9 Common Mold Problems That Can Be Avoided

1. Guide Pin Damage of Common Mold Problems

Guide pins primarily serve a guiding function in molds, ensuring that the core and cavity molding surfaces do not collide under any circumstances. They should not be used as load-bearing or positioning elements. During injection, large lateral offset forces can occur in two situations:

• When the plastic part has uneven wall thickness, the flow rate through the thick section is high, creating greater pressure there.
• When the part’s side is asymmetrical, such as in molds with stepped parting surfaces, the opposing sides experience unequal back pressure.

2. Difficulty in Removing the Gate

During injection molding, the gate may stick inside the gate sleeve, making removal difficult. This can cause cracks or damage to the product when the mold is opened. Operators often need to use a copper rod from the nozzle side to loosen it, which severely affects production efficiency. The main causes include poor smoothness of the gate taper hole, circumferential tool marks, or soft material leading to deformation or damage of the small end after a period of use. In addition, an insufficient nozzle curvature can result in a rivet-head effect at the gate.
To prevent this, it is best to use standard components for the gate sleeve. If self-machining is necessary, use or purchase a dedicated reamer. The taper hole should be polished to Ra0.4 or below. Additionally, a gate puller or gate ejection mechanism must be included.

3. Misalignment of Core and Cavity in Large Molds

Uneven filling rates in different directions and the weight of the mold during installation can cause core and cavity misalignment. In these cases, lateral offset forces act on the guide pins during injection, causing scratches, bending, or even breakage, and in severe cases, preventing mold opening.
The solution is to add high-strength positioning keys on all four sides of the parting surface—cylindrical keys are the simplest and most effective. The perpendicularity of the guide pin hole to the parting surface is critical. It is best to clamp the core and cavity together and bore the holes simultaneously on a boring machine to ensure concentricity and minimize verticality errors. Also, ensure the hardness of the guide pins and bushings meets design requirements.

4. Bending of the Moving Template

During injection, the molten plastic generates significant back pressure, typically 600–1000 kg/cm². Some mold makers underestimate this and change the original design dimensions or use low-strength steel plates for the moving template. In molds with long support spans, this can lead to downward bending during injection.
The moving template must be made of high-quality steel with sufficient thickness. Low-strength steel (e.g., A3) should not be used. If necessary, install support pillars or blocks beneath the moving template to increase load-bearing capacity.

5. Ejector Pin Bending, Breaking, or Leakage

Custom-made ejector pins generally have good quality but high processing costs, so standard parts are commonly used, with average quality. If the clearance between the pin and hole is too large, leakage may occur; if too small, the pin may expand due to mold temperature rise and get stuck during injection. A more dangerous situation is when the ejector pin partially ejects and then breaks, leaving a protruding section that cannot return and damages the cavity during the next mold closing.
To solve this, regrind the ejector pin, leaving a 10–15 mm fitting section at the front, and reduce the middle section by 0.2 mm. After assembly, strictly check the clearance, which should generally be 0.05–0.08 mm, ensuring smooth ejection and retraction.

6. Poor Cooling or Water Leakage

The cooling performance directly affects product quality and production efficiency. Poor cooling may cause excessive or uneven shrinkage, leading to warping and deformation. Overheating of the mold can halt production, and in severe cases, cause thermal expansion and seizure of moving parts such as ejector pins.
Cooling system design should be based on the product shape and must not be omitted due to complex mold structures or machining difficulties, especially for medium and large molds.

7. Insufficient Guide Slot Length

Some molds, limited by template size, have too short guide slots. After the core-pulling action is completed, the slider may protrude beyond the guide slot. This increases the risk of slider tilting during core pulling or early mold closing, causing slider damage or even bending.
Based on experience, after core pulling is completed, the remaining length inside the guide slot should not be less than two-thirds of the total guide slot length.

8. Failure of Fixed-Distance Tension Mechanisms

Fixed-distance latches are generally used in core-pulling or secondary ejection molds. These mechanisms are set in pairs on both sides of the mold and must operate synchronously—engaging simultaneously during mold closing and disengaging during mold opening. Loss of synchronization will cause template misalignment and damage.
These components require high rigidity and wear resistance, but are difficult to adjust and have a short service life. Avoid using them when possible. For low core-pulling forces, spring-loaded core-pulling is recommended. For high forces, use a sliding core mechanism activated by moving mold retraction or hydraulic cylinders in large molds.

9. Damage to Oblique Pin Slider Core-Pulling Mechanism

This mechanism commonly fails due to improper processing or undersized materials. Two main problems include:

• Excessive oblique pin angle (A): A larger angle creates a longer core-pulling distance in a short mold-opening stroke. However, when the extraction force (F) is constant, a larger angle increases the bending force (P = F / cosA) on the pin, leading to deformation and wear.
• Increased upward thrust on the slider (N = F × tanA): A larger angle increases this force, which raises friction in the guide slot, leading to poor sliding and wear.
  Based on experience, the oblique pin angle should not exceed 25°.