DIY Injection Molding

I recently completed a university brief exploring sustainability in materials and design. Specifically exploring various ways of addressing plastic waste. Looking at Precious Plastics and their range of tools and methods for creating usable products from plastic waste. As I am studying product design I wanted to end up with a product made from plastic waste that was attractive, durable and desirable to customers.

My plan was to create a series of cast aluminium moulds suitable for the injection moulding of recycled HDPE plastic made from shredded milk bottle lids. Unfortunately this only partially worked but the process will be explored below. Along the way, I discovered (Not necessarily a new method but new to me) a way of using hot glue in cast injection moulds to create stunning colourful rings. This works well as a proof of concept and hopefully will eventually lead to fully recycled rings.

I am currently studying my BA in Industrial Design at the University of Johannesburg in South Africa. This project and some other very interesting projects are available on the university's Instagram page. https://www.instagram.com/ujindustrialdesign/

I used a wide variety of tools, materials and methods throughout this project. While some of these are quite specialised most are easily available at a local makerspace, or afforadbaly from a hardware store. My foundry that I used to smelt the aluminium was hand made from fire bricks, 4mm angle iron and I created a forge burner from plumbing supplies from The KIng of Random's tutorial. (https://www.youtube.com/watch?v=eO8NwseRxSA).

Materials

• Hot glue- I managed to find clear, pink, black and white
• Polyeurethane resin
• Silicone- Iused Amsil 20
• Foam core
• 3D printing resin
• Beeswax
• 4 nuts, 4 bolts, 8 washers
• Aluminium/ aluminium ingots
• Small oven/ blow torch

Tools

• 3D printer- I used a resin printer that was available at my university. A 3D printer is not essential as the pattern can be made out of a wide variety of materials or methods. What ever you are comfortable with.
• Aluminium Foundry- Mine was hand made following various online tutorials. Eventualy I will make an instructable on it.

Safety Equipment

Aluminium smelting is very dangerous and correct safety gear is essential. A respirator, thick leather apron, closed shoes, welding gloves and a face mask are absolutely essential. A fire extinguisher is required for any processes using fire and a bucket of sand (NOT WATER) is required to contain any aluminium spills

When using any resin, investment or silicone a respirator and gloves are essential. These materials release toxic fumes, dust and particles that can be very dangerous when inhaled.

Never perform a task that you are not fully comfortable with and always use tools as intended. If possible always have someone near by who can assist/ call for help incase something goes wrong.

The ring, mould and sprue were all modelled in Solidworks and exported as STL files for 3D printing. I created a two-part mould with identical parts. This allowed me to print a single side and create a single mould. I modelled 4 through holes that can accommodate a 5mm bolt to perfectly line up both sides. My process from 3D printing to manufacturing is explored in the images above.

The mould pattern was 3D printed on a resin printer. I had to remove support material with pliers and a knife. I used acetone to clean the prints and put them in the sun to fully cure. The quality was superb so no sanding or paint was needed.

I sprayed the 3D prints with mould release before attaching them to a foam core base. I built walls around the pattern using hot glue and more foam core. I used Amsil 30 silicone (But any casting silicone would be suitable). Make sure to mix the silicone in one container and then pour it into another container for additional mixing to ensure the two parts are fully incorporated. Pour the mixed silicone into the mould from as high as possible in a single stream to try and reduce air bubbles. Additionally pour from a single corner and let the silicone slowly flow over the part to ensure a virtually bubble-free pour.

I used polyurethane two-part resin to cast the mould halves. Make sure to weigh each part accurately, mix thoroughly, use two mixing containers and scrape the sides and the bottom of each. I also cast beeswax in the same silicone moulds melting it in an old coffee tin in the oven at a VERY low temperature.

The same process for creating the silicone mould was virtually repeated for the investment mould. The beeswax patterns were screwed to a base and walls were created around them. An investment slurry was created and poured over the wax patterns. The wax was then burnt out using a blowtorch and an oven. Some of the investment cracked when the wax was melted out. So in the future, I am planning on drying the moulds out for longer to hopefully let more water leave.

I melted a huge amount of aluminium cans, broken window frames, and scrap aluminium pieces. I also had leftover aluminium ingots from a previous project (https://www.instructables.com/Workers-Stool-Made-From-Reclaimed-Materials/ ). Once the metal was molten I poured it into the investment moulds. Burning out any remaining wax or residue. I removed any excess slag/impurities from the aluminium with a spoon before pouring.

Unfortunately, nothing worked.

A wide variety of things went wrong. The aluminium was not hot enough so it cooled too quickly. Setting before all of the air could escape. The moulds were not heated enough so they cracked when the aluminium was poured. Finally, the investment wasn't set correctly so moisture escaped further messing up the casting.

After the aluminium casting didn't work I tried another method. I sprayed the resin cast moulds with mould release before bolting them together. I used colourful glue in my hot glue gun and injected it into the mould. Initially, experiments failed as there was no place for the air to escape making the glue unable to fully fill the mould. I carved a channel at the bottom of each mould to allow the air to escape. I injected each mould with hot glue.

The rings can be quickly cleaned up with a knife and I used a lighter to smooth out any bumps. This created some very smooth comfortable colourful rings.

Above are the final rings. In the future, I want to use an injection moulding machine to use recycled plastic to make these rings. I would like to revisit the aluminium mould in the future as I believe they will work much better and for far longer under the high heat of injection moulding.

Other glue colours are available including glitter and metallic options that could also make great rings.