Wall Thickness Principles for Injection Moulding Tool Design
Uniform wall thickness design balances polymer flow, prevents shrink defects, cuts cycle times, and reduces landed part cost in injection moulding tools.
Flow Length Ratio
Meaning
The capability of a molten thermoplastic resin to fill a mould cavity is represented by the maximum distance the flow can travel divided by the nominal wall thickness. This mouldability metric, known as flow length ratio, determines the tooling design limits and the required gate placement for a part. It varies based on the viscosity of the polymer and the injection pressure of the moulding machine.
Resins with a high flow length ratio, such as low-viscosity polyamides, can fill thin-walled sections without short-moulding.
Mould Design
Tooling designers use the flow length ratio to determine the number and location of injection gates. If the flow path exceeds the material limit, the melt cools and solidifies before filling the entire cavity. This situation produces incomplete parts, known as short shots.
By placing gates closer together, the maximum flow path is shortened, allowing the use of less flowable, higher-strength resins.
Material Selection
Polymer selection dictates the achievable flow length ratio for a specific wall thickness. Thermoplastics with high melt flow rates permit higher ratios, allowing thinner walls and lower part weight. However, high melt flow rates often come at the expense of mechanical properties, such as impact resistance.
Processors must balance these characteristics when choosing a material for large, thin-walled enclosures.
Processing Variable
Melt and mould temperatures directly influence the active flow length ratio during a run. Higher temperatures decrease melt viscosity. This extends flow reach.
Increasing injection speed also helps.