Plastics Part Design and Moldability

7/26/2019 Plastics Part Design and Moldability

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1/13/2015 PLASTICS PART DESIGN and MOLDABILITY

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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,

Light weight - low density,

Low conductivity of heatand 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 ensureuniform 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.
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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 volumeis 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

Materials S pecifi c volume

AT 20 degree C

Specific

volume AT 200

degree C

% age

change

cubic-cm / g cubic-cm / g

HDPE

(crystalline)

1.03 1.33 29 %


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PS (amorphous) 0.97 1.05 8%

Density Density

HDPE

(crystalline)

0.97 0.75 22.7%

PS (amorphous) 1.03 0.952 7.8%

To solve this problem, the corners should be smoothened with radius. Radius should be providedexternally 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.


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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 processhas 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.
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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.


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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 inbalanced manner.

When melt streams reach boundary at the same time it can be called balanced filling. When somestream 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.
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See various results of Moldflow Analysis.

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.
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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.


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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


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CHANGING WELD LINE POSITION


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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
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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 meanspoorer 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, betterthe 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
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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.


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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.


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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


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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.

Click here to see Understanding QUALITY

Let us understand the factors influencing qualityconsistency in processing and quality in

performance

Let us understand moulding problems.

Let us see the analysis of plastic part failurscarried out by RAPRA.

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Three Balances Moulded-in Stress Shrinkage Inconsistent Dimension Sink-mark Weld-lines

Moulding Problems CYCLE TIME Plastics Part Design Flow Analysis .. ..

Engineering Basics in

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Glossary of technical

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Myth Understanding Mould Cooling

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Ideal Moulding Shop
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Plastics Part Design and Moldability

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