Part Details Review
If strength is not critical and the weight reduction is the goal, thinning the part can sometimes be an option. There is, however, a limit to the amount a part can be thinned because of the flow length. As the wall thickness is reduced, the distance the material can flow is reduced due to faster cooling of the thin section. Injection pressures needed to fill the thinner cavity can become very high even with a small reduction in wall thickness. Filling pressure higher than traditional molding machines can generate can add significant equipment costs due to the need for specialized equipment such as pressure accumulators. During the past decade, technology and special equipment have been developed to injection mold articles with wall thickness as low as 0.040" and projected area of over 8 square inches. High performance molding equipment capable of achieving faster filling speeds and developing injection pressures to 35,000 psi are used for the purpose. As the new technology evolved, it was realized that several traditional rules of thumb, developed for conventional injection molding, did not apply in the same way to thin-wall molding. Thin-wall molding is even more sensitive to wall thickness variations than conventional molding; hence, uniformity in wall thickness is almost essential. Conventional wisdom suggests that rib thickness be 50% to 67% of nominal wall to minimize sink marks. Doing this in thin-wall molding would make rib thickness at the top so thin that it may not fill properly. Rib to wall-thickness ratio of 1:1 works well for a nominal wall of 0.050" thick or less. Use of fiber reinforced resin does help increase the structural rigidity of a thin-wall molding but not enough to counteract the effect of thinning. Adding 10% glass fiber to polycarbonate, for example, will increase the material modulus by 50%. Deflection of a 0.040" thick simply supported un-reinforced beam will thus be 1.5 times that of a reinforced beam. However, deflection of a 0.040" thick beam is 8 times more than that of a 0.080" thick beam. Also, remember that high injection rates are essential to fill the part before the material cools, and to produce the shear heating needed for filling. Hence, make sure that your processor is using the special equipment designed for thin-wall molding for your part. Learn more about thin wall technology on this Web Site. |