Spray-cast mould construction and function

Behind Every Plastic Part You Encounter in Daily Life Lies an Injection Mould That a Tool Maker has Designed, Machined and Assembled. The Mould is the Production Equipment Itself: It Determines the Part's Shape, Surface and Dimensions, and It Must be Able to Open and Close Thousands of Times Without Losing Precision. Part of the Education's Main Course is Precisely Being Able to Perform Testing and Adjustment of Tools in Injection Moulding Machines — so It's not Enough to Build the Mould, You Must also be Able to Get It to Run Clean in Production.

§Fixed and moving mold part.

An injection mold basically consists of two halves: a fixed mold part mounted on the machine's fixed table and a movable mold part that moves back and forth as the machine opens and closes. When the mold is closed the two halves form the cavity that the plastic melt is injected into.

§Cavity and core

The cavity is the cavity space that gives the workpiece its external form, while the core forms the workpiece's internal geometry. Because plastic shrinks as it cools and contracts around the core, the core is usually placed in the moving form part so that the workpiece remains there when the form opens.

§The exhaust system

• 01Pin ejector — the most common, simple round pins that push the part free
• 02Flat ejector — used where a round pin would leave too obvious a mark
• 03Pipe ejector — for workpieces with a central hole or a tap
• 04Blade ejector — for narrow, elongated parts where a round pin is insufficient

§Facility surface and division plan

Where the two halves of the mould meet is called the parting line or parting surface. The placement of the parting line is one of the most important design choices because it determines where on the item a visible parting line can form and whether the item can be removed from the mould at all without undercuts.

§Inlet system: cold channel and hot channel

The plastic melt must be guided from the machine's nozzle into the cavity via an inlet system. A cold channel system is simple and cheap but leaves a channel remainder that must be cut off or recycled. A hot channel system keeps the plastic flowing all the way to the gate using heated channels so no waste is formed — more expensive in the tool but often the preferred solution at high volumes.

§Cooling channels control cycle time

The form temperature must be controlled precisely because both too hot and too cold a form causes faults in the workpiece. Cooling channels drilled into the form steel conduct cooling water around the cavity so heat is removed evenly and the form can be opened as soon as the part is stiff enough — it is one of the factors with the greatest impact on how many parts the machine can produce per hour.