What are the common problems with plastic molds

Difficulty in material removal from the gate

Phenomenon: The sprue adheres to the sprue sleeve, causing the product to crack or requiring external force to knock and demold during mold opening.

Reason: Insufficient smoothness of the sprue taper hole (Ra ≥ 0.4), deformation caused by too soft material, and unreasonable curvature of the nozzle spherical surface.

Solution: Adopt standard sprue sleeve, grind the taper hole to Ra0.4 or below, and add sprue pulling rod or ejector mechanism.

Flying edge (rough edge)

Phenomenon: Plastic overflows from the parting surface of the mold or the gap between the inserts.

Reason: Insufficient clamping force, mold wear, high injection pressure or temperature.

Solution: Increase mold clamping force, repair mold gaps, reduce injection pressure and material temperature.

Lack of glue (short shot)

Phenomenon: The product is not full, with bubbles or shrinkage.

Reason: Insufficient injection pressure/speed, poor melt flowability, and poor exhaust.

Solution: Increase injection pressure/speed, optimize channel design, and add exhaust slots.

Weld mark

Phenomenon: Linear marks on the surface of the product, resulting in a decrease in mechanical properties.

Reason: Uneven melting temperature and unreasonable gate position.

Solution: Adjust the gate layout, increase the melt temperature, and optimize the cooling system.

Shrinkage (dents)

Phenomenon: Local depression on the surface of the product.

Reason: Insufficient pressure holding, uneven cooling, and significant differences in wall thickness.

Solution: Extend the holding time, optimize the layout of the cooling water channel, and design a uniform wall thickness. ‌

Damage to the guide column

Phenomenon: The guide column may become rough, bent, or even broken.

Reason: The guide column is subjected to lateral displacement forces (such as uneven wall thickness and mold self weight displacement).

Solution: Add a cylindrical positioning key, ensure concentricity by boring the guide column hole in one go, and improve the hardness of the guide column (HRC50-55).

Dynamic fixed model offset

Phenomenon: Inaccurate mold clamping of large molds and product size deviation.

Reason: Uneven filling rate and self weight of the mold cause misalignment of the parting surface.

Solution: Install high-strength positioning keys on the parting surface, and ensure the verticality of the dynamic/fixed mold holes through one-time processing.

Top pole issue

Phenomenon: The top rod is bent, cracked, or leaking material.

Reason: Inappropriate gap between the top rod and the hole (too large leakage, too small jamming).

Solution: Control the clearance of the top rod (0.02-0.05mm on one side) and use wear-resistant steel (such as SKD61). ‌

Cracking (residual stress)

Phenomenon: Cracks on the surface or inside of the product.

Reason: Excessive filling, high demolding resistance, and differences in thermal expansion of metal inserts.

Solution: Use multi-point gating, increase mold temperature, preheat inserts, and perform deep cold treatment to eliminate stress.

Warping and deformation

Phenomenon: The product is twisted or partially undulated after demolding.

Reason: uneven cooling, unbalanced demolding force, and differences in material shrinkage rate.

Solution: Optimize the cooling system (water flow rate 1-2m/s), increase the ejection point, and use low shrinkage materials.