Creating a plastic injection molding prototype is a critical step in product development—it lets you test design feasibility, validate performance, and avoid costly mistakes in mass production. Unlike mass-produced parts, prototypes prioritize speed, cost-efficiency, and adaptability, while still following core injection molding principles. Below is a complete, actionable breakdown of the prototype development process, from material pick to final application.
1. Material Selection for Prototypes
Choosing the right material is the first (and often make-or-break) step for prototypes. The goal is to balance material properties with prototype goals—whether you’re testing durability, appearance, or cost. Here’s how to navigate key choices:
| Material Category | Key Examples | Key Considerations for Prototypes |
| Thermoplastics | PP, ABS, PC, Nylon, PET | Most common for prototypes—melt and re-solidify, easy to adjust. Ideal for testing form, fit, and basic function. |
| Thermosets | Epoxy, Phenolic resins | Harden permanently after molding—good for high-heat or chemical-resistance tests. Less common for prototypes (hard to modify). |
| Additives | Fillers (glass fiber), UV stabilizers, flame retardants | Add only if the prototype needs to mimic final part performance (e.g., glass fiber for stiffness). Skip non-essential additives to cut costs. |
| Colorants | Liquid dyes, masterbatches | Use only if appearance testing is critical. Clear or natural resins save time and money for functional prototypes. |
Pro Tip: Prioritize cost-effectiveness for early-stage prototypes—opt for common resins like PP or ABS instead of high-end materials like PEEK. For supplier selection, choose vendors who offer small batch sizes (1-5 kg) to avoid waste. Also, check density (affects part weight) and flow rate (ensures the resin fills small prototype mold cavities easily).
2. Design Considerations for Prototype Success
Prototype design should be “mold-friendly” to speed up production and reduce defects. Even small design tweaks can save days of rework. Here’s a checklist of critical factors:
Core Design Elements & Tips
- Part Design: Keep it simple—avoid overcomplicating with unnecessary features (e.g., intricate logos) in early prototypes. Focus on testing the part’s core function.
- Wall Thickness: Aim for 1-3 mm (uniform across the part). Thinner walls (<1 mm) cause short shots; thicker walls (>3 mm) lead to sink marks. Use gradual transitions if thickness must change.
- Draft Angles: Add 1-3 degrees to all vertical surfaces. This lets the prototype eject smoothly from the mold—no more stuck parts or scratches.
- Ribs & Bosses: Ribs (for stiffness) should be 0.5x the wall thickness; bosses (for screws) should have a diameter 2x the screw size. Add fillets (radius = 0.5 mm) to avoid stress cracks.
- Undercuts: Minimize them! Undercuts (e.g., side grooves) require complex mold slides, which increase prototype cost and lead time. If needed, use temporary solutions like post-machining.
- Tolerances: Loosen tolerances for early prototypes (±0.1 mm is enough for fit tests). Tight tolerances (<±0.05 mm) add cost and slow production.
Design Validation Tools
Before finalizing the design:
- Use CAD Modelling (e.g., SolidWorks, Fusion 360) to create 3D models—share these with your mold maker to avoid miscommunication.
- Run Mold Flow Simulation (e.g., Autodesk Moldflow) to test resin flow. This catches issues like air traps or uneven filling early.
- For high-stress parts (e.g., automotive brackets), use Finite Element Analysis (FEA) to test strength—this avoids building prototypes that fail under load.
3. Mold Preparation for Prototypes
Prototype molds (called “soft tools”) are simpler and cheaper than mass-production molds. They’re often made from aluminum (instead of steel) to speed up machining. Here’s the key process:
Mold Components & Preparation Steps
| Component | Purpose | Prototype-Specific Tips |
| Mold Base | Provides structure for the mold | Use standard-sized aluminum bases (e.g., 150×150 mm) to cut costs. |
| Cavities & Cores | Shape the prototype (cavity = outer surface; core = inner surface) | For single-cavity molds (most prototypes), machine cavities directly into the aluminum—faster than multi-cavity molds. |
| Ejector Pins | Push the prototype out of the mold | Use 2-4 small pins (3-5 mm diameter) — place them near thick areas to avoid warping. |
| Cooling Channels | Cool the mold to set the resin | Drill simple straight channels (instead of complex curved ones) — aluminum cools quickly, so basic channels work. |
| Heating Elements | Warm the mold (for resins with high melting points) | Skip unless using resins like PC (melting point >220°C). Aluminum retains heat well, so extra heating is rarely needed. |
Mold Making Process
- Mold Machining: Use CNC milling for simple shapes; use EDM (Electrical Discharge Machining) only for fine details (e.g., small holes). Aluminum machines 5x faster than steel—perfect for quick prototypes.
- Mold Polishing: Polish cavities to a #4 finish (matte) for functional prototypes. High-gloss finishes (#8) are only needed for appearance tests.
- Mold Assembly: Assemble components loosely first—test fit with a dummy resin (e.g., wax) to ensure alignment. Tighten screws only after test fitting.
- Mold Testing: Run 5-10 test shots with scrap resin. Check for leaks, misalignment, or stuck parts—fix issues before running the actual prototype batch.
4. The Injection Molding Process for Prototypes
Prototype injection molding focuses on speed and flexibility—you’ll often run small batches (10-50 parts) and adjust parameters on the fly. Here’s how to execute it smoothly:
Key Machine Settings (for ABS Prototype Example)
| Parameter | Optimal Range | Why It Matters for Prototypes |
| Clamping Force | 50-100 tons | Lower force works for small prototypes—avoids damaging the aluminum mold. |
| Injection Pressure | 60-90 MPa | Too high = flash (excess resin); too low = short shots. Start low and increase if needed. |
| Melt Temperature | 210-240°C | Keep 10-15°C lower than mass production—prevents resin degradation in small batches. |
| Cycle Time | 30-60 seconds | Longer than mass production (gives aluminum molds time to cool). Rushing leads to warped parts. |
| Screw Speed | 60-100 rpm | Slow speed mixes resin evenly without generating excess heat. |
| Drying Process | 80°C for 2-3 hours (for ABS) | Critical for resins like nylon or PC—moisture causes bubbles. Skip only for dry resins like PP. |
Step-by-Step Molding Workflow
- Material Feeding: Load 1-2 kg of resin into the hopper (small batches reduce waste). Add a few pellets of colorant if needed.
- Nozzle Design: Use a small-diameter nozzle (3-5 mm) to fill narrow prototype cavities. Keep the nozzle 1-2 mm from the mold to avoid leaks.
- Injection Speed: Start at 40-60 mm/s. If the part has thin walls, increase to 70-80 mm/s to avoid short shots.
- Packing Pressure: Apply 80-90% of injection pressure for 2-3 seconds. This fills any small gaps in the prototype.
- Cooling Time: Let the mold cool for 15-25 seconds (aluminum cools fast!). Eject the part only when it’s cool to the touch.
Common Issue Fix: If the prototype has flash (excess resin), reduce injection pressure by 5-10 MPa. If it has short shots, increase melt temperature by 5-10°C.
5. Post-Processing and Finishing for Prototypes
Post-processing turns raw molded parts into usable prototypes. Focus on tasks that support your test goals—skip unnecessary steps to save time.
Essential vs. Optional Post-Processing
| Task | Purpose | When to Use |
| Deburring/Deflashing | Remove excess resin from edges/parting lines | Always do this—sharp burrs ruin fit tests. Use a hand file (for small batches) or rotary brush. |
| Trimming | Cut off runner systems (the plastic channels that feed resin) | Always do this—runners make prototypes hard to test. Use scissors (for soft resins) or a bandsaw. |
| Machining (Drilling/Tapping) | Add holes or threads for assembly | Only if testing assembly (e.g., attaching the prototype to another part). Use a handheld drill for small holes. |
| Painting/Plating | Improve appearance | Only for appearance tests (e.g., showing the prototype to stakeholders). Use spray paint (dries in 30 minutes) for quick results. |
| Assembly | Join multiple prototype parts | Use ultrasonic welding (fast, no adhesives) or adhesive bonding (low-cost) for small batches. Avoid rivets (permanent, hard to modify). |
Pro Tip: For functional prototypes, skip painting/plating—focus on deburring and trimming. For appearance prototypes, use printing (e.g., pad printing) for logos instead of expensive plating.
6. Applications and Uses of Injection Molding Prototypes
Prototypes are used across industries to de-risk product development. Here’s how different sectors leverage them:
Industry-Specific Uses
- Automotive Parts: Test fit of interior components (e.g., dashboard clips) or durability of small parts (e.g., door handles).
- Consumer Electronics: Validate the size of phone cases or the fit of charging port covers.
- Medical Devices: Test the ergonomics of syringes or the compatibility of plastic parts with liquids.
- Packaging: Check if a bottle prototype holds liquid without leaking or if a lid seals properly.
- Toys: Test safety (e.g., no small parts that break off) and durability (e.g., withstands dropping).
- Aerospace Components: Test lightweight parts (e.g., plastic brackets) for strength under low pressure.
Prototype Stages in Product Development
- Concept Prototype: Early-stage, low-cost (e.g., ABS parts) to test basic form.
- Functional Prototype: Mid-stage, uses final material (e.g., PC) to test performance.
- Pre-Production Prototype: Late-stage, identical to mass-produced parts—used for final validation.
Yigu Technology’s View
At Yigu Technology, we know prototype success hinges on balancing speed, cost, and clarity of goals. For plastic injection molding prototypes, we prioritize aluminum molds (fast, cost-effective) and common thermoplastics for early stages, then shift to final materials for functional tests. We integrate CAD, Mold Flow, and FEA to catch issues before molding, cutting rework time by 30%. Our focus is on delivering prototypes that solve real problems—whether it’s testing a fit, validating a design, or impressing stakeholders.
FAQs
- Q: How long does it take to make a plastic injection molding prototype?
A: 1-2 weeks for simple prototypes (aluminum mold + ABS parts). Complex prototypes (with undercuts or FEA testing) take 3-4 weeks.
- Q: Can I use the same mold for prototype and mass production?
A: Rarely—prototype molds are aluminum (soft, wears out after 1,000+ shots), while mass-production molds are steel (hard, lasts 100,000+ shots). Use the prototype mold to refine the design, then make a steel mold for production.
- Q: How much does a plastic injection molding prototype cost?
A: \(500-\)2,000 for a simple prototype (aluminum mold + 10-50 parts). Costs rise to \(3,000-\)5,000 for complex designs (EDM machining, FEA testing, or final materials like PC).