By Jim Bott, business development manager
Have you ever gotten a part fill like this? (fig. 1) … when you really expected this? (fig. 2)
In some cases, even after the expense of not only filling using hot runner technology, but also adding valve-gating, an unwanted and unexpected part filling pattern still can be experienced. That can cause big problems.
3 points of concern with unbalanced fill patterns
Unbalanced filling progressions will
- negatively impact the part quality due to core shift, trapped air and filler bias (if present).
- cause increases in molding cycle time to reduce or fix the problem.
- result in a high rate of scrap.
These impacts are seen at a higher rate in parts with an unbalanced fill rate – much more so than a part that fills at a balanced rate, period.
3 keys to avoiding quality issues
There are three keys that can be used to avoid part quality issues before they happen.
- First, the mold has to be of “sound” construction and design. No need to define “sound” in this article. The criteria includes the following: venting, cooling, follow the cooling rules and manufacturing tolerances.
- Second, ask, “What is this part doing, and what is it worth?” This fundamental question will determine if additional technology is required beyond that required to create a “sound” mold and a “quality” hot runner system. Another similar question would be, “How much are points 1, 2 and 3 really costing the molder?”
- Third, to avoid a problem before it happens, seek out a day or two of education on plastic characteristics of non-Newtonian flow, rheology and the science of plastic fill patterns. This refresher can add to an understanding of what could happen and why.
See the pattern here? Groups of three.
Especially for certain materials that are shear-sensitive (ABS and PC), points 1, 2 and 3 will happen with a “sound” mold and a “quality” hot runner system.
However, the magnitude of the effect will always differ. Based on the case, the effect could be negligible or severe; it could be known or not known. How is “not known” possible? Easy! All it takes is one technician to say, “We have always molded the part this way, using this technology, with these results.” Basically, we’re happy making money.
What if, after studying the third key to avoiding a problem before it happens and then applying the practice, you now have improved the customer’s part quality, reduced the cycle time and reduced scrap? Would the molder be happier making more money? Would the moldmaker be more satisfied with additional orders and referrals? I would think so. The challenge here is to seek continuous improvement. Doing things the same way as always will find you left far behind the competition.
A question easily could arise at this point in the conversation: How can a part fill problem be anything from negligible all the way to severe? How will you know in advance so it can be avoided in advance?”
The answer is simple: Ask the expert – your customer, the molder.
The molder very likely has experience seeing imbalances during short shot progressions on like, similar or duplicate parts. Unfortunately, in many cases, the molder does not always fully grasp the “three points.” However, the molder will be able to tell you – or better yet, show you – short shot progressions from past experiences. This is when the moldmaker can add up all of the POINTS and the KEYS to help make a decision about the amount of technology necessary for the mold and hot runner system to meet the objectives of the molder, avoiding part quality issues before they happen.
Fundamentally, all part filling imbalances are due to differences in pressure drops, either from cavity to cavity or within the cavity. A number of contributors can be blamed for the differences in pressure drop in any application deltas in
- tolerance sizes
- cooling/heat (homogenous distribution) and
- flow path lengths
Dang it – that’s four this time.
These four items are loosely termed “steel variations” by Dr. John Beaumont, Beaumont Technologies. I want to credit John and his team for originally bringing this to my attention and teaching me what I needed to know. All of these pressure drop contributors can be avoided with a “sound” mold and a “quality” hot runner system.
Unfortunately, there does lie a fifth nasty contributor – namely, viscosity variations. Variations in viscosity caused by shear cannot be avoided (all of the time). Correction using temperature manipulation and/or making varying gate diameters throughout a mold for parts that are exactly the same is “bad” for a couple of reasons.
- First, the temperature/gate size recipe is only good for that viscosity for that lot set (or reground) material. If the viscosity is different from lot set to lot set, the “recipe” will no longer be optimized.
- Second, another reason for the practice of varying nozzle temperatures and/or gate diameters is that this practice does nothing for the molecular orientation of the material and/or the bias of any filler within the part(s). This is a root cause for part dimensional variations cavity to cavity, as well as variances in part warpage cavity to cavity.
In summary, discuss the part expectations in further detail with the molder, review the science of plastics rheology and apply the required technology to the melt delivery system to achieve the lowest price/performance ratio.
Jim Bott is business development manager for INCOE Corporation. For more information, call 248.556.7857 or visit www.incoe.com.