3. Investment Casting

3D Mastering for Injection Moulding Project Author: Adeeb Anees Bhiwandiwala

Supervisor: Dr. Nikolaj Gadegaard

University of Glasgow, charity number SC004401

MSc. Mechanical Engineering

1. Introduction

2. Industrial Application

5. Injection Moulding

4. Design Considerations and CNC Machining

6. Conclusion and Recommendations

References

In the context of this project, investment casted inserts were not prepared due to time, budget

(outsource), technological (+1000ºC vacuumed furnace) and tool size constraints as well as lack of

experienced supervisory workshop staff. Injection moulded lobe impellers from CNC machined inlays

established that traditional manufacturing process was still promising for complex setups. However, a

suitable ejection system for a larger tool size was conceptualized and prototyped using the FDM

technology for the purposes of illustration.

Extensions concerning this investigative project would include:

• Machining and testing an ejection system for complex mould designs in the injection moulding tool.

• Feasibility study of inserts prepared from novel rapid prototyping technologies that amalgamate the

casting and machining processes, in particular Hybrid Rapid Prototyping.

Injection moulding is the driving technology in the manufacture of plastic components. There is a rapid

increase in the number of parts being manufactured in plastic as the technology is developing. This provides

for opportunities in replacing metal parts which are more costly to manufacture. With this trend come the

requirements for new tooling technologies to enable this translation from metallic parts to plastic

replacements. Traditional tooling is performed using CNC milling or spark erosion of metallic inserts. The aim

of this project is to explore the potential of using 3D printing to manufacture industrially relevant tooling inserts

for injection moulding. Figure 1: ENGEL Victory 28 injection moulder [1]

Investment casting (IC) is a manufacturing process that

links suitable RP tooling models with other

manufacturing processes. The IC process involves

dipping a wax model into ceramic slurry, followed by

firing in an autoclave to dissolve the wax and leave

behind a cavity for a suitable casting material. With the

advent of RP technology, the IC process has been

transformed into a

more rapid and

lucrative process. In

this context, the

combination IC and

RP technology

facilitate for the rapid

tool pattern

generation in various

casting materials.

Figure 3: Investment mixing procedure and a typical burnout cycle [4]

Figure 4: Overview of the Investment Casting process [5]

Table 1 summarizes the key parameters of the existing injection moulding machine. The

synchronization of the clamping unit and the injection unit, two constituents of the

moulding machine, facilitate in the melting, injecting, and solidifying of the plastic resin at

appropriate stages. The Polypropylene (PP) pellets are poured and stored within the

hopper before being gravitationally introduced into the cylindrical barrel via throat. The

Solidworks was used to develop CAD models of 24x24x8 mm inlays that were to

be inserted in the tooling frame. However, the existing setup, as illustrated in

Figure 5, revealed the formation of a flat squared platform (24 x 24 x 2 mm)

before the development of the patterned surface. This was observed due to the

internal tapering feature of the frame in which the inlay was accommodated. The

sprue was positioned at the bottom of the frame due to the eccentric position of

the injection nozzle on the fixed platen.

Modification to the tooling was classified as expensive, time consuming and

problematic for other ongoing applications. As such, the inlays were modified

with an upper cut to eliminate the flat plate formation and a couple of gates

additionally designed to facilitate uniform filling. One and three gates designed

impeller inlays were scrutinized under Solidworks mould flow analysis. Designs

were rectified accordingly to minimize observed defects before being machined.

Impeller shaft cavity was disregarded due to ejection issues and a drastic 6°

taper angle was incorporated to eject the remaining mould smoothly.

Figure 5: Cross-section of existing setup Figure 6: Illustration of flow

simulation indicating fill time

Figure 7: CNC machined inlays for successful injection moulded lobe impellers

Table 1: ENGEL Victory 28 specification summary [1]

barrel houses the Archimedean screw and several heaters that plasticize the

thermoplastic to its melt temperature (Tm) while facilitating uniform mixing for

homogenization of temperature, pressure and consistency. A non-return valve at

the screw end regulates the flow of the molten polymer into a cold cavity to cool

and solidity under high holding pressure before being ejected from the clamping

unit. The PP (Brand: Capilene 125) injection moulded lobe impeller constituted of

a fan gate overlapped with a semi-submarine gate and a ~4 cm sprue, as

illustrated in Figure 7.

The name of lobe pump is derived from the round shape of its rotor

radial surfaces. It is a fixed volume, positive displacement rotary

pump. The fluid is forced to flow around the interior of the casing and

not between the lobes .The lobes rotate in synchronism and do not

come in contact with one another as this is prevented via external

timing gears situated in the gearbox [2]. They are capable of working

with various high and low viscosity fluids and can even pump solids

(both soft and hard particles) without damaging the product with

minimal agitation or shear [3]. Petrochemical industries deal with lot

of chemicals & additives which are highly viscous (Monoethylene

Figure 2: Industrial

lobe pump [3]

[1] Stormonth-Darling, John Moir (2013, July). Fabrication of difficult nanostructures by injection moulding, PhD

thesis for University of Glasgow, 33-36

[2] Kang, Y.-H., & Vu, H.-H. (2013, October 26). A newly developed rotor profile for lobe pumps: Generation and

numerical performance assessment. Journal of Mechanical Science and Technology, 915-926.

[3] Viking Pump, Inc. (2014). Industrial Lobe Pump Series. Retrieved June 12, 2014,

http://vp.salesmrc.com/pdfs/925_industrialLobe_en.pdf

[4] Specialist Refractory Services Limited. Steelcast : investment powder for jewelry casting. Retrieved June 26,

2014, http://www.srs-ltd.co.uk/products_services/steelcast.html

[5] ACTech GmbH. Ceramic Shell Process: Investment Casting. Retrieved July 03, 2014,

http://www.actech.de/index.php?id=83&L=1

glycol, lubricating oils, etc) and corrosive (sulfuric acid, caustic, hydrochloric acid, etc).

Some processing chemicals can be hazardous and toxic for human exposure during

leakages. Conventional metallic impellers are not suitable for such application as plastic

(Polypropylene) lobe pumps are ideal for a combination of viscous and corrosive service.