Plastic molding is a broad set of methods, but it’s often referred to in all encompassing language that blurs the complexities and differences.
This article is one of a series intended to explain the relative benefits and drawbacks of the full range of methods that are used for molding and forming polymers.
In this article we focus on explaining the differences between thermoforming (often called vacuum forming) and injection molding, to show when each should be used and how the two processes relate to each other
I. Types of Molding Methods
Thermoplastics refers to materials that are widely used in a variety of manufacturing methods to produce an astonishing range of finished parts for consumer, medical, industrial and other uses.
Chemically, polymers are long chain molecules that exist in nature to a degree, but many more are synthesized, increasingly designed with particular properties in mind.
- Polymers offer various options in toughness, chemical durability, elasticity and low manufacturing and processing costs in making complex parts
- Melted thermoplastics are low viscosity, unreactive liquids that can conform to any imposed shape. At lower temperatures they become highly plastic solids – able to stretch and return to their original form unless cooled first
- Multiple coles of remelt and reshape can be tolerated, though properties may degrade more quickly for some polymers than others
- Forming complex shapes and recycling to form a new shape are the great strengths in polymer manufacture
Applicable manufacturing methods vary extensively in their ability to deliver precision, fine detail, hollow vessels and their applicability to families of materials such as thermosets, thermoplastics and rubbers/elastomers.
All forming methods use a cavity or former to impress a shape on the material, and this shape is retained by the polymers either because of cooling, or a catalytic completion of polymerization from soft/liquid to a solid state.
In the example discussed here, the processes of injection molding and thermoforming are both specifically used for thermoplastics.
The polymer is heated until it either loses its structure entirely (i.e. liquifies) or it becomes softened and subject to highly plastic behavior (i.e. it softens and can be stretched). In both cases, a new structure/form is imposed on the hot material and it is then cooled to lock it into the new shape.
II. What is Thermoforming?
Thermoforming is a shaping process that works with sheet materials, stretching them over a former (or into a cavity) by mechanical and air pressure means.
The material is heated, generally with a radiant heater on one side (and sometimes both sides) until the required degree of softening is achieved. This is a temperature that will vary according to the nature of the materials, former depth/complexity and machine air pressure.
With the material appropriately softened, the heater is usually retracted and the former pushed up against hot plastic. Either pressure, or vacuum is then applied to pull/push the softened sheet into close contact with the former.
With the shape achieved, pressure (both mechanical and pneumatic) is retained and the formed sheet is allowed to cool, preserving its memory of the new shape.
Once extracted from the forming machine, the remaining ‘flat’ sheet surrounding the required part is trimmed away, or punch cut, to release an essentially finished component and form any holes that are required in the product.
Some refining and finishing of these cut edges may then be necessary, to complete a deliverable and end use ready part/component/product.
III. What is Injection Molding?
Injection molding is as different from thermoforming as a process can be, but the purpose is identical; – transformation of polymer source material in a one stage process, from raw material to near-finished part requiring minimal or no post processing.
Polymer chips or pellets are melted in the machine barrel, which is both a preparation chamber and a hydraulic ram. This melting involves mixing using an integral archimedes screw to ensure good heat distribution and uniform properties.
Once the plastic is ready, the ram forces a charge of it into the tool through an injection gate. The liquid polymer entirely fills the cavity or cavities, displacing the air from the tool which leaks at the closure faces. The cooling process then starts, hardening the polymer in the shape of the cavity it fills.
When cooling from the melt temperature, most polymers shrink significantly – by as much as 15%. The mold cycle involves a sustained pressure period, so the shrinkage is counteracted by forcing additional plastic into the cavity.
After cooling, the polymer will be a faithful copy of the cavity it solidified in, dimensionally accurate and textured from the cavity surfaces.
As the tool opens, integral ejector pins push the part out and it drops or is taken from the tool, by hand or robot.
The finished part may have mold tool aspects connected to it, where the material was fed to the part. These aspects require manual or machine trimming, to complete the part manufacture.
IV. Thermoforming and Injection Molding: Which Method is Best for Your Application
There is no hard and fast rule about process selection in plastic components. A sequence of design decisions will be taken, starting right back at the concept stage, and these will drive the choice of tools/processes.
Some components are only practical when made by thermoforming. If the design task is a single use product package, or a food container clamshell, or a dome to cover a radar then thermoforming is likely the right process to select.
A sheet formed over a one sided former is a great way to enclose and protect – takeaway food containers, egg boxes, hanging display packages, low volume enclosures for electronics, racking and tote tray liners and much more.
Many component specifications cannot be met by a low detail, formed skin part but need the precision of a steel cavity as a former for molten plastic.
This decision is often made by classification – packaging and temporary use components are often made by thermoforming by default. But the process can achieve greater precision and more feature detail, allowing limited and simple components – such as machine covers, ventilation louvers, acoustic shells, auto roof boxes and more.
Some are products that could be made by other means, but the decision to use thermoforming often relates to the tool cost more than the component cost.
As a rule, large and simple parts may be feasibly made by thermoforming methods, but the component price will be considerably higher than by injection molding. This decision usually results from the expectation of production volumes – high volume products will tend more towards injection molding.
Thermoforming: Benefits and Applications
Thermoforming is a powerful method that allows the designer to achieve many of the benefits of thermoplastic polymer use, while avoiding the extreme tooling costs of injection molding;
- Very low cost tooling, often 2% of the cost of injection mold tooling, particularly significant for very large tools such as simple watercraft components like paddleboard skins and automotive roof boxes – both of which are commonly thermoformed
- Very low cost clamshell packaging that offers very high product protection levels. Plastic egg boxes were very widely thermoformed, although this application has moved to greater use of card slurry moldings.
- Product packaging for hanging display, using thermoformed PET and PETG is widely used, across most small consumer product areas. these can be one side thermoformed, for card backing or clam shell types
- Large weather covers for marine radar are often thermoformed, as the tooling cost is very low and the product volume is insufficient to justify that cost. Additionally, the product being packaged is high value, so the additional cost of finishing implied by thermoforming is acceptable
Injection Molding: Benefits and Applications
Injection molding is the go-to solution for such a huge range of applications, markets and products that the list grows faster than it can be written;
- Parts requiring low to moderate strength, good accuracy/cosmetics, low cost and high volume generally tend towards injection molding – making injection molding ideal as a manufacture method for most consumer products
- At large production volumes, injection molding equips the designer for imaginative solutions that reduce part count – making injection molding ideal in the automotive and engineering aorta sector
- A wide spectrum of materials with good compatibilities allows designers to alloy and overmold materials to add complex functionality like handgrips, integrated metal parts etc – making injection molding a preferred option for tools and instrument housings
- Plastics can have long environmental life combined with excellent toughness – making them ideal for street furniture and outdoor equipment
- Many polymers offer closed loop or near closed loop potential in manufacture, allowing aggressive collection/recycling procedures – much exploited by an increasing number of automotive brands, to moderate the environmental impact of their products
- While the environmental burden plastics represent should not be understated, smart selection of polymers (and a design process that accommodates this) can result in an almost circular economy for molding materials
V. Differences Between Thermoforming and Injection Molding
While the fundamental purpose remains the same for both processes – namely the manufacture of high quality plastic components for long and short term use across wide markets – every detail of the techniques is different.
Feedstocks, processing methods and finishing requirements all vary widely. Applications can, however, overlap to a surprising degree, making a process choice early in the product development arc a wide choice, as design criteria are very different.
Injection mold tools use a minimum two plate construction, generally using one plate as the ‘cavity’ side (into which the bulk of the part is molded) and a ‘force’ side with upstanding features that penetrate into the cavity and form the obverse side of the molding.
These two plates are mounted on linear bearings, allowing the tool to slide open in a low force and wear-resilient way. The force side of the tool usually includes the injection point (gate) and mechanically driven ejector pins that bottom out and push the part, as the tool opens.
This type of tooling can utilize more complexity than two simple plates; with mechanically (or hydraulic or electrically) driven ‘wedges’ which move sideways to allow undercuts; multiplate solutions for complex profiles; hydraulic or electrically driven screw thread formers; hand or robot loaded overmolded metal and plastic inserts; two stage tooling to overmold (for example) rubber onto rigid moldings; and many more features.
Typical tools are made from advanced alloy steels and either hardened after making or cut from pre hardened steel with carbide machining, spark erosion and wirecut tools. Tool parts are therefore slow to make and expensive.
Typical tools for simple parts start at ~US$5k and can be very much more expensive, for complex and high volume molding production.
Thermoforming tools, on the other hand, are very simple, being either an upstand former, or a cavity, with low detail and often made from wood. Vacuum is used to pull material down onto the former, sometimes assisted by air pressure applied to the other side.
In a few cases, a pressure side tool is also used, to encourage more aggressive forming to work well.
This tooling is extremely low cost – even when CNC machining is required it is simple and fast. In extreme cases tools can be made from Aluminum, but often they are made from hardwood or laminated MDF.
It is unusual for a thermoforming tool to cost more than a few hundred dollars, and good suppliers can often make them in days.
These are the commonly used materials for thermoforming AND injection molding:
- Polyethylene terephthalate (PET) – commonly used for making plastic bottles, trays, and containers for food and beverage products.
- High-density polyethylene (HDPE) – used for making products like fluid and industrial containers.
- Polyvinyl chloride (PVC) – used for making products like blister packaging, clamshells, and medical packaging.
- Polystyrene (PS) – used for making products like disposable cups, plates, and food packaging.
- Acrylonitrile butadiene styrene (ABS) – used for making products like automotive parts, luggage, and toys.
- Polycarbonate (PC) – used for making products like safety helmets, eyewear, and medical equipment.
- Polypropylene (PP) – used for making products like food containers, automotive parts, and medical packaging.
- Acrylic (PMMA) – used for making products like signage, displays, and lighting fixtures.
- Thermoformed foams – used for making products like cushioning material, insulation, and soundproofing.
These materials include various grades and formulations that provide varied properties, such as UV stability, impact resistance flexibility and color stability. The choice of material should be based on the specific requirements of the application.
A more extensive range of materials are available for injection molding than for thermoforming, because thermoforming requires extruded or calendered sheets, which form is not available for many engineering polymers.
Thermoforming involves heating a thermoplastic sheet to a temperature that renders it pliable but not liquid; forming it into a required shape by applying vacuum and a former; and then cooling it to create a rigid product.
- The first step in thermoforming is to select a thermoplastic sheet whose material and thickness of the sheet will depend on the desired final product.
- The sheet is preheated, clamped into the former, to a temperature that makes it soft and elastic. The temperature and duration of preheating depend on the type and thickness of the sheet.
- Once the sheet is appropriately heated, it is stretched over a mold or the mold is lifted under the sheet. Molds can be either positive (to create a shape) or negative (to create a cavity). The atmosphere below the sheet is then evacuated to pull it onto the mold. Additionally, a matching former and compressed air is used to press the sheet into the mold from above.
- Cooling then solidifies the sheet into the molded shape and the vacuum/pressure are released and the mold(s) removed.
- After the sheet is formed into the desired shape, excess material is trimmed off with a trimming tool or knife.
- Once the product is cooled and trimmed, it is inspected for any defects, such as warping, cracking, or other imperfections. Additional finishing processes, such as assembly, labeling, or painting, may also be required.
In contrast, these are the basic steps involved in the injection molding process:
- The first step in injection molding is to prepare the material. This involves melting plastic pellets or granules in a hopper, ready for injecting the molten plastic into the mold.
- The mold must be preheated to improve the flow of the molten plastic and reduce stress on the final product. In some cases, release agents are sprayed to coat the tool to aid ejection.
- Once the mold is prepared, the molten plastic is injected into the mold cavity through a nozzle under high pressure. The pressure is maintained until after the plastic fills the cavity and the gate and has cooled sufficiently to minimize shrinkage.
- After the plastic is injected, the mold is (water or oil) cooled to solidify the charge, so that it retains the cavity shape.
- Once the plastic has cooled and hardened, the mold cavity is opened and the part is ejected from the mold by ejector pins.
- Finished parts will then be gate and flash trimmed material.
- Finally the component is inspected for any defects, such as warping, cracking, short-shot, scorching and weld quality and then released for use or packaging and delivery.
Production capacities are moderate, in thermoforming. Where a single mold former is used and the table of the forming machine is filled, a cycle from loading to unloading might take three to four minutes.
On this basis, a basic single tool/machine setup can deliver around 20 finished moldings per hour. Continuous and automated systems can increase this, as can multiple tools per machine cycle.
Finishing and trimming operations are often labor intensive, so the entire process for continuous production of a component can require several people. Automation reduces this burden, but significantly increases equipment costs.
Production capacities in injection molding are equally part size and tooling dependent. A single cavity tool for a 50g part might have a cycle time of 30 seconds, making it faster than a thermoforming process. By adding cavities to the tool, this productivity can be increased – as it can with thermoforming.
Where injection molding wins in productivity is in molding small parts (1-20g). It is common to place 10-20 cavities for such parts in high volume production tooling, increasing productivity dramatically – at the cost of additional tooling.
Thermoforming is a moderately precise process where typical tolerances in fine features formed in thin (1mm or less) sheet can be +/-0.3mm.
Overall dimensions are often less precise, so part size in thin sheets can generally be held to +/-0.5mm.
However, dimensional accuracy depends heavily on sheet thickness, profile depth/complexity and operator skills. Trimming can add another layer of size variations, unless it is performed with precise press tools.
Injection molded components can be specified to very high tolerances, and where the part and material are suitable it is possible to maintain localized tolerances of +/-0.05mm on local features and +/-0.2mm on critical larger features.
Injection molding is also more repeatable and significantly less reliant on operator experience, once the equipment settings are stabilized.
Complexity of Models
There a several guiding principles that help in component and therefore tool design:
- As a general rule, models can have fine details on upward facing surfaces that are approximately one to two times the material thickness in X, Y and Z dimensions
- Simplicity is the key to a successful molding – avoid unnecessary complication and detail
- Where the part requires upward steps that are higher than the basic formed surface, these steps should be heavily drafted in the tooling and fileted at their root, to encourage good material formation. This is illustrated by a recess formed into the bottom of a thermoformed box half. The secondary recess should be tapered by at least 3° and more if possible and the upstand (or cavity) that molds the recess should be fileted into the next stage with as large a radius as practical – ideally 5-10 material thicknesses in radius
- Moldability depends heavily on aspect ratio, so consider ways to reduce the depth relative to the part size to reduce the degree of stretch that is required of the heated material
- Thinner materials give better results, so consider how to reduce material thickness specified. Rigidifying features can add as much or more stiffness as sheet thickness
Overall, there are few limitations on the dimensional or feature complexity of injection molded parts. A kayak hull or a
Lead Time Length
Lead times for simple parts can be surprisingly short. The longest lead time aspect is the tooling. Where this involves fine detail, it will require a precision manufacturing process such as CNC machining or potentially 3D printing.
As a rule, a capable CNC workshop can turn around a thermoforming tool in days, so it is common to see an order to product schedule of a week – and this can be reduced by mold simplicity and processes such as 3D printing of molds.
Injection molding is one of the longest lead time industrial services and the schedule can often become a burden on and the critical path aspect of pre-and-mass production.
A simple,single cavity tool for a component requires many process stages, each of which has intricacies and technical specializations that often require multiple services and sometimes even multiple suppliers:
- Tool design – typically 3 days, and this stage can take longer for complex tools with solutions to difficult gate/flow/welding and undercut issues
- Preparing bolster for detail work – typically 1 day
- Machining cavities, gates, galleries/runners, slides, waterways and ejectors – rarely less than 5 days
- Delivery of ancillaries
- Dry assembly and evaluation of tool – 1 day
- Finishing tasks – typically 2 days
- Assembly and first molding trail (T0) – 1 day
- Disassembly and refining of blanking (seal faces), flow, welding etc. – typically 2 days
- Assembly and tool trial (T1) – 1 day
- Release parts for client evaluation
- Undertake any client required alterations – minimum 1 day
- Apply textures to cavity surfaces by polishing, photo etch etc. – min 2 days
- Assemble and tool trial for pre production quantity – (1 day)
This results in a best case tooling schedule for a relatively simple, single part of 20 WORKING days to pre production parts. Then pre production often flags details in the tooling that need small attention.
Shape of Final Product
Thermoformed components have severe restrictions as to shape and must be quite simple.
This is increasingly true with material thickness, as thinner materials can be forced into greater feature compliance than thicker sheet:
- They must be essentially unidirectionally formed. This means that the tool withdrawal can only be impeded by simple ‘bump’ features that can stretch around the tool for ejection/release. Generalization of maximum feature sizes that can bump off the tool is not possible, as this depends heavily on the local area shape/flexibility and the toughness of the material being formed
- The ideal thermoformed component is a heavily tapered (5-8°) recess/bowl with few features on the ‘bottom’ face and no features on the ‘sides’.
- All corners need to be heavily fileted with radii that are several times the planned material thickness of the finished part.
- Rising side features can be complex to a degree, as long as the bump (tool withdrawal from undercuts) is not made too hard and the material thickness/feature size restriction is complied with
- Bottom features such as brand embossing, feet, stiffening ribs will form more easily than such features on the side, so fewer restrictions apply here.
Overall, thermoformed parts must be bowl or box like (including multiple such features in a single part like an egg-box) and stepped recesses work well as long as the material has enough space to conform to the pull-in that is implicit.
Injection molded components are almost unlimited in shape and complexity and can reproduce extremely fine details and extensive undercuts. These undercuts can be formed by blanking features from the other side of the tool making holes, or by sliding features that pull out of the undercut before ejection.
The main limitation in general shape is that hollow features can generally not be molded with necks narrower than the feature, so the tool can withdraw.
In exceptional cases, such aspects can be considered moldable, using a system called a collapsing core, that internally folds or slides to reduce its section enough for withdrawal.
Thermoformed parts require minimal finishing, beyond the edge being trimmed to release the final part from the sheet it originated from. The surface finish of the sheet source material is essentially preserved in the forming process, apart from any coarse texturing that is acquired from the face of the tool.
Trimming can be performed in custom tooling or completed by hand, where volumes do not justify the additional tool cost.
Injection moldings are similar in finish requirements, in that gate and sometimes flash (from poorly made or operated tooling leaking) must be trimmed. However, gate trimming is a simple task and can, with additional cost in the tooling, be either performed at ejection or in an automated post process
The cost to produce the first part in a thermoforming process might be $200-$500 including tooling. Subsequent parts will be moderately expensive, because the process uses a highly processed raw material (sheet polymer) and requires a high labor content for loading, molding and trimming parts.
The set up costs for an injection molded part will generally exceed $3,000 for the smallest part. The advantage comes in the mass production process, where large numbers of parts can be run 24/7 in a largely automated process with low variation.
It’s common to amortize the setup costs for an injection molding over several thousand components, in which process the setup costs per part are then negligible.
VI. Thermoforming vs. Injection Molding: An Overview of Each Molding Process
These two processes are closely related to each other in a general overview, but differ fundamentally in virtually every detail. There are times when a choice must be made as to which process to apply, as equivalent functions and forms can be achieved from the two processes.
However, the cost/volume relationships differ greatly, and this will generally be the deciding factor in process selection.
Thermoforming: Principle of Process
A thin sheet of polymer is heated to soften (but not melt) and then a form tool is pressed into the sheet. This causes a tent to form, framed by the tool.
Air is then evacuated from within the ‘tent’, to pull the sheet into surface conformance with the tool. Additional air pressure can be applied outside the ’tent’ to assist in this, and in some cases an additional tool is pressed onto the stack, to further stretch the heated polymer before or during vacuum/pressure application.
The sheet as stretched will not have a consistent thickness, as the stretching process in forming to the tool will reduce the thickness where the most movement is required.
Once cooled, the vacuum/pressure are released and the finished molding is pulled off the tool (or blown with air pressure) and trimmed to complete the part.
Injection Molding: Principle of Process
Injection molding differs from blow thermoforming in almost every detail.
Heated and liquified polymer is introduced into a mold cavity tool, in which volume the part is formed and solidified. This results in finished parts whose wall thickness and shape details are controlled by the two sides of the cavity tool. By this means, parts of almost unlimited complexity can be made repeatably and consistently..
The injection of plastic fills the cavity, then as the material cools it tends to shrink, particularly in thicker sections, causing a risk of poor conformance to the cavity dimensions. For this reason, a dwell time continuing the injection pressure is maintained, to feed more polymer into the cavity to counteract the shrinkage.
The cavity is then opened and the part ejected by integrally operated ejector pins, releasing the finished component for use.
VII. Should you Choose Thermoforming or Injection Molding
The decision to use thermoforming or injection molding is generally a straightforward one that is driven by some clear factors:
- Where volumes are small, the cost of establishing injection molding as a supply will usually be ruled out, rendering alternative and lower tool cost processes attractive
- Where part volumes are high and cost sensitivity is also a factor, injection molding is generally the clear winner
- In a limited number of cases, thermoforming also wins in the high volume applications, because of particular strengths in the process. Egg cartons and factory component tote trays are good examples, where an automated thermoforming process for high volumes of product can win over injection molding.
- Where part profile or complexity is high and varied wall thicknesses are required, injection molding wins
- Where varied and high cosmetic finishes are required, injection molding wins
- Where materials are required that are simply not available in sheet form, thermoforming is not an option
VIII. Choose Kemal for Your Injection Molding Needs
Whatever your molding requirements, Kemal can serve. From the initial specification of processes, tooling types, materials, through solving design issues to, low cost production.
For thermoforming, we offer premium services and we can supply cost effective tooling and parts.
For injection molding, we are confident that we know what we need to know to make your development and transfer to production easier. If it’s multistage or simple tooling, if it’s single cavity or multiple with hot runners, if it’s tricky materials or a basic masterbatch, we can help. And the earlier you invite us to help, the better we can advise on design for manufacture.
Kemal has a team that is well versed and skilled in all aspects of manufacture. We are the right service partner, whatever your product, whichever areas you require support in.
To know more about our services in injection molding, blow molding and so much more, contact us to discuss your project, meet our team and let us explore together how we can make your product, make it better, and make your life easier.