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PLASTICS PART DESIGN and MOULDABILITY
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.
SINK MARK IS INEVITABLE.
Temperature dependent change in volume - 29% in crystalline and 8% in amorphous-.
Compressibility of melt under pressure is 10-15%.
On falling temperature of melt in the mould, decrease in volume is more than the increase in volume on relaxation of pressure.
Therefore void can not be perfectly filled in. Hence sink mark is inevitable.
CHANGE IN VOLUME and DENSITY OF MATERIAL
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 for stifness consideration
Ribs in plastic part improve stiffness (relationship between load and part deflection) 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.
While designing plastic part, pitfalls in achieving quality, consistency and productivity must be considered. It is wrong to assume that shapes can be moulded successfully with out any defects. All shapes may not be 100% mouldable. To improve the mouldability injection moulding process has to be understood in depth.
Part design obviously has to be influenced by the intricacies of the process.
Filling phase of the process is influenced by type of gate, location of gate, number of gates, size of gate (also dependent on material viscosity). Gate should be located at such a position from where flow path to thickness ratio (flow ratio)is constant in all direction. The difference in flow ratio could be as small as possible. In some cases where thickness variation is unavoidable, melt must flow from thin section to thick section for better mouldability. Melt flow from thin to thick results in poor moulding. The size of gate should not result in excessive pressure drop across it. It should be adequate to handle flow rate required.
Resistance to flow and viscosity determines the filling pressure. Filling pressure variation should be gradual and not abrupt. It should be remembered that flow thinner section introduces shearing of melt, resulting in lowering of melt viscosity. This is the shear thinning nature of thermoplastics melt.
Filling phase is influenced by wall thickness variation as it introduces variation in resistance to flow in all directions from the gate. Melt is held in cylindrical shape in plasticating cylinder before injection. When the melt is injected through gate and runner system, melt streams move equally in all directions only when resistance to flow is equal in all direction.
It should be realised that variation in wall thickness, hole / slot, variation of mould surface temperature introduces variation in resistance to flow. Therefore melt moves in number of streams with different velocity in different direction and mould does not fill in balanced manner.
When melt streams reach boundary at the same time it can be called balanced filling. When some stream reaches the boundary early and some other streams reach late - this time lag to complete the filling of part results in induction of moulded-in stresses in the part.
Unbalancing flow can be corrected by using flow-leader / flow deflector and multiple gates so as to form the melt stream shape very close to the projected shape of the part.
Ideally all the melt streams should move with the same velocity till the mould is filled. Variation in cross section area (due to changes in wall thickness or slot)
introduces variation in melt stream velocity. Hence the freezing of melt can not be uniform through out the part. It should be realised that while freezing, cross section through which melt can flow
reduces thereby introducing increasing resistance to flow. When some stream freeze faster then other, faster freezing streams introduce increasing resistance to flow. Therefore, balance in filling
can not occure and moulded-in stresses are induced.
WELD LINE IN MOULDING
Weld line ocures when two melt streams join. Melt stream gets divided at cutout (core) in the part and they join at the other end of the cut out.
Normally weld line region is filled at the end of injection stroke or during pressure phase.
Strength of the weld line is weak when partially frozen melt front meet. The orientation at the joint remains perpendicular to direction of flow -a sign of weakness.
Weld line can form by melt stream flowing in same direction or in opposite direction.
It is not possible to eliminate weld line, but it can be made sufficiently stronger or its position can be altered.
MELT STREAM FROM OPPOSITE DIRECTION
CHANGING WELD LINE POSITION
Over cooled region can also freeze faster than lesser cooled region. When freezing is not uniform, melt moves through narrowing cross section of slow freezing stream and overpacks the slow slow freezing stream region. Hence uniform mould surface temperature distribution is very important. This has to be achieved through proper design of cooling channels for turbulent water flow.
Melt temperature is highest near the gate. Hence freezing likely to be slower near the gate. This happens near the gate during pressure phase of the process. Here over packing can be controlled through proper profiling of pressure - reducing with time.
Volumetric changes associated with changes in temperature and pressure should be understood well. Click here see pvT characteristics of thermoplastics.
Adequate draft angle, good surface finish, mechanism to handle undercut, stregic location of ejector pins etc should be the consideration of part designer.
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
Click here to see Understanding QUALITY
Let us understand the factors influencing quality consistency in processing and quality in performance
Let us understand moulding problems.
Let us see the analysis of plastic part failurs carried out by RAPRA.
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||Thermal stability||Balancing melt Flow||Balancing Heat||Freezing||Quality Problems|
||Moulded-in Stress||Shrinkage||Inconsistent Dimension||Sink-mark||Weld-lines|
||CYCLE TIME||Plastics Part Design||Flow Analysis||..||..|
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