INJECTION MOLDING DESIGN GUIDELINES

  INJECTION MOLDING DESIGN GUIDELINES

            The important guidelines required while designing the mold are

 

Section thickness: Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall, the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs. Also, thinner parts weight is less, which results in smaller amounts of the plastic used per part, thereby resulting in lower part costs. Use uniform wall thickness throughout the part. This is, because, the thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section. This builds stresses near the boundary of the thin section to thick section that leads to warping or twisting. Also uniform walled parts are easier to fill in the mold cavity, since the molten plastic does not face varying restrictions as it fills. Due to design limitations, when uniform walls are not possible, then the change in section should be as gradual as possible.

Recommended Radii: Sharp corners greatly increase the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts. In addition to reducing stresses, fillet radii provide streamlined flow paths for the molten plastic resulting in easier fills. Use generous radius at all corners. Typically, the inside radius is 0.5 x material thickness and the outside radius is 1.5 x material thickness. A bigger radius should be used if part design will allow it.

Draft angles: Design parts to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing. Draft angles as low as 0.25° per side is required for highly polished surfaces and as high as 1.5° per side is common. The amount of draft angle depends on the depth of the part in the mold, part design, complexity and its required end use function.

Draft (A) in mm for various draft angles (B) as a function of molding depth (C).

Ribs: Ribs are effective design features that add strength and often facilitate flow during filling. However, proper design is important as ribs sometimes cause sink marks or aesthetic irregularities. Ribs are also used to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs. Rib base thickness should not exceed 50% of the adjoining wall thickness. In addition, ribs should include proper draft and a base radius of at least 0.5mm.