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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,

  • Light weight - low density,
  • Low conductivity of heat and electricity - insulating properties,
  • Low hardness,
  • Lower strength than metals,
  • Ductile,
  • Dimensional stability- not as good as metal,

WALL THICKNESS

Solid shape moulding is not desired in injection moulding due to following reasons.

  • Cooling time is proportional to square of wall thickness. Large cooling time for solid will defeat the economy of mass production. (poor conductor of heat)
  • Thicker section shrink more than thinner section, thereby introduce differential shrinkage resulting in warpage or sink mark etc. (shrinkage characteristics of plastics)

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?

  • It must be thick and stiff enough for the job. Wall thickness could be 0.5 to 5mm.
  • It must also be thin enough to cool faster, resulting lower part weight and higher productivity.

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.

CORNERS

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

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.

  • Rib thickness should be between 0.5 to 0.6 times nominal wall thickness to avoid sink mark.
  • Rib height should be 2.5 to 3 times nominal wall thickness.
  • Rib should have 0.5 to 1.5-degree draft angle to facilitate ejection.
  • Rib base should have radius 0.25 to 0.4 times nominal wall thickness.
  • Distance between two ribs should be 2 to 3 times (or more) nominal wall thickness.

BOSSES

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.

  • The size of the bore relative to the screw is critical for resistance to thread stripping and screw pull out.
  • Boss outer diameter should be large enough to withstand hoop stresses due thread forming.
  • Bore has slightly larger diameter at entry recess for a short length. This helps in locating screw before driving in. It also reduces stresses at the open end of the boss.
  • Polymer manufacturers give guidelines for determining the dimension of boss for their materials. Screw manufacturers also give guidelines for the right bore size for the screw.
  • Care should be taken to ensure strong weld joints around the screw bore in boss.
  • Care should be taken to avoid moulded-in stress in boss as it can fail under the aggressive environment.
  • Bore in boss should be deeper than the thread depth.

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

  • Thread size and
  • Boss material.

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

  • Friction and
  • Ratio of bore size to screw diameter.

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

  • Boss material,
  • Thread dimensions and
  • Length of screw engagement.
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