There are many options when deciding on which manufacturing method to use. Each method, however, is dependent on a few factors.
This is an important consideration because every manufacturing method varies in cost.
For example, parts are generally cheaper to make using injection molds as opposed to casting. However, you also need to take into account the cost of the tooling. The cost of the tooling for injection mold is more expensive than cast tooling. As a result of the initial cost, this is also a very important factor.
Be sensible in deciding which material to use. Think about the application: how will it be used? Consider the weight: is it too heavy or too light? Most of all, think of the durability of the parts. Is this a one-time use product or should it last for a certain duration?
Avoid choosing a manufacturing method that produces an invincible, wonderful looking and expensive part if people are simply going to store it in a cupboard.
Remember, there are tricks to change the strength of a part by adding ribs, for instance, or even making the material stronger by adding another material to the mixture. Typically adding glass to a plastic part will increase its strength but be careful as the more glass you add the more brittle the part becomes. The same method of thought applies to alloys in metals during casting.
Your designer should have vast knowledge and experience on this matter. Discuss the options with them if you are having difficulty deciding on the best manufacturing method to use.