How to truncate sprite without changing the scale of the sprite?

For now, I use transform.localScale to design a HP bar that will keep decreasing HP when player get hit
but because of localScale issue I get the undesired result as image below. I dun want tat portion
also changing.
alt text

You will ask why dun just put a complete rectangle to do that but the UI layout is semi-transparent as image
below. So, I need a way to truncate the sprite without modifying the scale of sprite.

alt text

My desired output is decrease the HP bar without changing the frame layout of the HP UI as the sample image below.
alt text

Here is my code for modifying scale of the sprite:

using UnityEngine;
using System.Collections;

public class HPBar : MonoBehaviour 
    public float hp = 1800;
    public float damage = 510;

    float totalhp;
    float healthPercent = 100;
    SpriteRenderer hpBar;
    Vector3 hpScale;

    void Start()
        hpBar = GetComponent<SpriteRenderer>();
        hpScale = hpBar.transform.localScale;
        totalhp = hp;

    void Update()
        // Mouse left click 
        if (Input.GetMouseButtonDown(0))
            hp -= damage;
            if (hp > 0)
                healthPercent = hp / totalhp;
                healthPercent = 0;
            hpBar.transform.localScale = new Vector3(hpScale.x * healthPercent, 1, 1);

Write a custom shader capable of truncating alpha by mask and supplied scaling factor(as float)