三. Bottle Design Features Figure 6.4 Flat bottles Flat shapes (Figure 6.4) inherent problems. “bird swing” and “spike” defects. Spikes - glass projections inside the bottle Bird swing- glass thread joining the two walls Careful design to avoid stress points. angular shapes-difficult to form三. Bottle Design Features Figure 6.4 Flat bottles Flat shapes (Figure 6.4) inherent problems. “bird swing” and “spike” defects. Spikes - glass projections inside the bottle Bird swing- glass thread joining the two walls Careful design to avoid stress points. angular shapes-difficult to form