Classic plastic army men? Yep, they’re injection molded.
Sometimes there’s a tool you want to buy, but you can’t justify the cost. In that case, why not make it? For me, a plastic injection molding machine fell into that category — and it turns out they’re not hard to make.
I wanted to make solid plastic parts for some of my amateur science experiments. There are a number of ways to make things out of plastic, each with its advantages and disadvantages. Often just cutting raw material to the desired shape works best. Some plastics can be cast by pouring a liquid resin with hardener into a mold (see “DIY Rotocaster,” Make: Volume 41). Vacuum forming works well for making things out of thin sheets of plastic (see “Kitchen Floor Vacuum Former,” Make: Volume 11). I considered making a 3D printer, but for the few plastic parts I envisioned needing, it wasn’t worth the time and money.
Plastic injection molding has been around since thermoplastics were invented. It’s a great way to make many copies of a part quickly, and what I like best is that it’s easy to reuse old plastic objects to make new ones.
Build it Yourself and Save! Hobby-Vac Vacuum Forming Machine Plans $29.95. Construction plans for a tabletop vacuum forming machine that can mold flat plastic sheets into complex shapes using heat and vacuum. There are no comparable small machines currently being produced with this level of performance. Perfect for the serious hobbyist or small business. Vacuum Forming for the Hobbyist PDF eBook digital. There is an excellent booklet available entitled Do It Yourself Vacuum Forming for the Hobbyist by Douglas. Vacuum Forming for the Hobbyist PDF eBook digital download 12.95. Learn how to use heat and vacuum to mold flat plastic sheets into complex shapes. Build your own low cost equipment using hardware store items and your kitchen oven as a heat source.
So I built an injection molder based on the plans in Vincent R. Gingery’s book Secrets of Building a Plastic Injection Molding Machine. David Gingery could be considered a forerunner of the Maker Movement — he and his son Vincent have written a whole series of books on building tools for the machine shop.
This project should cost between $100 and $200. It depends a lot on where the metal is purchased. I had a lot of the metal already, left over from other projects. Try to find a friendly local iron dealer, rather than getting the metal online or from a hardware store. They’ll often let you pick through their cut-off pile and sell it for almost scrap prices.
I wouldn’t recommend this project for someone who has no metalworking experience. I made full use of my machine shop: mill drill, lathe, stick welder, horizontal band saw, ½” reamer, and various taps. You can get by with a drill press and various hand tools. I welded most of the assembly together but the book calls for screws, except for one weld (which you could have someone else do if you don’t have a welder).
My Build
Being an engineer, I couldn’t resist making improvements. The plans call for a 1″×1½”×4″ piece of cold rolled steel for the heater block (A), where the plastic is heated before being injected into the mold. I used a leftover piece of 1″×2″ hot rolled steel instead. The wider block allowed me to add a second cartridge heater (B), so my machine warms up quicker and can get hotter.
The frame (C) in the Gingery design is mostly angle and flat iron held together with bolts and nuts, but two connections needed to be welded. So I decided to weld most of the frame and avoid drilling so many bolt holes.
The injection lever (D) pivots on a½”-diameter steel rod (E). I beefed this up by drilling larger holes in the frame and turning some steel bushings (F) on my lathe to support the rod. The bushings also keep the lever centered over the injection piston (G).
My favorite fastener for projects is Allen head cap screws so I used #10-24 cap screws instead of 1/4-20 bolts to mount the heater block and guide block to the frame. They’re at least as strong as ungraded 1/4-20 bolts and should conduct a little less heat from the heater block to the frame.
(One modification I tried didn’t work out so well. The heater block and guide block need to be spaced out from the frame a bit, to isolate the hot heater from the frame and put the parts into proper alignment with the piston. The plans call for washers, but I tried using some leftover fiber ceramic insulation sheet instead. It was too soft — it allowed the heater block and guide block to move out of alignment, preventing the piston from operating smoothly.)
Temperature Control Upgrade
The major deviation from the plans is the temperature control. In the original design, you have to watch a dial thermometer while fiddling with a bimetal thermostat. Going with something more modern is well worth the effort.
I used an inexpensive digital temperature controller (H) from Auber Instruments (auberins.com). These controllers use PID (proportional-integral-derivative) feedback control and can bring the temperature up to the exact setting quickly without any overshoot. I mounted mine in a separate enclosure and added a solid-state relay to handle the high-power heaters. A thermocouple sensor (I) comes with the controller; it screws into a tapped hole in the heater block near the nozzle (J). It is really nice to be able to set the desired temperature on the controller, turn on the heaters and watch the temperature rapidly climb to the setting.
Using the Injection Molder
Using the machine is easy. The book recommends making a test mold that will make two ½”-diameter pellets of plastic. These pellets are then used to feed the machine when making real parts.
To try it out I cut some ⅜”-wide strips of polyethylene from an old plastic tote lid, set the controller to 380°F and fed the plastic strips into the cylinder. After the cylinder is filled with sufficient molten plastic, the mold is placed under the nozzle and raised into place. (If the mold in put in place while the plastic is being loaded some of the plastic will dribble out of the nozzle and clog the sprue of the mold.)
Then pull the lever and inject! Here’s the pellet mold and some successfully injected pellets.
Some Tips
![Vacuum forming hobby Vacuum forming hobby](/uploads/1/2/6/2/126263061/153927204.jpg)
- Wear leather gloves when working with the machine. The heater block is quite hot and the mold warms up with use.
- It’s easy to apply too much pressure when injecting the plastic. Stop when you first feel additional resistance. If you apply too much pressure, plastic will squirt out of the mold and you’ll have a lot of “flash” on your part.
- I put a little oil on the piston where it goes through the guide block. Plastic can build up and make it sticky.
Making Molds
Now I’m making test-tube caps for my science experiments. I started out by drawing a sketch for the mold in my project notebook. The test tubes are 16mm in diameter so I started with that dimension and chose arbitrary dimensions for the height of the cap and thickness of the plastic. I added a lip to the cap to make it easier to pull off.
Once the sketch is made, I’m off to the machine shop to cut some metal. Two little blocks of 6061 aluminum make up the halves of the mold. First, two 0.124″ holes are drilled through both blocks to hold guide pins that will ensure the mold halves are properly aligned. Two ⅛” guide pins are pressed into one half of the mold, and the other holes are reamed out just slightly to 0.126″. Then the parts are machined on the lathe to form the cavity for the cap. Finally a sprue hole is drilled and countersunk.
I tried out the cap mold with some of the tote lid plastic I used for the test pellets. The caps turned out nice, but they were a bit stiff and difficult to put on. A more flexible plastic was needed. Then I made some caps with LDPE (low-density polyethylene) lids from oatmeal boxes. The caps made with the LDPE work just fine.
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For now, I’m limited to simple molds I can make with a lathe or mill, but I’m adding CNC capability to my lathe, and thinking about a CNC router. That would open all sorts of possibilities.
Learn more: Secrets of Building a Plastic Injection Molding Machine by Vincent R. Gingery (David J. Gingery Publishing, 1997), ISBN 1-878087-19-3
A simple visualization of the forming process
Vacuum forming machine to produce inner liner/food liner of refrigerator
Vacuum forming is a simplified version of thermoforming, where a sheet of plastic is heated to a forming temperature, stretched onto a single-surface mold, and forced against the mold by a vacuum. This process can be used to form plastic into permanent objects such as turnpike signs and protective covers. Normally draft angles are present in the design of the mold (a recommended minimum of 3°) to ease removal of the formed plastic part from the mold.
Relatively deep parts can be formed if the formable sheet is mechanically or pneumatically stretched prior to bringing it into contact with the mold surface and applying vacuum.[1]
Suitable materials for use in vacuum forming are conventionally thermoplastics. The most common and easiest to use thermoplastic is high impact polystyrene sheeting (HIPS). This is molded around a wood, structural foam or cast or machined aluminium mold, and can form to almost any shape. This high impact material is hygienic and capable of retaining heat and its shape when warm water is applied and is commonly used to package taste and odor sensitive products.[2] Vacuum forming is also appropriate for transparent materials such as acrylic, which are widely used in applications for aerospace such as passenger cabin window canopies for military fixed wing aircraft and compartments for rotary wing aircraft. Vacuum forming is often used in low-level technology classes for an easy way to mold.
Typical applications[edit]
Vacuum formed vehicle part
Original equipment manufacturers (OEMs) utilize heavy gauge vacuum formed components for production quantities in the range of 250–3000 units per year. Vacuum-formed components can be used in place of complex fabricated sheet metal, fiberglass, or plastic injection molding. Typical industry examples besides product packaging include: fascias for outdoor kiosks and automated teller machines, enclosures for medical imaging and diagnostic equipment, engine covers in a truck cab or for construction equipment, and railcar interior trim and seat components.[3] Vacuum formers are also often used by hobbyists, for applications such as masks and remote control cars.
Common problems[edit]
There are some problems encountered in the vacuum forming process. Absorbed moisture can expand, forming bubbles within the plastic's inner layers. This significantly weakens the plastic. However, this can be solved by drying the plastic for an extended period at high but sub-melting temperature. Webs can form around the mold, which is due to overheating the plastic and so must be carefully monitored. Webbing can also occur when a mold is too large or parts of the mold are too close together. Finally, objects that are formed often stick to the mold, which is remedied by using a draft angle of three degrees or more in the mold.
Types of molds[edit]
There are numerous patterns one can make with vacuum forming. The most inventive way to use vacuum forming is to take any small item, replicate it many times and then vacuum for the new pattern to create a more cohesive form. The vacuum forming helps tie the individual pieces together and make one mold out of many pieces that can easily be replicated. From there, you can cast plaster, concrete, etc. into the plastic form.
Wood patterns are a common material to vacuum form as it is relatively inexpensive and allows the customer to make changes to the design easily. The number of samples that one is able to get from any pattern depends on the size of the part and the thickness of the material. Once the specifications of the part have been met, the pattern is then used to create a ceramic composite mold, or cast aluminum mold for regular production. Potentially, there are ways to create holes in plaster with a vacuum form if the replicated forms you make the vacuum form from are deep enough and you leave gaps between them for the plastic to form into. Then, once the plastic is used to cast a plaster mold, the deep plastic areas will leave holes if the mold is not completely filled.
Castaluminium molds are cast at a foundry and typically have temperature control lines running through them. This helps to set the heat of the plastic being formed as well as speed up the fabrication process. Aluminium molds can be male or female in nature, and can also be used in pressure forming applications. The main drawback with this type of mold is the cost.
Vacuum forming mold made from Aluminium (cavity) and Steel (frame)
Machined aluminium molds are like cast aluminium, but are cut out of a solid block of aluminium using a CNC machine and a CAD program. Typically, machined aluminium is used for shallow draw parts out of thin gauge material. Applications may include packaging and trays. Cost is a significant factor with this type of tooling.
Free plugin pro tools. Composite molds are a lower cost alternative to cast or machined aluminium molds. Composite molds are typically made from filled resins that start as a liquid and harden with time. Depending on the application, composite molds can last a very long time and produce high quality parts.
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Finishing methods[edit]
Vacuum Forming Hobby
A sheet of vacuum-formed plastic. The pieces needed (in this case parts for a model aeroplane) will need to be cut out of the sheet.
Once a vacuum forming has been created out of a sheet of plastic, a finishing operation will be needed in most cases to turn it into a usable product. Common vacuum forming finishing methods include:
Guillotining: The product is cut out of the sheet by pressing a blade through the product into a die underneath. This is a clean way of removing vacuum formed parts from the material sheet. It doesn't require a special cutting tool to be made for an individual product and is therefore suitable for low volumes of parts where straight lines are no problem. Cutting only straight lines and being a fairly slow approach compared to other finishing methods, guillotining can be expensive for projects with larger, more complex quantities.[4]
Drilling: If simple round holes are the required finish, manually drilling them is a good solution for small quantities. Drilling guides can be used to ensure holes can be drilled quickly in the right place. As this is a labour-intensive method, it is only suitable for small production quantities.
Roller cutting: Process whereby the vacuum formed product is placed on a custom made cutter and pushed through a roller cutter machine. An efficient way of cutting vacuum formed items from the original sheet of vacuum forming material. The cutter can also cut any necessary holes, such as cable or access holes, at the same time. Roller cutting is suitable for fairly large items, where precision alignment is not required. As the vacuum formings and the cutter are rolled sideways through the roller cutter machine, some misalignment can occur. Roller cutting can't be used for making holes or features in the sides of vacuum formings, as the cutter tool always cuts vertically from the bottom.
Press cutting: This is a very precise method of cutting which uses a press and a custom precision made cutting tool. This method is very suitable for items where the roller cutter process wouldn't achieve the precision required. It's often used for punching very small finished items out of a plastic sheet, rather than punching features into a product. Press cutting can't be used to make features in the sides of vacuum formings.
Cutting with a CNC machine: Machining is a very precise method of creating holes & features. The real advantage is that it can be used to create features in the sides of vacuum forming, e.g. guide rails for a tray that needs to slide onto shelving. It can also cut sidewalls of a different depth than the pockets—something which cannot be done with roller or press cutting.[5]
References[edit]
- ^J.L. Throne, Understanding Thermoforming, Hanser Gardner Publications, Inc., Cincinnati, OH, 1999
- ^Plastics, Eagle (2013-02-18). 'High Impact Polystyrene Sheeting (HIPS), What is it?'. Eagle Plastics Ltd. Retrieved 2018-04-30.
- ^'Vacuum Forming Services - Custom Vacuum Forming Plastic | Emco Industrial Plastics'. www.emcoplastics.com. Retrieved 2018-04-30.
- ^'Vacuum Forming - 5 Finishing Methods'. www.toolcraft.co.uk. Retrieved 2018-04-30.
- ^Webster, Stephen. Vacuum Forming Market, Stephen Webster Plastics Ltd. Retrieved 2018-04-30.
Further reading[edit]
- Soroka, W. Fundamentals of Packaging Technology, IoPP, 2002, ISBN1-930268-25-4
- Walsh, D. E. Do It Yourself Vacuum Forming for the Hobbyist (rev. ed.), Workshop Publishing, Lake Orion, MI, 2002, OCLC46798883
- Yam, K. L. Encyclopedia of Packaging Technology, John Wiley & Sons, 2009, ISBN978-0-470-08704-6
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