With the rise (fall, rise, plateau, small rise, little fall, and rise again) of 3D printing and its ability to let us create, basically, anything (except for money, which is illegal), it was only a matter of time before its close cousin, injection molding, got in on the accessible, mass part-producing action.
Formlabs recently released details on how to create your own injection molds using an SLA 3D printer. The proposed molds can be used to make a minimum of 10 and a maximum of 100 parts before having to be recycled and reprinted once again.
By placing the molds in aluminum frames which can either be bought from mold manufacturers or custom-made, you can prevent the molds from warping when making your parts.
As for the material which comes into contact with the plastic, Formlabs suggests using clear, high-temperature resin. This, coupled with a store-bought injection molder, allows you to inject your plastic into the mold (they used a non-stick, low-density polyethylene in their example).
With 3D printing behind it, injection molding not only becomes more accessible, but it allows manufacturers to make and customize their molds much faster and with fewer materials.
You can download the entire white paper on the Formlabs webpage, although you do have to fill out a form to have it emailed to you.