Improper Wall Thickness
Proper wall thickness is critical for the successful molding of plastic parts. Uneven wall thickness can lead to several issues during the injection molding process and defects in the final parts:
Uneven wall thickness prevents uniform cooling and material solidification. Thicker areas will cool and solidify more slowly than thinner areas, leading to internal stresses in the part.
Non-uniform wall thickness disrupts the even flow of molten plastic into the mold. Thicker areas require more material flow, which can cause thin areas to be underfilled.
Variations in wall thickness are a major cause of part defects like sinks, voids, and poor surface finish. Thicker areas will shrink more as they cool, pulling material away from adjacent thinner sections.
Non-uniform wall thickness also reduces the structural integrity of the part. Stress concentrates in areas with sudden transitions from thick to thin.
To avoid these issues, wall thickness should be optimized for uniform cooling and material flow. Gradual transitions between thicker and thinner sections are ideal. Analysis tools can help identify problem areas in the design. Communication with your injection molder is critical to determine the appropriate wall thickness for a given part design and material.
Gradual Transitions Between Thick and Thin Areas
When transitioning between thick and thin wall sections in a part design, it is important to do so gradually rather than abruptly. Sharply transitioning from a thick wall to a thin one can create stress concentrations in that area. The sudden change in thickness impedes the flow of material as it fills the mold. This can lead to incomplete filling and voids or sinks in the finished part.
To avoid these issues, designers should aim to transition between thick and thin sections with tapers or radii. The more gradual the change in wall thickness, the better the material flow will be through the transition. Using generous fillets or chamfers allows the material a smooth path between the different wall thicknesses. This eliminates turbulence in the melt flow, removes stress points, and promotes a fully filled part.
It is best practice to make thickness transitions using large radii or angles of at least 30 degrees. The exact values depend on the difference in wall thicknesses and the geometry involved. Coordinate with your injection molder to determine the proper transition designs for a given molding. Gradual transitions lead to a higher quality, more robust finished part.
Incorrect CAD File Formats
Choosing the right CAD file format is critical for clear communication and efficient mold design. Here are some key considerations when exporting your 3D model:
Use a format compatible with your molder's software. Most mold design software can import common formats like STEP and IGES. But if your molder uses more specialized software, check which formats they accept. Converting files can cause loss of data.
Consider the file size and level of detail. File formats like STEP preserve all CAD data, resulting in huge file sizes. For basic mold design, compressed formats like IGES or Parasolid may suffice. Avoid proprietary formats like SolidWorks or Inventor files.
Make sure all dimensions and tolerances are defined. The CAD file should fully specify all dimensions, geometries, surface finishes, and any other requirements. Don't assume the molder will infer design intent from an incomplete model.
Include product requirements and specifications. Supplement the CAD file with a product design specification sheet detailing material choices, product performance requirements, quality standards, etc.
Taking the time to export your CAD model in the optimal format will prevent headaches, errors, and delays for your molder. It shows respect for their process and will lead to a smoother manufacturing experience overall.
Unreasonable Fillet Radii
The fillet radii selected in your mechanical design can have a significant impact on part strength, appearance, and manufacturability. Choosing unreasonable fillet radii is a common mistake that should be avoided.
Larger fillet radii may seem beneficial for improving strength and aesthetics. However, excessively large radii can negatively affect the molten plastic flow during the injection molding process. Sharp corners allow for easier filling of the mold. Overly large radii restrict material flow, which can lead to short shots and unfilled sections.
On the other hand, fillet radii that are too small can concentrate stresses at sharp corners. This decreases the strength and durability of the part. Tiny radii may also make demolding more difficult and increase the chances of damage.
The ideal fillet radius depends on the specific wall thicknesses and geometries involved. Work closely with your injection molding partner to determine the appropriate radii for your application. Ask them to provide recommendations based on their experience producing similar parts.
Aim to use larger radii on the exterior visible surfaces and smaller radii internally. This maintains strength while enhancing appearance. Strategically placing large and small radii can prevent sink marks and other defects.
Fillet radii selection requires finding the right balance between strength, moldability, and aesthetics. Blindly applying generic radii without considering the design is a recipe for failure. Collaborate with your molder to define reasonable fillet radii tailored to your specific mechanical design.
Eliminate Undercuts if Possible
Undercuts in a molded part require side actions in the tooling which greatly increase costs compared to a straight pull mold. Undercuts make the tooling more complex, require additional components like lifters or sliders, and lengthen the mold manufacturing time.
Trying to fill an undercut area can also be very difficult for the material flow, often necessitating extra injection gates which further add to tooling costs. The material freezing off before completely filling the undercut can result in voids and other defects.
Undercuts reduce the strength of the final molded part as well. By creating a recess or indentation on the part, you are thinning the geometry and removing supportive material in that area. The part will be more prone to breaking or cracking at that undercut location.
For all these reasons, undercuts should be avoided in the design stage whenever possible. Instead, consider an alternative geometry that does not require undercuts yet still meets the functional requirements.
If some amount of undercut is absolutely necessary, work closely with your injection molder to minimize the severity of the undercut which will help control costs and ensure moldability. Carefully analyze the expected stresses on the part to make sure the undercut does not create a weak point. With good communication early in the design process, undercuts can sometimes be designed in a more budget-friendly manner.
Unnecessary Cosmetic Finishes
Cosmetic finishes like polishing, texturing, or other surface treatments add considerable cost to the molding process. These special finishes require extensive polishing and texturing of the mold itself to impart the desired appearance onto the plastic parts.
However, cosmetic finishes should only be specified on surfaces where appearance is critical. For example, a special brushed finish may be warranted on a plastic part that will be visible to the end user. But for internal plastic components or surfaces that will be hidden from view, a cosmetic finish adds unnecessary cost with no benefit.
Carefully consider each surface and determine if a cosmetic finish is absolutely required from a functional standpoint. In some cases, post-molding finishing like painting, pad printing, laser etching, or other secondary operations can achieve the desired appearance at a lower cost. Work closely with your molder and manufacturer to explore if any alternative finishing options exist.
In general, cosmetic mold finishes should be used sparingly and only where the appearance is truly critical. Eliminating unnecessary cosmetic specifications can reduce mold fabrication time and cost.
Inaccurate Dimensions
One of the most common yet avoidable mechanical design mistakes is having incorrect or inaccurate dimensions. It's crucial to double and triple-check that all dimensions in the drawings match the actual requirements and specifications of the customer. Having dimensions that are even slightly off can lead to parts being manufactured wrongly.
Here are some tips when adding dimensions to mechanical drawings:
Have multiple team members review the drawings to catch any erroneous dimensions. A second or third set of eyes can often catch subtle issues.
Trace every dimension back to the original customer specs to ensure accuracy. Don't rely on previous designs or assumptions.
Use your CAD software's built-in tools for model checking and collision detection. Look for any interferences based on the dimensions.
Add tolerance bands to the dimensions where possible. This provides some leeway in case a dimension is not 100% precise.
For critical features, add reference dimensions from multiple datums or surfaces to confirm accuracy.
Review drawings on a large monitor or print them out to detect small discrepancies.
Confirm that any changes made to the 3D model match up to the 2D drawings.
With so many complex components, it's key to take your time and carefully examine every dimension before sending drawings out for manufacturing. Investing effort upfront to eliminate inaccurate dimensions will prevent expensive errors down the line.
Failing to Design for Manufacturing
Designing a part without considering how it will be manufactured can lead to serious issues. It's crucial to optimize your mechanical design for both molding and assembly.
When designing parts for injection molding, you need to consider things like:
How the part will be gated and how the material will flow into the mold
The location and geometry of drafts to allow for part ejection
Avoiding sinks, voids, and other defects by designing uniform wall thicknesses
Eliminating undercuts that require complex side-actions and increase costs
Specifying reasonable fillet radii based on wall thickness
Minimizing parting lines on cosmetic surfaces
Failing to design with manufacturability in mind can result in parts that are difficult or impossible to mold without defects. This leads to increased scrap and costs.
It's also critical to design parts for efficient and error-proof assembly. Considerations include:
Minimizing the total number of parts
Designing self-locating features like tabs and chamfers
Avoiding hidden fasteners or adhesives
Eliminating operations like riveting, staking, or welding
Using common fasteners and standard sizes
Optimizing the mechanical design for assembly allows you to reduce labor costs and improve quality. Get input early from your manufacturing and assembly teams to avoid issues. Be willing to modify the design as needed to improve moldability and assembleability.
Over-Tolerancing
Specifying overly tight tolerances can significantly increase manufacturing costs and lead times without providing value. As a general rule, you only want to tolerance critical dimensions that are required for proper function and fit. Dimensions that do not impact performance should be left loose or not toleranced at all.
Work closely with your manufacturing partners to determine what tolerances are realistically achievable for each feature. In injection molding and other processes, looser tolerances lead to faster cycle times, lower scrap rates, and reduced tooling costs. The mold can be made and run more easily without holding super tight tolerances everywhere.
Be sure to communicate upfront which dimensions actually require precision versus which ones can vary. Focus your tolerancing efforts strategically on true engineering requirements. Eliminate arbitrary tolerances that sound good in theory but don’t contribute to performance. You’ll end up with a more robust design that can be manufactured faster and at lower cost.
Some additional tips when assigning tolerances:
Avoid tolerancing non-critical aesthetic features
Consider larger tolerances on secondary dimensions
Specify bilateral instead of unilateral tolerances when possible
Increase tolerances for low-volume or prototype runs
Loosen tolerances if they require expensive tooling or setups
Review tolerances to identify any that can be relaxed
By taking a critical look at your tolerance scheme, you can often refine it to reduce costs while still retaining full functionality. The result is a design that meets performance needs but is easier and more economical to produce.
Forgetting Safety Components
Mechanical equipment can pose serious hazards to operators if not properly safeguarded. It's critical to include all necessary safety components in your designs to prevent injuries.
Guards and shields should protect operators from pinch points, rotating parts, ejected debris, extreme temperatures, and other hazards. Make sure guards allow for proper maintenance access but prevent access during operation. Consider both fixed and interlocked guards where needed.
Sensors such as proximity sensors, light curtains, and e-stops will disable machinery when a human enters a hazardous area. Determine sensor spacing and placement to maximize detection.
Other safety components to consider include:
Lockout/tagout devices for power isolation
Warning labels for hazardous areas
Status indicators when machines are operational
Emergency stop buttons within easy reach
Audible and visual alarms to signal hazards
Antiskid surfaces in hazardous foot traffic areas
Work closely with your safety manager and machine operators to identify all necessary safeguards. An experienced system integrator can also help implement and validate safety features. Investing in adequate safeguards during design will prevent injuries and regulatory violations.
Comments