Many things can affect the end result of injection molding. Temperature and pressure are probably the two most common variables thought of to control the quality of injection. In truth, viscosity may be more critical to have success with injection molding. Controlling viscosity ensures a consistent and uniform product, but controlled viscosity isn’t the result of turning a knob or pushing a button. It’s a variable that takes skill and knowledge to control for consistent high quality with reduced operating costs.
What Variables Influence Viscosity in Injection Molding?
Viscosity, as defined as the measurement of a substance’s resistance to motion under an applied force, is one of the most important variables in injection molding. Think of two fluids on opposite ends of viscosity: thick syrup and water. Water flows easily in liquid form. Syrup, especially at colder temperatures, doesn’t flow well. Higher viscosity indicates a resistance to flow, such as the colder syrup. Low viscosity flow like water. Some of the other variables that influence viscosity are pressure, temperature, relative molecular weight, and shear rate.
Various plastics react in a different way to pressure. Some such as Polystyrene (PS) are easily compressed and forced into a mold under pressure due to the relatively large space between molecules. Others, such as high-density polyethylene (HDPE), have an opposite reaction. It has a higher viscosity and doesn’t flow as easily as a plastic resin with a low viscosity index. Low viscosity is easier to fill in a mold, and high viscosity is more resistant. Significant pressure can improve mold filling though, but it needs to be balanced with the result of performance loss and wear on molding equipment. The more pressure used to fill difficult molds, the more wear and tear will be the result. You may also find that excessive pressure leads to molding defects and flashing that will need extra rework for a quality part.
The influence of temperature on viscosity is directly related, as plastic melt increases exponentially with an increase of temperature. The hotter the plastic resin, the more viscous (the easier to flow) it becomes. That is true to a certain point as too high of a temperature ca cause the resin to breakdown and burn inside of the molding machine. Each plastic will have a different melt temperature, which will also have to be correlated to the mold design and the other influencing parameters noted. Temperature and pressure can be used in combination to fully fill a difficult mold, or they can be separately adjusted as needed to increase performance.
Relative Molecular Weight
Molecular weight is rarely a discussion point when picking a resin for a new product, but it may be a determining factor as the best option to use. Choosing a lower molecular weight resin can reduce the temperature needed to melt the resin for injection. That reduced temperature can add value by reducing molding cycle time (less time required to melt the resin), and by reducing power required for the molding machine. Higher molecular weight may also require higher injection molding pressure to completely fill the mold during processing.
In general, resin viscosity and shear rate are inversely correlated. The lower the shear rate, the higher the viscosity. That holds true for most resins, but for the cases of very low and very high shear rate, the viscosity will not change accordingly. Staying within a certain range, changing the shear rate can significantly change the viscosity of plastic and subsequently improve their ability to flow inside an injection mold. Best practices show that staying within the parameters of the resin will reduce defects and extra work needed to produce a quality product.
Can Viscosity Be Used as a Tool in Injection Molding?
There are many variables that can positively and negatively affect injection molding. The question is whether viscosity should be changed to improve the performance. Adjusting machine parameters is common practice to fine tune the molding operations as performance can slightly fluctuate with every batch of resin, surrounding temperatures, and a handful of other factors. Controlled viscosity won’t drastically change mold fill time. You can use extra energy to force molten resin under higher pressure to fill a mold, but reducing the viscosity of the material isn’t going to drastically reduce the cycle time.
As noted above, there are many situations where viscosity can be used as a tool to correct poor molding conditions. When standardizing the process, start off with lower level viscosity as determined by melt temperature, and then make adjustments using higher temperatures when viscosity appears to be directly related to molding issues. Viscosity can positively affect quality defects such as sinks, flashing, and knit lines. Adjusting viscosity may help reduce all of these defects, but viscosity isn’t the only variable that can fix problems. Be sure to adjust temperature and pressure accordingly to reduce defects, and study if molecular weight and shear rate can also benefit the operation if they are changed.
There are thousands of resins available for injection molding. Picking the correct one for your new product can utilize the advantages of low cost, decreased weight, and high strength to deliver the best results. Choosing which resin is the best option and which manufacturing process is best for your new project may be a tough decision to make without a lot of research. SEA-LECT Plastics prides itself as one of the industry leaders, and we’re here to help when you need advice on a future project. We have an elite team that designs for injection molding, rapid prototyping with metal and plastic, and produces world-class products under the same roof. Give us a call at (425) 339-0288 or email us at firstname.lastname@example.org. We look forward to offering support and advice on your next project. SEA-LECT Plastics is an ISO 9001:2015 certified facility that incorporates a Total Quality Management (TQM) atmosphere into our daily mindset. Your products will be made by an elite team with a single goal of exceeding your expectations.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.