Temperature is naturally a very important factor in the plastic injection molding process, and two particular temperature types hold an especially vital role: Melt and mold temperature. These are two different but related metrics, both of which speak volumes toward how your final injection molded products will look, feel and last.
At EnviroTech Custom Injection Molders, we’re happy to provide numerous plastic injection molding services, from over-molding and bulk molding to several other customized processes to meet your every need. We’re happy to detail some of the important parts of our process to you, including why melt and mold temperature matter for plastics during the machining process. What exactly are these metrics, what are they telling our manufacturers, and which other parts of your final product do they have the greatest impact on? Here’s a primer.
When we talk about melt temperature, as its name indicates, we’re discussing the actual temperature at which the material contained in your injection molding machine “melts” into its liquid form while leaving the nozzle and entering your mold. This is very important, since a good deal of the product design and development process for injection molded plastics comes down to understanding exactly what type of molten plastic state you need to get the look, feel and functionality that you want from a finished item.
For this reason, melt temperature is carefully monitored and controlled during the injection molding process. All of our machines come pre-set with standard melt temperatures, but these can differ between different types of plastics, even when they are made from the same basic formula. For example, ABS is one commonly used plastic in many household items, and its melt temperature is 220 degrees Celsius (428 degrees Fahrenheit). However, even this can change depending on the manufacturer; according to Plastics Technology, Sony uses ABS that has a melt temperature of 240 degrees Celsius (464 degrees Fahrenheit).
When possible, it’s important to note that different batches of plastic within the same formula may also have slightly different melt temperatures. This can be due to slight variances in their basic makeup, but it can also be due to environmental conditions, such as temperature and pressure. This means that there’s no exact way of telling what the melt temperature for your polymer will be based on either the recipe or its previous usage; you’ll need to test out different batches before committing.
How Melt Temperature Impacts Molded Products
Depending on the type of resin being used, there are a few different ways melt temperature may impact the final product that’s being created. These include:
- Resin viscosity: The higher the melt temperature, the lower the viscosity. If it’s too high, your resin will be overly thin and won’t be able to fill out your mold properly, resulting in an incomplete product.
- Resin molecular weight inside molded part: The higher the melt temperature, the lower the weight. Using a higher-weight resin means that more of it will be drawn into your final product, meaning that it’s stronger and more durable.
- Part shrinkage: When you’re talking about injection molding materials, “shrink” actually refers to expansion; drawing in too much material could result in your final product being thick and flimsy, while drawing in too little could leave your part hollow. If you’re planning on over-molding an existing product with a second resin to reinforce it, make sure that its melt temperature is lower; this will prevent damage or even explosion due to the excess pressure inside of the mold cavity.
For these reasons, injection molding processionals will consider several factors within the realm of melt temperature: The resin chosen, the residence time, screw and barrel design, and the final property requirements of the part.
Mold temperature, on the other hand, refers to the temperature of the mold surface itself. This plays a crucial role during the injection molding process, as it will determine how your product heats up and cools down, which itself has an impact on its final look, feel and quality.
Like melt temperature, mold temperature is carefully monitored throughout each stage of the injection molding process; this is especially true for plastic components with very specific property requirements.
For example, Plastics Design & Fabrication magazine noted that when you’re considering the mold temperature for injection molded parts, it needs to be carefully examined during all three stages of the process; beginning with the cooling down period known as “quenching,” then at the end of injection molding known as “annealing,” and finally during any post-processing steps like painting.
If mold temperature is improper, the risk of defects, warping and jetting all become much more prominent, which could lead to problems with the final molded product.
How Mold Temperature Impacts Molded Products
How does mold temperature play a role in the final properties of a molded product? In several ways, actually:
- Molded-in stress: Though it may not seem significant, the cooling process of an injection molded part can “set” any inherent stress that’s already in the polymer. This means that if it undergoes shock-cooling at certain points during production (due to incorrect mold temperature), there could be issues with its endurance and resilience.
- Creep and fatigue resistance: If a product is cooled down at certain points during the injection molding process, it will be less resistant to long-term forces and environmental factors. This means that if it’s being used as an automotive exterior component, for example, its resistance to wear may be compromised after just a few months of use on the road.
- Molecular weight: One of the most important aspects to take into consideration when it comes to mold temperature is the molecular weight of the material. This will determine not only how much stress can be applied, but also how long it takes for that stress to take effect.
- Wear resistance: Along with molecular weight, factors like mold temperature will play a critical role in determining how well it can resist the effects of friction and abrasion.
- Durability: If a product is cooled too quickly, not only does its resistance decrease, but also it could become very brittle and even crack under pressure from an impact or force.
For more on the role of melt and mold temperature in the quality of your plastic injection molded parts, or to learn about any of our plastic molding services, speak to the staff at EnviroTech Custom Injection Molders today.