The Five “Ms” of Molding—Part II: The Mold

A bad mold can cripple a process; learn what areas should be considered as a tool is evaluated and reviewed.

 

Tooling is a key foundation block when developing a work program. A poorly designed or improperly functioning mold can become the root cause for systematic failures. In addition, as continuous improvement projects are outlined, the mold should be reviewed for potential improvements through modification. The next section outlines major areas that should be considered as the tool is evaluated and reviewed:

 

Design: The very first step of successful molding is how well the mold has been designed to fit a particular application. Molds that do not perform well because of poor processing capabilities, frequent breakage or fluctuating molding conditions are crippling to a company’s productivity and efficiencies. Molding simulation software is a great tool for mold development. The ability to analyze mold temperature, pressure fluctuation and how the flowfront will likely perform greatly improves the designer’s ability to make adjustments to the tooling to counteract potential molding problems before they happen.

 

Qualification-Tooling: There are many different aspects to mold qualification. The primary goal of qualifying a mold is to develop tooling that consistently produces quality parts at an optimal cycle time. Here is a list of some of the primary areas that are important to mold qualification.

 

Balancing the runner system: The need to verify that the cavitation is balanced is paramount to a molder’s ability to control his process. Part weights should be consistent and equal. Parts that weigh more or less than the mean should be adjusted by making changes to the gates. Sprue, runners and gates should allow for adequate filling based on material properties.

 

Mold Temperature:  The ability to heat or cool the mold consistently is a vital part of building a consistent process. The mold faces should be checked in multiple areas to verify that temperatures are equal and consistent. Hot or cold spots can cause major inconsistencies. Adding or removing circuits might be needed to accomplish this. In addition, the mold circuitry should be clearly marked to prevent irregular set ups and assure turbulent flow and consistent direction. It is important to measure mold temperature variability in a running state to assure that mold temperature is consistent and not affecting process via hot or cold spots in the mold during molding.

 

Venting: Verify that the mold has adequate venting to meet the needs of the molding application.

 

Validation-Process: Process validation assures that once a process has been developed, the set-up of that process will be repeatable and consistent. Here is a list of some of the primary process controls that should be reviewed to assure that an established process is true and dependable:

 

Melt Temperature: Melt temperature should be verified to be within the recommended temperature window supplied within MSDS data by the material manufacturer.

 

Barrel Temperature: While the process is in a running state, compare actual temperatures to set points. Conditions that allow heats to ride above what the set points dictate create inconsistency in the process.

 

Velocity vs. Fill time: Injection speed should allow room for adjustment as determined by the fill time of a process. If increasing velocity set points does not decrease the fill time, injection speed is maxed out and the potential for process variance increases.

 

Cushion: Cushion should remain consistent to assure that the process is stable.

 

Peak Pressure: Pressure at cut-off should be verified as consistent, and must not be pressure limited by the maximum pressure limit setting. The pressure limit set point should generally be about 200 PSI higher than the peak pressure achieved.

 

Mold & Area Set up: There are a variety of situations where slight or even major set up variations can affect the ability to repeat a process. Here is a list of factors to consider when developing your mold set up plan:

 

Water: It is important to repeat your water set up consistently. Once process has been established, clearly identify supply and return lines to prevent circulation from changing one set to the next. Identify hoses using color and mark circuits

 

Hot Runner: Whenever possible, use the same hot runner box every time you run a mold.

 

Clamp Force: Record and verify that tonnage used stays consistent. Variability in tonnage setpoints can lead to poor venting or flash.

The 5 M’s of Molding—Part I: Man (labor)

The 5 M’s of Molding—Part I: Man (labour)

There are many areas in which personnel affect consistency and repeatability within a plastics operation.

As a plastics operation evolves there are numerous factors affect productivity, and these detractors can be small or large…consistent or sporadic…obvious or hidden. In any continuous improvement-minded facility, there is always a need for repeated analysis of each job being run. The overall success or failure of each individual operation hinges upon effective review of inconsistencies and system failures. The company then develops and implements improvements, approaches and/or corrections to address shortcomings that could potentially include the purchase of specialized tools or equipment to better equip personnel.

One of the key requirements for profitable continuous improvement is starting out with a solid base to build on. When a job is turned over from engineering to production, all facets of the production line need to be in stable working order. This helps to prevent costly down time and scrap directly related to poor set up, and will assure bad product does not reach the customer.

There are 5 key components that must not only be reviewed during engineering’s development of each work system, but also as the job matures through continuous improvement. The “5 M’s of Molding” make up the solid foundation upon which a company develops a successful molding operation. This article outlines these principles and suggests ways to use them for the evolution of a company’s production capabilities.

Man (Labor):
Labor is one of the most critical contributors to the success or failure of any production development. There are many areas in which personnel affect consistency and repeatability within a plastics operation as it evolves. Here are some of the primary points to consider when evaluating labor as an area for improvement:

Work area: Engineering does not end when parts have been removed from the mold. An evaluation must be made as to what steps need to be taken to provide the customer with a top quality part every time. Make sure the area is well lit and mark locations for tables, tools, scrap bin, etc. Area layout should be designed to maximize operator efficiency and great care should be taken to assure that waste-of-motion has been eliminated. Inspection, part preparation and packaging should flow smoothly allowing the operator both comfort and ample inspection time. It is important to remember that the more labor intensive a job becomes, increased quality problems directly related to human error could be the end result.

Tools: As continuous improvement efforts intensify, it is important to listen to the workers who are most involved with the production end. These are the people who day in and day out have their hands on the parts being produced. Listen to their concerns and suggestions for areas of improvement and provide them with the tools that best fits what needs to be accomplished, thoroughly and quickly.

Defects: It is important to note that quality should be molded in and not sorted. There are circumstances though where depending on operators is necessary. Be sure that they have been fully trained regarding what defects they are looking for, and whenever possible show them what area on the part a defect would normally be found. Track scrap data to identify what defects are most common and then look for solutions through mold modification, process change, etc. to eliminate or significantly reduce the defect.

Ergonomics: It is easy to overlook the importance of this category to the overall profitability of a company. Workplace injuries drive insurance costs, which inevitably reduce the overall profitability of every project on the floor. As you are developing work instructions, look for areas of the job that require frequent twisting, bending and turning. Evaluate methods and/or tools to improve the workflow of the station.

Work instructions: Operational instructions are a vitally important tool once the work pattern has been established. Great care should be put into providing all personnel responsible for the various tasks a job requires with very complete and concise directives. Pictures are a great tool that allow visual explanations of various components. Work instructions should be written in the simplest form possible with detailed explanations of all required information.

Human Error: As mistakes happen, review the fault for ways to eliminate failures from happening again. The “5 Why” system is an effective method for getting to the root cause of failures and developing solutions. Here is an example of this method:

Problem: 32 bad pieces were packed that had splay on the parts
 
Why? The operator did not catch the mistake
Why? It was a new defect that had never been seen on this part
Why? The dryer ran low on material which resulted in light splay
Why? The material handler did not check the material as frequently as he should
Why? He is new and needs retraining on the frequency of checking the hopper

Once the 5 why’s have been asked, it is now time to review the problems and establish solutions to prevent a reoccurrence. For instance, the operator missed the mistake because it had never happened before. To fix this we can implement a visual picture or defect part to be kept at the press in order to assure ourselves that the new problem has been passed along to everyone responsible for part inspection. The hopper also ran low due to an inexperienced material handler. The 5 why’s suggests that this person be retrained to prevent this situation from happening again.

Improve Pinch-off result of multi-layer blow molding

Focus on HDPE PE/EVOH based COEX bottle having drop test failure, Specially in Agrichemical / Pesticide bottles : For reference PA (Polyamide) Material from BASF taken and truly incorporated with new mold design :-

Focusing on PE-HD/Recycled/Tie/PA/ EVOH multi-layer blown molding only. Polyamide here provides good resistance to various chemicals therefore this multi-layer structure is widely used for agro-chemicals e.g. liquid pesticides which contain organic solvents.
[Determination of a “good pinch-off” or a “bad pinch-off” result]
A good pinch-off should to create a welding line which is rather smooth on the outside and forms a flat elevated line or a low bead inside, not a groove.

Extrusion blow molded parts often fail at the parison pinch-off seam of the mold parting line. Common forms of part failure at the pinch-off are cracking from impact, fatigue failure from flexing, or chemical stress cracking. Once the mode of failure is identified, the appropriate processing changes or pinch-off design modifications can be selected to optimize part performance and appearance.
Part failure along the parting line is related to material processing conditions, mold design, or a combination of these factors. Developing the optimal material shape inside the part at the pinch-off is a key to build parting-line integrity.
Selection of polymers

• Ultramid B40L and C40L/C40LX are recommended for multi-layer blow molding containers or bottles, C40L and C40LX provide better dart impact strength therefore suitable for finished product volume bigger than 500ml, while B40L is usually used for no more than 500ml bottles.
• Various tie materials are available in market, but to match low MI and high viscosity PA, strongly suggest to select low MI (no higher than 2.0) MAH-grafted LLDPE based tie material.
• Lower MI HDPE (higher molecular weight) definitely increases the impact strength but anyway its melt viscosity needs to match with polyamide under processing temperature. For example a MI-0.25 is just well matched to Ultramid C40L(X).
Parameters optimization
• Typical temperature profile for Ultramid B40L and C40L(X): 230C/250C/250C for extruder and 195-200C for the die, too low melt temperature of polyamide may cause melt fracture, poor thickness distribution and “bad” grooved pinch-up Weld line.
Mold design
Because of the comparatively high pressure and mechanical stress extered on the mold bottom when (in the closing step) it pinches one end of the parison together, the pinch-off in a nonferrous metal mold is frequently an insert made of hard, tough steel. The effect on the blown part always shows in the so-called weld line.
The pinch-off section does not cut off the excess parison tail. Its protruding edges cut nearly through, creating an airtight closure by pinching the parison along a straight line which makes it easy later to break off or otherwise remove the excess tail piece. A high quality pinch-off of a thick-walled parison is more difficult to obtain than that of a thin-walled parison. However, much depends on the construction of the pinch-off insert.




 The pinch-off should not be knife-edged, but, according to some molders, should be formed by lands about 0.1-0.5mm long. The total angle outward from the pinch-off should be acute, up to 15 degree. These two features combine to create a high quality welding line. A groove, which weakens the bottom along the seam, may be formed when these two features of the pinch-off are missing.
One method of obtaining more uniform weld lines is to build “dams” into the mold halves at the parison pin-off areas. These dams force some of the molten resin back into the mold cavities to produce strong, even weld lines.