MPM particles and rigid bodies

[xiabanjijifenzi]

Hello, I encountered an issue with the coupling between MPM (Material Point Method) particles and rigid bodies.
I created a URDF (Unified Robot Description Format) file for a Y-shaped pipe, which has a diameter of 6 mm and a thickness of 1 mm. The MPM particles have a diameter of 5e-4 meters (0.5 mm), and their initial position is set inside the pipe. However, as the simulation progresses over time, the particles pass through the pipe and fall vertically downward. (The same problem occurs when using an STL (STereoLithography) file instead of the URDF file.)
Attached below are the source code, the URDF file for the pipe, and a demonstration video for your reference.
I look forward to your valuable insights and solutions to this issue.

import genesis as gs
import gstaichi as ti
import numpy as np

gs.init(
    backend=gs.gpu,
    logging_level='warning',
    seed=0
)

scene = gs.Scene(
    sim_options=gs.options.SimOptions(
        dt=1e-3,
        substeps=20,
        gravity=(0, 0, -9.8),
    ),
    mpm_options=gs.options.MPMOptions(
        lower_bound=(-1, -1, 0.8),
        upper_bound=(1, 1,1.5),
        particle_size=5e-4,
    ),
    vis_options=gs.options.VisOptions(
        visualize_mpm_boundary=True,
        shadow=False,                 
        show_cameras=False, 
    ),
    viewer_options=gs.options.ViewerOptions(
        camera_fov=60,
        camera_pos=(0, -0.3, 1),   
        camera_lookat=(0, 0, 1), 
        max_FPS=120, 
    ),
    show_viewer=True,
)

plane = scene.add_entity(
    morph=gs.morphs.Plane(),
)
guandao = scene.add_entity(
    material=gs.materials.Rigid( 
        needs_coup=True,
        coup_friction=0.8,
        coup_softness=0.0001,
        sdf_cell_size=5e-5,
    ),
    morph=gs.morphs.URDF(
        file='/home/zhx/vs_code/11.27/y/urdf/y.urdf',
        fixed=True, 
        pos=(0, 0, 1),
        euler=(0, 0, 0),
        scale=1.0,
        merge_fixed_links=False,
        convexify= False,
    ),
)
cixinglizi = scene.add_entity(
    material=gs.materials.MPM.Sand(),
    morph=gs.morphs.Sphere(
        pos=(0.01,0, 1), 
        radius=0.0015,  
    ),
    surface=gs.surfaces.Default(
        color=(0.3, 0.3, 1.0),
        vis_mode='particle',
    ),  
)

scene.build()

horizon = 200
for i in range(horizon):
    scene.step()

video.mp4