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Injection Molding Design Guidelines注塑模具设计指导Much has been written regarding design guidelines for injection molding. Yet, the design guidelines can be summed up in just a few design rules.已经有许多关于设计注塑模具的书了,但是设计准则可以被概括以下几点:Use uniform wall thicknesses throughout the part. This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times.1.零件整体壁厚保持均匀.这样可以最小化缩坑,翘曲,强度减小,模具填充和循环时间.Use generous radius at all corners. The inside corner radius should be a minimum of one material thickness.2.在转角处大量采用圆角.圆角内部最小半径为一个壁厚.Use the least thickness compliant with the process, material, or product design requirements. Using the least wall thickness for the process ensures rapid cooling, short cycle times, and minimum shot weight. All these result in the least possible part cost. 3.根据工艺,材料,产品设计需求.采用最小壁厚.最小壁厚的应用可以确保快速的冷却,短循环时间和减少喷射重量.所有这些最终达到最小的产品成本.Design parts to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing.4.在模具开合方向设置拔模角便于设备的运做.Use ribs or gussets 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.5.为了提高产品强度抵抗弯曲应该加筋或肋.避免用厚的结构达到同样的效果,以此减少了产品重量,材料成本,循环时间成本.The use of ribs筋的应用Ribs increase the bending stiffness of a part. Without ribs, the thickness has to be increased to increase the bending stiffness. Adding ribs increases the moment of inertia, which increases the bending stiffness. Bending stiffness = E (Young's Modulus) x I (Moment of Inertia)如果没有筋来增加强度,厚度就必须增加(费话一堆).加强筋增加了转动惯量!The rib thickness should be less than the wall thickness-to keep sinking to a minimum. The thickness ranges from 40 to 60 % of the material thickness. In addition, the rib should be attached to the base with generous radiusing at the corners.筋的厚度要<壁厚,以降低缩坑.厚度推荐=40%~60%壁厚.另外筋应该在根部倒圆角.At rib intersections, the resulting thickness will be more than the thickness of each individual rib. Coring or some other means of removing material should be used to thin down the walls to avoid excessive sinking on the opposite side.在筋交叉的截面处,壁厚要>单独的筋,为了防止在反面产生缩坑,尽量出去交叉中心的材料.The height of the rib should be limited to less than 3 x thickness. It is better to have multiple ribs to increase the bending stiffness than one high rib.筋的高度<3倍的厚度.最好多加几条代替一个.The rib orientation is based on providing maximum bending stiffness. Depending on orientation of the bending load, with respect to the part geometry, ribs oriented one way increase stiffness. If oriented the wrong way there is no increase in stiffness.筋的方向布置以增加产品强度为准,参考弯曲载荷方向(其他为费话)Draft angles for ribs should be minimum of 0.25 to 0.5 degree of draft per side.If the surface is textured, additional 1.0 degree draft per 0.025 mm (0.001 inch) depth of texture should be provided.拔模角单边0.25~05度,表皮有皮纹,没增加0.025毫米+1度.Boss DesignBosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).为了配合或者为了连接如螺钉,或者有螺蚊的插入(注入,压接,超生地或热的插入) ,引入了柱. The wall thicknesses should be less than 60 % of nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.为了防止缩坑,厚度保持在60%的壁厚,如果柱在不可见部分,可以根据自攻螺钉的强度要求适当增加厚度.The base radius should be a minimum of 0.25 x thickness最小半径=0.25壁厚.The boss can be strengthened by gussets at the base, and by attaching it to nearby walls with connecting ribs.为了增强柱的强度,在根部加筋并且把它和周围的筋相连.Hoop stresses are imposed on the boss walls by press fitting or otherwise inserting inserts.周向应力会在配合或者插入时会被施加在柱上.The maximum insertion (or withdrawl) force Fmax and the maximum hoop stress, ocurring at the inner diameter of the boss, smax is given by最大的的插入力和周向应力发生在内直径.Failures of a boss are usually attributable to:失败的柱的原因归为以下:High hoop stresses caused because of too much interference of the internal diameter with the insert (or screw).由于内部直径的过多干涉造成了周向应力变大.Knit lines -these are cold lines of flow meeting at the boss from opposite sides, causing weak bonds. These can split easily when stress is applied.结合线-在柱相反方向相遇的形成大冷线,导致脆弱.Knit lines should be relocated away from the boss, if possible. If not possible, then a supporting gusset should be added near the knit line.把结合线移开,如果可能的话.如果实在不可行,在接结合线附近加筋.Snap Latches• Snaps allow an easy method of assembly and disassembly of plastic parts. Snaps consist of a cantilever beam with a bump that deflects and snaps into a groove or a slot in the mating part.对于塑料件卡扣提供了一个简便的拆装方法.卡扣由一个突起和凹槽或缝配合组成.• Snaps can have a uniform cross-section or a tapered cross section (with decreasing section height). The tapered cross-section results in a smaller strain compared to theuniform cross-section. Here we consider the general case of a beam tapering in both directions.卡扣可以有同意的截面或者成角度的.跟前者比后者有更小的应变.这里我们以便推荐两面有倾角的.When Rh=1 and Rb=1 , the above formula does not apply, L'Hospital's rule applies and the formula is simplified to the following:• Disassembly force. The disassembly force is a function of the coefficient of friction, which ranges from 0.3 to 0.6 for most plastics. The coefficient of friction also varies with the surface roughness. The rougher the surface, the higher the coefficient of friction.拆卸力.一般塑料表面摩擦系数为0.3~0.6.• There is an angle at which the mating parts cannot be pulled apart. This is known as the self-locking angle. If the angle of the snap is less than this angle, then the assembly can be disassembled by a certain force given by the above formula.The self-locking angle a = tan-1(1/µ)自锁角.卡扣的角度应该>自锁角.(记住公式就可以了)_where µ is the coefficient of friction which ranges from 0.3 to 0.6 for most plastics.摩擦系数就不说了.This computes to angles ranging from 73° for low coefficient of friction plastics to 59° for high coefficient of friction plastics.高的摩擦系数表面的自锁角为59,小的为73If this angle is exceeded then the snaps will not pull apart unless the snap beam is deflected by some other means such as a release tool.(本段大意就是要大于自锁角)This property can be used to advantage depending on the objective of using the snaps. If the snaps are to be used in the factory for assembly only (never to be disassembled by the end user), then the ramp angle the self-locking angle should be exceeded. If the user is expected to disassemble (to change batteries in a toy for example), then the angle should not be exceeded.如果不要拆卸就设计》自锁角,要是方便以后拆就《自锁角。
