Mould-sets : Design Guidelines

Mould set blanks		Custom machined mould tool

Mould-set Design for Low-pressure Moulding
Introduction
Any experienced mould maker can manufacture Mold-sets for low-pressure moulding. These guidelines will point out some aspects of mould making for this technology that differs from “high pressure Mould-set” manufacturing.
Materials
Mould-sets for low-pressure moulding are normally manufactured from 7075 Aluminium. The polyamide molding materials are non-abrasive. This combined with the low injection pressures results in negligible wear on the Mould-set. Aluminium is cost effective to machine. It is excellent at dissipating heat resulting in shorter cycle times. High wear areas where components are inserted should have steel inserts. Mould-sets for very high production rates can be manufactured from P-20 material or any other conventional tool steel.
Dimensions
Please refer to specific drawings for the different machines. Mould-set guides shall be approximately 35 mm long with 6 mm long tapered lead. Two different diameters shall be used such as 9 mm and 10 mm respectively.
Cavity design
The design and surface texturing of the individual Mould-set cavities follow conventional mould making guidelines. It is important to position the parting line so that the moulded components stay in the lower mould half, when the Mould-set is opened. This may require a stepped parting line for some applications. It is often possible to avoid the use of locating pins as the low-pressure fill of the cavity is unlikely to dislocate components. When circuit boards are over-moulded, these can be positioned by locating pins if necessary.
For some applications where holes from locating pins are unacceptable in the moulded parts, a two-step over-mould should be considered. One side/portion of the component is over-moulded first, then the other side/portion. These Mould-sets will have two different cavities and components are moved from the first to the second cavity for completion.
Venting
The air vents should be 0.05 mm deep and up to 2.5 mm wide. Larger parts will require several air vents from each cavity. The vents should generally be located at the top of the cavity opposite to the gate. For components that protrude more than 6 mm above the parting-line, use inserts or stepped parting line to allow for venting from top of cavity.
Shut offs
When shutting off around cables and wires, guides are required to facilitate positioning of the components. Shut-offs around wires and cables shall normally be in the parting line. Shut off around wires is normally 6 to 8 mm long and around cables 8 to 12 mm long. Wire/cable guides are often best made as a trough, which is two cable diameters deep and positioned “outboards” to the actual shut off. Use steel inserts for shutting off around steel components, such as connectors.
Runners and gates
The polyamide materials can contract some 10% when they go from liquid to solid state. It is critical that the runner / gate system allows packing of material during this phase change.
The runners shall be slightly oval to release easier. Typical runner is made with 6 mm ball end mill 2.7 mm deep in each mould half.
Normal gate to be 1.5 to 2.0 mm diameter by 0.5 mm long. Gates should taper towards the runner to ensure that gate stays on runner after removal.
Ejector systems
Most production Mould-sets utilizes ejector systems. The design follows conventional guidelines: Guide-pins and ejector-pins are sandwiched in between a base plate and a retainer plate. This sandwiched arrangement slides on shoulder bolts with return springs. Typical ejector stroke ranges from 3 to 10 mm depending on the application. Refer to Mould-set drawings for centre of machine ejector system.
Cooling
Mould-sets for Moldman™ machines do not require any cross drilling for cooling. The Aluminium mould platens are prepared for water cooling via an auxiliary chiller.
Release agent
A release agent is required for the Mould-sets. A new Mould-set may require application of this release agent several times hourly or daily. Extensive research has gone into selecting the optimum release agent for this technology. Call us for more advice on this.

Frequently asked questions on Mould-sets [FAQ]