Why Do the Hole Diameters in Perforated Blocks Always Get Smaller and Smaller?
In municipal, water conservancy, and communications engineering, the dimensional accuracy of the hole diameters in perforated blocks (such as conduit bricks and drainage slope protection bricks) is critical. If the hole diameter is too large, the inserted pipe may be loose or leak; if the hole diameter is too small, the contractor's cables or rebar simply cannot be inserted. Many block factories make molds with hole diameters that fully meet the drawing requirements, but after production has run for a period of time, customers frequently complain that "the holes have become smaller."
Where does the problem lie? It is not the mix design, nor concrete shrinkage, but the wear and material build-up on the forming machine's core rods.
1. "Paste Build-Up" on the Core Rods: An Invisible Increase in Thickness
During demolding of perforated blocks, the core rods (the mandrels that form the holes) rub violently against the highly abrasive dry-hard concrete. If the surface finish of the core rods is insufficient, or if the release agent is not applied evenly, a small amount of cement paste will adhere to the core rod surface. Once this "paste build-up" hardens, it effectively increases the diameter of the core rod, causing the hole diameter produced in each cycle to shrink imperceptibly. Over repeated cycles, the hole diameter moves from out-of-tolerance to fatally nonconforming.
2. Wear and Deformation of Core Rods: From "Slight Shrinkage" to "Deflection"
In addition to surface build-up, long-term pressing and vibration also cause wear to the core rod body itself. If the core rods are made of ordinary steel, their wear resistance is insufficient, and after tens of thousands of cycles, the lower part of the core rod (the area under the greatest pressure) will undergo slight wear deformation. More seriously, if the core rod fixing method is not sufficiently secure, micro-displacement or deflection will occur under strong vibration. This not only deforms the hole diameter but can even tear the hole wall during demolding, producing rejects.
3. The Solution: From "After-the-Fact Correction" to "Material and Process Prevention"
To solve the problem of shrinking hole diameters, scraping the core rods with a scraper afterward is too late.
First, the core rod material must be upgraded. High-end molds typically use high-strength alloy steel and undergo cryogenic treatment and hard chrome plating on the surface, which not only provides extremely high hardness but also an extremely smooth surface, greatly reducing the adhesion rate of concrete.
Second, the coordination of the demolding system is crucial. During demolding, the forming machine must be matched with perfect vertical vibration to ensure that the core rods are "pulled out smoothly" rather than "dragged out with hard rubbing." In addition, regularly cleaning the core rods with a dedicated mold cleaning agent and checking the verticality of the core rods are daily essentials for maintaining hole diameter accuracy.
4. The Commercial Cost of Dimensional Accuracy
When blocks with undersized holes are delivered to the construction site, the contractor will either reject them or be forced to enlarge the holes manually with a sledgehammer and steel chisel. The former causes the factory to lose orders, while the latter increases construction safety hazards. For producers of perforated blocks, controlling hole diameter accuracy is not only a matter of mold maintenance but also a core link in safeguarding the factory's commercial reputation.
