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,

• 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 and pvT characteristics)

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.

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 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.

MOULDABILITY consideration

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.

COOLING consideration

Volumetric changes associated with changes in temperature and pressure should be understood well. Click here see pvT characteristics of thermoplastics.

Dimensional variation of mould cavity and core during moulding, moulded part before ejection and after ejection and thereafter sever hours later is described in this figure.

Balance in heat exchange during a cycle time ensures the consistency moulding.

EJECTION consideration.

Adequate draft angle, good surface finish, mechanism to handle undercut, stregic location of ejector pins etc should be the consideration of part designer.

SUMMARY

Design Factors	To improve mouldability, understand the following;
Gate	Ideally at geometric center of the part. Melt stream shape is similar to projected shape of the part by multiple gate or suitable type and size of the gate. Locate gate at thickess section so that melt flow from thick to thin section.
Wall Thickness	No variation in wall thickness. Larger the variation means poorer mouldability. Rib thickness 50 –60% of wall thickness.
Pressure drop in runner system	Runner system should be designed for high pressure drop, thus minimising material in runner, in order to give low runner to part weight ratio.
Flow pattern	Distance (L/T ratio) from gate to boundary in all direction, if not same, provide flow leaders or flow deflectors to balance the flow to improve mouldability. Lower the difference in L/T ratios in different direction, better the mouldability.
Melt temperature variation in side mould	Variation of melt temperature should be with in 10 degree centigrade. Shearing through narrow wall increases melt temperature.
Filling Pressure	The good mouldability occur when pressure gradient i.e. pressure drop per unit length, is constant along the flow path.
Maximum Shear Stress	The shear stress during filling should be less than a critical value. This critical value depends on material and application.This data is available with Moldflow software.
Melt stream velocity	Ideally, all melt streams move at same velocity.This can ensure same cooling time for all melt streams. Difference in velocities as less as possible for better mouldability
Avoid hesitation effect	Melt flow from thick to thin section is better for mouldability.
Weld-lines	Weld-line distance from gate should be as less as possible for better mouldability. Weld line can be shifted by using frame of suitable thickness.
Hold-on pressure (not desin factor but processing factor)	Multi steps with reducing pressure with time to avoid moulded-in stress near the gate.
Thermal shut off of runners.	The runners must be sized for thermal shut off when the cavity is just filled and sufficiently packed, to avoid overpack or reverse flow, in and out of cavity, after the mould is filled.
Heat exchange	Consistent mould temperature can only be ensured when there is balance between heat in and heat out during moulding cycle time. Cooling channels must be designed with the help of MoldFlow software.This should ensure uniform cooling time to enhance mouldability.
Core and Cavity dimensions	Core and cavity Dimensions computed taking into consideration mould-makers tolerance, mould shrinkage and post moulding shrinkage.
Easy ejection	Proper taper on the part and smooth polished mould surface facilitate easy part ejection.

MECHANICAL consideration.

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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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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