Polymer investment molding is a technique that combines injection molding and investment casting to create hollow microscale parts. It involves placing a sacrificial element into a micromachined insert, securing the insert in a mold, injection molding plastic around the element, and then dissolving the element to leave a hollow part. The document describes creating microneedles using this technique with different insert designs fabricated via RIE and KOH etching. Aligned and KOH inserts produced the best results, including triangular cross-section needles 300 microns long with a 35 micron hollow tip. While the technique offers a way to create hollow plastic microparts without post-processing, further refinement is still needed to maintain needle tip sharpness during

1. Polymer investment molding
Method for fabricating hollow
microscale parts

2. Introduction
• Polymer investment molding combines
traditional injection molding with investment
casting to create hollow parts on the micro
scale.
Engine block

3. • The major draw back with current techniques is
that making hollow parts difficult.
• Drawback is especially evident in the
manufacture of hollow microneedles for painless
drug delivery.
Needle
Problem

4. 1. Laser - ablation
To make hollow parts several post-
fabrication process have been developed
such as
Laser - ablation

5. 2. X – Ray exposure
X – Ray exposure

6. 3. Bonding of two parts after molding.
Bonding of two parts after molding

7. Solution for making of hollow parts
• To overcome the problems of making hollow
parts using molding techniques, we have
developed a method, investment molding,
that combined investment casting with
traditional injection molding.
• To demonstrate the potential of this method,
we investigate the fabrication of in-plane
microneedles.

8. Investment molding overview
• Investment molding is a
three-step process
1. Sacrificial Element
(Investment) is placed into a
micromachined insert.
2. The insert is secured into a
mould assembly containing
pathways for plastic to
reach the microfeatures.
3. The plastic part is removed
from the mold, and
immersed in etchant that
dissolves the investment
leaving a hollow plastic part.
Investment casting steps.

9. RIE insert
• RIE was used to fabricate two insert types
Straight-channel insert fabrication process.
1. Straight-channel
• The straight-channel
design is fabricated using a
single-mask
photolithography process
Astandard 4’’.
• Siliconwafer is patterned
using 2mof Fuji/Arch OiR
897-10i i-line PR and a Karl
Suss MA6 contact printer
(KS printer).
• The channels are etched in
a STS.

10. RIE insert
2. Aligned insert
• The aligned insert requires a
dual-mask process to achieve the
two level alignment structure Fig.
• The first masking layer is defined
in a 1m layer of thermal oxide on
a standard silicon wafer.
• A 2µm PR layer is spun-on and
patterned using the KS printer.
The pattern is transferred into
the oxide with a 10 min wet etch
in 10:1 hydrofluoric acid.
• The wafer is diced into two
inserts in the same fashion as the
straight-channel insert. Aligned insert fabrication process.

11. KOH (Potassium hydroxide) insert
• For the KOH insert
process, the anisotropic
nature of the KOH is
utilized to create a needle
body with exit vent and
aligner in one step.
• The overall depth and
length of the needle body
is determined by the etch
time.
• After etching, the wafer is
diced into two inserts in
the same manner as the
RIE process. Progress of KOH etch during insert formation. As
the KOH etch progresses the nitride mask is
undercut forming a tapered needle body and an
alignment feature, simultaneously.

12. Part fabrication
• 32 µm-diameter aluminum bond wire
(98% Al, 2% Si) is used as the
investment.
• A plastic part is fabricated by placing the
prepared insert into the mold assembly
which is installed in a 30-tonne FANUC -
30iA Roboshot injection molding press.
• Cyclic Olefin Copolymer (Ticona Topas®)
is injection molded into the insert.
Topas was selected for its balance of
strength and easy of manufacturing, as
well as its biocompatibility.
• The injection molded part is removed
from the mold and immersed in a 50 ◦C
bath of Transene Co. Aluminum etchant
(Type A). This etchant exhibits excellent
selectivity between aluminum and most
polymers, including Topas®.
• After approximately 12 h the bond wire
is dissolved, and the now hollow part is
removed and rinsed with DI water.
Micrograph of KOH insert with investment.

13. Results and discussion
1. The first results to discuss are
the inserts themselves. As Fig.
shows, there is a small
misalignment between the first
and second mask in the aligned
insert. This is a minor issue
however, since this difference is
less then a micron. The KOH
insert on the other hand shows a
significant defect.
2. The nitride mask was not able to
protect the alignment features
sufficiently so they did not
develop.
3. A thicker nitride layer and a
larger C structure should help
improve this result.
Micrograph of aligned insert with investment installed.

14. Straight-channel insert results
• With the injection molding
process optimized to deliver
sufficiently long rods,
investment casting was
attempted in the straight-
channel inserts. Fig. shows a
typical result.
• The length over which the
investment is surrounded by
plastic is approximately
200µm, within the length
scale of a microneedle.
Clearly, what is required is
better alignment of the
investment during molding.
Micrograph of typical result from straight-channel insert.

15. Aligned insert results
• The aligned inserts
delivered better
results. Fig. shows a
thin-bodied
microneedle structure
after removal of the
investment.
Thin-bodied microneedle from aligned insert.

16. Aligned insert results
• A thick-bodied needle with
tapered inlet. The needle is
also fully formed with a
hollow tip, however its tip
has been bent. This
appears to have happened
during the removal of the
part from the mold. This
suggests that greater care
must be taken in the
removal step to ensure
sharp needle points are
maintained. Thick-bodied microneedle from aligned insert.

17. KOH insert results
• The shaft is 300µm long
(base-to-tip) with a 35µm
wide hole at the tip.
• At the needles thickest
point, the cross-section is
approximately an
isosceles triangle having
two 105µm sides and one
150µm side.
• As with the RIE needles
the tip sharpness is hard
to characterize because of
the presence of the hole.

18. Conclusion
• Investment molding offers a
way to create hollow, inplane,
plastic parts in an injection
molding machine without
complicated post-processing
steps. This method has
delivered hollow
microneedles with lengths of
280µm and cross-sections of
130µm×100µm and
160µm×100µm. Triangular
crosssection needles, 300µm
long with two 105µm sides
and one 150µm side, and
inner diameters of 35µm,
were also produced.
Front side of needle tip before and after
aluminum etch.

19. Thank You
By: S. Arun Kumar
Roll No.: 135A5A0309.