When it comes to pipes, PVC is the first word that comes to most people’s minds these days. PVC pipes have been in use for over 50 years since their first manufacture. Plastics offer several advantages over metal pipes. They are lightweight, they do not rust, and are better insulators.…
When it comes to pipes, PVC is the first word that comes to most people’s minds these days. PVC pipes have been in use for over 50 years since their first manufacture. Plastics offer several advantages over metal pipes. They are lightweight, they do not rust, and are better insulators. The fact that they get processed at a much lower temperature than metals makes them cheaper. They are long-lasting and hardly need replacement during their useful life. PVC pipes are the first choice for plastic pipes and fittings. The most practical method for producing pipe fittings is injection molding. Manufacturing PVC pipe fittings by extrusion is not a practical option. This is due to the complex shape of the pipe fittings. Injection molding of pipe fittings has its unique challenges. This article explains the process of injection molding pipe fittings. In doing so we cover the properties of PVC. We look at how these properties come to play in the application of PVC pipes. We also look at how they play out in the injection molding of PVC pipes.
PVC (polyvinyl chloride) is a vinyl polymer. It has a repeating unit with two carbons, three hydrogens, and chlorine. The bane for manufacturers is the toxicity of PVC. Under the right condition, little stops the chlorine from reacting with hydrogen. It does so to form hydrochloric acid (HCl). This compound is acidic and can cause corrosion. So despite its many desirable properties, PVC is corrosive. This causes some challenges in its processing and application. PVC has excellent resistance to water and most everyday liquids. It is soluble in tetrahydrofuran, cyclohexane, and cyclopentanone. So when using PVC fittings consider the type of fluids that goes down the drain. For example PVC pipes and fittings in a laboratory.
At room temperature PVC is non-corrosive and is great for use in water pipes. They should not get used to hot water pipes. These hot water pipes get close to the boiling point of water. This risks the release of toxins into the water. Especially bad if the water is for human use. PVC melts at a temperature of between 100 and 260oC. This depends a lot on the type of PVC. PVC’s typical service temperature is below 60oC. This means most of its applications are room temperature applications. Although used for household piping. Hot water piping uses a more temperature tolerant plastic. Some people love their hot showers somewhere around the high 60s.
To meet different requirements, pipings need to bend in different ways and angles. This could be to divert the entire flow or part of the flow. Pipe fittings get used to connecting pipes at different angles. They can connect 2 to 4 pipes together. Pipe and their fittings get used in many ways. Examples are sewage drainage, water supply, and irrigation. The introduction of PVC pipes was a significant change in the home and industry. Today many homes and industries are transitioning from metal pipes to PVC pipes. The PVC pipes last longer. They do not rust and can withstand flow pressures. Thanks to large scale production processes like injection molding, they are cheaper. Below are some examples of injection molded pipe fittings.
Examples of PVC pipe fittings.
The injection molding process begins with the PVC in the form of granules or pellets. The smaller granules are better for more even heat distribution. This reduces the chances of overheating any. Overheating leads to undesirable outcomes. The granules go into the machine through the hopper. Depending on the scale the feed is automatic. This controls the rate at which the materials flow through the process. A controlled feed rate is important to prevent accumulation. It also ensures even blend and plastics moving at the rate they get melted. The granules pass through a cylindrical barrel. In the middle of the barrel is a reciprocating screw. The screw rotates and at the same time moves backward. The gradual backward rotating motion pushes the melted plastic into the melt chamber. Once the melt chamber is full the screw reaches its limit. This gets controlled by the setting of the limit switch by the operator. At this point the screw gets pushed forward, powered by a hydraulic cylinder. This action forces molten PVC out through the nozzle, sprue, and runners into the mold.
The screw must push with enough pressure to get the melt from the nozzle to the mold. The mold must get filled with enough pressure to fill every gap. This pressure holds to ensure the shape remains while PVC cools. Cooling occurs by passing a cooling fluid through channels around the mold. This can be water or other cooling fluid. Once cooled, the product gets ejected. During cooling the screw begins to rotate and move backward again. The beginning of another cycle.
This process is pretty much standard for injection molding of most plastics. But there are a few peculiar points with PVC. There are also some for pipe fittings in general. The injection molding of PVC is by conventional injection molding machines. PVC gets processed anywhere between 100oC and 260oC. It is often mixed with all sorts of additives. PVC achieves a range of physical properties. You can have a very flexible PVC used in films. You can also have rigid PVC used in electrical socket covers. They achieve these various forms using different PVC additives. Hence why the processing temperature varies to such an extent. The presence of these additives also poses some challenges in the process. This gets explained in more detail further down the article.
Despite the ranges of melting point possible, PVC is heat sensitive. So for a specific batch of PVC, it is necessary to adhere to the processing temperature. While different PVC samples have different melting temperatures. This is not the same as being able to process any specific sample at a broad range of temperatures. The heat input for different sections of the barrel varies. At the feed throat, no heating occurs. This part remains cool to prevent blockage by premature melting. Further down heat input occurs to reach the melting point. As the melt travels further, it experiences friction. This generates some heat. The heat input must get adjusted to maintain the temperature of PVC.
The production of PVC pipes and indeed most plastic pipes is through extrusion. This process is like the injection section of the injection molding machine. The difference between the two is the reciprocating screw and mold. Pipe extrusion is a continuous process that does not make use of molds. Rather it uses a die. Injection molding is a cyclic process and uses molds. Pipes can get extruded because of their continuous shape and uniform cross-section. But pipe fittings have a more complex non-uniform cross-section. The pipe fittings form connections between flow lines. They form the points where flows join, divert, or bend. There are several types of pipe fittings. Examples are tee and elbow-shaped pipe fittings. The larger pipe can fittings get produced using other molding methods. Smaller pipe fittings less than about 12 inches get produced by injection molding.
One good thing about pipes and pipe fittings is that the sizes are global standards. This means a mold gets produced for a particular type of fitting. This is the same size needed globally. This gives the PVC pipe fitting manufacturer a diverse customer base. With the same mold, the PVC manufacturer can produce for different markets.
Given the properties of PVC certain factors are important in their injection molding. The injection molding of PVC requires it to get exposed to high temperatures. Given the chemical and physical properties of PVC, this might put some strain on the process. The following are some considerations in the injection molding of PVC pipe fittings.
PVC is temperature-sensitive. Not so sensitive that it destroys the material. But the concern is what gets released from PVC at high temperatures. Chlorine can form hydrochloric acid. There are also a lot of additives added to PVC. These help to achieve varied properties of PVC. For example, there are additives to make it more flexible. There are also additives to make it more rigid. The additives used to depend on the intended application. At high temperatures, these get released and can be toxic. For this reason, scrap should be minimal. PV recycling occurs either in size reduction and remelting. Other methods like solvent treatment or pyrolysis also get used. Other thermoplastics like HDPE can go into regrind and recycle. This does not work with PVC despite it being a thermoplastic. Melting PVC is a major health hazard. The more heat the PVC gets exposed to the easier it becomes for toxins to get released. The best a PVC manufacturer can do is to reduce the scrap generated. This is achieved by making the runner as short as possible. Since this is the only factor that the manufacturer can tweak here. Scraps also come from rejected parts. So making sure the PVC pipe fitting mold design gets used is accurate for the purpose.
The best option for mold fabrication for PVC is anti-corrosion stainless steel. This should be well polished hardened steel. There is a high potential for the release of HCl during the production of PVC pipe fittings. This is even more so with the PVC in the molten state. Any chlorine in the gaseous form is likely to condense upon hitting the mold. This exposes the mold to corrosion. Although it will happen, using high-quality metal reduces the likelihood. This extends the useful life of the mold. So do not go cheap when it comes to choosing mold material. For PVC pipe injection molding, go for the best metal you can get.
Designing a mold for complex solid shapes is complex. Designing a mold for PVC pipe fittings takes complexity up a notch. The mold cavity is not a simple cut out of a solid shape and gates. The mold is a rather complex assembly. It requires an expert in mold design and mold manufacture. Looking at the shape of a pipe fitting. Take for example an elbow pipe fitting. The mold assembly gets designed in such a way that allows the filling of the pipe body. But this occurs without filling the hollow region. This gets done with consideration for product ejection and release. Typical designs need a multiple-part mold. This can be up to 4 part molds. This is unlike simple solid structures that can get made with two-part molds. So for PVC pipe fittings seek mold engineers with experience with this type of mold. Below is an example of a PVC pipe fitting mold.
Example of a PVC pipe fitting mold.
For injection molding of PVC pipe fittings, cold runners work is less complicated. This limitation gets attributed to the complex design of the mold. It is more difficult to work a hot runner system into the PVC pipe fittings mold assembly. This means PVC pipe fitting production with a cold runner leads to more scrap. More hot runner systems for PVC pipe fittings are becoming available. As technology advances more solutions get developed. Some of the challenges of a hot runner system for PVC pipe fitting injection molding is warping. For cold runners, it is the generation of scrap.
Thus far the article explains the process for injection molding of PVC pipe fittings. The key challenges lie in the complexity of the mold needed and the heat sensitivity of PVC. These get addressed by taking necessary precautions. The process of PVC injection molding is well established in the industry. Engaging the right skill in mold engineering and process design will ensure success.