Automotive injection molding components require a vast array of knowledge in order to make parts that hit tight tolerances and pass strict testing requirements. The main elements that we focus on are typically the product design, material, tooling, and manufacturing. But what can easily be overlooked is how some materials react to their environment after the part is molded.
In this post, we will specifically focus on Nylon (popular thermoplastic material used in a variety of automotive applications), moisture conditioning methods, design elements to focus on, and some of the basics of moisture conditioning in general.
Basics of Nylon
Polyamide or Nylon (which is Du Pont's trade name) is a synthetic thermoplastic that is frequently used in an effort to replace metal components (metal-to-plastic conversion) due to its high tensile strength properties, repeated impact toughness, abrasion resistance, temperature resistance, and chemical compatibility.
Nylon 6 and Nylon 6/6 are the most widely used forms of nylon (approximately 90% of the nylon market). The numbers (6 and 6/6) describe the type and quantity of polymer chains in their chemical structure.
Nylon 6/6 is made of monomers, each containing 6 carbon atoms, hexamethylenediamine, and adipic acid. This is the most widely used nylon for many reasons. Nylon 6/6 is stronger, stiffer, harder than nylon 6 (because it's more crystalline), more resistant to hydrolysis, and it also crystallizes faster (resulting in shorter cycle times compared to nylon 6)
Nylon 6 is made of a single monomer in combination with itself and has a higher water absorption rate compared to Nylon 6/6. One of the big pros of using Nylon 6 is its lower shrink rate
Nylon is Hygroscopic
To put it in basic terms, hygroscopic means that nylon is able to absorb water from its surroundings. The drawback to this is that high moisture pickup results in changes to its dimensional and mechanical properties.
What that means is that when the moisture molecules contact the nylon structure, they diffuse through the material and force the polymer chains apart (source). This is why nylon parts end up swelling and softening after being exposed to moisture.
So, what is moisture conditioning? When it comes to nylon, moisture conditioning is the method of increasing the polymer's moisture content. To better help us define this, let's first start from the beginning of this process.
The nylon material must be dried prior to processing. Injection molders typically want to use nylon resin that is dried to below 0.2% moisture content. Once processed, the material is at its driest point. If it is not dried properly during processing, you're going to end up with a weak part.
The moisture does act as a lubricant that helps increase the flow of the material during the molding process. If it is dried beyond that 0.2%, the mold flow decreases, which can make it more difficult to fill the mold's cavity. Injection molders will condition their nylon parts soon after they're ejected from the tool to shorten the time frame that is required for the components to reach equilibrium from weeks/months, down to just a few days.
Moisture Conditioning Methods
- Packaging In Bag Containing Water: This is the most economical option where conditioning will occur during shipment/storage. This is commonly used for high-production items, like automotive clips. (Another alternative we've seen has been by placing saturated paper towels into the package, allowing the moisture to migrate from the paper towel over to the the nylon components)
- Steam: Fastest method, but has several disadvantages that make it an unfavorable option. Moisture conditioning via steam can result in discoloration from oxidation, part warpage, uneven distribution of moisture (exterior will be more conditioned than the inside of the material)
- Hot Water Storage: This method requires hot water to condition the plastic, but will result in similar issues to what you'll see with the steam method. But, this method is faster than cold water storage
- Cold Water Storage: Simple method of conditioning nylon, but will take more time compared to using hot water or steam
- High Humidity Climate: Most effective method as it will result in a thorough and even distribution of moisture. But, the drawback here is the expense of the equipment.
- Aqueous Potassium Acetate: Process of rapidly condition nylon parts to a uniform moisture level by immersing the parts in boiling aqueous potassium acetate solution in order to attain 50% relative humidity condition. This process should only be used on test pieces with thicknesses of 0.25" or less as you may run into issues from prolonged boiling required for thicker parts.
Injection Molded Automotive Nylon Clips & Moisture Conditioning
When it comes to automotive applications, where moisture and nylon must be emphasized is with components that have living hinges and snap features. The material must be dried prior to processing. Once processed, the material is at its driest point. If it is not dried properly during processing you will end up with a weak part.
With that said, when designing a nylon clip, you should never use the "dry-as-molded" properties of nylon. Instead, you'll need to use 50% relative humidity properties of nylon when calculating product design.
Packaging for these components typically requires the molded parts to be placed into a box that has a bag liner with approximately a cup of water. Conditioning of these components will occur during storage and shipment.
Living Hinge Basics
A living hinge is a thin piece of plastic that allows a part to fold or bend from 1 to 180 degrees. The main concern with living hinges on nylon clips is that it creates tensile stress.
Moisture conditioning is critical in reducing the risk of failure on this feature. If it's kept in a cold, dry climate, your assembly team will most likely run into multiple clips snapping during installation.
Plastic Snap Feature Basics
Snap fits are injection molded integrated design features used to easily install and secure plastic components. Snap fits are a great way to replace or reduce the need for screws, bolts, etc.
The way they work is there is a small protrusion that is deflected during the joining/installation process and then "snaps" back open to its original unstressed state catch onto a feature in the mating component.
The stress/strain caused by the installation process can become a major issue if the nylon clip is not conditioned properly.
Impact of Moisture Conditioning on Automotive Clips
Let's look into what kind of impact moisture has on nylon components, such as automotive underbody clips. Water packing nylon clips is critical towards ensuring you have a "tough" part that is flexible and strong. So, let's see what pros and cons come from this in order to get a better feel for the impact moisture conditioning plays.
- The living hinge on an unconditioned clip will be very stiff and more difficult to close in comparison to a much more flexible conditioned version (This is even more true if the clips were stored in a cold environment)
- Assembly: When clips are conditioned (resulting in more flexibility), assembly becomes much easier, reducing the risk of injury and parts breaking
- Less risk of brittle failure
- Significant increase in clip's impact strength and ability to absorb energy
- Increase in elongation and toughness
- Decrease in yield stress, tensile modulus, and hardness decrease
- If the clip is placed in a cold, dry environment before final installation, the clip will return to a weaker, less flexible state
- Results in dimensional changes to the clips (Need to factor this into the part design)
- Increase in weight
To sum it all up, nylon is an excellent material but requires a molding expert who understands the impact moisture plays in a part's life. It's not just about packaging, it's also knowing how to design a part by factoring in the dimensions
When looking into sourcing, designing, molding, and installing an automotive nylon part, just remember to always keep the following in consideration:
- Designing the part to factor in dimensional changes caused by moisture conditioning
- Ensuring the material is dry before processing
- What type of nylon you'll be using will react differently to moisture conditioning (Nylon 6 vs. Nylon 66 vs. Glass-filled Nylon)
- Moisture conditioning method
- Storage of components (i.e. if a tier-1 supplier in Michigan is storing clips in a cold, dry climate without moisture, they'll most likely run into issues)
Keep in mind that sometimes conditioning a nylon component won't solve all brittleness issues if there is an underlying issue still present, such as material quality or design flaws. If you're currently having issues with your nylon components, need a quote, or just have a question you'd like to ask, reach out to our team with the form below.