5 Methods of Joining Plastic Parts to Each Other

Making plastic parts is one of the most efficient ways to design and manufacture parts. In numerous cases these days, it is necessary to join two or more plastic parts together. Fortunately there are a wide variety of ways you can do this. I want to show you five of the best ways to accomplish this just like the pros and not like the other guys.

Before we discuss the exact methods you will need some information about what you are trying to accomplish. Why is this important? To choose the best method to join two parts you must know exactly why the parts need to be joined. There is a big difference depending on your design needs how the part should go together. Here some examples of things to look at on your list of requirements.

  • Are the parts permanently or temporarily joined (example: a door for batteries
  • How often do the parts need to be separated (if not permanent)
  • How much dust resistance and/ or waterproofness are needed if any?
  • Is the product used where it will touch sensitive areas of the human body? (medical instruments, toys or eating utensils)
  • Will the part be subjected to heating and cooling? (like in a car parked outside)
  • How much time is required to assemble the parts?
  • Will the parts be machine assembled?

Make a list of your requirements which will help you find the best way to find out how to proceed. Now let us get to some solutions.

Using screws is a very common way to assemble plastic parts. The advantages are that anyone with the ability to turn a screw can assemble the parts. Screws can be used to join plastic parts to different materials like metals and composites. This is also easy for smaller volumes. That said one disadvantage is that it costs more time, money and labor as the number of parts increases. Assembly labor or expensive machinery is required for high volumes. Screws do allow for disassembly but wear out the screw pocket if the product is disassembled frequently. Using some adhesive will lock in the screw.

The general practice is to have the hole molded on one part for the screw to go through. The part to be joined to the first will have a pilot hole. The pilot hole is smaller than the screw threads. When the screw is first driven into the pilot hole it will cut into it. This gives a strong joint for the screw to clamp both parts together. Using a metal insert can increase strength of the joint and allow more frequent disassembly.

As a general rule use screws with fat heads like a pan head or button head screw. Flat head screws should not be used as the cone shape head will over stress the plastic. Be cautious if you are joining rubber parts with screws.

One of the most popular methods for joining two plastic pieces is using hooks. A hook, sometimes called a snap, looks like a barb on a fishing hook. This hook snaps into a slot in the part it is being joined to. A hook can be setup for either a permanent or temporary joint.

If the hook is shaped like a fish hook with a very pointed barb, and the slot is not accessible that is ideal for a permanent joint. Toys and small appliances frequently use this attachment method.

If you want a removable joint, like a battery cover, the hook should have no back angle so it acts more like a detent. A detent is better if the parts must be removed a put back together. One good example of this is a Fastex buckle which can be found at here. You probably have seen these on luggage, baby seats and outdoor gear.

Adhesives can be used to join parts easily as well. Adhesives, like epoxy, are not only good at bonding plastics but can be used to bond plastics to metal, composite, or ceramic. If correctly selected, the joint can also seal water, dust and other contaminates out. Since each adhesive is different, follow the directions and warnings given by the adhesive manufacturer. You should consider using adhesives as permanent.

Be careful when selecting adhesives if the product will experience hot and cold temperatures in regular cycles. Some adhesives will be better choices depending on the conditions the product is subjected to. Be sure to look at which materials you are bonding and make sure the adhesive will work with all parts to be bonded. Special care should be taken with adhesives if the parts they are used on come in contact with sensitive body parts.

On approach that gets over looked is to not have separate parts at all. Storage boxes can use a living hinge to allow the box and lid to be molded as one part. One example is this battery box. Living hinges are a great way to simplify no matter if you plan to make a 100 or 1 million parts. Material selection is very important with living hinges so do your homework. Living hinges are generally a good way to attach parts but special steps must be taken if the hinge is permanently bent.

In a high volume production setting, sonic welding can be a fast and inexpensive method to joint two parts. In many cases the two parts are mirrored (they often are the left and right halves of some larger part) giving the parts a natural joining area. Sonic welding requires both parts to be specially designed for the task and requires a sonic welder. Once you have the proper machinery and setup, welding the material takes a few seconds. As with other methods, check to see if the materials will join together. Sonic welding makes good joints but can sometimes not align the parts exactly. The designer should make allowances for any misalignment that may occur.

Now that you know about these five methods of joining plastics part together and understand what you need to know before you choose one of them let review what I have said here. Make sure you know whether a joint is permanent or temporary. Check to see what the other requirements are. Select the method that best suits your needs.

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