Additive manufacturing (AM) revolutionized modern production and tooling, as it can
help speed up the development process. This is also true for injection moulding. For
instance, polymeric low-volume injection moulds are easy to produce by material jetting
(MJ) technologies like PolyJet™. The downside of these MJ printed moulds is their
relatively low glass transition temperature, which can result in unacceptably low
stiffness and increased creep compliance in the operational temperature range. Different
post-curing techniques like high-energy irradiation can enhance the degree of cross-linking
in MJ-printed photopolymer parts. We applied MJ (PolyJet™ technology) to produce specimens
for mechanical and morphological characterizations and low-volume injection moulds.
After printing, we subjected the specimens and the inserts to high-energy gamma irradiation
with doses of 50 kGy, 100 kGy, 150 kGy and 200 kGy. Dynamic mechanical analysis (DMA)
showed the effects of irradiation on material properties: the glass transition temperature
of the photopolymer rose by almost 10 °C from 70.8 °C of the untreated insert to 81.6
°C of the specimen irradiated with 200 kGy. The creep time temperature superposition
(TTS) tests proved that the increasing irradiation doses significantly reduced creep
compliance, which resulted in considerably lower mould insert deformations. Creep
compliance measured at 35 °C fell from 1920 μm2/N of the untreated specimen to 518
μm2/N of the specimen irradiated with 200 kGy. After the material tests, we applied
an elaborated comprehensive state monitoring system (operational strain, cavity pressure
and temperature measurements) to highlight the fundamental effect that the irradiation
has on the operational behaviour of the MJ-printed mould inserts. Injection moulding
tests showed that the increasing irradiation doses resulted in significantly decreased
operational deformations. Maximal operational strain of the mould inserts fell from
1 % measured on the untreated (0 kGy) insert to 0.5 % measured on the insert irradiated
with 200 kGy. It is highly desirable because product dimensional accuracy is also
increased. Irradiation also significantly increased mould life (the number of products
that can be manufactured), which is a crucial advantage from an economic point of
view. We proved that post-curing by gamma radiation is a feasible way to enhance the
applicability and the dimensional stability of photopolymer injection mould inserts.
This is a definitely novel way to enhance the applicability of MJ-printed low-volume
injection moulds.
