Vacuum Forming Vs Injection Molding, What is difference?
Plastics have been the industry standard for small to large parts, products, and prototypes. They are cheap, offer mass production, and are all around us. Yet, not many people know the manufacturing processes of plastic goods. It may sound surprising, but there aren’t more than a few production methods for plastics. It is worth understanding the differences between the most popular methods to decide which will achieve the complexity of your design eventually.
In vacuum forming or thermoforming, a plastic sheet is stretched and clamped in a frame, where it is heated to its forming temperature (the temperature at which the plastic softens), and a vacuum is created under the plastic sheet that allows atmospheric pressure to stretch the sheet into the mold where it cools. In injection molding, like a syringe, melted plastic is pressured into a mold cavity until it fills all crevices and is allowed to cool, forming the final product.
Vacuum forming and injection molding can be used to make many complex designs, but there are many differences that start with cost, production time, raw material, and details of a product for which you should take a detailed look at both processes.
What is Vacuum Forming?
Vacuum forming is a method to produce plastics using a soft heated plastic sheet that covers the mold surface and traps air inside. Tiny vents inside the mold remove the air producing a vacuum and atmospheric pressure forces the plastic sheet to cover the mold completely. Once cooled, the mold is separated from the product. This process is also called pressure forming and utilizes a female mold.
In vacuum forming processes, where a male mold is used, the plastic sheet is heated and draped over the mold. The mold is raised and the sheet covers all sides forming an airtight connection. A vacuum is created using vents and the atmospheric pressure presses the sheet on the sides of the mold, giving it the projections and details of the final product.
- Cheaper production costs
- Repeated batches can be made with
- Consistency Efficient prototyping
What is Injection Molding?
Injection molding is a mass-production method that works by injecting molten plastic into a mold and cooling it down to conform the plastic to the shape of the mold. The mold opens and the product is ejected.
At the face of it, this might seem very simple, but there are many considerations that need to be discussed when injection molding.
- Low production costs. Economies of scale
- Great for producing identical products
- Little to no material is wasted
- Quick production rates
7 Difference Between Vacuum Forming and Injection Molding?
Vacuum forming and injection molding are both widely used plastic-making techniques. They are both suited for different purposes and dominate different industries. However, injection molding is a far more popular plastic manufacturing method and accounts for almost $269 billion worth of the plastic market compared to a $14 billion market cap of vacuum forming. This goes without saying, most plastic products come from injection molding, but is it right for your project?
We’ll now cover the 7 key differences between vacuum forming and injection molding.
1. Product Development Cycle
In vacuum forming the product development cycle starts at the design stage. After an accurate model is built, considerations about the tooling are known. The amount of plastic sheet and whether male or female molds are to be used is decided. The tooling stage usually involves aluminum molds. These molds can be repeatedly used for thousands of production runs. Cut from a CNC machine, the range of error is quite small and almost every part produced is identical. Next, is the vacuum forming process, which an experienced individual does manually or for large production, controlled by a machine. Finally, the part is cut using a die cut or CNC for final packaging.
The product development cycle of injection molded parts begins at the design stage. The required design is made and a prototype is developed using a 3D printer to assess, the form and fit of the part. After approval, the tooling process begins. Here, the tooling is made from a precision CNC machine and instead of the mold having any raised projections for the plastic sheet, a mold cavity is constructed and the tooling is always in two parts. Next, the injection molding machine uses a heated mechanism and plunger to force molten plastic through a nozzle into the mold. The part cools and an ejector mechanism releases the part when the mold is opened. At the final stage, runners and gates which are not needed are cut off from the part.
2. Process Capabilities
Vacuum forming and injection molding have different capabilities. While both processes can develop your designed product, one might be better suited than the other in the long run. Here’s a table comparing the process capabilities of vacuum forming and injection molding that takes into account everything from scratch.
|Process Capability||Vacuum Forming||Injection Molding|
|Efficient Design Stage||✔||✘|
|Versatile Product Range||✔||✔|
|Repeat Jobs s||✘||✔|
|Adapts to Changes Easily||✔||✘|
Injection Molding vs Vacuum Forming Process Capabilities
The stage which defines your choice is the tooling and detail on your part. Injection molding requires attention to detail at the tooling stage. This is where it is better than vacuum forming as the tooling can be made as intricate as possible with tiny cavities, threads, cores, holes, and places for other composite structures. It is possible to achieve details in the micro-scale with injection molding. The same details are reflected on your part and depending on the size of the job, a small part is made within seconds. The high repeatability of a job is also a reason, you should consider injection molding.
The melted plastic is forced into the mold through a screw which ensures it is molten throughout compared to vacuum forming where the sheet is not at an even temperature. This results in identical parts in injection molding but a chance of discrepancy in thermoforming.
3. Tooling (mold)
Tooling is simply the process of making the mold.
Tooling for vacuum forming is fairly less complex than other plastic manufacturing processes. Vacuum forming only requires details on one side of the tool which results in quicker and more cost-effective tooling. The raw material cost is also halved as you’re using only a one-sided tool. It can be made from aluminum or a cheaper aluminum alloy as it isn’t intended to last for thousands of batch runs.
A major consideration for the tooling is the cooling process. As more and more plastic sheets are molded, the temperature of the tool rises and this can result in long wait times for the tool to cool in between production runs. For this reason, cooling the tool can also affect or limit the design intricacies. The tooling in vacuum forming is best for large parts and becomes more cost-effective as size increases.
The tooling for injection molding is much more time-consuming and has a higher cost. This is because, the tooling can achieve much more internal complexities, and details to make the final part. The tooling can have these tiny cores and threads because the large pressure through which molten plastic is forced into the two-sided mold can deliver the plastic into all crevices and holes. Other than the hardened steel or beryllium-copper alloy which increase the cost of tooling, the vents inside the cavity to release air are under 40 microns in diameter, which can only be made by special drills.
The choice of plastic that can be used for vacuum forming or injection molding is vast and cover a range of properties from color, toughness, durability, melting points, etc. For both of these plastic-making processes, thermoplastics are used. Thermoplastics can be recycled and reheated repeatedly without causing any change in their chemical structure. If you plan on using thermosets, which are much tougher and resist weight, these methods might not be your best choice.
Vacuum Forming VS Injection Molding Material Range
Vacuum forming has a smaller range of raw materials than injection molding. This is because, in injection molding, the plastic is completely melted and evenly distributed to the mold. But, in vacuum forming, the plastic is neither evenly heated nor has the same thickness across the part.
|Vacuum Forming||Injection Molding||Use|
|High Impact Polystyrene (HIPS)||Polyacetal / Nylon||Beverage cups / Gears|
|High Density Polythene (HDPE)||Polystyrene||Shampoo bottles / foam packaging for food|
|Acrylonitrile Styrene Butadiene (ABS)||Acrylonitrile Styrene Butadiene (ABS)||Pipes and Fitting, luggage cases|
|Polyvinyl Chloride (PVC)||Polyethylene||Garden hoses / grocery bags|
|Acrylic (PMMA)||Acrylic (PMMA)||Medical uses|
5. Precision & tolerances
Injection molding is a very controlled plastic part manufacturing process. Everything from the raw material to the final finish can be altered to suit requirements. The real tolerances of this process come from the tooling. With CNC-machined molds, that are sophisticatedly cut from multi-axial tools, you can expect a tolerance of around one-tenth of a millimeter (0.1 mm). This results in identical parts being manufactured within minutes.
Vacuum forming has many aspects that cannot be controlled. For this reason, it’s reasonable to expect a greater tolerance and smaller precision in the parts. For instance, when the plastic sheet is stretched over the mold, it’s not possible to stretch the film evenly maintaining a uniform thickness. You can expect a tolerance of around 5 mm with most parts.
6. Surface finish
Surface finish is extremely important for the final part. This describes the aesthetic, feel, grip, and more importantly, stops unwanted surface marks and impurities from disrupting the fit or paint of the part.
Injection molded parts have generally a higher quality surface finish, as the mold is accurately cut and polished before use. The cavity image affects the final surface finish. The choice of raw materials also changes the surface considerably. Resin-based materials provide a glossy finish while others give a grainy rough feeling. Injection speed and cooling times also affect the surface finish.
Vacuum-formed parts have a poorer surface finish as it is exposed to the atmosphere. Vacuum-formed parts aren’t generally made for their aesthetics, but their surface can be improved by painting and printing. You would commonly see this on food plates and plastic packaging.
With these key differences, you might be wondering which one and how do you choose them for your project.
Here’s a quick guide that can help you decide where to start and how to finish.
How to choose them for your project?
Automotive engine, transmission connector, intake manifold, etc. In addition, aluminum can also be used in the car lid, doors and front bumper, aluminum body will be a future development trend.
The plastic pellets you choose for injection molding offer a wider variety of choices in mechanical properties, colorant that can be mixed with the pellets, and the final surface finish.
With vacuum forming your raw material choices are limited to plastic sheets made from ABS or other common plastics. If you’re looking for variety in your parts, injection molding is the better option.
Details in the part
Injection molded parts are superior in terms of detail, tiny tolerances, cavity, holes, screws, and bosses. It is not possible to achieve this with a single-sided mold in vacuum forming.
Injection molding is a completely controlled process and produces identical parts. If you’re looking to make complex parts with internal details, injection molding is the only option.
Without a doubt, for small jobs and bigger tolerances (smaller precision) vacuum forming is the industry leader. The cost of tooling and production is much smaller compared to completely melting the pellets and using a sophisticated plunger mechanism.
However, it’s worth mentioning for larger repeated jobs injection molding can lower the eventual cost of production and get a faster production rate.
Type of part
Sometimes it all boils down to the type of part you are designing. Even though, injection molding might seem like the better option, vacuum forming can make the same thing with a much simpler process. For example, consider helmets made from polycarbonate.
Both processes can manufacture polycarbonate parts, but the shell of a bike helmet can be easily made within a reasonable range of tolerance using pressure forming. So, why bother with spending extra on tooling and injection molding equipment?
There are many parting lines between vacuum forming and injection molding. It all boils down to the size, cost, and complexity of your part. Injection molding is a much better production method but has a higher cost. If time is of the essence and there’s common ground between both production methods for your project, vacuum forming is the better choice.
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.