Bioglass 45S5 is well-known for its bioactivity, but it possesses poor sintering behaviour owing to viscous flow being inhibited by the crystallisation of sodium calcium silicate phases. Mg or Zn were partially (0, 25, 50, 75%) or fully (100%) substituted for Ca on a molar base, and thermal properties (differential scanning calorimetry, dilatometry) and sintering (heating microscopy, SEM and X-ray diffraction) were investigated. Here we show that sintering can be improved significantly by partial or complete substitution of Mg or Zn for Ca, owing to a pronounced decrease in crystallisation tendency. Glass transition temperature and dilatometric softening point went through minima for mixed compositions, with random mixing of Mg/Ca or Zn/Ca ions in the glass structure and the resulting effect on configurational entropy being a likely explanation. As the onset of crystallisation did not vary much with substitution, substituted glasses possessed a wider temperature range for sintering, resulting in up to 57% and 27% sample height reduction for Mg and Zn substituted glasses, respectively, compared to only 3% height reduction for Bioglass 45S5. Taken together, these results suggest that using a combination of modifiers, particularly alkaline earths or zinc, may be a promising approach for improving the sintering of Bioglass 45S5.