How to Troubleshoot Common Block Mould Issues: Sticking, Cracking, and Uneven Demoulding

In concrete block production, mould issues are the leading cause of downtime and defective product. Three problems dominate field service calls: blocks sticking to the mould during demoulding, cracks or hairline fractures on block surfaces, and uneven demoulding across cavities. Each issue has identifiable causes and proven solutions. This guide provides a systematic troubleshooting approach based on UNIK's experience across 28 export markets .

Problem 1: Blocks Stick to the Mould During Demoulding

Description: Blocks fail to release cleanly from the mould cavity. Sticking can manifest as partial adhesion, complete jamming, or surface tearing.

Likely Causes

Release Agent Issues:

Insufficient or uneven release oil application

Low-quality or degraded release agent

Release agent that does not match the concrete type

Mould Condition:

Cavity surface roughness due to wear (surface finish > Ra 1.6)

Insufficient draft angle in mould design (should be 1–3° per side)

Worn ejector pins creating friction points

Process Factors:

Demoulding timing delayed beyond the optimal window (should be 10–15 seconds post-vibration)

Overly stiff or sticky concrete mix

Improper moisture content

Solutions

Immediate Fixes:

Apply a thin, even layer of professional-grade release agent across all cavity surfaces before each production run

Ensure the release agent is appropriate for your mould material and concrete mix (water-based agents often produce superior surface quality compared to oil-based alternatives)

Medium-Term Improvements:

Check that your mould maintains a polished cavity finish. UNIK recommends Ra 1.6 or better-re-polishing can reduce friction by up to 30%

Maintain a consistent demoulding time of 10–15 seconds after the vibration cycle completes

Adjust your concrete mix to improve flowability-proper moisture control is critical

Long-Term Solutions:

Verify the mould includes adequate draft angles (1–3° per side)

Consider upgrading to a mould with precision-polished cavities

Replace worn ejector pins and cavity inserts

Problem 2: Cracks or Hairline Fractures on Block Surfaces

Description: Blocks show cracks, corner damage, or hairline fractures after demoulding.

Likely Causes

Demoulding Forces:

Excessive force required due to sticking

Blocks wedged in the cavity from insufficient draft angle

Ejector pins not synchronized, creating uneven lifting forces

Mould Design:

Sharp 90-degree corners creating stress concentration points

Insufficient corner radii (UNIK recommends 3–5 mm)

Uneven ejection force distribution across the mould plate

Material or Process:

Concrete too dry, resulting in low green strength

Too much vibration causing aggregate segregation

Blocks handled too soon after demoulding

Solutions

Immediate Fixes:

Reduce ejection force by addressing sticking causes first

Adjust vibration settings to achieve uniform compaction

Ensure demoulding timing is consistent (10–15 seconds post-vibration)

Design Verification:

Check that internal corners have radii of 3–5 mm

Verify draft angle is sufficient for the block height and shape

Confirm ejector pins are synchronized and operating smoothly

Material Adjustments:

Optimize concrete mix for adequate green strength

Avoid over-vibration that can cause segregation

Ensure consistent moisture content

Problem 3: Uneven Demoulding Across Cavities

Description: In multi-cavity moulds, some cavities release blocks cleanly while others struggle or produce defective blocks.

Likely Causes

Mechanical Issues:

Uneven ejector pin wear across cavities

Bent or misaligned ejector pins

Uneven presser foot clearance (should be 0.2–0.4 mm unilateral clearance)

Filling Imbalances:

Uneven concrete distribution across cavities

Inconsistent compaction from variations in vibration transfer

Partial blockage in some cavities

Mould Condition:

Differential wear-cavities that face the flow of concrete wear faster than opposite walls

Uneven surface finish across cavities

Uneven release agent application

Solutions

Systematic Checks:

Measure presser foot clearance across all cavities-should be 0.2–0.4 mm per side

Inspect ejector pins for wear, bending, or binding. Replace pins in sets, not individually

Verify mould plate flatness ≤0.5 mm

Filling and Compaction:

Check material distribution system for even flow to all cavities

Verify vibration motors are synchronized and transmitting energy uniformly

Ensure all cavities are clean and free of material buildup

Maintenance:

Regularly rotate or refurbish cavity inserts to compensate for directional wear

Apply release agent evenly across all cavities

Replace worn components proactively-do not wait for failure

Prevention Through Design: The UNIK Approach

Many demoulding issues can be prevented through proper mould design and material selection. UNIK integrates these principles into every mould :

Draft Angle (1–3° per side)

A proper taper on cavity walls is essential for clean demoulding. Without it, blocks will stick regardless of release agent quality. The exact angle is calculated based on block height, surface finish, and concrete mix.

Corner Radius (3–5 mm)

Sharp 90-degree corners are stress points where cracking initiates. UNIK designs incorporate 3–5 mm radiused corners to eliminate stress risers and ensure product integrity .

Surface Finish (Ra 1.6)

A polished cavity surface reduces friction and improves demoulding performance by up to 30%. This is achieved through precision CNC machining and hand polishing .

Heat Treatment (HRC 58–62 Surface, HRC 35–40 Core)

Proper carburizing achieves surface hardness for wear resistance while maintaining core toughness to prevent cracking under vibration . This dual-property design ensures the mould maintains its surface quality across 80,000–100,000 cycles.

Steel Grade

Using high-grade materials such as 42CrMo or tool-grade steels ensures the mould maintains its shape and surface quality over its service life .

Operational Best Practices for Reliable Demoulding

Preventive maintenance and process consistency are as important as good mould design :

Daily Cleaning: Remove concrete residue from mould surfaces after each shift using soft, non-abrasive tools. Hardened buildup is a primary cause of surface wear and sticking.

Consistent Release Agent Use: Apply a thin, even layer of high-quality release agent before every cycle. Water-based agents are often preferred for their superior surface finish and reduced residue.

Standardized Vibration Settings: Run the machine within the recommended parameters for the mould and mix. Inconsistent vibration leads to inconsistent demoulding results.

Regular Inspection: Check ejector pins, guide bushings, and cavity surfaces for wear. Replace worn components proactively.

Moisture Control: Ensure the concrete mix is consistently within the target moisture range. Semi-dry mixes require precise control to achieve good compaction without causing sticking.

Demoulding Timing: Maintain a consistent demoulding time. For most standard production, the window is within 10–15 seconds after the vibration cycle completes.

Quick Troubleshooting Reference

When to Involve Your Mould Manufacturer

If internal troubleshooting does not resolve the issue, involve your mould supplier. UNIK supports clients with :

Precision-Engineered Moulds: Designed with optimal draft angles, corner radii, and surface finishes to minimize release problems from the start

Technical Support: Responding to inquiries on the same business day, with video conference troubleshooting available

Refurbishment Services: Including re-machining and re-heat-treatment to restore worn moulds

Spare Parts Supply: For rapid replacement of wear components like ejector pins and cavity inserts

Conclusion

Most demoulding problems are predictable and preventable. By systematically identifying the root cause-whether in mix design, vibration settings, release agent application, or mould condition-you can implement targeted solutions that reduce waste and downtime. A well-maintained mould can last 80,000–100,000 cycles while maintaining perfect block surface quality and operating smoothly on high-speed automatic lines.

For persistent issues, consult your mould supplier. UNIK's technical support team provides same-day diagnostic assistance via video call and can recommend design modifications or refurbishment to restore mould performance .

Fujian Unik Mould Technology Co., Ltd.
Website: www.unikmould.com
Location: Quanzhou, Fujian Province, China
Founded: 2010