Multilayer sheets were used for thermoforming non thermoformable polymers in order to face the challenge of semi-crystalline polymers like low melt-stiffness. Mono- and 2-layer sheets consisting of different polypropylene (PP) homopolymers were extruded on a twin screw extruder. The PP viscosity as was measured by melt flow rate (MFR) 3 g/10 min., 12 g/10 min., 22 g/10 min. and 50 g/10 min. The layer ratio was varied between the equal layer ratio (A50/B50) of the individual layers and the low (A30/B70) or high viscosity ratio (A70/B30). The extrusion results show that for extreme viscosity differences (MFR3 and MFR50), the critical layer ratios known from the literature are only valid to a limited extent. The critical viscosity ratio < 4, which is known from the literature, is much lower here and should be less than 3.

The investigation of thermoformability on laboratory scale of the extruded PP sheets with different viscosities showed that the low viscous layer position has only a marginal influence on the general thermoformability. Thermoforming of materials that are not thermoformable, with a storage modulus of less than 10³ Pa and a ratio between storage and loss modulus (tan δ) greater than 1, becomes possible using a multilayer sheet independant of the layer ratio. If the layer with higher viscosity acts as a stabilizing layer, thermoforming is possible.

多层片材用于热成型不可热成型的聚合物，以应对半结晶聚合物的挑战，如低熔体刚度。由不同聚丙烯 (PP) 均聚物组成的单层和 2 层片材在双螺杆挤出机上挤出。通过熔体流动速率(MFR)测量的PP粘度为3g/10min.、12g/10min.、22g/10min.。和 50 克/10 分钟。层比在各层的等层比 (A50/B50) 和低 (A30/B70) 或高粘度比 (A70/B30) 之间变化。挤出结果表明，对于极端的粘度差异（MFR3 和 MFR50），文献中已知的临界层比率仅在有限的范围内有效。从文献中得知的临界粘度比 < 4 在这里要低得多，应该小于 3。

在实验室规模上对具有不同粘度的挤出 PP 片材的热成型性进行研究表明，低粘度层位置对一般热成型性的影响很小。对于储能模量小于 10³ Pa 且储能模量与损耗模量之比 (tan δ) 大于 1 的不可热成型材料进行热成型，可以使用与层比无关的多层片材。如果具有较高粘度的层用作稳定层，则热成型是可能的。