Glass fibers combine the properties of glass with those of plastics.  This makes injection molding possible. But the injection molding of glass fibers is quite different from that of plastics. In this article, you’ll learn about glass fibers and how they are injection molded. 

The injection molding of glass fiber is an interesting area. Glass fibers have transformed the processing and application of glass. It has done the same for plastics. The field of glass molding may seem to have lost a lot of attention to plastics in the past years. Today plastics such as polycarbonate and PMMA have replaced glass in some uses. Examples are in applications such as automotive lighting and transparent screens. Glass seems to be making a come back in recent years. Part of this is down to the increasing demand for more alternatives to plastic. Glass is reusable and withstands sterilization. It also still does command more aesthetic appeal than plastics. It also has better structural stability and heat tolerance. But the processing of glass isn’t as easy as that of polymers. This is where glass fibers come in. 

Injection molding is the industry standard for the production of complex parts. The injection molding of glass poses several challenges. Most of these challenges are concerning the high softening temperature of the glass. Modifying the glass to decrease its softening temperature addresses these challenges. The thermal resistance of glass might reduce after such modifications. These are better in applications where the glass is not exposed to high temperatures. For example in camera lenses and vision aids. In such applications, only the optical and mechanical properties of glass matters.  To expand the application of glass it needs some modifications. One such modification is to convert them into glass fibers. The image below illustrates a glass fiber material. It is generally a composite material which has a polymer as the matrix and glass fibers as the filler. This specific example is of long glass fiber. There are also short glass fiber reinforced composites.

Image illustrating a long glass fiber material

Within the scope of this article glass fiber herein refers to a composite of glass and polymer. In particular, thermoplastics serve as the polymer. The article explains the process of glass fiber injection molding. This is with a focus on long glass fibers. To give more insight into the process, we explore the injection molding of glass. The article also explains how glass fibers make for more processible material. Especially when compared to glass or plastics in their neat form.

Process of Glass Fibre Injection Moulding

To understand the essence of glass fiber injection molding, let’s begin with glass.  The raw material for glass production is silica. Which is what sand is; purified sand in the form of silicate glass. In the conventional injection molding process, feed goes into the hopper. This then gets melted by the rotating action of a reciprocating screw within a heated barrel. This is then fed into a cool mold within which the desired shape forms. For glass to melt the temperature increases to somewhere between 800 and 1000oC. If you’ve ever seen molten glass, it forms a molten sometimes glowing gob semi-solid. In this form, it is malleable and forms into different shapes. 

The main challenge in injection molding of glass is the high softening temperature. Most plastics like polyethylene or polypropylene that melt at temperatures well below 200oC. These low melting materials can cool within seconds. But glass needs longer cooling time. This is because it heats to a much higher temperature before softening. The heat input to soften is almost the same amount of heat that gets removed to harden.

In a more crude method, the glass gets heated to the melting point in a furnace. This is then scooped and placed in a mold. The mold shuts and the formed glass cools under pressure. Once the glass cools past its glass transition temperature, the mold opens. The glass product formed is then released.

One imagines that it takes quite a lot of time to cool an object from around 1000oC to room temperature. To allow for the longer cooling time that glass needs, several molds are necessary. This reduces the idle time during cooling time. But this also means more investment in molds. After the molding, annealing follows.  This is to make up for the stress generated in the glass due to the high drop in temperature. This is the process of heating the glass to a high temperature and allowing it to cool at a slow rate. This process helps to realign the molecules of the glass. This reduces the built-in stresses. This process improves the physical properties of the material. The slow cooling makes annealing a rather time-consuming process.

To make the injection molding of glass possible some modifications are necessary. These are for the main purpose of reducing the temperature required for softening. One of these approaches is particle size reduction. This is by using the glass in powdered form. This reduction in particle size improves heat transfer. So the heating element does not need to get as hot to transfer heat to the glass. Most of the equipment parts are metals, much of which is stainless steel. Some manufacturers even use aluminum molds where possible to reduce costs. Heating to between 800 and 1000oC makes the metals vulnerable to corrosion. There is also a thermal expansion to consider. This would cause parts to expand when heated. Because the glass is usually still very hot even when it has hardened. 

Reducing the particle size does not change the chemistry of the material. The same amount of energy is still required to break secondary bonds and cause the material to flow. So using glass in powdered form does not change the melting point. Another approach is the use of composite in the form of glass fiber. This is a composite of glass in the fiber form as a filler within a polymer matrix. A well-dispersed composite with good compatibility is best. It combines the properties of the glass with that of the polymer. For a given type of matrix, the properties of the final material depend on the length of the fiber. So the injection molding process should maintain the fiber length throughout.

The injection molding machines used for plastics are also applicable to glass fibers. Except plastics melt at a much lower temperature than that required for the molding of glass. The injection molding of glass fiber needs special considerations. In particular when it comes to tools and equipment design and operating conditions.    

Equipment and Operating Conditions In Glass FIber Injection Molding

The operating conditions affect the properties of products from glass fiber injection molding. The properties obtained after injection molding can vary for the same material. So let’s consider a case where two manufacturers use the same glass fiber material. Let’s say they were from the same suppliers and made into the same end product. The properties of the end product each manufacturer obtains will be different. This is because they use different operating conditions. Let’s look at some operating conditions crucial to glass fiber injection molding.

• Gate Type
• Temperature
• Mold Design
• Nozzle and runners
• Screw and barrel dimensions
• Screw speed
• Recycle regrind

Before discussing further you should have an understanding of injection molding in general. This helps to better grasp how these factors affect the injection molding of glass fiber. Let us take a look at the general outline of the injection molding process. The image below is an illustration.

Image showing a general outline of the injection molding process.

Much of these operating parameters link to the shearing of the material. The type of gate affects the orientation of the fibers in the melt and the final products. It also affects the number of shear melt experiences. A particular type of gate does not work for all injection molding of glass fiber products. For example, the edge gate gives better flexural strength than a double-edge gate. But this is specific for long fiberglass reinforced thermoplastic. It is also for one in transverse direction using PET as the plastic.  Fibre Orientation and Distribution is important in the properties of the product. As the melt moves from the feed to the mold, the fiber orientation may change. Shearing is due to factors like the rpm of the screw, the mold filling velocity, and the screw type. The fibers sometimes get broken under shear or get redistributed. The redistribution of fibers affects the dispersity. The more even dispersion of the fibers is within the matrix, the better the material. 

The nozzle, sprue, and runners are also points where the melt experiences shear. Reducing sharp edges at these points keeps shear low. The runners should be wide enough within acceptable limits and with round walls. Reducing taper and orifice flows avoids pressure drops. So a flute type nozzle works better than a tapered one. Gates that feed through the mold edge work better. This avoids the head on the drop into the mold.

The screw-type used should be one that exerts the least compression on the melt. Usually, a constant tapper screw is manageable. But one with a longer metering section and shorter compression section is better. This way the melt only experiences compression after it has reached the least viscosity. The screw speed should also be moderate. This reduces the chance of fiber breakage.

You generally want to decrease the viscosity of the melt as much as possible. This allows better flow and dispersion of the fibers within the matrix. A lower viscosity also means less sharing of the fiber and less chance of breakage. When the fibers break within the matrix it gives it uneven properties. This means some parts have long fibers while others have short ones. Remember that the property of the material is dependent on the length of the fiber so you want to maintain it. Shrinkage of matrix polymer around the glass fiber increases compressive strength. So the shrinkage that occurs within the mold is acceptable. The shrinkage of course depends on the type of plastic used.

Advantages of Glass Fiber 

To see how glass fiber combines the advantage of glass and polymer let’s look at some of the advantages of glass. We then look at the limitations and how glass fibers overcome those limitations. Heat resistance is one of the desirable properties of glass. This is because it makes it suitable for sterilization. Unlike plastics like PET and PE which soften at a lower temperature. If you’re ever poured the hot water in a PET bottle or made the mistake of dipping a plastic spoon in hot oil you’ll know. To kill bacteria, the temperature in thermal sterilization is around 180oC. The objects must withstand this temperature for some minutes. This is why the alternative to glass would be polycarbonate. They have a softening point above 180oC. The heat resistance of glass also makes them useful as substrates. In such applications, they hold other materials during processing. An example is in the production of solar cells. The glass gets exposed to different conditions like annealing. Other general thermal applications of glass include appliances like ovens and stirring rods.

Other properties of glass include solvent and scratch resistance. Glass fiber combines the properties of the polymer matrix with that of glass. The flexibility and lower melting point of plastics introduce processability. The glass gives the structural and thermal stability to form strong stable products. Hence we have a material that has the advantages of glass and plastics. The list below summarizes the desirable properties of fiberglass:

• Lightweight
• Electrical insulator
• Chemical inertness
• Low thermal Conductivity
• Good mechanical strength and structural stability
• Thermal stability

Applications of Fiber Glass Injection Molded Products

Fiberglass gets applied in a variety of injection molded products. Below are some examples.

• Bathtubs
• Equipment casings
• Storage tanks and 
• Safety cupboards
• Kiosks
• Outdoor furniture.
• Rooftop covering panels
• Kitchenware
• Pipe fittings

Images of some glass fiber products

Conclusion 

Glass fiber, in particular, long glass fiber materials combine plastic and glass properties. This is without needing the high processing temperature of the glass. Injection molding of glass fiber is like that of common plastics. Except for necessary modifications of equipment and operating conditions.  

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