Precision plastic prototyping ensures success when transitioning to full-scale injection molding production. Even small oversights or mistakes in the design phase can lead to significant issues down the line, from part defects and quality problems to production delays and increased costs.
By understanding common pitfalls and missteps early in the design process, product developers can save immense amounts of time, money, and headaches. Careful consideration of material selection, manufacturability principles, tolerances and fits, ejection requirements, gate placement strategies, and robust prototype testing and iteration is fundamental for success.
Avoiding commonly overlooked details in plastic part design allows companies to get to market faster with a higher-quality and more cost-effective product. Taking the time upfront to design for manufacturability and test plastic prototypes properly sets up projects for efficient, smooth-running production rather than nasty surprises after production tooling is manufactured. By embracing an iterative approach and learning critical design principles, engineers can feel confident that their innovative plastic part designs will transition successfully from prototype to full-scale manufacturing.
Understanding Material Properties
The foundation of any injection molding project is material selection. Material properties directly impact the functionality, dimensional accuracy, and cosmetic appearance of molded plastic parts.
Key factors, such as how a material shrinks as it cools or how it flows under high pressure, must be considered to avoid problems such as warping, short shots, and deviation from critical dimensions. Failing to account for the specific material shrink rate or viscosity, or relying on ballpark guesses or outdated data from previous projects can compromise part quality down the line.
To avoid such issues, engineers should:
- Conduct thorough research when first evaluating candidate resins for an application.
- Collaborate early with material suppliers to select optimized grades and formulations.
- Engage in extensive material testing under expected processing conditions prior to production mold fabrication.
This approach helps engineers predict and address possible defects related to the characteristics of a specific resin.
Designing for Manufacturability
Molded parts should be designed with manufacturability in mind when fabricating prototypes. By overlooking fundamental Design for Manufacturability (DFM) principles, engineers risk seriously complicating mold fabrication and injection molding processes when transferring a design to production.
Common mistakes include specifying inadequate or inconsistent wall thicknesses and incorporating over-complex geometries. Thin walls often warp, and thick walls develop visible sink marks as material flows unevenly into the mold. Anything overly complex, like small features or features that can’t be created with the pull axis of the mold, can cause issues with part filling, increase mold machining costs, and create issues with multi-cavity scalability.
Following DFM guidelines to maintain uniform wall thickness and minimum radii throughout the plastic part design enhances moldability. Strategically simplifying geometry where possible also reduces machining costs for mold components. While engineers should ensure any simplifications do not sacrifice overall part functionality, simpler parts tend to be more robust, less costly, and increase tool life.
Considering Tolerances and Fits
Specifying appropriate tolerances on injection molded plastic parts is necessary for ensuring components assemble and function properly. Excessively loose or tight tolerances lead to alignment and fit issues; ignoring tolerance stack up potentially compromises overall product function and quality.
Engineers often neglect to conduct comprehensive tolerance analysis on their designs, overlooking potential stack-up issues across mating parts. Typical molders will agree to +/- 0.005” on molded part dimensions, which, if stacked up in an assembly, can be too much slop. However, in an attempt to improve upon this fit issue, a common mistake is assigning unnecessarily stringent tolerances without regard for manufacturing capabilities. This dramatically increases machining costs for mold components and molded part costs.
The ideal approach balances tight tolerances on critical dimensions with realistic manufacturing limits. Using proven statistical tolerance analysis methods can prevent nasty surprises once prototypes are tested. This strategy incrementally verifies acceptable variance levels across all part features as individual components are made. A better approach may be to actually injection mold prototype parts to fully understand how the part will shrink based on geometry and material choice. Once molded, the dimensions are very stable and can be counted on with very little variation, reducing uncertainty in tolerance analyses.
Draft Angles and Ejection Considerations
Proper draft angles and ejection design are imperative to smoothly eject finished plastic parts from injection molds. Insufficient draft or poorly placed ejector pins lead to parts sticking in cavities, distorted features, and surface defects.
A common mistake is neglecting draft requirements based on part geometry, usually to maintain aesthetic appearance. Without angling vertical faces to assist demolding, plastic will not cleanly release from the tooling and can frequently cause unintended part damage.
To avoid these problems, engineers should:
- Incorporate draft angles between 1-3 degrees on all vertical walls, depending on factors such as height, material selection and surface finish needs. While greater draft eases ejection, it may also impact certain cosmetic details or part function.
- Strategically plan even ejection locations to prevent delicate areas—such as side walls or tabs—from cracking or becoming distorted.
Gate Placement and Flow Analysis
Gate placement greatly influences how evenly and efficiently plastic fills an injection mold. Poorly located gates lead to visible flow marks, air traps, and uneven plastic density as material packs out the cavity. Failing to simulate flow with analysis software early in design also heightens the risk of defects.
Engineers often position gates based solely on convenience or symmetry without considering internal flow dynamics. This causes the material to preferentially pack in certain regions while leaving others starved. This uneven packing can warp and distort the part as it cools. Flow simulation can predict fill imbalances and correct them digitally before molds are fabricated.
Strategic gate placement should factor in part geometry, material viscosity, desired cosmetic properties, and cavity dynamics. Locating gates to encourage laminar flow best prevents voids and visible knit/flow lines. Complementary mold filling simulations further validate gate locations before committing to steel tooling. Digitally troubleshooting any undesirable fill patterns can help prevent physical prototyping missteps.
Testing and Iteration
Prototyping and testing are invaluable for confirming plastic parts meet all functional, quality, and manufacturing requirements before high-volume production. Skipping this verification process invites expensive, time-consuming production downtime and mold reworks.
Due to timeline restraints, product developers often forgo prototyping altogether or fail to implement thorough testing protocols. This risks overlooking critical flaws in part geometry, material performance, or moldability. Even with prototypes, rapidly progressing to production without iterative refinement and validation leaves latent issues unaddressed.
To avoid such pitfalls, a rigorous prototyping phase should test proposed designs under real-world conditions. As data reveals room for improvement, revisions to initial concepts should be embraced rather than rushing to market. This iterative approach maximizes learnings from physical, production-equivalent parts before committing to full production.
Take the Next Step with Protoshop
If you are motivated to mitigate potential issues with your current or upcoming project, Protoshop is here to help. As an industry leader in rapid prototype injection molding with over 25 years of experience, Protoshop offers superior part quality, personalized service, and rapid turnaround times.
Don’t leave your prototype project up to chance. Partner with the prototype molding experts at Protoshop to ensure high quality, functional plastic prototype parts that accurately replicate end-use production performance. The time savings and confidence boost our services provide are invaluable during the product development process.
