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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.

Constant wall thickness

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 marks and corner


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.



Specific volume AT 20 degree C

Specific volume AT 200 degree C

% age change


cubic-cm / g

cubic-cm / g


HDPE (crystalline)



29 %

PS (amorphous)








HDPE (crystalline)




PS (amorphous)




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.

sink mark under the ribs

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.

Rib - dimensions

Ribs should have following dimensions.

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.

pressure drop accorss the moulding

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.

Flow disc

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.

fill-time to reach shorter boundery path
fill time to reach longest boundary path

pressure plot

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.

Flow leader for balanced filling

Multiple gates for balanced filling


Melt stream velocity

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.

Melt flow and freeze simultaneously in the mould


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.


Weld lines in Moulding


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.


Design Factors

To improve mouldability, understand the following;


  • 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.It should also be lower than the crtical value for a given material.

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-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.


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.