TechArchive

Original article date: February 1999

How do you achieve an enclosure with an injection moulded finish in volumes which preclude the use of the process? Bafbox has an answer as Alan Quinn discovers.

When it come to deciding how to house the product for many designers and professionals involved in the medium-volume capital goods market there is often a difficult choice to be made. If cost were no object the favoured choice would likely be injection moulding. However with physically large parts and production runs sometimes not even in the hundreds the cost-effectiveness of injection moulding rapidly diminishes. Typical applications which fall into this category include information displays where display sizes frequently alter requiring changes in enclosure aperture sizes and fixing positions.

The Bafbox Hiform high pressure forming process launched in 1998 delivers injection moulding quality without the up-front investment and lead times required for tooling. It combines 3D solid modelling with computer-aided machining to deliver plastic enclosures and covers that to all intents and purposes look and feel as if they are injection-moulded. There is also the flexibility to change moulding features during the life of the product – practically any design modification can be incorporated into the design at relatively low cost without the cost penalties associated with retooling an injection moulding. Prototypes are usually available within four six weeks and production quantities available six weeks later.

The 3D software package Solidworks is used to design and draw up the housing and a comprehensive schedule of design tooling prototype and production run costs are produced. Once the design is approved the 3D information is transferred to a CNC-controlled router that machines the forms from which the moulds are made.

Then using an innovative aluminium deposition process the mould is then created. Details of the aluminium deposition process are few but it is known that the resin male form is first coated in water-soluble PVA release agent. Aluminium or carbon is then deposited onto the male form using high current electric arc and a 2-3mm rod in a process which resembles arc welding. It leads to a coating of aluminium or carbon about 1mm in thickness. Deposition normally takes between ten and twenty minutes.

The male resin form is then reversed into a second resin block and removed leaving behind the aluminium/carbon shell as a female form – the mould. All the accurate detail machined in initially to the resin block is now on the inside of the mould. Typically this mould is heated to between 20 and 80degC depending on the complexity of the product. A heated plastic sheet is then placed over the mould and sealed. The plastic sheet is formed into place using an initial vacuum followed by 7-12 bar air pressure from above forcing the plastic into the detail of the mould. The male form having similar shape as the female mould is then pushed into the female mould targeting areas of potential thinning to create a generally uniform wall thickness.

Now the fine detail reproduced on the outside of the component giving an injection moulded quality of finish. This is totally different to vacuum forming where plastic is drawn over the form to make the part. Materials used include polystyrene and ABS as well as polycarbonate for applications which must be vandal-resistant.

The formed enclosure shrinks by a factor of 1.009 which is pre-built into the original Solidworks model and it pops out of the mould. The moulds are found to be good for up to 10,000 cycles.

Once the raw part is formed it is then post-machined on CNC routers. Webs pillars and grooves can be added or modified at a later date if required using a bonding process and without affecting the tooling process.

Bafbox then applies the desired paint finish and the enclosure is completed with screen printing if required. Recently the company has launched a new range of paint finishes for the Hiform moulding process. They encompass metallic soft-feel and duo-tone flip-flop types.

A successful example of the use of Hiform process is ocular blood flow measuring equipment manufactured by OBF Labs. The OBF Tonometer is used for detecting and monitoring glaucoma. Research indicates the importance of ocular blood flow and the Tonometer measures not only intra ocular pressure but also pulsatile blood flow in a single simple test. This effectively doubles the detection rate. The machine has a resolution of 0.01mm Hg and automatically takes 200 readings per second. Test results are displayed instantaneously on an integral LCD screen. IN all the test takes only 12s per eye.

The housing was designed to be produced initially in small batches to help establish the product. Once awareness was established the design allows cost-effective production in much larger batches. Product designer Mike Ayre who runs an industrial design company in Wantage managed the transition of the conceptual design through model making and prototyping to production at Bafbox.

• Bafbox
• 01280 705777
• Chris Mills

February 1999