Introduction: The Importance of Intelligent Design for Injection Molding Success
Gaining the knowledge of creating a part for injection molding means you must learn the so-called Design for Manufacturability (DFM) rules. This is a set of rules that enables you to create durable parts that are cheaper and easily produced in mass quantities. Successful project design begins with a solid plan and very good design.
A part with a proper design allows the plastic to flow in a uniform manner. The part cools symmetrically and comes out of the mold easily. Consequently, you get excellent parts, fast production, and less final cost.
Poor design is the main culprit. It can cause a lot of expensive mold rework, long waiting times, and a blown budget. The typical part defects such as warping, sink marks, and weak points involve design defects. However, not just engineers should be aware of them. Anyone who is involved in the actions to bring a physical product to market should know their application.
This guide will show you step by step the rules that you must observe. We will deal with the fundamental principles of design, common pitfalls to steer clear of, and a functional workflow. You will become proficient in transforming your design from a CAD model to an actual product.
The 10 Rules of Design for Injection Molding
Adhering to these ten guidelines is a major step in the learning curve of injection molding design. You can view this as a checklist to go through before you send parts for manufacturing.
1. Uniform Wall Thickness
The first rule is to maintain an even wall thickness for your parts. When the molten plastic cools, it shrinks. If there is an area that is too thick in comparison to the rest, the thick area will cool more slowly and, as a result, it will own more shrinkage. This can lead to defects like sink marks (tiny dips) and warping (twisting).
The general value for most plastics is the wall thickness which should be in the range of 1.2mm to 3mm. It is possible that too thin walls will not be filled completely, while too thick walls will not only waste material but also increase the cycle time.
| Matériau | Recommended Wall Thickness (mm) |
|---|---|
| ABS | 1.2 – 3.5 mm |
| Polycarbonate (PC) | 1.5 – 3.5 mm |
| Nylon (PA) | 1.0 – 3.0 mm |
| Polypropylène (PP) | 0.8 – 3.8 mm |
| Acétal (POM) | 1.0 – 3.0 mm |
If it’s impossible to avoid thickness variations, then make the passage smooth. A smooth transition of the plastic flow should be achieved by using chamfers or fillets.
2. Use Draft Angles
A draft angle is a small taper applied to the walls of the part. These walls are in parallel with the direction of pull of the mold. The draft is an essential angle because it gives the part the ability to be stripped from the mold easily without damage.
If there is no draft, the part surfaces will rub against the mold, which causes the appearance of scrape marks on the surface. As a result, in extreme cases, the part could be entirely stuck. A common draft angle of 1 to 2 degrees is generally adequate for most parts.
In cases where your part has a textured surface, you will have to use more draft. A good guideline is to add an additional 1 degree of draft for every 0.025mm (0.001 in) of texture depth. This key will ensure that the texture remains intact during the ejection process.
3. Not All Corners Should be Sharp
Plastic parts with sharp corners are more likely to be exposed to high-stress concentrations. Such corners can make the part weak and even in some cases lead to possible cracks under load. They can also be an obstruction for the plastic getting through to fill the mold cavity.
To solve this, you should add rounded corners, or radii, to all inside and outside edges. A useful suggestion is to make the inner radius at least half the thickness of the wall. In turn, the outer radius should be formed by the inner radius plus the wall thickness. This ensures that the corners will not affect the wall thickness consistency.
4. Clever Ribs & Gussets Design
To achieve the desired rigidity in a part, do not only refer to thickening it. Besides increasing cycle time, which in turn costs more money, thickening walls can also create sink marks. Instead, use ribs and gussets to add strength and stiffness efficiently.
Ribs are thin wall-like structures which are designed just for that. In order to avoid sink marks on the part’s opposite side, the rib’s thickness should be in the range of 40-60% of that of the main wall. The height of the rib has to be less than three times the thickness of the wall. This condition will improve the filling process of the rib and will inhibit it from causing a cooling problem.
5. Correctly Design the Bosses
Bosses are round protrusions having the function of mechanical fastening points for screws or special elements. Their presence is frequent, but they need to be thought out to eliminate defects. The difference from the typical situation is that a single boss with a thick wall will create that prominent sink mark.
In this case, the solution is to connect bosses with the main wall or to rely on ribs or gussets for support. As seen in the comprehensive guides such as Injection molding: The manufacturing & design guide, the outer diameter of a boss should be about twice the diameter of the screw or fastener it will hold. This design achieves the needed stability without excessive thickness.
6. Exclude Undercuts or Plan for Them
An undercut is any feature that prevents the part from being directly ejected from the mold. Simple examples include side holes, clips, or snap-fit latches. These features get “stuck” behind the steel of the mold.
Undercuts are a major cost driver because they require complex mold mechanisms. These mechanisms, called side-actions or lifters, physically move parts of the mold to release the undercut feature. This adds significant cost and complexity to the tool. A good approach for how to design for injection molding is to avoid undercuts whenever possible by changing the design.
7. Consider Gate Locations and Ejector Pins
The gate is the part where molten plastic enters the mold. Ejector pins are the things that push the finished part out. They will leave small marks in the part. Usually, it is the mold maker who chooses the exact placement, but you as the designer should also help.
Pinpoint the “cosmetic” surfaces of your part. These are the parts where the marks should not occur. By communicating these upfront, you assist the tool designer to place gates and pins on hidden or non-critical surfaces.
8. Make the Right Material Selection
The type of plastic material you choose is decisive for the strength of the part and the final dimensions. Different plastics cool at different formulars, and the molds have to be prepared for a specific shrinkage. The first step of material selection is very crucial because of this fact as it is illustrated in several Design Tips for Injection Molding.
The selection of material also determines the stiffness, heat or chemical resistance, and flexibility the part will have. Consult your manufacturing partner to find the best material that meets your part’s requirements. A variety of products are available from professional moulage par injection de plastique service providers.
9. Provide Realistic Tolerances
Tolerances are sets of limits specifying how much manufacturing processes and engineering can deviate from a specified design. Realistic targets of getting tight tolerances are expensive and difficult to accomplish. They involve having a more precise mold, tighter process control, and inspecting more frequently.
If you want to keep the costs in check, you have to specify the tight tolerances only on the critical features. These include areas that mate with other components. On the non-critical dimensions, you can set them looser, standard machining tolerances. The final result is the product that is cheaper and easier to make.
10. Specify Surface Finish
The surface finish is responsible for how your part will look and feel after production. Finishes can go from a very shiny, glossy surface highly polished to a rough, matte texture. The finish is directly applied to the mold surface and consequently, it is transferred into every part.
More polished and smoother surfaces are more costly since they require more work to prepare the mold. Textured surfaces can help to hide some minor imperfections but require bigger draft angles for clean part ejection. You have to be honest with your aesthetic and functional needs from the beginning.
Avoiding the Pitfalls: 4 Costly Design Mistakes and How to Fix Them
In our long years of manufacturing, we can highlight only a few common design oversights. Hardly these issues fail to cause a delay and increase a cost. Knowing how to design for injection molding also encompasses knowing what should be avoided. Here is the way you can avoid these apparent mistakes.
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Mistake #1: The “Zero-Draft” Presumption.
- Problem: A designer creates a part with perfectly vertical walls, believing it is correct.
- Consequence: The part scratches the mold during ejection by drag marks. In some situations, it can get stuck, and then add a costly lifter mechanism to the mold.
- Solution: It is necessary to add at least 1 degree of draft at all times to any walls that are parallel to the mold’s opening direction, without exception. 0.5 degrees is even better than nothing.
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Mistake #2: The “Thick is Strong” Belief.
- Problem: A designer attempts to improve the strength of a component by making a wall or feature much thicker than the rest of the part.
- Consequence: It leads to lousy sink marks severely on the surface which were there at the beginning. The entire time this wastes plastic material and increases cycle time.
- Solution: The correct design of ribs and gussets. They add strength and stiffness efficiently without the downsides of thick sections.
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Mistake #3: The Undesired Undercut.
- Problem: A snap-fit hook or clip is designed in such a way that it becomes trapped in a simple, two-part mold.
- Consequence: This forces the need for a side-action, a complex mechanism that can increase the mold’s cost by 15-30%. It also adds another potential point of failure.
- Solution: Redesign the feature. Can the hook be moved to an outer edge? Can you use a pass-through core or a “shutoff” to create the feature without trapping it? Always look for a simpler alternative first.
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Mistake #4: Material Shrinkage Ignored.
- Problem: A CAD model that is being designed for exact dimensions will not take into account that the plastic shrinks when it cools.
- Consequence: The final part comes from the mold smaller and therefore out of tolerance.
- Solution: This is a vital factor. Always check the material datasheet for the specific shrinkage rate. Your manufacturing partner will apply this rate for molding the cavity up. Consequently, the part will be reduced to the desired dimensions.
From CAD to Production: A Practical Workflow for DFM
The utilization of these principles is manageable if you adhere to a structured process. A practical workflow is here for your guide, which shows how your initial designing gets you to a manufacturable part.
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Conceptual Design & Self-Audit.
Create your initial CAD model in 3D. Before going any further, you should check with the 10-point checklist in this guide and do a self-review for your design. Check for the uniform walls, draft angles, and radii. This preliminary investigation can catch a lot of basic mistakes. -
Virtual Prototyping & Mold Flow Analysis.
Prior to cutting any steel, simulate the injection process. Mold flow analysis is a software simulation that shows how plastic will fill your part’s geometry. It can predict potential problems like weld lines, air traps, or areas that won’t fill completely. This way you can deal with issues before they become costly physical problems. -
Physical Prototyping.
A prototype is a must to test the fit, form, and function of your design. A prototype that is close to the final parts in terms of material properties can be achieved with high-precision China CNC machining services. These services can be used to create a model from a production-grade plastic. Thus, you’ll gain confidence in your design before making an investment in tooling. -
Engage Your Manufacturer for DFM Review.
This is the key step. Share your refined design with your manufacturing partner. Their engineers will review your part from the perspective of their specific equipment and experience. They provide crucial feedback for successful fabrication de moules par injection. This collaborative DFM process, as highlighted in many expert guides like this Injection Molding Design Guide, is the bridge between a good design and a great product.
Your Partner in Manufacturing
Learning how to design for injection molding is a mix of following established rules and working closely with experienced professionals. You do not have to be an expert in every aspect of manufacturing. The best results come from a partnership built on clear communication and shared expertise.
A successful project depends on finding a partner who can guide you through the process. This includes design feedback to final production. Choosing a partner with a proven track record, like Mékalite, ensures your well-designed part is manufactured to your exact specifications, on time and on budget.
Frequently Asked Questions (FAQ) about Injection Molding Design
1. What is the single most important rule in design for injection molding?
While all rules are important, maintaining a uniform wall thickness is arguably the most critical. It prevents a cascade of common defects like sink, warp, and voids. This rule has the biggest impact on overall part quality and the efficiency of the manufacturing cycle.
2. How much draft angle is really enough?
A general rule is 1 to 2 degrees of draft for most parts with a smooth finish. However, for parts with a textured surface, you should add an extra 1 to 1.5 degrees for every 0.025mm of texture depth. This extra angle is needed to ensure a clean release without damaging the textured finish.
3. Can you injection mold parts with zero draft (vertical walls)?
It is highly discouraged. A zero-draft wall will almost always cause drag marks or get stuck in the mold. If a vertical wall is absolutely required for your design, it can be done. But it requires a lifter or a side-action in the mold. These mechanisms significantly increase tooling cost, complexity, and maintenance.
4. What is the difference between the “core” and “cavity” of a mold?
The core and cavity are the two halves of the mold that form the shape of your part. The cavity is the hollow, female side that typically forms the outer, cosmetic surface of the part (the “A-side”). The core is the convex, male side that forms the inner, functional features like ribs and bosses (the “B-side”). The part shrinks onto the core and is pushed off by ejector pins.
5. How does my material choice affect my part design?
Material choice is fundamental to how to design for injection molding. Every plastic has a unique shrink rate, and the mold must be built larger to compensate for it. Materials also have different strengths, flow properties, and temperature limits. All of these influence design rules like recommended wall thickness and how features like snap-fits should be designed.
