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PLASTICS PART DESIGN
Injection moulding is popular manufacturing method because of its high-speed production capability. Performance of plastics part is limited by its properties which is not as strong (as good) as metal. There are applications where the available properties of the plastics can be useful. The strength of plastics can be improved with reinforcement of glass fiber, mica, talk etc.
Plastics generally have following characteristics,
Solid shape moulding is not desired in injection moulding due to following reasons.
Therefore we have basic rule for plastic part design; as far as possible wall thickness should be uniform or constant through out the part. This wall thickness is called nominal wall thickness.
If there is any solid section in the part, it should be made hollow by introducing core. This should ensure uniform wall thickness around the core.
What are the considerations for deciding wall thickness?
Any variation in wall thickness should be kept as minimum as possible.
A plastic part with varying wall thickness will experience differing cooling rates and different shrinkage. In such case achieving close tolerance becomes very difficult and many times impossible. Where wall thickness variation is essential, the transition between the two should be gradual.
When two surfaces meet, it forms a corner. At corner, wall thickness increases to 1.4 times the nominal wall thickness. This results in differential shrinkage and moulded-in stress and longer cooling time. Therefore, risk of failure in service increases at sharp corners.
To solve this problem, the corners should be smoothened with radius. Radius should be provided externally as well as internally. Never have internal sharp corner as it promotes crack. Radius should be such that they confirm to constant wall thickness rule. It is preferable to have radius of 0.6 to 0.75 times wall thickness at the corners. Never have internal sharp corner as it promotes crack.
Ribs improve stiffness of the part and increases rigidity. It also enhances mouldability as they hasten melt flow in the direction of the rib.
Ribs are placed along the direction of maximum stress and deflection on non-appearance surfaces of the part. Mould filling, shrinkage and ejection should also influence rib placement decisions.
Ribs that do not join with vertical wall should not end abruptly. Gradual transition to nominal wall should reduce the risk for stress concentration.
Ribs should have following dimensions.
The boss is required for fixing or mounting some other part with screw. It is cylindrical in shape. The boss may be linked at base with the mother part or it may be linked at side. Linking on side may results in thick section of plastic, which is not desirable as it can cause sink mark and increase cooling time. This problem can be solved by linking boss through a rib to the side wall as shown in the sketch. Boss can be made rigid by providing buttress ribs as shown in the sketch.
Screw is used on the boss to fasten some other part. There are thread forming type of screws and tread cutting type of screws. Thread forming screws are used on thermoplastics and thread cutting screws are used on inelastic thermoset plastic parts.
Thread forming screws produce female threads on internal wall of boss by cold flow - plastic is locally deformed rather than cut.
Screw boss must proper dimensions to withstand screw insertion forces and the load placed on the screw in service.
Quality of screw connection in plastics
Screw connection would obviously be successful only if driving torque is less than the stripping torque. Torque required to drive in the screw is driving torque. The torque required to tear away the internal thread is called stripping torque. Boss should be designed with factor of safety higher than 2. The ratio of stripping torque to driving torque should be more than 2 and preferably 5.
Stripping torque depends on
Stripping torque increases as screw penetrates and tends to level off when the screw engagement is about 2.5 times screw pitch.
Driving torque depends on
When force required to hold something down exceeds the screw pull out force, the screw thread in the plastics boss will shear off .
Pull out force depends on
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