The pros of solvent-based adhesives

The main advantage of a solvent-based adhesive is the ease with which a strong bond is achieved. In solvent-based adhesives, the solvent dissolves and carries the adhesive, making it easier to spread across a surface and providing quicker wetting and stronger initial bonding.

A strong solvent is also able to roughen the surface by etching it as it is applied, providing a stronger bond and possibly removing the need for an extra etching step to prepare the substrate. Additionally, the solvents in these adhesives often evaporate quickly, providing faster drying times.

Thanks to the properties of the solvents they contain, solvent-based adhesives also offer better temperature stability, chemical resistance, and weather resistance than many solvent-free adhesives.

The cons of solvent-based adhesives

Most solvent-based adhesives use volatile organic compounds (or VOCs) as solvents. These solvents evaporate quickly and easily, which provides fast drying, but causes many other disadvantages.

Because they readily evaporate, the VOCs found in solvent-based adhesives are easily inhaled and can create flammable fumes and unpleasant odors, presenting a significant health hazard and fire risk. Not all VOCs are equally dangerous in toxicity or flammability, but most require safe handling precautions.

VOC solvents also contribute to climate change by combining with nitrogen oxides in the atmosphere to form smog. In addition to the harmful effects of their VOC vapors, the natural etching effect of solvent-based adhesives may damage the surface of some substrates.

Additionally, some countries regulate products that contain VOCs more strictly than others. In these countries, importing, exporting, or working with solvent-based adhesives may be more expensive or intensive than working with solvent-free adhesives.

Tips for applying solvent-based adhesives

The key to working safely with solvent-based adhesives is ventilation. Most solvent-based adhesives produce toxic and flammable fumes as they dry. To remove these fumes, make sure there is proper ventilation—especially when working in confined areas.

To ensure that you are taking all safety precautions and getting the most out of your adhesive, make sure to follow the manufacturer’s application guidelines. While these precautions won’t prevent the solvent from polluting the environment, they will ensure the safety of anyone working with them.

What are solvent-free adhesives?

To be considered solvent-free, an adhesive must contain less than 5% hazardous solvents. Some adhesives classified as solvent-free do still contain a small percentage of hazardous solvents.

To create an effective adhesive without the use of solvents, solvent-free products may use alternative, non-hazardous solvents such as water to carry their adhesives. They may also avoid solvents entirely, such as in hot-melt adhesives and solvent-free glues.

Types of solvent-free adhesives

There are three main types of solvent-free adhesives currently available.

Water-based dispersions are water-based, solvent-free glues. In these products, water carries the adhesive, which then cures as the water evaporates. These water-based glues are the traditional approach to solvent-free adhesives.

Solventless adhesives are high-viscosity, non-liquid products. Through their high viscosity, they are capable of holding two surfaces together. Although they were once only used in low-demand applications, such as candy wrappers, advances have made solventless adhesives a popular choice for any lamination.

Hot-melt adhesives are fully solid polymer adhesives. At high temperatures, these thermoplastics melt into liquids with excellent flow, before cooling in seconds to form strong bonds. Hot-melt adhesives do not require drying, which makes the production more efficient.

The pros of solvent-free adhesives

The main benefits of solvent-free adhesives stem from their hugely reduced levels of VOCs.

Because these adhesives contain negligible amounts of VOCs, very few dangerous fumes will be produced, and you won’t need to ventilate or take other safety precautions to prevent fire or health hazards. This also means that you can lessen the burden of managing solvent-related regulations or restrictions when importing or exporting the adhesive. This advantage is especially important in several countries that have strict standards for products containing VOCs.

Beyond eliminating strong odors and hazards in the workplace, solvent-free adhesives are also safer for the environment, as they produce almost no VOC vapors that could contribute to smog.

KURARITY™: Our material for solvent-free adhesives

KURARITY™, the transparent TPE, is the Kuraray solution for a solvent-free adhesive. KURARITY™ is an acrylic block copolymer produced by living anionic polymerization and utilized as a solvent-free, hot-melt, pressure-sensitive adhesive.

The all-acrylic structure of KURARITY™ gives it optical clearness and high weather resistance. Thanks to its narrow molecular weight distribution, KURARITY™ exhibits low viscosity and good removability.

Best of all, thanks to its block copolymer structure, KURARITY™ is capable of self-assembly with physical crosslinking, allowing you to omit the aging process.

With KURARITY™, run a simpler process with no UV curing, no drying process, and minimal VOC and odor.

Interested in learning more about KURARITY™? Contact us to get in touch with our experts.

Injection molding machine

An injection molding machine consists of three main parts: the injection unit, the mold, and the clamping or ejection unit.

Within the injection unit, a hopper feeds the plastic into an injection ram or screw-type plunger and heating unit. Once the required amount of molten plastic has accumulated, the injection process is started.

The material is melted by heat/shear and injected into the mold with high pressure. There, the injection molded parts are formed.

The clamping unit or ejection unit has the task of opening and closing the mold and ejecting the molded products. Toggle type clamping units are the most common which consist of platens to hold the mold. These platens, usually arranged vertically on the frame of the injection molding machine, can be compared to an internal press that exerts a strong clamping force. Alternatively, straight-hydraulic type clamping units use a hydraulic cylinder to exert the clamping force on the mold.

Clamping force

The clamping force can range from a few tons to several thousand tons. Because of this wide spectrum, presses are commonly classified by their tonnage, which indicates the clamping force that the machine can exert. The required tonnage to mold a specific part depends on its projected area.

The rule of thumb is usually 5 tons per square inch for most products. However, exceptions apply for very stiff plastic materials as they require a higher injection pressure and clamping force to keep the mold closed. Larger parts also require a higher clamping force.

Mold

The mold, sometimes simply referred to as “tool”, is the heart of the entire process. It is a hollow metal block into which the molten plastic is injected to create the desired shape. The mold has many holes in it for temperature control and venting. Temperature is controlled using water, oil or a heater.

In its simplest form, the mold consists of two halves:

1. The cavity (the front) and
2. the core (the back).

Conversely, injection molded parts usually have two sides:

1. The A-side of the part, usually the more visually appealing one, faces the cavity.
2. The B-side, also called the functional side, usually contains the hidden structural elements of the part. Its surface is therefore often rough and shows traces of the ejector pins.

The molten plastic flows through a sprue into the mold and fills its cavities. After cooling and solidification, which usually account for about half of the injection molding cycle, the mold is opened and the molded parts are ejected.

Mold design

The mold can be compared to the negative of a photo, where the surface structure and geometry of the mold is transferred to the finished part. It also includes features such as the gating system for material flow and internal cooling channels.

To produce thousands or even millions of parts with precision and repeatability, mold makers need an enormous amount of expertise. That’s because a mold can be very complex with various consideration such as wall thickness, corners, ribs, undercuts and threads. The high development effort is also the reason why the mold usually accounts for the largest part of the start-up costs in injection molding.

The cost of molds can vary greatly: Straight drawn molds with simple geometries for small production runs are relatively easy to develop and cost a few thousand dollars. More advanced molds for large-scale production and with complex geometries come at a much higher cost because they require retractable cores or inserts. These movable elements are inserted into the mold from above or below to produce parts with overhangs, such as an opening or cavity.

Mold design is often done in-house by machine manufacturers or specialized companies and requires consideration of raw materials and the use of finite element analysis. Moldmakers, or toolmakers, make the molds from metal, usually steel or aluminum, and precisely CNC mill them to shape the desired features. Recent advances in 3D printing materials allow for the production of molds for smaller batches at a lower cost compared to traditional methods.

The gating system, also known as runner system, controls the flow and pressure and funnels the molten plastic into the mold. The molten plastic flows through the following channels as it enters the mold:

1. The sprue,
2. the main runner,
3. the sub-runner and
4. the gate or “feeding port”, which is the narrow channel between the sub-runner and the mold cavity.

In addition, a cold slag well commonly collects the cold slag in the injection molding process to prevent a blockage in the sub-runner or gate, so that the remaining hot material can flow into the mold cavity without complications.

The production of multiple parts often requires multiple sprues. The sprue systems are separated from the parts after ejection. Injection molding is a low-waste manufacturing method, with the sprue systems being the only material waste. A portion of this waste can even be recycled or reused, depending on the material.

Mold makers can choose among several types of gates depending on various considerations, such as part orientation (A-side vs. B-side), ease of mold production, material selection, material flow, part size, trimming behavior and scrap cost. For more information, refer to the injection molding gating guide by Basilius.

Molds also include a closing system with two main purposes:

1. Keep the two parts of the mold firmly closed during injection and
2. push the injection molded part out of the mold after opening.

The ejected part then falls onto a conveyor belt or into a container and is stored or assembled.

Because the various moving parts of the mold can never be aligned 100 percent, two defects can be seen on almost every injection-molded part:

1. Parting lines, which are visible where the two halves of the mold meet, and
2. ejector marks, which are caused by the ejector pins.

Step 1: Clamping

Before the material is injected into the mold, the two halves of the mold must be securely closed by the clamping unit. The clamping unit presses the mold halves firmly together while the material is injected. Larger machines with higher clamping forces require more time to close.

Step 2: Injection

The raw material, usually plastic granules, is conveyed from the injection unit to the mold, thereby it is melted by heat and pressure. The molten plastic, the shot, is then injected through the sprue system into the mold, where it fills the entire cavity.

Step 3: Cooling

In the mold, the molten plastic begins to cool as soon as it comes into contact with the inner mold surfaces. When the material cools, it solidifies again and takes shape. The mold remains closed until the required cooling time has elapsed.

Step 4: Ejection

The cooled part is ejected from the mold using the ejector system: When the mold is opened, the molded part is pushed out of the mold by a mechanism. Depending on the gate design, the part is either separated automatically from the rest of the molding, or by cutting.

After ejection, a conveyor belt moves the molded part to storage, assembly or post-processing. Simultaneously, the mold is closed again for the next shot and the injection molding process is repeated. In some cases, the injection molded parts are ready for immediate use, while others require varying degrees of post-processing.

Equipment-dependent defects

Equipment-dependent defects are due to technical reasons. For example, excess molten material can escape from the mold if the injection pressure is too high or the clamping force of the mold is too low, a defect called flash or flashing.

Uneven cooling can cause parts to warp. If the mold design or the molding process do not correctly account for the shrinkage that normally occurs during cooling, parts can warp.

Bubbles on the molded parts appear when the mold or the material are too hot. This defect is usually machine-related and caused by a lack of cooling around the mold or a heater not working as intended.

Material-dependent defects

Polymer degradation is an example for a material-dependent defect. If the material has gone through hydrolysis, oxidation or another cause of polymer degradation, the molded parts will show it in the form of cracks, discoloration or similar defects.

Another material-dependent defect: Short shots. If there is not enough material in the injection molding machine, the molded part will have unfilled sections. Alternatively, this can also be causes by insufficient flow rate.

Equipment and material-dependent defects

As equipment and material affect each other, it is common to have defects caused by both. For example, sink marks can be an equipment-dependent defect if the cooling rate is too high. This can make the surface of the part solidify before the material in the center has time to flow in place. Alternatively, they can be a material-related defect as some materials, such as filled or reinforced resins, have a higher tendency to shrink and display sink marks.

Examples of equipment-related defects that cause ejector marks are poor mold design (especially regarding gates), poor ejector pin design or inadequate mold maintenance. On the other hand, material-related defects such as brittleness or low impact resistance could also lead to ejector marks.

Again, this list of defects and their reasons is incomplete. We are merely presenting some examples.

Injection molding materials

All thermoplastics, as well as some thermosets and elastomers are suitable materials for injection molding.

The most frequently used thermoplastics in injection molding worldwide are polypropylene (PP), acrylonitrile butadiene styrene (ABS), polyethylene (PE) and polystyrene (PS).

As “soft” materials for 2K and 3K injection molding, thermoplastic elastomers (TPEs) are most commonly processed.

It is rather uncommon for thermosets to be used in injection molding. This is because of their cross-linking density and their inability to be re-melted and reshaped. However, some thermosets are used as additives in certain applications such as improving the properties of thermoplastic parts.

The materials are usually available as small pellets or as a fine powder. In some cases, they are liquid. By adding various additives, the properties of the molded parts or the processability of the material can be improved. Examples of additives include colorants for color or glass fiber for increased stiffness. However, there are also more advanced additives, such as the specialty elastomers from Kuraray.

Each material requires different parameters during processing to get the desired molded parts. These include injection temperature and pressure, mold and ejection temperature as well as cycle time. Depending on how these parameters are set, the appearance, dimensions and mechanical properties of the molded parts change considerably. In addition to the appropriate technology, a great deal of experience is required.

Thermoplastic elastomers

Thermoplastic elastomers (TPEs) are materials with a rubber-like elasticity and the processability of thermoplastics. In other words, they can be stretched like a rubber band and melted and molded into a wide variety of shapes and sizes. After cooling, they keep their elasticity. If needed, they can be re-melted and re-molded again later on, which also implies less material waste in processing.

The processability of TPEs makes them an excellent material for injection molding. Unlike rubber, they do not require vulcanization, which is a time-intensive rubber processing method. Due to their diverse applications, they are a well-suited material for many industries and purposes.

To learn more about the material properties of different kinds of TPEs, please refer to the following table.

SEPTON™

SEPTON™ is a series of styrenic thermoplastic elastomers developed by Kuraray. The Hydrogenated Styrenic Block Copolymers (HSBCs) consist of styrene-based hard blocks and a hydrogenated diene soft block. HSBCs exhibit rubber-like elasticity since the hard block acts as a crosslinking point below the glass transition temperature of polystyrene and the soft block provides elasticity. Hydrogenation generates excellent heat and weather resistance.

SEPTON™ BIO-series

With SEPTON™ BIO-series, Kuraray offers a unique hydrogenated styrene farnesene block copolymer (HSFC) – which makes Kuraray the first and only manufacturer of bio-based HSBC materials on the market. SEPTON™ BIO-series thermoplastic elastomer represents a new solution for manufacturers that enables new compounds and end-uses with a high bio-based content to expand existing market areas and open up new ones.

For additional benefits, please refer back to the previous section about SEPTON™ or contact sales.

HYBRAR™

HYBRAR™ is a truly unique triblock copolymer consisting of polystyrene end blocks and a vinyl bonded rich poly-diene mid-block. Due to its peak tan delta near room temperature, HYBRAR™ exhibits high vibration damping and shock absorption properties – even without integrated plasticizer. These TPEs are available as durable hydrogenated and non-hydrogenated grades.

KURARITY™, the transparent TPE

KURARITY™ is a new series of acrylic block copolymers that are produced using Kuraray’s unique living anionic polymerization technology which combines various (meth)acrylates into A-B or A-B-A type block copolymers. Due to its structure, KURARITY™ thermoplastic elastomers exhibit a variety of properties such as excellent transparency, weather resistance, self-adhesion and good compatibility with other polar materials.

Injection molding is possible with KURARITY™ itself. However, KURARITY™ can also be used as an additive to polar plastic resins such as PC, PC/ABS, PLA, HSBC, PVC, et cetera to improve flexibility and melt flow. Blends with KURARITY™ feature high impact resistance compared to low molecular weight resins. Compared to plasticizers, bleed-out is reduced.

KURARAY LIQUID RUBBER

KURARAY LIQUID RUBBER is a cross-linkable liquid rubber. It comprises liquid butadiene rubber (L-BR), liquid isoprene rubber (L-IR) and liquid styrene-butadiene rubber (L-SBR). The liquid rubbers are colorless, transparent, almost completely odorless and have low VOC values. The polymers of butadiene, isoprene and styrene have a low molecular weight which is between solid rubber and plasticizer.

KURARAY LIQUID RUBBER products are designed as “reactive plasticizers”, meaning: In the time-consuming and labor-intensive rubber mixing process, they help reduce Mooney viscosity and facilitate the mixing process. This results in increased flowability of the compound as well as reduced time requirements and lower processing costs.

In addition, KURARAY LIQUID RUBBER products are co-vulcanizable with the base rubber to prevent migration. The significantly reduced migration massively improves the shelf life and durability of the products.

KURARAY LIQUID RUBBER is well-suited for the injection molding of rubber compounds.

Liquid farnesene rubber

Do you value sustainability?

Liquid farnese rubber (LFR) is based on natural and renewable raw materials: It contains a polymerized form of β-farnesene, a renewable monomer. Through established fermentation processes, proprietary yeast strains convert sugar sources such as sugarcane into β-farnesene.

Liquid farnesene rubber functions as a reactive plasticizer with a higher molecular weight than standard plasticizer. Liquid farnesene rubber is co-vulcanizable and reduces migration significantly which improves the durability of rubber compounds.

Would you like to learn more about our products and their excellent processing capabilities? Then please contact your Kuraray representative.

History

The inventor of injection molding is the American John Wesley Hyatt. He patented the first injection molding machine together with his brother Isaiah in 1871. At that time, it was still a fairly simple machine in which plastic was injected through a heated cylinder into a mold with the aid of a piston. It was used to manufacture products such as buttons, combs and billiard balls.

In 1903, German chemists Arthur Eichengrün and Theodore Becker invented the first soluble forms of cellulose acetate which could be easily injection molded in powder form. It was also Eichengrün who developed the first injection molding press in 1919 and patented the injection molding of plasticized cellulose acetate two decades later.

During the Second World War, the industry expanded strongly due to the high demand for inexpensive mass products. In 1946, the US American James Watson Hendry made a groundbreaking development with the first screw injection molding machine. It not only allowed much more precise control of the injection speed and quality of the molded parts, but also allowed the material to be mixed before injection.

Another Hendry development from the 1970s, the gas-assisted injection molding process, enabled the production of complex, hollow articles and greatly improved the design flexibility, strength and surface finish of injection molded parts.

Today, injection molding is a global market with annual sales of hundreds of billions of dollars. Worldwide, around 55 million tons of plastics are processed with it each year.