When possible, use JSON models instead. The result will be much better, more stable and better compatible with Minecraft. OBJ support in Minecraft is not native and is added by Forge.
MCreator supports custom OBJ models for item and block based mod elements.
Importing OBJ model
Go to Workspace tab, in Resouces sub tab and select 3D models and texture mappings section. Click import OBJ model button. Select your OBJ model and the MTL material file of the model. If your model has textures, MCreator will ask you for texture mappings too (from 1.8.3 onwards). To use the model for block or item, simply select it as the model when creating the block or item.
OBJ model limitations
The OBJ model must meet all the general OBJ model standards.
All faces must be triangulated, this can be done by choosing the Triangulate Faces option when exporting the model from your select 3D modeling software.
All parts of the model must be joined together into one mesh.
No S references are allowed in the obj file.
MTL material limitations
Only very basic materials are supported. Only basic transparency is supported for the time of writing. It is said that this support is to be improved in the future, but this depends on when Minecraft adds support for this as we use their model loader. If more advanced material is used, they will be reduced to more simple implementation. All advanced material settings like metalness or roughness will be ignored.
UV texture limitations
From MCreator 1.8.3 onwards, support for UV textured OBJ models was introduced.
There are two main limitations:
- Textures need to be in PNG file format and need to be square! If the texture you use is not square, just stretch it so its aspect ratio will be 1:1. This will not affect how your model looks as it will be down stretched later, but Minecraft requires square textures.
- All textures can be mapped only on the scale from 0 to 1. This means that no vt parameter (UV texture coordinates) in OBJ can be larger than 1 or smaller than 0. You need to consider this when mapping the textures on the model in your OBJ modeler.
If these do not fix your issues, please read this and the see also section of this wiki article.