Some injection moulded parts we've made range widely through many industries...Medical, Electronic, Industrial, Chemical, Security and others.
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Things to consider are, life expectancy, quantity, strength, and environment encountered by the component. In some cases, 3D Printing will be suitable for small quantities as the strength and finish available will be more than adequate.
A Moulding tool often worth many thousands of dollars is required and will last for many years producing quality parts. It's important to consider when planning to make plastic parts what processes are suitable.