Many of the challenges faced in molding can be addressed by returning to the basics of how a material moves through the nozzle, gate, and mold. In the first part of this two-part series, we take you through the physics of resin behavior. Stay tuned for part 2 next month, when weâll give you the not-so-secret formulas for calculating shear rates.
The process of molding thermoplastics requires shearing the resin to make it melt and to get it to flow into the mold. Shearing occurs throughout the process and, done correctly, makes for successful molding. Done incorrectly, it can lead to resin degradation, part failure, poor cosmetics, and excessive mold corrosion, among other things.
A great number of molding problems can be avoided with a basic knowledge of plastic resin shear. The gate is generally the highest-shear area and many problems start there. From sizing gates to selecting proper machine nozzle sizes, a little understanding and a few calculations can save you a lot of grief.
How often are you relying on experienced people to pick the gate sizes? How often do you sample molds only to find the gate size is incorrect? The purpose of this two-part article is to provide you with the knowledge needed to troubleshoot this straightforward area and help size the gates right the first time.
Back to basics
Letâs start with a few definitions:
What is shear?
Shear in injection molding occurs when layers of molten resin flow relative to each other. During injection, molten plastic generally flows through the melt delivery channel (nozzle, sprue, runner, and gate) and then into the mold like a fountain. Fountain flow means the material moves through the flow channelâs center and then outward to the walls. The flow channelâs outer layers flow more slowly in relation to the material in the center. For visualization purposes, imagine the plastic separating into many different layers as it flows, each layer moving at a different rate relative to an adjacent layer, or shearing.
What is shear rate?
How fast shear occurs is referred to as the shear rate. Shear rates are important to the design and molding of plastic products. As shear rates rise, the molecular chains that make up the polymer are stressed more. If a polymerâs shear rate is too high, molecular chainsâor the glass fibers they might containâcan be torn apart. This can reduce the mechanical properties of a product.
The highest shear rates occur where the relative movement between layers is the greatest. This is generally near the wall and at the gate. The shear rate at the center of flow is much lower because there is little difference between the center layerâs speed relative to the other center layers. The units of shear rate are reciprocal seconds, also shown as sec-1. Maximum suggested shear rates can range from acetal at 20,000 sec-1 to PP at 100,000 sec-1. For our purposes, the higher numbers simply mean more cubic inches of material can go through a gate in a certain time.
What is shear stress?
Shear stress is a measure of the tension created between molecules within the plastic, caused by the plastic layers flowing relative to each other and tugging on each other. Too much stress causes the molecules to break. The maximum shear stress a material can withstand is generally estimated at 1% of tensile strength. Shear rate and shear stress are directly related. Mechanical properties are lost when the average molecular size decreases.
How are water and plastic different?
The study of how plastics flow falls under the science of rheology. Comparing plastic to water is the most common distinction for how plastics flow. Water and plastics behave differently when sheared at different rates. Water is referred to as a Newtonian fluid because its viscosity does not change as the shear rate changes; its consistency does not get thicker or thinner. Plastics, on the other hand, are non-Newtonian, or shear-thinning, because the viscosity drops (flows easier) as the shear rate (fill rate) increases. As a side note, there are materials that actually get more viscous at higher shears. These are referred to as dilatant materials.
Why are shear rates important?
With plastics, we need to pay attention to how we process the material so we donât degrade it. While plastics can be degraded in a number of ways, we are concerning ourselves here with injecting too fast and overshearing the material through a gate. Fortunately, the highest-shear-rate areasâgates, runners, and nozzle tipsâhave primarily simple geometries and lend themselves well to basic calculations. Therefore, we focus our calculations of shear rates on these areas.
How much is too much?
Hereâs a great question with many answers. In one sense, it is a question of how much degradation you can live with or what surface imperfections are acceptable. What loss of properties can you live with? Different research shows different results. You will hear terms such as âmelt fracture.â Some polymers process fine above the melt fracture regions. Maintaining the chain length of a molecule is very important to maintaining good mechanical properties in the finished parts. Remember, high shear rates cause high stress, which causes the molecules to break.
For glass-filled resins, itâs a little more clear how much is too much. We depend on a minimum glass fiber length to maintain physical properties. If the fibers break, the physical properties suffer. A number of major resin suppliers indicate 40,000 sec-1 to be a good maximum allowable shear rate for glass-filled resins.
For resins that have different components (colorants, rubber modifiers, and so forth), a phenomenon called multiphase stratification can occur. Cool term, but what does it mean? It appears as plating out, delamination, or streaking. âMultiâ means more than one and âphaseâ simply refers to the different compounds that may exist in a resin; âstratificationâ refers to separation. So, it means the components donât stay mixed together. The phases (or different components of the resin) react differently to the high shear rates and separate.
The following are some actual case studies in which a focus on shear rate helped solve the problem at hand.
Case Study #1. Problem: Poor appearance of overmolded thermoplastic vulcanizate (TPV) on the partâs surface. In this application, the soft-touch TPV flowed over a rigid substrate. The melt delivery was a hot tip feeding a cold runner. Unacceptable surface defects, primarily cold slugs, drove the scrap rate higher than 10%. Despite adding ample cold slug wells, the cold slugs still persisted. TPVs like to see high shear rates for proper processing. A quick calculation of the hot-tip gate showed that it was probably too big. Simply advancing the spreader tip into the gateâbasically changing the round orifice to a much higher-shear annular flowâeliminated the problem.
Case Study #2. Problem: Nonhalogenated polypropylene degradation. Overshearing nonhalogenated PP leads to readily visible degradation streaks. We were building some new molds and hoped to avoid an existing partâs processing issues. The existing partâs processing window was extremely narrowâinject too quickly and degradation resulted, inject too slowly and the part would not fill. The resin compounder was able to share some experience on another part using that resin where gates were opened up to make acceptable product. Knowing that partâs weight and the injection time, the fill rate (in3/sec) was easily determined. The resin compounder provided the gate size that did not work and the new, larger gate size that worked. Using a shear rate formula (given in next monthâs article), we were able to calculate what shear rate worked. We then provided this maximum shear rate to the flow analysis provider and made sure that no part of the melt delivery system approached this shear rate in the new part.
Cast Study #3. Problem: Weak vibration welds on toughened-nylon parts. Making a two-piece nylon assembly required vibration welding together two parts made from toughened nylon. One part in particular typically had trouble achieving acceptable weld strengths. Toughened nylon has modifiers to give it its impact properties and these additives can be separated from the resin under higher shear rates. Using the formulas to be outlined in next monthâs article, it was determined the troublesome part had a distinctly higher shear rate at the gate than parts for other welded-part assemblies using the same resin. The solution was to open the gate on the troublesome part to be equal or less than the shear rate on parts in successful welded assemblies.
Part 2 of this series shows how to calculate shear rates. Author Mike Miller (firstname.lastname@example.org) manages two business units for tool and diemaker Carlson Tool & Mfg. (Cedarburg, WI; www.carlsontool.com). He wrote this two-part article as part of a continuing effort to correlate what textbooks say to the issues he faces in practice.