels.
As a result, parts become lighter, less prone to warping, and cool faster. This works well for parts with thick handles or ribs. However, it needs special equipment and material selection. Overall, savings from using less material and faster production can reduce the higher setup costs.
Overmolding adds a second layer of plastic or rubber onto a base part. This creates soft-touch grips, multi-colored components, or parts with rigid and flexible sections. Similarly, insert molding places components into the mold before injection.
Both techniques prevent assembly steps but need more complex molds. When planning for overmolding, ensure materials will bond well. The right combination creates high-quality parts with unique properties that are impossible with single-material molding.
Metal inserts provide stronger threaded connections than plastic alone. These can be placed in the mold before injection or installed after molding. Molded-in inserts save assembly time but complicate production.
Additionally, they create stress points where molding defects like cracks can form. Design inserts with features for a better plastic grip. Leave enough material around each insert to prevent cracking. When done right, metal inserts improve part strength.
The world's best injection molding manufacturer:
Protolabs
Protobals was founded in 1999 by Larry Lukis, with headquarters and manufacturing facilities in Maple Plains, Minnesota, specializing in the rapid manufacture of custom injection molded parts.
Xometry
Xometry was founded in 2013 by Altschuler and Laurence Zuriff. It is based in Derwood, Maryland.
Aria ( China plastic injection molding manufacturer )
Aria was founded in 2010 by Lance Lin and is headquartered in Dong Guan, China.
It is an online manufacturing platform offering a range of manufacturing services, including injection molding, Mold Making, sheet metal, CNC machining services.
3DHubs (Buyout by Protolabs)
3DHubs was founded by Bram de Zwart and Brian Garret in 2013 and is headquartered in Amsterdam, The Netherlands. Protolabs acquired it in January 2021. And renamed Hubs.