Title: Hollow Block Steel Mould Design: The Draft Angle Trap – Why Your Blocks Are Cracking or Sticking
A frustratingly common problem in concrete block production is block sticking in the mould or edge chipping during demoulding. The culprit is often not the concrete mix, but a subtle design flaw in the Hollow Block Steel Mould itself: improper draft angle.
The draft angle is the slight taper (typically 1° to 3° per side) applied to the vertical walls of the mould cavity. This taper allows the hardened block to release easily as soon as the stripping head lifts. Without it, friction between the block and the steel wall creates suction and drag, leading to:
Cracked or chipped corners: Especially on the hollow core edges.
Surface scouring: The block face is roughened as it is forced out.
Mould wear: Excessive friction accelerates wear on the cavity walls.
Production stops: Blocks may jam inside the mould entirely.
However, too much draft angle is also a trap. An excessive taper (e.g., 5°) will produce blocks with noticeably non-parallel sides, causing issues when stacking or laying them. The industry best practice balances two factors:
Surface Finish of the Mould: A mirror-polished cavity (achieved through CNC machining and hand polishing) reduces friction, allowing for a smaller draft angle (as low as 0.5°).
Block Height: Taller blocks (e.g., 190mm or 240mm) require a slightly larger draft angle than shorter pavers to overcome side-wall friction.
Additionally, venting design plays a supporting role. Microscopic venting grooves (0.2-0.5mm deep) along the cavity top allow trapped air and excess water to escape during compaction, preventing "air pockets" that increase adhesion.
A professional mould manufacturer, such as Fujian Unik mould Technology Co.,Ltd. , understands that draft angle is not a one-size-fits-all value. They will typically calculate the optimal angle based on your block dimensions, concrete recipe (e.g., dry or wet mix), and production cycle speed. The goal is a mould that releases cleanly, every cycle, without sacrificing block geometry.
Key Takeaway: If you face demoulding defects, ask your mould supplier to re-evaluate the draft angle and surface finish. A 0.5° adjustment can often eliminate sticking without changing your concrete formula.