Understanding the Basics of Plastic Machining!

Nylon Machining - Chizel

When we talk about machining, it is evident from the majority response that most people think in a single direction that is of the metal components.  Machining, also known as Subtractive Manufacturing, is actually a process of working a piece of raw material, into any desired shape and size by certain material-removal processes. While it is mainly performed for metal products, it can also be used for plenty of other types of products that include plastic. Plastic Machining is a concept that is not as popular as it should be. Due to this, even if some manufacturers know about it, it isn’t acknowledged as much. It is also an widely unacknowledged fact how plastic coupled with CNC Machining, which stands for Computer Network Control machining, have become a boon for the manufacturers.

In this article, we are mainly going to go through everything we need to about Plastic Machining. We are going to study Plastic Machining in more detail and go into the technical depth on why it is not being used as much as it should be. So, keep reading!

Nylon Machining

Source: Chizel

Material Selection for Plastic Machining

We need a separate section on Material Selection for plastic machining because of the ranges of varieties and qualities that you find in this field. With the range of application and degree variations that a plastic carries, that expands its importance as well as makes it tougher to select which plastic would be of best use to you.

Material selection is the base of your entire project and hence needs to be done very carefully considering all the important points. We will go into depth of problems you might face during material selection point by point.

Quality: The quality of plastic can often be judged by how soft or hard it actually is. Some points that you must keep in mind are that poor quality plastics are usually soft. Also, you can find color variations, voids and poor annealing in a plastic of subordinate characteristics. You have to keep these points in mind to completely reject any of the plastics that fall in such category.

To take help in the machine selection process, you can choose to go to machining specialists who can guide your team on what plastic you may need depending on the type of project you are aiming for. It is very important to get your own research right as well. You can take suggestions for such specialists or distributors and then add your own research to it in order to decide on the perfect material for your project.

Stock Size: With plastics, the size is another factor that you have to consider before the final decision on the material. In this case, we are dealing with certain materials that might be delivered in a size larger than required. They can further be cut into the exact size that we need. So, it is important for you to research the size availability of the plastic component you’ve chosen for your project.

We can consider Acetal rods for example. They are purposely delivered oversized varying from 0.005 inches for small diameters to 0.03 inches for large diameters. Apart from acetal rods, we can also take the example of Acrylic sheets. They are usually available in metric sizes accompanied with 10% tolerance. Different materials have different values for stock sizes and tolerances which need to be considered on priority during the research and decision.

It is very important for an engineer to consider quality and stock size of each material that they consider for their product. When you finally decide on a plastic, additional importance should be given to obtaining a material certificate from the manufacturer for guarantee which proves helpful in case of any malfunction.

Common Material Problem

Now after you finally select a material, the next part you need to know about is acquiring the material and actually putting them to use. In this section, we are going to talk about the major problems you may face while getting the desired materials and while using them so you can be ready for them, perform a better research and tackle these problems before they waste any of your important time.

Choosing Between Different Types: When it comes to plastics, almost every material comes with a number of variations that you will have to carefully consider. If we talk about an example material like Acrylic, we will see two major variations in Extruded and Cast Acrylic. Where Extruded Acrylic is more affordable, it cannot be used for dimensional stability where Cast Acrylic plays an important role. Extruded Acrylic, however, is generally used for display-type work.

You may see numerous variations in the material that you would’ve chosen for your project. It is important to carefully consider all the important properties of each type and variation for deciding and moving on with one.

Color: The plastic’s color can also go on to pose multiple problems in your project. There are multiple plastics that have the color variation properties. We can talk about Polypropylene, a plastic that has the color varying property. If you compare two Polypropylene samples from different manufacturers, it is possible that one of them might depict a characteristic opaque white color while the other might be depicting a yellowish or a translucent beige color.

It is important to choose a material that can maintain the color uniformity and color repeatability properties. These color variations are often dependent on the manufacturers and the procedure they use to process or manufacture the plastic. So it is recommended that you specifically discuss this issue with the manufacturer or machining expert that you are working with.

Beware of Imported Material: It is often being realized that a lot of imported plastic material is not of acceptable quality. Importing of such materials is being done only for cost-controlling reasons. While we can accept trading some quality to cost in the display or electronics industry where cost-cutting is very important to business, such practices cannot be accepted in industries such as high-end medical industries.

We can take the example of imported Acrylic, which is much cheaper than domestically produced Acrylic. On testing these imported Acrylic materials, it has been found out that these are not as stable as they are required to be. Apart from this, problems such as larger internal stress and thinner sheet thickness at the bottom of tolerance band are prevalent in imported Acrylic.

That’s why it is important for you to find a domestic manufacturing source to ensure greater stability and even better machinability.

Contamination: Just like the machine counterparts, the plastic components are also exposed to loss in quality, durability and appearance quality due to contamination. This contamination can arise due to different reasons and hence you should be aware of beforehand.

It is very important to maintain the proper plastic material compatible coolants. Care should be taken to make sure that metal coolants are not used because they can work in a negative way for plastic and cause stress cracking. Another important issue to be addressed is that when we use a machine for certain plastic material, very often some little pieces of that plastic remain in the machine after the job is done. If the machine is not cleaned before putting it to use for another material, contamination is caused in the latter material.

While the plastic machining experts would be experienced of taking care of this from their own side but you should be aware of the problems and stay alert for such situations.

Check for Stability: One of the most important tests to perform before finalizing a decision is to check for the stability of the material. There are three characteristics of stability which give you the idea of how stable material is.

Thermal Expansion: It refers to the tendency of a material to change its shape, area or volume in reaction to change in temperature. In best case scenarios, you would not want your material to change its shape due to external factors. Thermal Expansion is material dependent so you need to research for the properties of the material you have selected. For example, we can talk about Torlon which is a resistant material to thermal expansion whereas polypropylenes are comparatively more susceptible.

Outgassing: It refers to the situation in which a plastic material continues to emit a gas even after it has been processed. The major problem this causes is the reduction of mass of some components which leads to them shrinking. We can take the example of DuPont’s Delrin Acetal which, if allowed to stay undisturbed in a sealed container for a few weeks will release a strong formaldehyde smell.

Material Stress: It refers to the internal forces being exerted by neighboring particles of a continuous material. You can think of a rolled plastic sheet which when cut and placed on the table, has a tendency to curl. This has been resolved by the plastic machine providers by making use of special techniques that help in the special handling of such components and avoid the problems.

Material Handling: This is one of the most basic points that you’d need to take care of whilst arranging your plastic material. With the kind of sensitivity that plastics have, it is common for them to strike against each other resulting into dents in surfaces. This damage proves rather disastrous in the long run. Therefore, it is recommended for you to take care that the material transport and handling is being done properly.

Tolerance Issues: When it comes to tolerance, it is very clear that it is comparatively tougher to achieve tight tolerances in plastics as compared to their metal counterparts. Plastic tolerances are often affected by factors like material type and part configuration. This makes it rather difficult for plastics to achieve a certain high level of tolerance. This point should be kept in mind while selecting the right kind of material.

Processing Considerations for Plastic Machining

Now that you have better knowledge on selecting, acquiring and guidelines to follow on using the plastic materials, our next step is to learn some points to keep in mind while we pursue the processing of our plastic materials. In this section, we are going to cover three major processing considerations.

Polishing: One of the first steps that you take in the processing of your plastic part is Polishing. It is important to know that there are three methods of polishing that come with their own advantages and disadvantages. It goes without saying that the skill of the operator working on polishing of product makes a great impact on how the final product will turn out.

Machined Nylon

        Source: Chizel

Mechanical Polishing: It refers to putting plastic through certain machines for grinding, buffing and eventually polishing. It’s the most common method for plastic polishing. Apart from that, another positive for mechanical polishing is that it can be used for any plastic. There is one drawback associated with this method as sometimes, the final product can be left with some very fine scratches on its surface.

Vapour Polishing: It refers to the polishing of a material by exposing it to the action of a chemical vapour which reacts with the plastic surface and results in smoothening of the surface. Vapour polishing is majorly known for polishing of plastics like polycarbonate, acrylic, polysulfone and polyetherimide(known as Ultem). The proper implementation of vapour polishing results in a visually appealing product. It is also important to talk about the health hazards that it carries. Due to harmful chemical vapours, the operator must be equipped with proper equipment while performing vapour polishing.

It is very important to note that the result of polishing is very much dependent on the machining of the product. The products with correct machining will be enhanced after being put through any of the three polishing methods.

Annealing: In the previous section of our article, we covered one of the major problems that we face with plastics, that of internal stress. This problem can be fixed with Annealing. It refers to the process of strengthening a material using heat treatment in order to reduce internal stress.

Annealing is a very important step of the process as without this step, some plastic materials can deteriorate and suffer from stress cracking due to the stresses a part must face during the machining and polishing processes. It is important to note that while annealing is very effective as a standalone treatment, some critical applications also require Stress Relieving to ensure maximum plastic stability.

Finishing: Finishing is the final step to cap off the entire procedure of plastic processing. It basically refers to the approaches made for enhancing the overall appearance of materials. Some methods are included to improve the protection of the material from certain environmental factors.

It is important to note that while most of the plastics adapt to the process of finishing, Teflon very often produces a surface that is porous in nature. This limits Teflon’s adeptness as a plastic that can be finished well. Finishing is a property that is largely dependent on the application need, the material being used as well as the configurations of the actual part that has been manufactured out of that material.

Conclusion

I would like to repeat what I mentioned at the beginning of this article. The fact that machined plastic components are still finding more and more significant applications despite the knowledge and information available on machined plastic is limited. This alone should highlight the growing importance of Plastic Machining.

Plastic Machining, along with the technology of CNC Machining which automates the major portion of the process, should be brought to the notice of as many manufacturers as possible. It’s high time that manufacturers take a look at the humongous scope offered in the field of Plastic Machining and invest their skills in it.

In this article, we have covered all the basics that you would have needed to learn about Plastic Machining. You have to ensure thorough research and contact with a trusted and highly rated plastic machining shops or distributors. Your elaborate research along the lines of all that we have discussed in this article would help your project to be without any mishaps.

I hope you learned ample of new knowledge about plastic machining from this article. Please do subscribe for more content like this.

 

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