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 . 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.