In the production of cement blocks, the moulding process directly determines the final product's strength, dimensional accuracy, and surface appearance. A poorly designed or manufactured mould will produce unqualified blocks even with good concrete. Here are 5 critical parameters to evaluate when selecting a mould for cement blocks.
The mould directly withstands concrete abrasion and high-frequency vibration. Low-grade soft steel leads to rapid cavity wear, resulting in uneven wall thickness and rounded edges. Recommended practice: Choose heat-treated (quenched and tempered) mould steel such as 45# steel or tool steel, achieving a surface hardness of 50-55 HRC. This ensures dimensional stability over hundreds of thousands of cycles.
The draft angle is the slight taper (typically 1°–3° per side) applied to cavity walls. Too small a draft angle: blocks stick or chip at corners. Too large a draft angle: block sides become non-parallel, affecting stacking. Professional manufacturers calculate the optimal angle based on block height, concrete moisture, and surface finish. With CNC machining and polishing, some suppliers (such as UNIK) achieve draft angles as low as 0.5°, balancing easy release and block verticality.
A smooth cavity surface (Ra ≤ 0.8μm) greatly reduces friction between concrete and steel walls. This results in:
Lower demoulding resistance, reducing core pin bending
A dense, fine surface appearance without extra finishing
Longer intervals between mould cleaning, raising productivity
For hollow blocks, core pins form the voids. Common problem: misaligned pins cause uneven wall thickness, weakening thin walls. Solution: CNC-machine the cavity and core pin retainer plate together, holding alignment tolerance to ±0.05mm. Core pins can be carburized or hard-chrome plated to resist cement paste corrosion and abrasion.
During compaction, air and water vapor in the concrete must escape. Poor venting leads to:
"Honeycombing" or pinholes on the block surface
Local suction during demoulding, causing cracks Professional design: Micro venting grooves (0.2-0.5mm deep) machined at the cavity top or core pin roots, or small vent holes. This significantly improves surface density, especially for zero-slump concrete.
Summary: When selecting a mould for cement blocks, don't only look at the price. Ask your supplier for specific data on steel grade, heat treatment hardness, draft angle, surface roughness (Ra value), core pin alignment tolerance, and venting method. A well-parameterized mould has a higher upfront cost, but it pays back within months by reducing rejects, extending mould life, and raising block quality.
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