As material prices skyrocket to new heights and forecasters see no end to this development any participant in the blow mould value chain has to consider cost-cutting measures. Extrusion blow molding offers unique opportunities for material savings compared to injection molding for example. In EBM the weight of the product can be adjusted with no or very simple tool modifications and at little cost. Sadly enough, many managers in the industry are not fully aware of all the possibilities the process offers. This article will explore all available strategies as they apply to converters and brand-owners.
Material Saving Strategies in Extrusion Blow Molding
Let’s start with reviewing the basics of the process and relevant terminology.
In EBM plastic pellets are extruded into a hollow tube called a parison. For this purpose, the plastic exits the extrusion head via a male pin (also called mandrel) and a female die (also called bushing). The parison is then clamped between two mold-halves and air enters through a blow pin or blow needle into the cavity forcing the parison onto the cooled mold walls where it rapidly cools and solidifies into the desired shape of container. There is always flash at the top and bottom of the container that must be trimmed, ground, and fed back to the extruder as regrind.
Most machines are nowadays equipped with a wall thickness controller or programmer that moves either pin or die during extrusion thereby allowing the thickness of the parison to be varied. This programmer may use 20 to 400 points to control parison thickness during extrusion. Material can so distributed to various container sections that require different amounts of material in a more uniform way. Bottle shoulders for example always need less material than bottle bodies and this can easily accomplished by using the programmer. For the programmer to work properly it is paramount that the length of the parison is always the same and that all parisons in a multi-cavity setup are of the same length. This is true because the programmed thin and thick spots must be precisely end up in the container sections they are destined for and varying parison length will move these spots up or down.
To ensure this virgin plastic, regrind, and possibly color should be fed via gravimetric material feeding system that weighs each lot of material before dispensing. Many companies use these systems but unfortunately many of them are not calibrated properly and are therefore not delivering consistent batch amounts. This is often due to the fact that material handlers are not aware of the importance of this or are not trained to performing this operation. Another complication arises from the fact that regrind levels are not constant. During startup and when adjustments become necessary during a production run, inadequate bottles are thrown onto the flash conveyor and ground. This increases the amount of regrind and fills bins up rapidly. Material handlers that have not been instructed otherwise then routinely increase the regrind percentage on the material feeding systems dramatically to work off the excess regrind. However, regrind processes differently and the result of increased regrind percentages is longer and wider parisons. The longer parisons change the positions of important program points and the result can be out of spec containers. Savvy operators are well aware of this fact and program their parisons in anticipation by stretching thick parison sections over more points of the program than necessary. This way, they will still make a sellable part even with a different parison length. As a result of this situation many companies make parts that are heavier than they need to be.
Suggestion #1: Train material handlers in calibrating gravimetric feeding systems and establish procedures to deal with upsurges in regrind. I also recommend taping a piece of tail flash with the proper length to the safety gate so an operator can quickly check if the parison length is what it should be.
Staying with this topic, I find that many operators give up too early in adjusting parison length when it comes to multi-cavity systems. To get all 4, 8, or 12 parisons extruding out of a head to the same length requires skill and patience. Small mechanical adjustments are necessary and each requires stopping the machine which operators are reluctant to do since it takes away from production time. For very short production runs it may not be suitable to spend too much time on parison length but every run over one week should be considered as deserving.
Suggestion #2: Allow setup personnel time to properly adjust parison length when warranted and have them tighten up thick sections in the extrusion program.
The next suggestion concerns the programmer itself. In order to move material in places where it is needed the programmer must be responsive and allow good control over the parison. Unfortunately, may companies still run programmers that offer only 20 or 25 points of programming points during one extrusion cycle. While this is sufficient for the short parison of a 20 oz. bottle, bottles over 32 oz. would do better with 64 points and a parison of 2′ length or above should be programmed with 100 points or more (programmers with up to 400 points are available now). Only with sufficient programming points is it possible to direct material effectively into container areas that need it. The servo valve(s) that control the movement of the hydraulic cylinder(s). for programming also need to be responsive enough to follow sharp program transitions. All too often a servo valve only comes under scrutiny when it fails. However, it should be adjusted on a monthly or so basis for proper gain, a procedure that takes less than 5 minutes.
Suggestions #3: Review programmer needs and plant equipment and replace inadequate programmers. Have setup personnel adjust servo valve gains regularly.
When it comes to oblong bottle shapes there are additional options. Either pin or die may be ovalized with the die being the preferred tool part. This may be necessary to add strength to corners or to avoid thick ribs across the panel area of a flat oval container. This is usually accomplished by scalloping parts of the die out in some specified fashion to allow plastic to flow into these areas, which will then in turn add material where it is needed in the container. It is of great benefit to a producer to have this capability in-house rather than buying ovalized tooling from outside. That is because more often than not small adjustments can only be made after an initial run and many companies hesitate to send tools out for rework because of the ensuing production delays. As a result, insufficiently ovalized tools lead to weight settings higher than would be necessary had personnel gone the extra mile.
Suggestion # 4: Build die ovalization expertise in-house and strive to perfect tools before going in production to net long-term gains.
The situation is different for converters and brand owners. The latter have the highest interest in lowest-weight containers as it directly affects the price they are paying for the package. The challenge is the same whether production is in-house or through a supplier. Smaller companies can learn from large groups and develop their own sets of testing procedures that adequately reflect the product life cycle. All too often assumptions are being made about what top load the container must have or what drop test conditions are adequate. It takes a close look what the container is exposed to in the distribution chain and at the consumer to come up with testing regimens that will give a high confidence level that the product will not fail. Because EBM allows lightweighing without much cost long-term tests can be made while in production having built in some safety margin in the container. If it turns out that a lighter container will do the job this can be implemented at any time. When lightweighing engineers should know that the brimful capacity of the container will most likely go up, not just because of thinner container walls but because these thinner walls bulge out more after filling. It is therefore recommended to run initial containers at the lower end of the capacity tolerance.
Suggestion #5 for brand owners: Because some testing involves long-term procedures and in order to speed up time to market, manufacture initial containers with a wider safety margin. Run lower-weight samples and test them during production. Reduce weight when testing showed lower-weight containers will perform.
For converters the situation is slightly different. They are usually given a weight specification with tolerance. Let’s say a container may be run at 27 g +- 1.5 g. Very often there is also a minimum wall thickness specified, usually for the bottom corner. Because operators do not like to have their production quarantined by quality control for insufficient wall thickness their tendency is to run the container at 27.5 to 28 g. However, if they had followed the suggestions above they might be comfortable in running it at 26 to 26.5 g. This would result in a 1.5 g weight saving per container and translate into Dollar savings of $2,000 or more (depending on the material used) per million containers produced. The ability to run containers consistently to tight specifications will build a reputation and attract business to the converter
Suggestion #6 for converters: Implement all other suggestions and run suitable product at or slightly below the target weight for additional savings.
I should also mention that tightening up on machine functions with new programmers and properly adjusted material feeders results in more consistent container dimensions, weights, and test results. This then translates into higher process capability factors when the numbers are run through statistical analysis. Large brand owners tend to want to do more business with companies that can provide these numbers!
About the Author
|Ottmar Brandau, President
OB Plastics Consulting
Mahone Bay NS B0J-2E0 Canada
Fax: 419-844-7759Email: email@example.com
|Ottmar Brandau is principal of OB Plastics Consulting, which specializes in extrusion and stretch blow molding. Mr. Brandau started out as service engineer for Bekum, Germany and has worked for Husky Injection Molding Systems, Canada and the distributor for Magic mpa, Italy, He has started his own consulting firm in 1997. Among services offered are training of operators, product development, plant audits and improvement programs, process troubleshooting, and new plant layouts and startups.|
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