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Core Design Principles
The foundation of a successful molded part lies in its design. These core principles are essential for creating high-quality, manufacturable, and cost-effective components. Click on each principle to explore the detailed guidelines and recommendations.
1. Uniform Wall Thickness
Ensures even cooling and prevents defects like warping and sink marks.
2. Draft Angles
A slight taper on vertical walls for easy part ejection from the mold.
3. Rounded Corners
Avoids stress concentration and improves material flow for stronger parts.
4. Ribs & Gussets
Adds strength and rigidity without increasing wall thickness.
5. Coring Out
Hollowing out thick sections to save material, reduce weight, and cycle time.
Click a principle to see details here.
Guideline: Uniform Wall Thickness
Why? Uniform thickness is critical for controlling how the part cools. If one area is thicker than another, it will cool slower, causing internal stresses that can lead to warping. It also prevents cosmetic defects like sink marks, where the surface dips inward over a thick section. A consistent thickness ensures the molten plastic flows predictably and fills the entire mold cavity.
Recommendation: Strive for consistency. If thickness variations are unavoidable, make the transition smooth and gradual. A good rule of thumb is to keep any change in thickness to no more than 10-15% of the adjacent wall’s thickness.
Guideline: Draft Angles
Why? As plastic cools, it shrinks and grips onto the mold core. Without a draft angle (a slight taper), ejecting the part would require significant force, potentially causing drag marks, scratches, or even breaking the part or the mold. Draft makes ejection smooth and efficient.
Recommendation: A minimum of 1° of draft per side is a standard starting point. For textured surfaces, you’ll need more (e.g., an additional 1.5° per 0.025mm of texture depth) to prevent scraping the finish during ejection.
Guideline: Rounded Corners (Radii)
Why? Sharp, 90° corners are stress concentrators. They create weak points where a part is most likely to crack or fail under load. Rounded corners (fillets on the inside, rounds on the outside) distribute stress over a larger area, resulting in a much stronger part. They also significantly improve the flow of molten plastic through the mold, preventing flow defects.
Recommendation: The radius of an inside corner should be at least 0.5 times the wall thickness. The outside corner radius should be the inside radius plus the wall thickness (or at least 1.5 times the wall thickness).
Guideline: Ribs & Gussets
Why? Instead of making a wall thick and heavy to achieve stiffness, you can use thin, perpendicular supports called ribs. This provides excellent structural integrity while keeping the wall thickness uniform, saving material, reducing weight, and minimizing cooling time. Gussets are small triangular ribs that reinforce features like bosses or walls.
Recommendation: To avoid sink marks on the opposite surface, the thickness of a rib should be between 40-60% of the wall it’s attached to. The height should be no more than 3 times the wall thickness.
Guideline: Coring Out
Why? Solid, thick sections of plastic are inefficient. They use excess material, take a very long time to cool (increasing cycle time and cost), and are prone to defects like sink marks and voids (internal air pockets). Coring out these sections to create a hollow part with uniform walls solves all these problems.
Recommendation: Analyze your part for any section that is significantly thicker than the nominal wall. Design it to be hollow, using ribs for support if necessary, to maintain a consistent thickness throughout.
Material Selection
Choosing the right plastic is critical to your part’s performance. Each material offers a unique profile of strength, flexibility, temperature resistance, and cost. Use the chart below to compare the properties of common injection molding plastics. Select a property to see how they stack up.
The Molding Process
Injection molding is a cyclical process where plastic is melted, injected, cooled, and ejected. Understanding these steps is key to optimizing production and troubleshooting issues. Click each step below to learn more about what happens at each stage.
Clamping
The two halves of the mold are securely closed by the clamping unit of the molding machine. This unit must exert enough force to keep the mold shut against the high pressure of the injection phase to prevent material from leaking out (flash).
Injection
Plastic pellets are melted in the machine’s barrel and injected under high pressure into the closed mold. The “shot” of molten plastic fills the cavities of the mold, taking on its shape. The speed and pressure are carefully controlled to ensure a complete fill without defects.
Dwelling (Packing)
After the mold is filled, pressure is maintained for a short period. This “dwelling” or “packing” phase forces more material into the mold to compensate for shrinkage as the plastic begins to cool, ensuring the part is dense and accurately shaped.
Cooling
The molten plastic inside the mold begins to cool and solidify as it makes contact with the mold surfaces. This is often the longest part of the cycle and is critical for the part’s final dimensions and stability. The mold has internal channels for water or oil to regulate this cooling process.
Mold Opening
Once the part has cooled sufficiently and is solid, the clamping unit opens the two halves of the mold, exposing the finished part.
Ejection
Ejector pins or plates push the solidified part out of the mold cavity. The machine is now ready to begin the next cycle by closing the mold again.
Defect Troubleshooting Guide
Even with perfect design, issues can arise during molding. This gallery helps you identify common defects and understand their causes and solutions. Click on a defect to see how to prevent it.
Warping
Sink Marks
Flow Lines
Short Shot
Flash
Burn Marks
Weld Lines
Select a defect above to view details.
Warping
Description: Unintended twisting or bending of the part as it cools.
Prevention: The primary cause is non-uniform cooling. Ensure your design has a uniform wall thickness. During molding, increase the cooling time to allow stresses to relax, and ensure the mold’s cooling channels are working effectively to cool the part evenly.
Sink Marks
Description: Small depressions or dimples on the surface, typically found over thick sections like ribs or bosses.
Prevention: This happens when the inside of a thick section cools and shrinks after the surface has already solidified. The best design solution is to core out thick sections and use thinner ribs for support. In processing, increasing the holding pressure and time can help pack out the area before it solidifies.
Flow Lines
Description: Wavy or streaky patterns on the part’s surface, indicating the path of the molten plastic.
Prevention: These are caused by variations in cooling speed as the plastic flows through the mold. Increasing the injection speed, pressure, and melt temperature can help the plastic fill the mold more uniformly before it starts to cool. Relocating the gate to a thicker section can also improve flow.
Short Shot
Description: An incomplete part, where the molten plastic failed to fill the entire mold cavity.
Prevention: This is a flow problem. Ensure the material is hot and fluid enough by increasing the melt temperature. Increase injection speed and pressure to push the material to the farthest reaches of the mold. Check that gates and runners are not too small or blocked.
Flash
Description: A thin layer of excess plastic that squeezes out of the mold at the parting line or ejector pin locations.
Prevention: This occurs when the mold can’t stay fully closed against the injection pressure. Ensure the machine’s clamping force is sufficient. Also, inspect the mold for wear and tear on the parting line surfaces that could create gaps.
Burn Marks
Description: Black or brown discoloration, usually at the end of the flow path, caused by trapped, superheated air.
Prevention: The trapped air gets compressed so rapidly that it ignites, burning the plastic. The solution is to improve venting in the mold to allow this air to escape. Reducing the injection speed can also give the air more time to get out before pressures get too high.
Weld Lines
Description: A visible line where two or more fronts of molten plastic have met and failed to fuse together perfectly.
Prevention: This creates a cosmetic flaw and a structural weak point. To improve fusion, increase the melt temperature and injection speed so the flow fronts are hotter when they meet. Relocating the gate can also change the flow pattern to move the weld line to a less critical or less visible area.