Authored by: Christie Jones Market Development Manager Spirol International Corp. Danielson, Conn. Edited by Kenneth J. Korane Hinges are ubiquitous mechanical components and, typically, engineers are most familiar with those with a solid pin.
Many may not know, however, that a leading use of coiled-spring pins is in hinges because they match or exceed the performance of solid pins while lowering costs. Here are some simple guidelines for using coiled pins in hinge designs. Basic types In general, hinges come in two basic types: Free fit. A free-fit hinge has little to no friction or drag when the latch or handle rotates. Hinge components are free to rotate independent of one another. Friction fit. A friction-fit hinge requires interference to prevent free rotation of components relative to one another.
Depending on design intent, resistance can vary from a slight drag to a level sufficient to hold components in a fixed position anywhere along their full range of rotation. Coiled-spring pins are well suited for both hinge types. But regardless of the type of pin, engineers should minimize the gap between hinged components to reduce clearance and avoid bending the pin. Spanish step working guide for na. Free fit For free-fit hinges, the coiled pin’s preinstalled diameter is of minor importance because the retaining, or smallest, holes in the pawl or handle determine the final pin diameter. Coiled pins are functional springs, so designers must consider recovery and retention in free-fit applications.
The amount of recovery/retention depends on the diameter of the retaining (tight) hole and the “free span” of the pin. Free span is the distance a pin passes through a free-fit component.
As free span increases, the pin diameter will also increase as it “recovers” a portion of its preinstalled diameter. See the accompanying “Free-fit hinge” graphic for more details. For better load distribution and closer-tolerance hinges, the tight fit of the coiled pin should be in the outer members of the hinge (pawl section).
The minimum thickness of the outer members should be 1 to 1.5 times the diameter of the pin. If the thickness of the outer members is less than the pin diameter, then the tight fit should be in the inside hole. To design a free-fit hinge, first establish maximum hole size in the retaining component for a tight fit. Insert the coiled pin into the retaining component and measure the free diameter of the pin at the center of the span. Add a clearance factor to provide room for the rotating member, usually 0.001 in. (0.02 mm) to determine the minimum diameter of the free hole.
Then add the required production tolerance to assign the maximum diameter of the free hole. If the tight fit is on the inside member of the assembly, the pin’s two ends become sized and unsized as it is installed (handle section). The end of the pin that does not pass through the hole is larger than the end that has been sized by the hole. Therefore, measure the diameter of the unsized end to determine the minimum diameter of the free hole in the outside members. Friction fit In a friction-fit hinge, all holes should be sized identically within the assigned tolerances. If the manufacturer is unable to maintain the same hole size within each component, split the tolerance between the components. Common practice is to assign the smaller half of the tolerance to the outside holes and larger half to the inside hole.
A coiled pin simplifies design as there is no need to accommodate misalignment between holes to ensure friction, as is the case with rigid, solid pins. Nonetheless, coiled pins perform best when installed in straight, properly aligned holes. The coiled pin’s spring characteristics ensure consistent performance and maintain desired fit and function throughout the life of the product. To ensure a friction fit with solid pins, on the other hand, the designer has to purposely offset or misalign the holes to get a “tight” feel. Then, because the pin is rigid, what started out as a tight fit will, over time, gradually loosen (referred to among hinge engineers as a “flopper”). This is particularly true in plastic hinges.
Because coiled pins can absorb wide hole tolerances, hinge manufacturers can loosen their tolerance requirements, which results in lower costs. In addition, coiled pins are conducive for automatic feeding and assembly and tend to be easier to insert than solid pins.
Finally, if the hinge sees cyclic or dynamic loads, the coiled pin is the only press-fit pin that can absorb shock and vibration and isolate the hinged components from these loads. This results in fewer maintenance headaches and longer assembly life. Taken together, coiled pins can significantly improve hinge performance as well as generate savings in overall manufacturing and operating costs.
Hinges & Snaps Optimized part design will help to reduce additional expenses when you face the need for fastening your plastic parts or require additional hardware installation such as hinges or fastening mechanisms. Hinges and snap-fit joints can be incorporated into your plastic parts in order reduce or eliminate the need for traditional fasteners such as screws, nuts, washers and spacers. A part designed with molded-in hinges can replace metal ones while still performing the same function and reducing your products overall cost. When you reduce required hardware, you can lessen the material and assembly cost while also simplifying your design.
Snap joints should be considered during the development of your custom plastic components that need to be secured to other components. Versatile and cost-effective, snap joints and hinges often reduce the cost of secondary hardware expenses and the labor of final assembly.
The Ultimate Guide to Living Hinge Design. 1. Living Hinge Design The Ultimate Guide. What is a living hinge?
A living hinge is a thin section of plastic that connects two plastic bodies together. The use of a living hinge allow you to keep the two bodies connected while allowing rotational movement. Living hinge example? A common example of a living hinge is a ketchup cap. The two sides are connected by a thin hinge.
Structural Plastic Hinge Design
The cap can rotate out of the way while still being connected. Living hinge. What are living hinges used for?. Makeup compacts. Candy container caps (Tic Tac mints). Condiments. Shampoo and liquid soap.
Fishing tackle boxes. What are the benefits?. Eliminate assembly- Conventional hinges need to be assembled. Living hinges do not.
Lower Cost- Lower cost though eliminating assembly steps and eliminate hardware. Durability- properly design living hinges can last indefinitely.
Conventional Hinges. Reason not to use a living hinge?. Limited material selection- Living hinges require the use of Polypropylene PP or Polyethylene PE.
Limited load bearing- Living hinges can not handle much load parallel to the hinge. Added development time- Living hinges take effort and time to design. Living hinge design considerations. Typical living hinge designs. Typical living hinge designs. Flat hinge – the strait hinge is common and It consist of a landing in both sides and a thin hinge section.
Double Hinge – A double hinge has two strait hinges separated by a narrow landing section. Butterfly hinge – A butterfly hinge is commonly used on dispensing caps. Bi-stable Hinge – this is very similar to the butterfly hinge only instead of one uniform living hinge there are three separate hinge sections. Living hinge geometry.
Living hinge geometry. Web thickness- Should range between 0.007” – 0.015” thick. No sharp corners – Every corner should have a radius in your design. Gate placement – RevPart will help you with this. The gate placement is very critical. Hinge Material. Polypropylene aka PP- is the best resin by far for living hinges.
Use Homopolymer PP when possible. Polyethylene – Every corner should have a radius in your design.
Gate placement – RevPart will help you with this. The gate placement is very critical. Fillers Fillers are commonly added to plastic to meet physical or aesthetic requirements. Most fillers are not recommended for living hinges.
Molecular Orientation The key to a quality living hinge is getting all the resin molecules to line oriented in the same direction. This direction is in parallel to the flow though the living hinge. Environmental Factors. UV exposure- UV light is brutal to all plastics in varying degrees.
Living hinges will not last very long with degraded plastic and fissures. UV additive is recommended in all outdoor applications. Chemical Exposure- It is important to take into account what chemicals the living hinge will come in contact with and see if the selected resin is applicable. Prototyping a living hinge. CNC Machining- RevPart has the expertise to CNC machine your living hinges out of Polypropylene. Prototype injection molding- RevPart can produce inexpensive prototype mold that let you test the real deal living hinge. This allow you to make revisions and ensure your design works before going to full scale production.
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. Great product designers are always on the lookout for better ways to reduce cost and improve the user experience. Living hinge design is method that can accomplish both these tasks handily by replacing conventional hinge design methods with a flexible thin web of plastic. Due to the unique flow dynamics of plastic two bodies of plastic can be connected with a resilient hinge can be cycled indefinitely.
What is the key to getting a quality plastic living hinge? It’s all about the design. Ketchup Cap What is a living hinge?
A is a thin section of plastic that connects two plastic bodies together. Since it is very thin it enables the two bodies to move rotationally to angles of 180 degrees and greater.
Plastic Living Hinge Design Guidelines
They are very durable and with good design can last for millions of cycles. A familiar example would be a ketchup cap. The purpose of the hinge on a ketchup cap is to keep the cap connected to the main body but also allowing it to move out of the way for dispensing. What are living hinges used for? When you need something that connects two sides together and allow rotational movement a living hinge will likely do the trick. It can take the place of a traditional hinge in most cases and do it with less parts and no assembly.
You will find living hinges on plastic parts like caps and closures. Common examples include:. Makeup compacts. Candy container caps (for example Tic Tac mints). Look in your fridge sauces, condiments and more have living hinges. Shampoo and liquid soap bottles. Fishing tackle and lure boxes What benefits will I get by using a living hinge over other types of hinges?
2 piece hinge Eliminate assembly and lower cost – Plain and simple this is the biggest advantage of a living hinge. Most require some type of pin like a screw or metal rod be inserted for the hinge to turn. Since a living hinge is a uni-body design you can eliminate those parts off the bill of material. Not only do you eliminate the hardware but you get rid of the hassle and cost of assembling the hinge. Eliminate the need for multiple cavities – Without the living hinge the two halves would be separate parts which would require separate cavities, gates and runners.
Since a living hinge connects both half the cavity becomes one and will typically reduce the cost to make the mold. Durability – A properly designed and applied living hinge can last well over a million cycles. Most cycle test usually give up around the million cycle mark so it is conceivable that they can last many more cycles. Reasons not to use living hinges? Limited material selection – The best material for a living hinge is. Can be used in some situations. Even when using PP and PE you are limited on the types of additives and fillers you can use.
If you need to use an engineering grade plastic such as polycarbonate or ABS then a traditional hinge would be a better fit. Limited load bearing – Living hinges are not very good at bearing a load parallel to the hinge. A good example would be a door. A door has significant weight and places a load on the hinges it is attached to. The hinges must bear the load of the door and swing open and closed. A living hinge is not good for an application that places this kind of load on them.
Added development time – Living hinges require some time and effort to design. Each application is different and presents a unique set of challenges. In my experience after the injection mold is made the living hinge feature often requires some type of adjustment to get it working perfectly.
Living hinge design considerations When it’s time to design your living hinge it is very important to follow some basic design guidelines to ensure the hinge will function as intended. Typical living hinge designs. Flat hinge – the strait hinge is the most common type of living hinge. It consist of a landing in both sides and a thin hinge section. Double Hinge – A double hinge has two strait hinges separated by a narrow landing section.
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Double hinges are useful when you need to A) Create a gap or space between the two folded parts or B) You need a 360 degree rotation. Butterfly hinge – A butterfly hinge is commonly used on dispensing caps. The big advantage of a butterfly hinge is its flipping action. Once the hinge is opened/closed past a certain angle the hinge will spring to the other position and stay there. So the cap will flip to the open or closed position. Bi-stable Hinge – this is very similar to the butterfly hinge only instead of one uniform living hinge there are three separate hinge sections. This design will result in strong opening and closing action.
The hinge is in equilibrium in the open and close position thus the name bistable hinge Living Hinge Design Guidelines for Injection Molding Living hinge design is no doubt an art as well as a science that comes with experience. It is common to make a few changes to the mold after testing the first parts and with small tweaks the design is normally honed in to a functional hinge. While there are many different types living hinges and probably some yet to be discovered we will go over the basics of what makes a living hinge work.
Web Thickness the thickness should range between 0.007” – 0.015” thick. Anything thinner than this and the plastic may not fill the hinge section properly in the mold. Stewart calculu solutions manual. Anything thicker and the molecules will not orient in the right direction which will effect cycle durability. No Sharp Corners It’s a good idea to radius sharp corners when designing for injection molding. It’s even more imperative to radius sharp corners in and around the living hinge. The increased stress in sharp corners will reduce durability.
Gate placement Ask RevPart for your gate location and we can give you a CAD drawing of your mold. You want to make sure that the flow of plastic will evenly distribute along the hinge. This typically will require a gate that is centered on the hinge.
Hinge Material Selecting the right material is critical to good hinge design. A good resin choice should flow well though thin areas and should not stress crack when flexed. Only two types of plastic are acceptable: Polypropylene PP is the best resin choice for living hinges. When reviewing your PP material options look for a Homopolymer PP. This type of PP constantly performs well and produces durable living hinges. Random Copolymer PP is the second best choice and will perform very well. Impact Copolymer is the least desirable for hinges and should only be used if you have a low temperature or impact strength requirement.
Polyethylene PE the second choice but PE but is seldom used. PE should only be used is PP isn’t an option.
Flake Additive in Plastic Resin Fillers and hinge life impact Adding fillers is a common way to achieve certain requirements for the resin. For example you could add glass fiber to increase tensile strength or add Perl flakes for a metallic appearance. Additives like Glass, talc and fiber may negatively impact the durability and functionality of your hinge design.
It is not suggested to use any fillers and if it is determined to be necessary then it is suggested to R&D the hinge with a prototype mold. Mold Flow a living hinge at the molecular level The key to getting a durable living hinge is getting all the plastic molecules to align instead of being randomly oriented.
When molten plastic flows though the mold cavity it is forced through the small area of the hinge. The flow direction of the plastic and increased speed will create a molecular alignment along the hinge. The molecules with be oriented perpendicular to the flex direction which will extend the durability of the hinge. Another important consideration in hinge design is having uniform flow along the hinge. Flow that is not uniform increases the chance of having weld lines or defects which could compromise durability. No need to worry at RevPart we do a flow analysis and choose the best gate entry point for your mold. This ensures you will get uniform flow along the hinge and get durable parts as a result.
Environmental factors to consider UV exposure – Polypropylene like most plastics will degrade with UV/sunlight exposure. If you have UV exposure requirements it is highly recommended to add UV additives or a UV protective coating. Prolonged UV exposure tends to create fractures in plastic which will affect the longevity of a living hinge. Chemical exposure – If the part is going to be exposed to chemical then it is important to make sure that the selected resin will not be degraded. Post processing A common post process named cold drawing is to flex the hinge while it is still hot. The hinge is usually flexed a few times as soon as it comes out of the mold.
By flexing the part while it is still hot further orients the molecules in a direction that increases durability. One caveat is that it is very time consuming to do this and it will increase manufacturing cost. If your cycle requirements are less stringent then you can often skip this post process and the cost associated with it. Prototyping living hinges. CNC milled living hinge by RevPart is a valuable tool that allow you to revise your part without a large investment Using Traditional methods, prototyping a living hinge is not possible due to its strict dimension and material requirements.
However due to RevPart’s proprietary processes we are able to CNC machine many types of living hinge designs out of Polypropylene. Hinges come out fully functional and have the ability to be cycled hundreds of times. Another option is to open a to R&D your living hinge design. Check out the SlideShare Final tip Design your living hinge for tool-safe changes. This means you’ll design your hinge slightly thinner than you actually need. This way, you can make tooling changes without much impact to the mold itself.
Injection Molding Design Guidelines Much has been written regarding design guidelines for. Yet, the design guidelines can be summed up in just a few design rules. 1 Use uniform wall thicknesses throughout the part. This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times. 2 Use generous radius at all corners. The inside corner radius should be a minimum of one material thickness. 3 Use the least thickness compliant with the process, material, or product design requirements.
Using the least wall thickness for the process ensures rapid cooling, short cycle times, and minimum shot weight. All these result in the least possible part cost. 4 Design parts to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing. 5 Use ribs or gussets to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs.