Plastic injection molding is a widely used manufacturing process, but producing high-quality parts requires attention to every detail. From material selection to post-processing, each step has its own precautions that directly affect the final product’s performance, appearance, and cost. This article will break down the key precautions for each stage of plastic injection molded parts production to help you avoid common pitfalls and improve product quality.
1. Material Selection Precautions
Choosing the right material is the foundation of making excellent plastic injection molded parts. The material must match the part’s intended use, performance requirements, and cost budget. Here’s a detailed breakdown of the key factors to consider:
| Key Factor | Description | Why It Matters |
| Resin type | Different resins (e.g., PP, ABS, PC) have unique characteristics. PP is lightweight and chemical-resistant; ABS has good impact strength; PC is transparent and heat-resistant. | Using the wrong resin type can make the part unable to meet basic performance needs. For example, using PP for a part that needs high transparency will fail. |
| Thermal properties | Include melting point, heat deflection temperature, and thermal stability. | It determines if the material can withstand the injection molding process temperature and the part’s working environment temperature. Poor thermal stability leads to material degradation during molding. |
| Mechanical properties | Cover tensile strength, impact strength, hardness, and flexibility. | Relate to the part’s ability to resist external forces. A part used in a high-impact scenario needs good impact strength; otherwise, it will break easily. |
| Chemical resistance | The material’s ability to resist corrosion from chemicals like oils, solvents, and acids. | Critical for parts used in chemical-related fields. For instance, a fuel tank part must be resistant to fuel corrosion. |
| Moisture absorption | Some materials (e.g., nylon) absorb moisture easily. | Moisture in the material causes bubbles and defects in the molded part. So, materials with high moisture absorption need pre-drying. |
| Recycled content | The proportion of recycled plastic in the material. | Affects cost and environmental friendliness, but too much recycled content may reduce material performance. |
| Cost-effectiveness | Balance between material performance and cost. | High-performance materials are often expensive. For non-critical parts, choosing a cost-effective material can reduce production costs. |
2. Design Considerations
A reasonable part design not only simplifies the injection molding process but also improves the part’s quality and reduces defects. Here are the key design precautions:
- Wall thickness uniformity: Uneven wall thickness causes uneven cooling and shrinkage, leading to warpage and sink marks. Keep the wall thickness as uniform as possible. If thickness variation is necessary, use gradual transitions (e.g., tapers) instead of sudden changes.
- Draft angles: Add draft angles (usually 1-5 degrees) to the part’s vertical surfaces. This helps the part eject smoothly from the mold and prevents scratches on the part surface. Without draft angles, the part may get stuck in the mold.
- Rib design: Ribs are used to enhance the part’s stiffness without increasing wall thickness. The height of the rib should not exceed 3 times the wall thickness, and the width should be 0.5-0.7 times the wall thickness. Also, add fillets at the rib’s root to avoid stress concentration.
- Bosses and supports: Bosses are used for assembly (e.g., screw holes). The diameter of the boss should be 2-3 times the diameter of the screw, and the height should not exceed 2 times the diameter. Add supporting ribs around the boss to prevent it from breaking.
- Undercuts and side actions: Undercuts (e.g., grooves on the side of the part) make the part hard to eject. If undercuts are necessary, use side actions or slides in the mold. But this increases mold complexity and cost, so minimize undercuts when possible.
- Parting lines: The parting line is where the two halves of the mold meet. Design the parting line to be easy to process and remove flash. Avoid placing the parting line on the visible surface of the part to improve appearance.
- Gate and runner design: The gate is the channel through which the molten plastic enters the mold cavity. Choose the right gate type (e.g., edge gate, sprue gate) and size. The runner should be designed to ensure uniform flow of molten plastic to each cavity. Shorten the runner length to reduce material waste.
3. Mold Maintenance and Inspection
The mold is the core tool of injection molding, and its condition directly affects the part’s quality and production efficiency. Regular maintenance and inspection are essential:
Routine Maintenance Tasks
- Mold cleaning: Clean the mold cavity, runner, and vent every 8-12 hours of production. Use a soft cloth or brush to remove plastic residues and dirt. Avoid using hard tools that may scratch the mold surface.
- Corrosion prevention: Apply anti-rust oil to the mold surface when it’s not in use for a long time. For molds used in humid environments, use corrosion-resistant materials (e.g., stainless steel) for key components.
- Venting check: Check the mold vents regularly. Clogged vents cause air trapping in the mold cavity, leading to bubbles and incomplete filling. Clean the vents with a small drill or needle if necessary.
- Ejector pin maintenance: Lubricate the ejector pins every day to ensure smooth movement. Check for wear and bending of the ejector pins. Replace worn or bent pins in time to avoid damaging the part or the mold.
Periodic Inspection Items
| Inspection Item | Inspection Frequency | Inspection Method |
| Cooling system checks | Every month | Check if the cooling water channels are blocked. Use a pressure tester to test the water pressure. Ensure uniform cooling of the mold. |
| Mold alignment | Every 3 months | Use a dial indicator to measure the parallelism and perpendicularity of the mold halves. Adjust the mold clamping force if there’s misalignment. |
| Wear and tear assessment | Every 6 months | Inspect the mold cavity, core, and other key components for wear. Measure the dimensions of the mold parts and compare them with the original design. Replace severely worn parts. |
4. Processing Parameters
Proper setting of processing parameters is crucial for obtaining high-quality parts. Even small deviations in parameters can cause defects. Here’s a guide to the key processing parameters:
Key Parameters and Their Effects
| Parameter | Optimal Range (Example for ABS) | Effect of Too High | Effect of Too Low |
| Injection speed | 50-80 mm/s | Causes jetting (irregular flow marks) and flash. | Leads to incomplete filling of the mold cavity. |
| Injection pressure | 80-120 MPa | Results in excessive stress in the part, causing warpage and cracking. | Can’t fill the mold completely, especially for thin-walled parts. |
| Melt temperature | 220-250 °C | Degrades the plastic, producing black spots and poor mechanical properties. | The plastic is not fully melted, leading to uneven flow and cold joints. |
| Cooling time | 15-30 s | Increases production cycle time and reduces efficiency. | The part is not fully cooled, causing deformation when ejected. |
| Cycle time optimization | Balance of filling, cooling, and ejection time | Over-optimization (too short) leads to defects; under-optimization (too long) increases cost. | – |
| Drying conditions | For nylon: 80-100 °C, 4-6 hours | Over-drying makes the material brittle. | Insufficient drying causes bubbles in the part. |
| Screw speed | 80-120 rpm | High speed generates too much heat, degrading the material. | Low speed leads to incomplete plastic melting and mixing. |
A common question: How to adjust processing parameters when facing warpage? First, check if the cooling time is sufficient. If not, extend it. Then, reduce the injection pressure and melt temperature to reduce internal stress. Also, ensure uniform wall thickness in the part design.
5. Quality Control and Testing
Quality control and testing throughout the production process ensure that only qualified parts leave the factory. Here’s the key process:
Testing Items and Methods
- Dimensional accuracy: Use calipers, micrometers, and coordinate measuring machines (CMM) to measure the part’s key dimensions. Compare the measured values with the design tolerances. Sampling frequency: 5 parts per hour for initial production, 2 parts per hour for stable production.
- Surface finish: Inspect the part surface with the naked eye and a magnifying glass (10x). Check for defects like scratches, bubbles, flash, and sink marks. Reject parts with obvious surface defects.
- Warpage and shrinkage: Place the part on a flat surface to check warpage. Use a feeler gauge to measure the gap. For shrinkage, measure the part’s dimensions and calculate the shrinkage rate (compared to the mold size). The acceptable shrinkage rate varies by material (e.g., PP: 1.5-2.5%, ABS: 0.5-0.8%).
- Flash and burrs: Check the part’s edges and parting lines for flash and burrs. Use a deburring tool to remove small burrs; reject parts with large flash.
- Material flow analysis: Use simulation software (e.g., Moldflow) to analyze the molten plastic flow in the mold cavity before production. Identify potential problems like short shots and uneven flow, and adjust the mold or processing parameters accordingly.
- Non-destructive testing: For critical parts (e.g., automotive and medical parts), use ultrasonic testing and X-ray testing to check for internal defects like voids and delamination.
6. Post-Processing and Finishing
Post-processing improves the part’s appearance, performance, and assembly ability. Here are the key precautions:
- Deburring: Use manual deburring (files, sandpaper), mechanical deburring (rotary brushes), or ultrasonic deburring. For precision parts, avoid excessive force during deburring to prevent deformation.
- Painting and coating: Clean the part surface thoroughly before painting to remove oil and dust. Choose the right paint (e.g., acrylic paint for good adhesion, enamel paint for durability). Control the paint thickness (15-30 μm) to avoid drips and uneven coating.
- Assembly considerations: Ensure the part’s assembly features (e.g., bosses, holes) meet the assembly requirements. Check the fit between parts; avoid over-tight or over-loose fits. Use appropriate assembly tools to prevent damaging the parts.
- Heat treatment: Some materials (e.g., nylon) need heat treatment (annealing) to reduce internal stress. Annealing conditions: Heat the part to 80-100 °C, hold for 1-2 hours, then cool slowly. This prevents warpage and cracking during use.
- Ultrasonic welding: Used to join two plastic parts. Control the welding time (0.5-2 s) and pressure (10-30 N) to ensure a strong weld without damaging the parts. The weld strength should be at least 80% of the base material strength.
- Laser marking: Used for marking logos, serial numbers, and other information. Adjust the laser power (10-30 W) and speed (50-100 mm/s) to get clear and durable marks. Avoid marking on the part’s stress concentration areas.
- Surface treatment: For parts needing better wear resistance or aesthetics, use surface treatments like electroplating (for metal-like appearance) and shot peening (for improved surface hardness). Choose the right treatment method based on the material and requirements.
Yigu Technology’s View
At Yigu Technology, we believe that high-quality plastic injection molded parts come from strict control of every production link. Material selection should balance performance and cost; design should be mold-friendly; mold maintenance ensures stable production; processing parameters need precise adjustment; quality testing guarantees product reliability; post-processing enhances added value. We integrate these precautions into our production process, using advanced simulation tools and testing equipment to provide customers with high-quality parts.
FAQs
- Q: What should I do if the plastic injection molded part has bubbles?
A: First, check if the material is fully dried (especially for moisture-absorbing materials). Then, inspect the mold vents for clogging and clean them if needed. Also, reduce the melt temperature and increase the injection speed slightly to avoid air trapping.
- Q: How to prevent warpage of plastic injection molded parts?
A: Ensure uniform wall thickness in the part design. Extend the cooling time to make the part cool evenly. Reduce the injection pressure and melt temperature to reduce internal stress. Also, check the mold alignment and adjust if necessary.
- Q: What factors affect the cost of plastic injection molded parts?
A: Key factors include material cost (resin type and recycled content), mold cost (complexity and material), processing cost (cycle time and energy consumption), and post-processing cost (painting, welding, etc.). Optimizing design (reducing undercuts) and processing parameters (shortening cycle time) can reduce costs.