Finite element analysis Archives

Tag: Finite element analysis

Is Using MFR the Best method for Material Selection?

By Bozilla

December 1, 2022

When a material selection comes down to flow rate, is using the (Mass Flow Rate) MFR or Melt Index (MI) the best choice? To answer this, we need to understand why the Melt Index test initially came about

The Origin of the Melt Index Test Method (ASTM D-1238)

ASTM D 1238: Test Method for Flow Rates of Thermoplastics by Extrusion Plastometer

Before there were standards to test polymers, there was a need to determine the differences in how polymers would flow when melted A method was created to keep all polymers on the same level playing field This method places the material in an Extrusion Plastometer or Melt Indexer

The standard has the barrel of the melt indexer heated to a specific temperature The user would obtain a resin sample and place it in the barrel where a piston would be inserted A specific load would be placed on the piston, and the melted polymer would be extruded through a capillary die (with a particular orifice size) The extrusion would take place for 10 minutes, and the amount of polymer would be weighed in grams yielding an output in g/10 minutes

Having MFR data for all materials allows one to compare them side-by-side, giving a respective idea of how each will flow with the other

The limitation of this test method is that it is, in fact, one point on the viscosity curve and is at a shear rate of nearly zero, which is not indicative of the injection molding process

When materials experience shear during injection molding, shear rates may be experienced up to and possibly exceeding 100,000 1/sec Some materials become more viscous at higher shear rates, but these are uncommon

So how do we compare materials at these higher shear rates?

Since the inception of the melt indexer (1950s), a much more accurate test method was designed using a Dual Capillary Rheometer

A dual capillary rheometer can produce a series of viscosity data points over a range of shear rates, such as the image below

A Rheology curve provides exact viscosity data based on specific shear rates at specifically tested temperatures Notice how the Melt Index MFR point does not provide any data relating to the injection molding process A curve like this will allow one to understand the exact behavior of the material and shear rate

Using Aluminum for Injection Molds?

By Bozilla

July 14, 2022

Aluminum tooling has significant benefits when compared to steel tooling because of the cost of aluminum as well as the ease of manufacturing Aluminum tools can be cut much quicker than steel saving a lot of time and money These benefits can result in shorter cycle times and price-per-part savings However, there are some drawbacks of aluminum such as being a softer metal which can cause mold deflection and also having a fatigue limit which can be catastrophic In order to better understand the advantages and disadvantages of utilizing aluminum for injection molds we will take a closer look at the properties of both aluminum and steel

Let’s compare the properties of aluminum to steel

Table of Aluminum v Steel properties

The above table compares the properties of aluminum and steel Based on this table, we can determine the following

Density: Aluminum is 279 times less dense than Steel

Hardness: Aluminum is much softer than steel

Thermal Conductivity: Aluminum is 655 times more conductive than Steel

Thermal Diffusivity: Aluminum has 79 times the thermal diffusivity than Steel

Yield Strength: Aluminum has almost half of the yield strength of Steel

Poissons Ratio: Aluminum deforms more than Steel

Advantages of Using Aluminum for Injection Molds

Now that we’ve compared aluminum to steel, we can take a look at the advantages of using aluminum for injection molds based on the above properties

Density:

Aluminum is 279 times less dense than steel resulting in a lighter end product As the cost of shipping is increasing dramatically, the weight of any product will have a direct impact on shipping costs and must be kept low as possible

Hardness:

The softer aluminum reduces machining hours and wear and tear on machining components

Thermal Conductivity and Diffusivity:

Thermal conductivity and diffusivity is extremely important in injection molding as it directly impacts the cycle time Aluminum is 655 times more conductive and 799 times the diffusivity of steel which results in a significant reduction in cycle time, faster start-up times, reduction in response times to process temperature changes, and mold change times

The faster thermal recovery of aluminum also modulates the cyclic ‘highs-and lows’ of the tool temperature during processing As the melt is injected into the mold, the heat must be removed as quickly as possible Aluminum is able to process the heat out of the tool much faster than steel resulting in a more stable mold temperature and thus a more stable process

Disadvantages of Using Aluminum for Injection Molds

Poissons Ratio:

The Poissons Ratio of Aluminum is 033 and Steel

Homopolymer vs. Copolymer

By Bozilla

May 24, 2022

Material selection for an injection molding application can sometimes prove to be very challenging What happens if you identify a material then find that it can be supplied as a homopolymer or a random copolymer Is there a difference? The answer is YES The choice made for your project can affect part quality

The Homopolymer:

A homopolymer has the same base unit which causes the molecular chain to have a high degree of consistency and size However, length can vary depending on how long the polymerization process is allowed to occur

The high degree of consistency in a homopolymer creates a high degree of regularity When many of these changes flow and combine, they are able to create a very tight entanglement and when they cool and shrink, they also have a high degree of crystallinity which increases shrink

The Copolymer:

A copolymer, as shown in the image above, has more than one base unit and each base unit is a different size There can be more than two base units Due to the variation in size of the base units, the copolymer chains will be spaced much further from each other and have a higher degree of irregularity And similar to the homopolymer, the length of the molecule will depend on how long the polymerization process is allowed to occur

The high degree of irregularity does not allow the polymer chains to form a tight structure, leaving a lot of space between the molecular chains Therefore, when the polymer flows, there can be alignment but there will be more irregularity and not as tight of a structure which prevents excessive shrinkage

When comparing the two types of polymers, assuming each is the same length (same molecular weight, per se) the homopolymer will be much more organized and structured therefore creating more mechanical strength and chemical resistance but have high shrinkage The copolymer will have more random orientation which will create space between the molecules allowing for easier chemical attack and less mechanical strength and also have lower shrinkage Of course, we could discuss these comparisons in much more detail but we will stick to the basics for now

As material selection relates to injection molding, the properties of the material is a crucial factor

The major properties when comparing homopolymers to copolymers are:

shrinkage
chemical resistance
mechanical strength

Each of these properties must be considered with regards to the outcome of part quality

For example, when injection molding

Creating a Virtual Plastics Injection Molding Window

By Bozilla

April 27, 2022

Since the inception of plastic injection molding, creating a robust injection molding process has always been a challenge As time has progressed, the design of plastic parts has become more detailed and intricate, the tolerances have become tighter and the boundaries of injection molding standards are being pushed to their limits The combination of each one of these factors is making it more and more difficult to create and maintain a robust molding process

Initially, it wasn’t difficult to design a basic injection molding window that would result in a robust molding process However, with the advent of increasingly demanding factors it has become more difficult to design a process molding window that is large enough to be robust and create consistently good parts As a matter of fact, not only is it difficult to create a wide process molding window, it’s nearly impossible to create a suitable molding window- Period We will discuss how and why it is necessary to first create a virtual injection molding window and how that data can be translated to the floor in order to have the best injection molding window possible

Let’s begin with understanding what a molding window (or process window) is Typically, a molding window is comprised of three major factors: Fill time (or fill speed), Mold Temperature and Melt Temperature Each of these factors has the greatest impact on the injection molding process

Graphs below will illustrate the impact of each

The influence of each factor:

Fill Time (fill speed): As fill speed decreases, the material moves into and through the cavity slowly which allows the cooling effects of the tool steel to have more time to influence and cool the temperature of the plastic resulting in a higher viscosity response and a greater pressure to fill the cavity Conversely, as the fill speed increases, the material will shear thin (the viscosity will decrease) significantly, but ultimately the plastic will resist filling the cavity and require a greater pressure to fill the cavity Somewhere between filling extremely slow and filling extremely fast is a sweet spot that requires low pressure to fill the cavity If plotted out in a graph, it will be a u-shaped curve where the lowest point is typically a good fill speed

Mold Temperature: The mold temperature is highly influential with regards to having the material fill the cavity The thickness of the part relative to the flow length is an important relationship with regards to the impact of the mold

Accuracy of Flow Simulation on Plastics Injection Molding machine

By Bozilla

March 15, 2022

I have recently been exposed to multiple articles and discussions regarding the implementation of flow simulation on injection molding machines This is an intriguing topic which I advocate Currently, this new combo method is proving difficult to line up the results from the analysis with the actual process on the injection molding machine In order to successfully utilize this capability on an injection molding machine, several factors must be understood

For those who have read my previous articles, you may appreciate that I strongly promote having the right person for the right job In other words, the simulation engineer must have a complete and comprehensive understanding of plastics in order to be able to properly simulate the plastics injection molding process

My suggestion is to always have a degreed Plastics Engineer with floor experience perform simulations on plastic parts The reason for this advocation is to ensure that the simulation engineer virtually takes on the role of the process engineer This makes certain that the simulation will properly emulate the injection molding machine process on the part being molded Unfortunately, this is not always the case and the articles I have recently read do not touch on this very important factor A virtual simulation cannot simply be executed, have the results taken to the floor, input into the injection molding machine and expect the molded part to perfectly match the simulation results It’s not that easy due to many variables which must be considered

For example, the simulation engineer (with the proper degree and experience) will understand the limitations and boundaries of the intended injection molding machine for that particular simulated part It is not always necessary to know every specification of the machine and to input that specific data into the simulation Most machines have a wide variation of capabilities that the simulation engineer will take into consideration The simulation will then be executed with all of the necessary variables factored in for the injection molding machine, thus maintaining a high degree of accuracy between the simulation and the floor

The difficult task is discovering those unintended variables that affect the process on the floor, eg material batch changes, colorant issues in the polymer, tool temperature variations that the press cannot record, physical changes within the tool such as polymer sticking to action within the tool, cold gates not opening and flowing when desired, cosmetic issues at the gate or

Key Factors in a Reliable Plastics Injection Molding Simulation Report

By Bozilla

November 30, 2021

I have written in a prior post about the key factors necessary in a plastics injection molding optimization analyst Now, I would like to discuss the importance of a skillfully assembled simulation report Jennifer Schmidt spoke of the key ingredients of a trustworthy injection molding simulation report in her talk at the Plastics Technology Molding 2021 conference In this brief, I will discuss the valuable information she provided and add additional feedback If you want a successful outcome for your tool, these key components are essential to consider

1Software

What version is being used and is the software up to date? Look for signs that the analyst is using an older version of software which will alter the results on the report, and ultimately the floor results

Typically, the output file(s) of the software contain the release version of the software It might not be the absolute latest release of the software, but is should be a proven release which is typically a year old or less

2 Type of Mesh used: Consider the type of mesh that used for the part and the runner

Is the mesh type appropriate for the part geometry?
Is it precise enough in critical areas to capture important details?
Is the correct technology being used for the part geometry/runner combination, ie midplane, Dual-Domain, 3D or a specialty mesh used?
Will the report allow access to display the mesh?
Does the filling animation, weld lines and sink marks reveal insights into the mesh quality?

Consider: Simulations of the same part with the same material and same mesh density, but different mesh types for the part and runner, may produce different results for pressure at the fill-to-pack switchover point, which could make quite a difference in what occurs in an actual molding environment

There are many factors to consider and only a seasoned user with the proper education in the software will be able to make these determinations in order to provide the best analytical outcome

3 Material Data: An accurate molding prediction requires good material data

What was the material data in the simulation based on?
Was data on the actual material available?
Was the data a substitute?-a resin of the same generic family but has a

Is this the correct Injection Molding Machine for your Tool?

By Bozilla

November 8, 2021

To start this discussion I’d have to first state that the size of the tool plays a large role when selecting an injection molding machine More specifically, it is the projected area that is of concern and how the projected area, along with the pressure distribution over that projected area, creates clamp force

Selecting a machine based on clamp force (tonnage) is more common when you have a part with a large projected area; ie multi-cavity tools, bumper fascias, housewares and many other items

In today’s economic climate, it’s more important than ever to conserve energy Many believe that using the smallest IMM is the best way to achieve this cost savings However, there are reasons why a smaller machine isn’t always the most efficient machine

Reason 1: If an optimum process is the objective, select a machine that does not allow the tool to exceed the clamp force and flash the tool (blowing open) during an ‘optimized’ process

We have had many concerned customers consult with us about the process Their questions are directed at finding out why the part is warping or exhibiting cosmetic defects Once I dig into the process, I typically find that the part is not packed sufficiently due to the tool blowing open In order to keep the tool closed, they must pack with very little pressure for a very short time Packing with too little pressure, too little time, or both can cause a loss of control with dimensional stability and/or cosmetic issues due to excessive shrinkage These issues create problems that are caused because the tool is in an IMM that doesn’t have the proper clamp force requirement

In the image below the clamp force required to fill and make the part is 250 Tons However, in order to pack the part out sufficiently and make a good part (meets tolerances, minimal cosmetic defects, minimal deflection, etc) the clamp force required during 2^nd stage pack is 1,450 Tons That’s a very big difference

Reason 2: You are able to make parts but the process window is so small that staying within the process window is difficult or impossible to maintain

The inability to stay within a process window could be caused by several issues, especially since there are so many variables in the molding process However, if the machine does not have sufficient clamp force to stay closed during an optimum molding process, concessions will be made and

How Experienced And Knowledgeable Is Your Analyst?

By Bozilla

August 4, 2021

In this economic climate, it is critical to create higher quality processes and parts while keeping costs as low as possible This typically means getting analysts involved in the beginning phases of a project Because a mold typically costs in the range of $12,000 to above $200,000, spending less money up front with analytical work will save you from costly tool re-work at the end of a project

More businesses are turning to mold flow analysts to provide in-depth, technical knowledge before, during, and after the course of their project Whether the analyst is internal or is a hired consultant, it is extremely important to know what type of experience and knowledge they have in order to take full advantage of their expertise

What basic requirements should an Injection mold flow analyst (example: Autodesk Moldflow) have when analyzing the injection molding process

Injection molding knowledge: What type of focus does your analyst have in the injection molding sector? For instance, are they knowledgeable about mold design, polymers, and flow, etc?

Having an analyst that fully understands the scope of their position is an absolute necessity In order to have a comprehensive understanding of their job, analysts must be knowledgeable in all aspects of the injection molding process This includes injection molding processing, mold design, part design, and polymer chemistry and properties(eg a plastics engineer)

Plastics Knowledge: Is the analyst a Plastics Engineer or will a Plastics Engineer be involved?

Without a full understanding of polymers, it is extremely difficult to understand polymer behavior during processing This understanding begins at a molecular level and extends far beyond standard processing knowledge A Plastics Engineer is able to identify the differences between polymers and each polymer’s flow characteristics This information can be used in conjunction with the simulation software to optimize the analyzation process Therefore, it is crucial to have a Plastics Engineer involved with plastics processing

Injection Molding Experience: Has the analyst ever run an injection molding machine or been formally trained on one? Does he/she understand ancillary equipment such as thermolators/chillers?

Without injection molding experience, it is difficult to properly analyze such a process

A mold flow analyst is typically required to identify and understand polymer flow behaviors within the injection mold Frequently, this can involve analyzing the 1st stage, 2nd stage, and cooling stages of the injection molding process If your analyst must survey these phases of the

Finite Element Analysis(FEA) – On Every Tool?

By Bozilla

March 3, 2021

Finite Element Analysis (FEA) has been available to the injection molding industry since approximately 1978 when Moldflow pty Ltd produced the first simulation software to be used to optimize all phases of design and production processes for injection molded plastic parts

Over the years, FEA has proven to be a successful, cost-saving optimization method used for injection molded tool manufacturing

injection molding trouble shooting

Prior to FEA, a typical method used to refine product and process was to cut tool steel and design a feed system based on experience with older, comparable tools This method is also known as the trial and error method

This type of “guessing” process has cost manufacturers thousands of dollars in re-tooling and time delays that could have been prevented if they had first utilized FEA to troubleshoot the product and process

The question remains, “Should FEA be used for every tool?” The answer is YES! Whether it is a new tool replacing an old tool running the same part, a New Tool being built for a new part, or a new mold for an existing part, FEA can positively refine both the quality of mold and the process

FEA is a gift to plastics manufacturers in that it provides an inside look at both the product and process before any steel is cut or altered The refinement of product and process allows the manufacturer to perfect their part and save both time and money by limiting or preventing future rework

Contact Bozilla Corporation to assist you in achieving a successful part based on your budget and timing goals for your next project We will provide you with detailed project data empowering you to make the most informed decisions to create a high quality mold

Do you remember the cost of your last mold? If the answer is “yes”, do you really want to pay for it again because of rework/redesign?

The plastic injection molding experts at Bozilla Corporation have over 20 years of experience with Autodesk Moldflow software, feed system design and field experience We provide the highest degree of professionalism, knowledge and quality to every project Contact Bozilla Corporation Today and Let’s get started!

A Deeper Understanding Of Conformal Cooling For Injection Molds

By Bozilla

November 22, 2019

Conformal cooling has become the latest trend in the injection molding industry As it compares to traditional gun-drilled cooling lines, it appears to be better, but is that always the case? We will discuss the comparisons and application of both traditional cooling versus conformal cooling as it applies to heat removal of the polymer/part

When designing cooling for injection molds, several factors must be understood with regards to the properties of the tool steel such as the type of steel and its corresponding properties ie thermal conductivity, thermal diffusivity, specific heat capacity and density Those same properties must be considered for the coolant along with Reynolds number and flow rate

Thermal conductivity:

A measure of a materials ability to conduct heat as shown below:

Thermal Conductivity

Thermal diffusivity:

The thermal conductivity divided by the density and specific heat capacity (at constant pressure) as shown below:

Thermal diffusivity

In order to take advantage of these properties, certain guidelines should be considered such as the spacing between adjacent cooling circuits, thickness or cross section of the circuits and the spacing between cooling circuits and the cavity The flow rate must be sufficient enough such that the coolant is turbulent (Reynolds number above 8000) in all regions of the circuits in order to have maximum heat removal

Conventional Cooling (gun-drilling):

Gun drilled cooling channels are straight holes cut through the tool steel Because these are straight holes, it limits the regions in which holes can be cut such as in any action within the tool or small or difficult regions near the cavity of the tool Implements can be used such as heat pipes (thermal pins), bubblers, baffles and small circuits and high heat-transfer materials However, there are still regions within the tool that are difficult to implement cooling and these regions are typically accepted They can cause issues within the mold such as parts sticking or controlling cycle time

Gun drilled cooling channels are very adequate when sized and spaced correctly Basic guidelines will take advantage of the properties of both the coolant and the cooling channels:

The spacing of the cooling channel from center-line to center-line (pitch) should be no more than 3 times the diameter (3D)
The distance from the cooling channel to the cavity surfaces should be no more than 15 times the diameter (15D)

If these simple guidelines are maintained, there will be adequate

Plastics Engineering, Simulation and Consultation

Tag: Finite element analysis

Is Using MFR the Best method for Material Selection?

Using Aluminum for Injection Molds?

Homopolymer vs. Copolymer

Creating a Virtual Plastics Injection Molding Window

Accuracy of Flow Simulation on Plastics Injection Molding machine

Key Factors in a Reliable Plastics Injection Molding Simulation Report

Is this the correct Injection Molding Machine for your Tool?

How Experienced And Knowledgeable Is Your Analyst?

Finite Element Analysis(FEA) – On Every Tool?

A Deeper Understanding Of Conformal Cooling For Injection Molds

Thermal conductivity:

Thermal diffusivity:

Conventional Cooling (gun-drilling):

Recent Post

Subject Tags

Archives