Blow Molding and Injection Molding: Which Method is Best for Your Application

The term plastic molding describes a group of different technologies used for manufacturing finished parts from various polymers. These processes are not always well understood, and lack of differentiation can reduce discussions about the subject to confusion and mistakes.

This is an article aiming to explain the range of tools available for plastic component manufacturers and illustrate the differences between them – it is one of a series.

This article is intended to make clear the differences and similarities between the processes of injection molding and blow molding, to assist in the development and transfer to production of components, assisting in choosing the right tools early in the development process.

I. Types of Molding Methods

Thermoplastic polymers are a broad family of materials with some common properties and useful characteristics. They are formed from chains of simple chemistries that are polymerized into chains, generally by high temperature/pressure processes and catalysis.

• The most important properties of widely used plastics are their toughness, chemical resilience, flexibility and low cost. This allows them to serve in a staggering array of tasks and applications, from milk bottles to sewage pipes, from zippers to car body panels.
• When heated to moderate temperatures, thermoplastics soften and plasticize and then melt to low viscosity, generally unreactive liquid forms.
• In the softening process, the materials literally plasticize, and reform undamaged, i.e when hot they can undergo shape change that is retained when cooled and is permanent until heat is reapplied.
• This allows a variety of complex shapes to be formed by various methods, providing for a flexibility of design and intricacy of functional and aesthetic detail that can only be matched by significantly more costly materials and tools.

The range of forming methods varies widely in detail and can deliver a spectrum of degrees of complexity of features. Various forming methods are used, but they have in common that they use a cavity or a former to impose a shape on the softened polymer, which is then ‘frozen’ into the material as it cools.

The degree of shape complexity and precision results form the degree to which the polymer is heated and the pressures available in processing. Blow molding, in the final formation stage, heats the polymer until it softens but does not completely liquify.

The materials generally become highly plasticized, mildly elastic solids when heated correctly. A cavity former can be used to impose a shape onto the inflated, softened polymer, which it retains a ‘memory’ of when cooled.

The other primary processing method is injection molding, which raises the temperature until the polymer chains lose their weak atomic coupling forces and become liquid. A cavity is then filled with the liquified polymer and cooled, so again it retains a strong memory of the new shape, as the chains recouple when returned to ambient temperature.

II. What is Blow Molding?

Blow molding is a forming process that works by expanding either a pre molded blank or an extrusion of a blank (both called a parison), around which the cavity closes. Expansion then continues by applying internal air pressure to the hot parison.

Blow moulding is only suited to the manufacture of hollow forms, generally with a single opening. This can include handles, which are often integrated into the design as co-processed features in a single stage.

The two approaches to blow molding offer relative benefits. A continuous extrusion method allows the parison to form as plastic is extruded from the barrel, using gravity to stretch the parison to approximate size. This results in poorer control of wall thickness but a faster overall production rate.

Alternatively, the barrel feeds an accumulator chamber which is hydraulically extruded into the parison more quickly, reducing the uncertainty of material distribution as gravity has less influence.

An increasingly common improvement in the blow moulding process has an injection molding pre-stage, where an injection molded parison is formed in a first stage tool, and then moved while hot to a blow mold stage. This allows the formation of more complex and precise ‘neck’ and thread features that are increasingly demanded for good quality cap fit.

With the mass of the parison formed, the tool closes. In simple vessels, the parison is expanded into the tool cavity and cooled to finish the part, which is then ejected as the cavity opens.

When the parison is used to form an integral and tubular handle in the component, a two stage process partially expands the parison and then the handle is formed by a hydraulically operated pair of slides that pinch the soft material and form the handle as a distinct feature, after which expansion is completed to finish the part.

As either the top, or the top and bottom of the resulting molding are pinched off by the closing of the tool, some trim/cleanup will be required in the simpler process types.

For an injection molded parison, the neck features are fully formed before blow molding and the bottom will be fully formed by pinching off, so no post processing is required.

III. What is Injection Molding?

Injection molding differs in every way from blow molding, and yet the goal is the same – to turn ‘raw’ material into a finished product or part with an essentially single stage process that freezes an imposed shape into a polymer, to make a resultant net shape that is robust and fit for purpose.

The first step involves melting pellet polymer material in a barrel (a static hydraulic cylinder that switches jobs later). This melting can be achieved by thermal means alone, but it works better when there is also a mechanical shear applied by an archimedes screw, heating by friction and ensuring very uniform heat distribution.

With the plastic appropriately liquified to the desired temperature/viscosity point, the piston of the barrel then applies pressure to the contents and forces it through an entry port (gate and sprue) or gallery distribution system (runners) built into the tooling cavity or cavities.

The plastic fills the pre heated tool, driving out the air which passes through the cavity split line (where the tool splits to pen) and then the cooling cycle commences, beginning to solidify the polymer.

As the polymer cools, it undergoes shrinkage that can be as high as 10-15% by volume. In an uncontrolled process, the plastic will then shrink away from the walls and form only a bad approximation of the cavity. To counteract this, barrel pressure is maintained during cooling, so new plastic is injected to compensate for the shrinkage.

When fully cooled, the part will remain in close contact with every surface of the molding cavity. It is an important aspect of tool design that the gate(s) can continue to supply molten polymer during cooling, so chill isolated volumes are co-fed generally, through secondary gates.

When the tool opens and the part is ejected (pushed out by mechanical (ejector) pins that are integral with the cavity surface) it will be a faithful reproduction of the tool in all features/dimensions.

At this point there are generally reproductions of the gate/sprue and galleries attached to the part, so these must be hand or press tool trimmed and the finished part is then ready for use.

IV. Blow Molding and Injection Molding: Which Method is Best for Your Application

There is no hard and fast rule about process selection in plastic components, but there is a very clear divide in selection of blow molding. Injection moulding requires the inner core of the tool to be steel which is withdrawn from the component. This core (or force) can be a collapsing core that accommodates internal complexity, but it cannot form a narrow neck bottle.

A sequence of design decisions will be taken, starting right back at the concept stage, and these will drive the choice of tools and methods – but bottles (and similar smaller hollow forms) must be blow molded.

Some problems lend themselves perfectly to blow molding and no doubts about that choice ever intrude into the development and manufacturing handover.

Other part and assembly specifications cannot possibly be achieved without a precision cavity being filled with liquified polymer.

This decision is often made by classification – single use vessels and nearly closed hollow forms are made by blow moulding by default. These parts lend themselves to blow moulding as they are often complex but low detail 3D shapes with uniform or near uniform wall thickness.

Undercuts are a critically important and challenging feature in tooled plastic parts, as they add considerable tooling complexity. Tool features to form undercuts must retract laterally (across the tool axis) before the entire tool can retract axially to release the part.

This is not possible in blow molding tooling, where all internal features are formed by air pressure expanding the parison to meet an external cavity.

The choice to injection mold a part or parts can be driven by any and all of a range of factors;

• Complex parts, with fine details and/or undercuts and with variable wall thickness are injection molded
• Parts intended for other than single use are generally injection molded, although many short/single use parts and wide neck vessels are still injection molded
• Parts requiring high precision and/or high cosmetic standards are generally injection molded, though Injection Blow Molding (IBM) allows these complexities (such as cap threads/vents) to be pre-formed before blow molding
• Parts that must fit into an assembly with other components are injection molded.
• Parts with fine surface texture or fine text/branding can only be injection molded
• Parts made from exotic materials must be injection molded
• Injection molding is a volume decision – parts required in high volume will be MUCH lower cost via injection molding than by any other possible production methods.

Blow Molding : Benefits and Applications

Blow molding is a critically important manufacturing process that gives designers great solutions to hollow body design requirements;

• Delivering low cost, thin walled products that are near closed, narrow neck vessels
• Offering low cost tooling approaches, as the final stage molding pressures are low and the materials very non aggressive, as there is no liquid flow
• The method can easily be executed in near turnkey automation
• complex and large components can be delivered at little more than material costs, once equipment is amortized
• Compact equipment can deliver high productivity with low labor costs
• There isn’t a practical alternative in making closed or nearly closed, thin walled vessels

Applications for blow molding are quite narrow and specific;

• Consumer product bottles. Plastic containers for consumer products from milk to bleach, laundry soap to maple syrup are almost universally made by blow molding
• Toys. Children’s toys – baseball bats, toy animals, ride on toys, balls and many more products are generally made by blow molding
• Automotive. Virtually all simple fluid reservoirs in cars and trucks are blow molded – screenwash, coolant overflow, suspension bladders (rubber) and other parts are often blow molded
• Appliances. Fluid reservoirs for laundry and dish washers are generally blow molded
• Aquatic and marine products. Floats and buoys, fuel tanks and more are blow molded
• Industrial and shipping vessels. Extensive use is made of single use and reusable fluid containers that are blow molded

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;

• High precision and low cost converge perfectly in injection molding
• Where volumes are high enough to justify the cost of tooling, a reliable and highly repeatable process frees the designer from many design/manufacture constraints in complexity of feature and shape
• The broad range of materials available allow for near metal levels of properties, all the way through to soft rubbers to be processed in the same equipment, once small tooling optimizations are considered
• The durability of plastic parts is remarkable, once appropriate design and material selection optimize the component
• The highest cosmetic standards can be achieved with relative ease and maintained reliably, through million quantity production runs in injected parts
• 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

There is no industrial sector that does not make use of injection molding. It is the single most commonly used manufacturing technique, a pre-eminence that is still growing as costs drop and methods expand and improve.

• Automotive: An increasing proportion of car components are injection molded – from structural components like bumpers and body panels, through exterior and interior cosmetic components/surfaces, to seat parts, controls and lighting components
• Consumer: Consumer products are synonymous with injection molding – from headphones to kitchenware, from storage to wearables – no area is immune to the cost/design/cosmetic/functional benefits that injection molding brings
• Medical: Few products in the medical space fail to use injection molded components. Single use and long terms use devices such as syringes, catheters, masks, identification tags, splints, stents, hernia meshes, drug release devices, joint prosthesis bearings and much more are injection molded. No machine housing, bed, wheelchair, suction device or waste container fails to make some use of injection molding
• Architecture: Most built environments make extensive use of fixings and finishings that are injection molded. Window and door furniture, bathroom components, waste pipe fittings, electrical fittings and much more are made this way
• Aviation: While aircraft make little use of injection molded parts for structural or aerodynamic parts, much of the flight controls and interior space is made of injected components
• Marine: Engine and fuel handling parts, fixtures and fittings, seating and controls all contain injection molded plastics
• Toys: Children’s toys are almost entirely made of injection  molded parts

V. Blow Molding vs Injection Molding: Common Traits and Characteristics

Blow molding is performed on a limited range of polymers, to make a huge range of components/parts for most sectors of product manufacture. Injection molding is performed on a much wider range of polymers and has an even greater spread across all market sectors. These processes exploit the properties of polymers and the effect of the processing methods to deliver;

• Custom color:
  • The polymers that are typically blow molded are very capable of carrying coloring agents to deliver basically any color that can be defined. Some material options can allow for either transparent or opaque color, where others lack the amorphous property that makes for transparency
  • Injection molded polymers can generally be colored similarly, and for most consumer product manufacture a narrow selection of materials is used and these are highly color tolerant. For industrial applications, materials are often molded in their ‘natural’ color but there are no widely used polymers that cannot be manufactured (‘masterbatched’) to any color required.

• Low labor costs:
  • Vessels, bottles, toys and other components made by blow molding can be very low cost, such that a container generally represents a few percent of the value of the contents. In high volume manufacture, the labor cost in production of, for example, a typical milk container, is essentially zero as equipment is fast, automated and lightly supervised for quality purposes. Less intensive production methods can use lower levels of automation, but the process of blow molding is essentially hands-off, so the maximum labor required for processing is a single person for unload and trim.
  • Injection molded components range in size from less than 1 gram to tens of kg. There is no one pattern of molding machine, nor tooling nor processing setup. However, most moderate volume molding of small (up to 100 gram) components run on tool cycles of a few seconds to a minute and parts are ejected and dropped into chutes to be boxed or trimmed. Molding machines require supervision and tuning to maintain quality, so some moderately skilled labor is required.

• Reduced Part Costs
  • Any comparable method for making closed or narrow neck vessels is slower (rotamolding), or much more complex (injection molding 2 parts and joining them), or much more expensive (superforming, hydroforming or pressing and welding). No process can compete with blow molding for intricacy, quality and production capacity – both in plastics and in glass, where the process originated as a manual skill in the Roman period.
  • The net-shape production of injection molded parts transfers all of the high effort precision into tooling. That tooling is hard to make, must be durable and efficient and it must allow fast production. Once the tools are made (and amortized), the cost of parts is not significantly higher than the cost of raw materials, since labor cost and machine time are both. Polymers range from <US$8.0 per kg for high volume materials such as polyethylene (PE) to >US$50 per kg for exotic materials such as liquid crystal polymer (LCP)

VI. Differences Between Blow Molding and Injection Molding

While their end purpose is the same, in grossly simplified terms, – i.e. the manufacture of high quality plastic parts – blow molding and injection molding differ in almost every functional detail.

Materials

Blow molding is limited to a relatively narrow range of materials;

• Acrylonitrile Butadiene Styrene (ABS) is a tough plastic capable of delivering high cosmetic quality, though it lacks the chemical resilience of many blow molding polymers. It is a copolymer of polystyrene and two synthetic rubbers that toughen it
• High-Density Polyethylene (HDPE) offers good moldability, chemical resistance, temperature stability and strength to weight ratio. HDPE is among the most used blow molding materials
• Low-Density Polyethylene (LDPE) is similar in moldability and chemical resistance to HDPE but much softer and more flexible, making it suitable for squeeze dispense containers.
• Polypropylene (PP) is also widely used and can be considered as interchangeable with HDPE in most applications, though it has a slightly different chemical resistance spectrum. However, it’s main benefit over HDPE is higher temperature resilience
• Polyurethane (PU) is easy to mold and widely used in industrial and marine applications. It results in rigid and tough vessels.
• Santoprene is a thermoplastic rubber (TPR). It is used for making bladders and soft, elastic vessels. Its chemical and temperature resilience is moderate but for (moderately) inflatable components it can be a good option.
• Kostrate offers good impact and heat resistance and it is used in food storage applications. It’s also widely used in toys.

Injection molding can make use of the full array of thermoplastics. There are general use polymers, engineering polymers, high temperature polymers, solvent resistant polymers, self lubricating polymers and so many more classifications.

Here’s a few examples of key engineering polymers for you;

• Acetal, or Delrin or polyoxymethylene (POM) is a widely used high strength and highly chemical stability engineering polymer that comes in homopolymer and copolymer forms. It’s not suited to cosmetic uses.
• Polyphenylene Sulfide (PPS) is a high strength, very heat and chemical polymer widely used in automotive and aerospace
• Polybutylene Terephthalate (PET) is extensively used in bearing applications because of it’s low coefficient of friction and dimensional stability
• Nylon (Nylon 6, 6/6, 11 etc) is a group of engineering polymers that are also commonly used as fibers. The family offers high strength/abrasion resistance, chemical resilience and low friction. Nylons are widely used as a replacement for metal parts,particularly in automotive applications
• Polyetheretherketone (PEEK) offers high tolerance to chemical attack, very high temperature resilience and radiation resistance. It is used to replace metal components in automotive, medical and industrial applications.

And here’s some consumer/commercial grade polymers that are widely used;

• Poly Vinyl Chloride (PVC) is durable, stiff and dimensionally stable. It’s widely used for installed piping and for exterior building components as it has a long environmental life and good resistance to fire, chemical attack and abrasion
• Polycarbonate (PC) is valued for optical clarity and high impact resistance, although its scratch resistance is poor. It’s used in automotive for light covers and lenses, though it requires its UV resistance booting (with vitamin C as an additive) to reduce yellowing.
• ABS (same grades as for blow molding)
• High Impact Polystyrene (HIPS) is similar to ABS but grey transparent and a little less strong, using only one of the synthetic rubber coploymers
• Polystyrene (PS) is a low cost, weak/brittle transparent material that’s typically used for low stress components in electrical equipment and for school rulers!
• Polyethylene and high density polyethylene (same grades as for blow molding)

Process

The injection molding process is used to manufacture high volume and often detailed/precise parts. Internal features are made with a tooled core, so parts must have a nearly open interior rather than a narrow neck and a near-closed inner surface. The steps are;

• The resin beads are fed into a hopper, where they are melted and can be mixed with colorants or other additives.
• With the plastic at the correct temperature, a hydraulic ram pushes molten plastic into the tool.
• Either a direct injection point, or internal galleries then supply this plastic into the cavity, from which the air is driven at the tooling split line
• Plastic fills the cavity and conforms to its details precisely
• As the tool is then cooled, the plastic solidifies and shrinks by 5-15%
• The shrinkage is countered by continued hydraulic ram pressure, feeding the shrinkage space and keeping the part full size
• When molding and cooling are complete, the tool opens and integral pins at strategic points push out or eject the finished molding
• Feeders and galleries are then trimmed, to complete the part

The blow molding process is used to produce hollow plastic parts such as bottles, toys, floats, bladders and other hollow items. These are the steps in basic blow molding;

• The resin beads are fed into a hopper, where they are melted and can be mixed with colorants or other additives.
• Once the plastic is suitably heated and prepared, some is extruded through a die to form a tube of plastic, the parison.
• The mold tool then closes around the parison.
• The parison is then inflated using high-pressure air, which forces it to expand and conform to the shape of the mold.
• Once the moulding is blown, it is cooled using either water or air, to solidify the plastic so it retains the shape of the cavity.
• The mold is then opened, and the finished part is ejected. Excess plastic is trimmed off, and the finished product is inspected for quality.

In the Injection Blow Molding (IBM) process, the parison is injection molded before being moved into the blow molding stage, allowing much greater detail in, for example the neck and threads of a bottle. This pre-molded parison can be delivered hot from the injection stage, or reheated in place. In either case, the blow mold tooling will contain features that support the more precisely pre-molded areas, so that this detail is retained in the reheating.

Production Capacity

Injection molding cycle times can range from a few seconds per shot (for very small parts) to several minutes for large moldings.

Multiple cavity tooling slightly increases cycle times, but when 10 parts are molded in one shot, the machine time advantages are significant.

Addition of hot runners, which keep the galleries liquid, allow faster cycles (and less material use).

Addition of ‘tab’ gates and stripper plates can remove the need for any post mold trimming.

Blow molding cycle times are generally short, potentially as low as 2-5 seconds for a single shot machine. Multi tool setups, running tools in parallel, reduce cycle times but significantly increase equipment costs.

Precision

Injection molding is ideal for high precision and high speed production of large quantities of parts. It is common to hold tolerances of +/-0.1mm on critical dimensions, where general tolerancing of +/- 0.25mm is regularly achieved.

Blow molding is by its nature less precise, because the type of vessels that are blow molded rarely fit into complex assemblies, so they are free of the precision required in assembly parts. General tolerances of +/-0.5 mm in blow molding are considered sufficient. Where a blow molded part must fit precisely to other (often injection molded) parts, an IBM process allows blow molded parts with limited areas of injection molded precision.

Complexity of Models

There is essentially no limit to the complexity of injection molded parts, as long as they can follow a split line (where the tool parts, to open) with only minor undercuts (both in number and size). Any degree of non-undercut curvature and complexity required will have little or no effect on moldability.

Features as small as 0.2mm can be faithfully reproduced, irrespective of the size of the part they appear on. High aspect ratio sections of 10:1 and greater can be molded reliably, allowing the forming of springs and snaps.

Precision undercuts of similar proportions can be molded, where a slide can allow the steel of the tool to withdraw from ‘above’ the undercut before ejection.

Textures on 20µm, and even smaller in some material/tooling setups, can be faithfully reproduced, allowing large areas to be uniformly cosmetic textured by photochemical and laser processes. text and logo/branding of similar scale is very reliably reproduced.

Blow molding is less suited to the high precision outcomes of injection molding, but complex and accurately produced parts can be made that reliably reproduce features as small as 0.2mm. Some texturing is also possible.

One direction of undercut is generally considered simple in blow molding – for example the forming of integrated (hollow) handles on bottles. These handles are formed as the two sides of the tool pinch the expanding parison, as they close.

Lead Time Length

Injection mold tooling is usually delivered from design to first trial in around 40-60 working days, sometimes longer for particularly complex or involved tooling.

This lead-time can be shortened by; choice of 7 day, double or triple shift suppliers; by simplification of the parts, allowing simpler tooling; and by rapid tooling processes and suppliers (which increases cost, sometimes considerably).

With the first tool trial completed (Trial 0 or T0), any cavity/gate corrections/adjustments are made and tooled surfaces can then be polished/textured. Final production quality parts can generally be expected from T3, but the T0 to T3 lead time is HIGHLY variable, from days to weeks.

The simplicity of blow mold tools generally allows them to be ready for trial in 10-15 working days, with a realistic expectation that the trial parts will be correct and the mass production can commence.

VII. Blow Molding vs Injection Molding: An Overview of The Molding Processes

As described above, these two processes are interrelated and complementary to each other It is rare that a decision must be taken after the specification of the part is defined, to have to  select between these processes.

The parts’ general shape and purpose usually make this decision at the problem definition stage of development.

Blow Molding: Principle of Process

Blow molding uses an extruded or injection molded parison as the start point, inflating the molten plastic like a balloon, to form a uniform wall thickness bottle form inside a two part cavity.

As the cavity is cooled, the plastic hardens and retains as its outside shape, the inside surface of the cavity in which it was inflated.

Injection Molding: Principle of Process

Though similar in principles, injection molding differs from blow molding in almost every detail.

Molten plastic is injected into a cavity mold tool that forms the part. Such parts are solid (not twin-skinned hollow).

Their form is essentially unlimited, as long as the tool component that forms the ‘inner’ surface of a part is able to withdraw – therefor the parts cannot in any way be described as bottle in topography, as they cannot, by definition, narrow to a ‘neck’.

As the molten plastic is injected, it cools and shrinks, so further material is injected to counter this, until the tool contents are solidified and accurately reflect the cavity with high precision. The tool then opens and the part is ejected.

VIII. Should you Choose Injection Molding or Blow Molding

Whatever your molding needs, we can assist. At the earliest stages, we are happy to help in specification of processes, tooling types, materials and in solving design issues to make easy, low cost production.

For blow molding, we have great expertise and equipment and we can offer premium services – and surprisingly low volume options that are cost effective.

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 its 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.

IX. Choose Kemal for Your Injection Molding Needs

Kemal is a company that operates at the leading edge of the technology areas we operate in. Our range of services is extensive and we are consummate professionals in our operation

• Molding Manufacturing
  • Mold Design
  • Mold Manufacturing
• CNC machining
  • CNC machining service
  • CNC milling
  • CNC turning
• Injection molding
  • Injection molding service
  • Plastic injection molding
  • Clean room injection molding
  • Insert molding
• 3D printing
  • Online 3D printing service
• Metal parts
  • Die casting
  • Metal stamping
• R&D and manufacturing solutions
  • Rapid Prototyping
  • Low-Volume Manufacturing
  • Surfaces Finishing

Conclusion

Kemal is a professional organization that is ready to meet your varied needs in manufacturing. We have the skills, know how and service philosophy to be your partner of choice in the full spectrum of component and product manufacture.

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.