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A Brief Introduction to Pultrusion Mold Design Requirements

As a company with years of experience in producing pultrusion molds, we understand that the mold is a critical tool in the pultrusion molding process. Let me introduce you to the basic requirements of mold design:

  1. Section Area Ratio: The ratio of the mold’s cross-sectional area to the product’s cross-sectional area should generally be greater than or equal to 10. This ensures that the mold has adequate strength and stiffness. Additionally, it allows for uniform and stable heat distribution when the mold is heated.
  2. Mold Length: The length of a pultrusion mold is influenced by the resin curing speed, mold heating conditions, and the pultrusion molding speed. The primary goal is to ensure that the product reaches a certain degree of curing as it is pulled out of the mold. Mold lengths can range from 500 to 1500mm. For most pultrusion products, except thin-walled items and small rods, molds are typically designed to be 800-900mm in length to meet standard production requirements.

In the mold processing stage, emphasis should be placed on the precision of the cavity machining, which includes:

  1. Parting Line Deviation: This is a critical factor affecting the quality of the product’s parting line. The general requirement is less than ±0.05mm.
  2. Internal Cavity Roughness: This directly impacts the appearance of the product. For pultrusion molds, the roughness requirement should be above 0.2. Finished molds should appear bright and smooth to the eye, without obvious scratches, machining marks, black spots, and should feel smooth to the touch, with no burrs on the edges.
  3. Cavity Hardness: Generally, the hardness should be greater than HRC50. This directly affects the mold’s lifespan; higher hardness means the mold can produce more products. Typically, a pultrusion mold should have a lifespan of 50,000 to 80,000 meters.
  4. Cavity Straightness: The requirement is less than ±0.55mm. This must be strictly controlled to ensure smooth product extrusion from the mold. Molds with inadequate straightness often experience unexpected blockages under normal process parameters, usually due to the mold outlet being thinner than the inlet, causing severe compression and surface wear of the product near the mold outlet, and in extreme cases, preventing the product from being released from the mold.
  5. Cavity Size and Shrinkage: During design, consider the shrinkage ratio and proportionately increase the size. As the shrinkage rate varies with different formulas and process parameters, this value cannot be generalized. It should be determined based on the product’s precision requirements and specific process parameters, in combination with the operator’s experience. The basic dimensional tolerances of the cavity should also be established.
  6. Additional Considerations: Besides these crucial factors, designers should also consider mold temperature measurement holes, parting line positions, clamping groove settings, and rounded corners at the mold entrances and exits. These design elements should be specifically tailored based on the product’s process parameters and production equipment conditions.

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