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Things to know about Mouldability and Part design

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.

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.

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