Injection Mold Design and Manufacturing

Key Considerations for Injection Mold Design and Manufacturing with Nylon Materials

Nylon materials, as commonly used engineering plastics, impose specific requirements on molds during the injection molding process. Ensuring product quality and production efficiency requires a comprehensive consideration of mold design, material selection, and processing technology.

I. Critical Aspects of Mold Design

The gating system design requires special attention. Due to the low melt viscosity , good fluidity , and short solidification time of nylon , gate location is crucial . A runner diameter of 3-6 mm is recommended , with a gate diameter being two-thirds to three-quarters of the wall thickness, but never less than 0.8mm . For hot runner systems, the gate size should be slightly smaller than conventional runners . For submarine gates, the minimum diameter should be 0.75mm .

The venting system is another critical element. The flash value of nylon resin is about 0.03mm , so vent gaps should be controlled below 0.025mm . Vent hole diameters are typically 1.5-1mm , usually placed opposite the gate , to ensure timely escape of air during filling and prevent defects like bubbles or burns in the part .

The cooling system design directly affects product crystallinity and cycle time. Mold temperature should be adjusted based on product structure: for thin-walled parts under 3mm, a mold temperature of 20-40°C is suggested; for 3-6mm thickness, 40-60°C; for 6-10mm thickness, 60-90°C; and for thick-walled parts over 10mm, around 100°C .

Ejection system design also requires careful consideration. Nylon parts have high shrinkage and are easy to demold , with a recommended draft angle between 40 minutes and 1 degree 40 minutes . The ejection system needs to be balanced to prevent part deformation.

II. Mold Material Selection

The choice of core and cavity material is crucial for mold life. Since nylon materials may contain glass fibers or other reinforcements , which can cause significant mold wear , the use of advanced mold steels like S136  and 718  is recommended due to their excellent wear and corrosion resistance. For high-precision molds, consider using cemented carbide or applying surface treatments, such as TD treatment (vanadium carbide coating) , to extend mold life.

Mold base materials require sufficient strength and stiffness. P20 mold steel is a common choice , offering excellent wear resistance, toughness, machinability, and the ability to withstand higher pressures and temperatures .

III. Injection Process Control

Material pretreatment is the first step to ensure product quality. Nylon is hygroscopic , and moisture absorption affects the processing , necessitating drying before molding . Vacuum drying at 85-95°C for 4-6 hours is recommended . If vacuum drying is unavailable, atmospheric hot air drying at 90-100°C for 8-10 hours can be used . Dried nylon material should not be exposed to air for extended periods (not exceeding 1-3 hours) .

Molding temperature control needs precise management. For example, PA6 requires a barrel temperature range of 220-300°C ; PA66 requires 260-320°C . Due to nylon's relatively narrow processing temperature window , barrel temperature must be strictly controlled  to prevent material degradation . Mold temperature significantly affects the degree of crystallization and molding shrinkage rate , generally controlled between 80-120°C , depending on product requirements .

Setting injection parameters is also crucial. Injection pressure typically ranges between 750-1250 bar , depending on the material and product design . High injection speeds are generally used , but should be slightly reduced for reinforced grades . A medium screw rotation speed is appropriate , and a back pressure of 0.5-1.0 MPa is recommended , kept as low as possible while ensuring part quality .

IV. Special Considerations

For glass fiber reinforced nylon, as the fibers tend to orient along the flow direction during injection , leading to increased mechanical properties and shrinkage in the orientation direction , special attention must be paid to gate location and runner design . When glass fiber is added to nylon, it increases wear on the plasticizing components of the injection molding machine . Therefore, it is advisable to use bimetallic screws and barrels .

For parts with metal inserts, due to nylon's coefficient of thermal expansion being 9-10 times greater than that of steel , the metal inserts hinder nylon shrinkage , which can cause cracking . The wall thickness around the insert should be no less than the diameter of the insert metal .

V. Mold Maintenance and Care

Daily mold maintenance should not be overlooked. Small amounts of release agent can sometimes help improve or eliminate defects like bubbles . Zinc stearate and white oil can be chosen as release agents for nylon products , but they must be used sparingly and evenly . The screw should be purged during shutdowns to prevent breakage in subsequent production runs .

Conclusion

The design and manufacturing of injection molds for nylon materials is a systematic project that requires comprehensive consideration of material properties, product structure, mold design, and process parameters. By optimizing the gating system, improving venting design, rationally selecting mold materials, and precisely controlling process parameters, product quality and production efficiency can be significantly improved, and mold service life extended. With the continuous emergence of new materials, mold designers need to constantly update their knowledge to meet increasingly demanding production requirements.