Head tooling – extrusion blow molding

>> Wednesday, 19 August 2009

Mandrel and die dimensions are estimated based on container dimensional data, container symmetry, blow-up ratio, targeted container weight, neck finish requirements and the type of material (degree of parison swell) that will be used to produce the container.

An initial blow-up ratio must be calculated using the container design dimensions and the required parison diameter.
The required parison diameter will depend on the relative size of the container, the container design (handle or no handle) and the container neck finish requirements. Initial blow-up ratios may be calculated using the following equation.

Blow up ratio = Bd / Nd

where:
Bd = Bottle diameter, in
Nd = Minimum neck diameter, in

The blow-up ratio is compared with the maximum recommended blow-up ratio of the selected material.
Blow up ratios of 2 or 3 to 1 are considered normal when molding commodity resins such as polyethylene.
A blow-up ratio as high as 4:1 is a practical upper limit.
The blow up ratio for large containers with a small neck, is generally extended to 7:1 so that the parison fits within the neck and so that there is no mold parting line mark on the neck finish.
Blow up ratios for a containers with a handles are generally in the 3 or 5 to 1 range as the die diameter must be larger to allow the handle to be blown.

Figure shows a typical blow molded container with dimension and design nomenclature for reference


In order to properly estimate and ‘size’ mandrel and die geometry for the blow mold(s), and to effectively control the process, a thorough understanding of parison swell and draw down phenomena is required. Parison swell is a combination of diameter swell and weight swell. It is a difficult blow molding property to estimate and to control. The parison diameter swell is a complex function of the weight swell, the rate of swell, and the melt strength.

Parison swell behavior varies significantly depending on material type, material processing conditions, machine processing parameters, basic die design (diverging vs. converging), container geometry (required parison diameter), container weight (shuttle process) and type of blow molding process. Some of the wheel type blow molding processes clamp (pinch off) and hold the parison at both ends during the blowing sequence in the process. The parison swell effects are normally more readily controlled on the wheel process compared with the shuttle process.

Parison swell data for a given material is often not available for mandrel and die calculations. The alternative is to proceed in a stepwise approximation towards the desired mandrel and die dimensions, and through trial and error, towards the targeted container weight with the aid of an interchangeable set of dies.

Internal die design dimensions including approach angles and land lengths vary significantly with blow molding machine capabilities and machinery manufacturers experience. Calculations for these dimensions are beyond the scope of this document and will not be discussed here.

However, as a rule of thumb, when blow molding commodity materials (PE, PP), a die land length of at least 8 times the annulus gap (die gap) is typical.

A simplified approach for calculating and estimating mandrel and die dimensions is presented here to serve as a general guide. The following equations may be applied in cases where the container geometry is symmetrical and there is no handle on the container.

Case A
When the neck size of a container or the smallest diameter of the container is the controlling feature (as when the parison must be contained within the smallest diameter) , the following approximations may be used to calculate the dimensions of the mandrel and die. Use of these equations assume a free-falling parison and they can be used with most PE blow molding materials.

Dd = 0.5 N d (Equation 1)

Pd = ( D d2 - 2Bdt + 2t2)1/2 (Equation 2)

where:
Dd = Diameter of die bushing, in
Nd = Minimum neck diameter, in
Pd = Mandrel diameter, in
Bd = Bottle diameter, in
t = Bottle thickness at Bd, in

Case B
When the container weight is specified instead of the wall thickness for a process using inside-the-neck blowing, the following equations may be applied:

(The equation may be applied for a free-falling parison, and is applicable to most irregularly shaped containers.)

Pd = (Dd 2 - 2 (W/T2) Ld)1/2 (Equation 3)

where:
Pd = Mandrel diameter, in
Dd = Diameter of die bushing, in
W = Weight of container, g
L = Length of container, in
d = Density of material, g/cc
T = Wall thickness, in

Case C
When a parison is partially controlled by tension, i.e., the rotary wheel blow molding process, the following relationships may be used. The assumption here is that the parison is not free-falling.

Dd = 0.9 N d (Equation 4)

Pd = ( Dd 2 - 3.6 Bd t + 3.6 t 2)1/2 (Equation 5)

Pd = ( Dd 2 - 3.6 ( W / T 2 ) Ld) 1 / 2 (Equation 6)

where:
Pd = Mandrel diameter, in
Nd = Minimum neck diameter, in
Dd = Diameter of die bushing, in
W = Weight of container, g
L = Length of container, in
d = Density of material, g/cc
T = Wall thickness, in
Bd = Bottle diameter, in
t = Bottle thickness at Bd, in

Case D
Estimating head tooling dimensions for containers with handles requires an empirical method based on container size and geometry. When the container has a molded handle use the following equations to determine the estimated dimensions for the head tooling die and mandrel.

The die diameter can be estimated using equation 7 with the container dimension data.

D = ( 0.8 Z ) / 3 (Equation 7)

Where:
Z = Maximum container width or diameter
D = Diameter of die bushing, in.

The die diameter, D, is then substituted in equation 3, equation 5 or equation 6 depending on available container data and the blow molding process that will be used.

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PET drying

A fault with the dryer will lead to hydrolysis of the material in the barrel of the machine. This will cause lowered I.V. of the material. Lowered I.V. is the major cause of bad quality in preforms and bottles. More than 60% of all PET processing related faults can be traced back to the dryer.
Maintaining the dryer in optimum condition will allow the molding machine to perform at maximum efficiency and quality.

Check the basics!
Airflow - The most important parameter, there should be nothing causing a restriction in the process and regeneration air flow.
Temperature - Process Temperature should be in the region of 145~170ºC Depending on the resin supplier and the drying time. Also check the Regeneration Temperature which should be around 200~230ºC depending on the maker.
Time - Calculate (or measure) the time the material is in the hopper, this should be at least 3½ hours.
Dewpoint - Correct dewpoint may vary according to the manufacturer of the dryer, consult the maker's manual before assuming an error exists.

How Do I Do This?
Airflow - Most common problem is a blocked process filter, but also check blowers are operating correctly and the delivery hoses have not been squashed.
Temperature - Confirm process air temperature is correct on the display panel. The regeneration air temperature is not so easy to check since most regeneration controllers are hidden away in the electrical panel.
Time - If you know the capacity of the hopper in liters, you can calculate the PET quantity using a figure of 0.84kg/L as the bulk density. Compare this figure (kilograms of PET) against the current consumption of PET by the machine in kg/hr. If you are not sure, the safest method is to switch off the hopper loader and check the time necessary for the material to be consumed.
Dewpoint - Use a commercially available dewpoint meter to sample the air coming from the desiccant chamber. Typical vales may be anything between -20ºC and -50ºC. Check the manual for correct specification. In machines with more than one desiccant chamber, be sure to check all of them.
Check the correct function of all heaters on a regular basis using a clip-on ammeter. If one or more has broken, the others will be taking extra load.
Always check the calibration of instrumentation and thermocouples before assuming a problem exists

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Calculating the consumption of polymer

What Is It?
A simple calculation that tells you the consumption of poly(ethylene terephthalate), PET by an injection molding machine.

The Calculation Is...

kg / hr = W x C x S

Where;
W = Weight of the container in kg. (e.g. 45 grams = 0.045 kg)
C = Number of injection cavities
S = Number of injection cycles per hour (shots)
(3,600 / Cycle Time (sec) for one set of preforms)

Example

A machine with a cycle time of 14 seconds making a 35 gram bottle
W = 0.035
C = 8
S = (3,600 / 14) = 257.1
kg / hr = 0.035 x 8 x 257.1 = 72 kg / hr

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1. Mold certification - Validation procedure for injection molds

>> Sunday, 16 August 2009

The first step in validating an injection mold according to injection mold validation flow chart is mold certification.

Purpose:
The purpose of the mold certification is to ensure the mold is built according to specification before the mold validation procedures begin, e.g., critical dimensions on the first mold set are cut "Steel Safe", core/cavity stack up dimensions are correct, cavity to cavity spacing is correct, cooling circuit layout, etc. The tool builder is required to provide verification of all critical metal tolerances on core and cavity ‘fits’ to be +/- 0,005 mm. The critical part dimension metal tolerances must be less than 10% of the plastics part dimension tolerances. In many cases, it will be necessary to go tighter than 10%. The mold certification should be performed at the tool builder while the mold is being fabricated to expedite any needed adjustments to the mold. Mold certification needs to be agreed upon when the mold order is placed. A copy of the mold certification should be included with the final mold validation report.

Variation observed during the process stability test, balance of fill analysis, and commissioning (multi-cavity analysis) are often a result of variation observed in the mold metal dimensions. Some molds, such as a four-face stack mold with a part thickness of approximately 0,80 mm are extremely sensitive to such variation, and may not pass the process stability test as a result. It is important for this step to be completed first so any correlation between variation observed during the remaining tests can be determined without pulling the mold from the press and disassembling it. Ensuring consistent cavity to cavity mold metal dimensions before running the mold will significantly decrease the time required to complete the validation procedure.

Data obtained from the mold certification for critical dimensions can be plotted on a control chart along with the critical part dimension obtained during the to determine if the two correlate to each other. A correlation between part dimension and steel dimension variation between cavities can be seen in figure: Correlation between steel and part dimensions
Correlation between steel and part dimensionsTable: Steel certification summary

Steel certification summarysummarize the metal certification data for this stack-up in figure: Critical core/cavity stack-up metal dimensionsCritical core/cavity stack-up metal dimensionsThe mold builder did not achieve their pre-determined mold metal tolerances. In some instances this may be acceptable. As shown for the outside diameter (dimension No. 5). A more constructive method to view the metal certification data is to control chart (X-MR) the data. The mold builder should know their measurement variation when performing metal certification. If not, they should perform a Gage repeatability & reproducibility (R&R) test. Ideally, the mold builder will machine the dimensions in control and be well within the agreed upon specifications.

The further steps are required in validating a injection mold according to injection mold validation flow chart:

2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
6. Balance of fill analysis
7. Gate freeze test
8. Commissioning (multi-cavity analysis)
9. Design of experiments
10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

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Reinforced Plastics – Free Subscription!

>> Tuesday, 11 August 2009

Free magazines, eBooks, publications, newsletter review from the polymer processing industry. Celebrating over 50 years of service to the composites industry worldwide.
For over 50 years Reinforced Plastics has been the first choice of moulders, fabricators and end-users as a comprehensive source of impartial and informed comment on the global composites industry. Their unique partnership with all the main industry players enables them to empower their readers with up-to-the-minute news on all aspects of the industry. They are totally committed to being the best in the business and maintaining the support of their readers and advertisers in the continuing challenges of their industry.

Geographic Eligibility: Select International

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Modern Plastics Worldwide – Free Subscription!

Modern Plastics WorldwideFree magazines, eBooks, publications, newsletter review from the polymer processing industry. Modern Plastics Worldwide is the only plastics publication dedicated to covering global business management issues, key technologies, and the latest manufacturing management trends for the international plastics industry.
Topics range across all processes, including injection, extrusion, blowmolding, thermoforming, blown film, cast film, and compounding. Modern Plastics Worldwide also covers the latest technology developments in materials, design, equipment, processing, and applications from a business strategy perspective, marketing information on new markets, industry trends, economic influences, forecasts and more.
Geographic Eligibility: USA, Canada, Mexico, Selected International (Print or Digital version)

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Canon Plastics Group - the Voice of an Industry

Canon Communications LLC is a strategic partner to the plastics industry. Both of our publications, Injection Molding Magazine and Modern Plastics Worldwide, address the plastics industry from a unique viewpoint. Each one offers in-depth coverage of its subject matter that a generalist publication just can’t accomplish. Each viewpoint helps readers stay on top of the fast-paced plastics industry. Together, Canon’s two plastics magazines add up to comprehensive coverage that gives industry professionals a 360° look at running a successful business. This is carried over online with our plastics industry community site, plasticstoday.com.

Corporate Information
Founded in 1978, Canon Communications has grown from a one-magazine publishing firm to a diversified multimedia communications company. While the company continues to grow and steadily expand its scope of markets, Canon remains committed to providing the most relevant, vital information to professionals in all areas of the complex industries it serves.
Canon now publishes more than a dozen magazines, all of which serve as authoritative sources of information in their respective industries. In addition to plastics, the packaging, medical manufacturing, and design industries are among the well-served business communities that rely on Canon's wide variety of publications for the latest in industry news, innovations, and market trends. Offering unique insight into each industry sector its magazines target, Canon publications are renowned for their unparalleled commitment to quality.

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Injection Molding Magazine – Free Subscription!

Free magazines, eBooks, publications, newsletter review from the polymer processing industry. IMM is the only publication devoted exclusively to the injection molding marketplace, consisting of custom and captive molders, contract manufacturers, OEM companies, part designers, moldmakers, and key members of the supply channel. Readers prefer IMM's editorial content as it provides them with "how-to" advice that contributes to their success.
Geographic Eligibility: USA, Canada, Mexico (Print Edition Only), Selected International (Digital Edition Only)

Request Free!

Canon Plastics Group - the Voice of an Industry

Canon Communications LLC is a strategic partner to the plastics industry. Both of our publications, Injection Molding Magazine and Modern Plastics Worldwide, address the plastics industry from a unique viewpoint. Each one offers in-depth coverage of its subject matter that a generalist publication just can’t accomplish. Each viewpoint helps readers stay on top of the fast-paced plastics industry. Together, Canon’s two plastics magazines add up to comprehensive coverage that gives industry professionals a 360° look at running a successful business. This is carried over online with our plastics industry community site, plasticstoday.com.

Corporate Information
Founded in 1978, Canon Communications has grown from a one-magazine publishing firm to a diversified multimedia communications company. While the company continues to grow and steadily expand its scope of markets, Canon remains committed to providing the most relevant, vital information to professionals in all areas of the complex industries it serves.
Canon now publishes more than a dozen magazines, all of which serve as authoritative sources of information in their respective industries. In addition to plastics, the packaging, medical manufacturing, and design industries are among the well-served business communities that rely on Canon's wide variety of publications for the latest in industry news, innovations, and market trends. Offering unique insight into each industry sector its magazines target, Canon publications are renowned for their unparalleled commitment to quality.

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Injection mold validation procedure

To validate an injection mold, there are numerous items one must take into consideration before the mold is completed and work is ready to begin. First, it is advisable to hold a mold design review meeting with all involved parties prior to tool manufacture. Invite the mold builder, hot runner supplier, steel supplier, press supplier, resin supplier, and a strategic molder for the design review. Involving everyone early on in the process and keeping them informed throughout the building of the mold avoids needless complications.

Once all the suppliers have been identified it is vital that they are all given training on the injection mold validation procedure, IMVP. The training should be done well in advance of the first mold qualification, if you have a pilot tool stage this is an ideal time. It could be done whilst the production tools are being manufactured. It is strongly advised the tool builder is made aware of the requirements of mold certification (steel measurements) before tool manufacture is commenced.
It is recommended the entire mold validation be performed on the production injection molding machine.
Well before the mold is ready to be validated, begin to identify resources and equipment to measure the molded parts accurately and efficiently. A majority of the time to qualify molds is based upon how fast the parts can be measured after proper conditioning. In addition, identify the best means to capture the measurement data and analyze the results. Working these issues beforehand can dramatically reduce the amount of time required to follow this process. For an alpha injection mold (first production mold of a series) one should target two weeks to fully qualify the mold. For repeat molds of the same design (beta molds), the target should be three days.

Purpose:
The purpose of the injection mold validation procedure is to identify a capable mold-process which will achieve key part dimensions and tolerances. It is also the purpose of the injection mold validation procedure to establish a mold processing-window for the injection mold. This document provides a detailed description of the steps which will deliver the types of results a validation process is designed for.
The injection mold validation procedure will determine the molding process conditions necessary to give production personnel maximized process capability and also process adjustment range while maintaining a capable, controlled process, and while molding parts to specifications.

Safety:
Equipment is operated and tests performed only by trained and qualified personnel.

Comply with safety requirements as defined by ANSI/SPI B 151.1-1990. A copy of the "Horizontal Injection Molding Machines-Safety Requirements, Care, and Use" can be obtained from:

American National Standards Institute
1430 Broadway, New York, NY 10018
(212) 642-4900

Comply with quality assurance and good manufacturing practice as defined by ANSI.

Procedures:
The following steps are required in validating an injection mold according to injection mold validation flow chart.

1. Mold certification
2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
6. Balance of fill analysis
7. Gate freeze test
8. Commissioning (multi-cavity analysis)
9. Design of experiments (DOX)
10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

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Injection mold validation flow chart

The following steps are required in validating a injection mold according to injection mold validation flow chart.

1. Mold certification
2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
6. Balance of fill analysis
7. Gate freeze test
8. Commissioning (multi-cavity analysis)
9. Design of experiments
10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

injection mold validation flow chart

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It's K time 2010

Companies from the world’s plastics and rubber industry put their faith in K 2010 in Düsseldorf. Some 3,000 exhibitors will step out at the No. 1 event for the industry from 27 October to 3 November 2010. No other trade fair in the industry compares with K 2010 for the panorama it will provide on offerings from all over the globe. The extensive palette on display is once again assured by exhibitors’ international reach. Companies from every continent have signed up to showcase their innovations in the following categories:
- Raw materials, auxiliaries,
- Semi-finished products, technical parts and reinforced plastics,
- Machinery and equipment for the plastics and rubber industry.
Once again, suppliers from Germany, Italy, Austria, Switzerland and the USA as well as Asian manufacturers from China, Taiwan and India will be out in force. Among them will be not only the global market leaders but also a wealth of exciting newcomers.

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