Cost Effective Injection Molding Tips from a Design and Engineering Firm

Injection molding is one of the most popular and effective manufacturing processes, because it is capable of producing high quality parts in large numbers, and generally very quickly. In a nutshell, injection molding is when a material (generally plastic) is heated until pliable, forced into a mold made for a specific purpose, allowed to cool and harden, and then ejected from the mold. Depending on how the mold is made (and what material is being injection molded), this process can be repeated over and over in order to create large numbers of a given product. Although popular and effective, injection molding is not a cheap process; projects typically cost between $10,000 and a few hundred thousand dollars to manufacture. Let’s look at some ways to ensure that cost effective injection molding is a reality for your project.

Tips for cost effective injection molding from the Creative Mechanisms team

Make sure you’re using the right material. Did you know that there are hundreds of plastics (let alone other materials) that can be injection molded? It’s important to consider what function you want a particular piece to accomplish, and which material is most appropriate to make that happen. Does a piece need to be pliable or rigid? Will it be exposed to heat or extreme temperature deviations? How does Factor of Safety affect the materials required for design? It’s a common mistake to assume that a state-of-the-art, top-of-the-line material is the right one to utilize, but if its good qualities aren’t pertinent to your project, then they are essentially useless – and may cost more money overall. For instance, why use a 40% glass filled nylon when polyethylene would do the trick just as well? The best material for injection molding is the one that best fits your requirements and is not simply the better material overall.

Identify where processes can be consolidated. There are a lot of secondary processes involved in producing a part from scratch. Such processes (like custom inserts, label printing, painting, etc.) can prove to be time consuming, as they require extensive setup – and in injection molding, time is money. All those extra costs – and the time that could have been saved with better production management – ultimately drive the part price up. The best practice is to try to combine all of these processes into one single robust process. Read more about the Creative Mechanisms process, and how we design for defect prevention in injection molding. Core-Competencies-Injection-Molding.png

Be selective when choosing who does the injection molding.Like most industries, the injection molding industry is full of small, mid-size, and large companies. One or the other may be more appropriate depending on your project. Smaller companies will generally offer more flexibility and lower costs, whereas prices may be driven up with large companies due to higher overhead, higher salaries, and sometimes more advanced technology. In general, it’s best to choose a company that has experience molding your type of product, as it will save time during the research and development part of the project. Remember that bigger and more expensive does not necessarily equal better quality. Creative Mechanisms is not an injection molding company, but over the years, we have built a reliable network of high quality, responsive manufacturing companies that have helped to turn ideas into reality.

Consider bulk production. Molding operations are rarely personal projects or projects that will produce small numbers. In the research and development phase alone, some projects produce hundreds or thousands of prototypes, as many benefits come from extensive testing and feedback. When the product moves into the production phase, it is even more important to be able to mold as many parts in one shot as possible. Molds for production should also have as many cavities as possible without compromising the quality of the parts produced. In a competitive market, a product must be the best it can be while also being affordable. That is why it is advisable to produce as many parts as possible at one time – because it spreads the setup cost out over more parts, thus leaving you with a lower price per piece. You can now sell your product in a competitive market. 

Is the mold design optimized for cost effective injection molding? In mold design, as in bulk production, it is beneficial if you can produce as many parts as possible in a single shot. For mold design, it is also very important to be able to eject the plastic product quickly and to be ready for the next shot without wasting movements. Rods, an air blast, or a plate are typically used for the ejection stage of injection molding. Every second in the injection molding process translates into money, so it is critical to minimize the mechanisms of molding to as few and as fast as possible. A design and engineering firm that is familiar with the nuances of injection molding will create parts that will lend themselves to optimized mold design.

Optimize product design and materials. You can save a considerable amount of money, especially in material consumption, with an optimized product design. Using ribs and gussets to reinforce a product, for example, will save on material consumption, as well as ensuring that the product has uniform wall thickness that is neither too thin nor too thick. Incorporating adequate draft is also essential, as it allows for quick ejection of the product from the mold, saving time and money. If there is a need for a mechanism in the product, there are quite a few to choose from. Many can be incorporated into the molding process without the need for secondary processes or machining. Some mechanisms, such as living hinges, take advantage of the properties of the material that was used to mold the plastic part. These mechanisms can be made directly from the molding process versus spending extra time and money on other processes, such as stamping.

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Everything You Need To Know About Injection Molding

What is Injection Molding:

Injection Molding is a manufacturing process for producing parts in large volume. It is most typically used in mass-production processes where the same part is being created thousands or even millions of times in succession.

Injection Molding Machine

Why Use Injection Molding:

The principal advantage of injection molding is the ability to scale production en masse. Once the initial costs have been paid the price per unit during injection molded manufacturing is extremely low. The price also tends to drop drastically as more parts are produced. Other advantages include the following:

  • Injection Molding produces low scrap rates relative to traditional manufacturing processes like CNC machining which cut away substantial percentages of an original plastic block or sheet. This however can be a negative relative to additive manufacturing processes like 3D printing that have even lower scrap rates. Note: waste plastic from injection molding manufacturing typically comes consistently from four areas: the sprue, the runners, the gate locations, and any overflow material that leaks out of the part cavity itself (a condition called “flash”).

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Image From Ferris.edu

A sprue is simply the channel that guides molten plastic from the nozzle of the injection molding machine to the entry point for the entire injection mold tool. It is a separate part from the mold tool itself. A runner is a system of channels that meet up with the sprue, typically within or as part of the mold tool, that guides the molten plastic into the part cavities within the mold tool. There are two principal categories of runners (hot and cold) which you can read about here. Lastly, the gate is the part of the channel after the runner that leads directly into the part cavity. After an injection mold cycle (typically only seconds long) the entirety of the molten plastic will cool leaving solid plastic in the sprue, runners, gates, part cavities themselves, as well as a little bit of overflow potentially on the edges of the parts (if the seal isn’t 100% right).

Thermoset material, such as an epoxy resin that cures once exposed to air, is a material that cures and would burn after curing if one attempt is made to melt it. Thermoplastic material by contrast, is a plastic material that can be melted, cool and solidify, and then be melted again without burning. With thermoplastic materials the material can be recycled are used again. Sometimes this happens right on the factory floor. They grind up the sprues/runners and any reject parts. Then they add that material back into the raw material that goes into the injection molding press. This material is referred to as “re-grind”. Typically, quality control departments will limit the amount of regrind that is allowed to be placed back into the press. (Some performance properties of the plastic can degrade as it is molded over and over). Or, if they have a lot of it, a factory can sell this re-grind to some other factory who can use it. Typically regrind material is used for low-quality parts that don’t need high performance properties.

  • Injection Molding is very repeatable. That is, the second part you produce is going to be practically identical to the first one etc. This is a wonderful characteristic when trying to produce brand consistency and part reliability in high volume production.Injection_molding_plastic prototypes design and engineering firm

What Is The Downside To Injection Molding:

Up front costs tend to be very high due to design, testing, and tooling requirements. If you are going to produce parts in high volumes you want to make sure you get the design right the first time. That is more complicated than you might think. Getting the design right includes:

  • Designing and then prototyping the part itself to specification
    • Initial prototype development is typically completed on a 3D printer and often in a different material (such as ABS plastic) than the final part will be constructed in
  • Designing an injection mold tool for an initial production round
    • Typically generating 300-1000 injection molded prototypes in the production material requires the development of an injection mold tool.
  • Refining any and all details in the injection mold tool prior to mass-production in an injection mold manufacturing plant.

Potentially negative aspects of injection molding include the following:

  • Two of the major disadvantages to injection molding are the high tooling costs and large required lead times. Tooling is almost a project in and of itself and only one phase of the entire injection molding process. Before you can produce an injection molded part you first have to design and prototype a part (probably via CNC or 3D printing), then you have to design and prototype a mold tool that can produce replicas of the part in volume. Lastly, and typically after extensive testing in both of the aforementioned stages, you get to injection mold a part. As you can imagine, all of the iteration required to get the tool correct prior to mass production requires both time and money. It is rare that you would prototype an injection molding tool. It does happen though, especially for parts that will be made in a multi-cavity tool. For example, let’s say we were going to injection mold a new shampoo bottle cap. That cap would likely have threads to attach it to the bottle, a living hinge, a snap closure, and potentially some overmolding too. A company may choose to make a single cavity tool of that part to make sure all of the features will mold as desired. Upon approval, they will make a new tool, that is capable of molding, for example, 16 caps at a time. They do the single cavity tool first so if there are any issues, they don’t have to pay and wait for it to be fixed 16 times for each cavity.
  • Because tools are typically made out of steel (a very hard material) or aluminum it can be difficult to make changes. If you want to add plastic to the part you can always make the tool cavity larger by cutting away steel or aluminum. But if you are trying to take away plastic you need to decrease the size of the tool cavity by adding aluminum or metal to it. This is extremely difficult and in many cases might mean needing to scrap the tool (or part of the tool) entirely and start over. In other cases you might be able to weld metal into the cavity that is undesired.
  • Injection molding necessitates uniform wall thickness. If you were to cut a cross-section of the Panasonic mold above you would notice that the wall thickness is approximately 2-3mm thick throughout. Keeping walls from being too thick is important to prevent inconsistencies in the cooling process resulting in defects like sink marks. A good rule of thumb is to keep walls less than or equal to 4mm thick. The thicker the walls the more material you will use, the longer the cycle time will be and the higher your cost per part will be. Conversely, if wall thickness is any thinner than 1mm or so you might experience trouble filling the mold tool (resulting in gaps or short shots). Designers can compensate for this potentiality by using a material with a higher melt flow index like Nylon which is often suitable for walls as thin as 0.5mm. Different manufacturing techniques like CNC don’t require uniform wall thickness at all.
  • Oftentimes large parts cannot be produced via injection molding as a single piece. This is due to the size limitations of injection mold machines and the mold tools themselves. For example of a large injection molded part consider the shopping carts at Target. Although the machinery exists to mold very large pieces (e.g. 1000 ton presses roughly the size of a train’s caboose), using it is very expensive. For this reason, objects that are larger than a typical injection molding machine’s capability are most often created in multiple pieces. CNC machines have similar limitations regarding product size while 3D printing has even more limitations. CNC is limited to the travel and size of the bed in the milling machine while large 3D printed parts often need to be printed in multiple pieces and then bonded together.
  • Large undercuts require experienced design to avoid and can often add costs to the project.

What Are Some of The Considerations For Injection Molding:

Before you endeavor to produce a part via injection molding consider a few of the following things:

  1. Financial Considerations
    1. Entry Cost: Preparing a product for injection molded manufacturing requires a large initial investment. Make sure you understand this crucial point up front.
    2. Production Quantity
      1. Determine the number of parts produced at which injection molding becomes the most cost effective method of manufacturing
      2. Determine the number of parts produced at which you expect to break even on your investment (consider the costs of design, testing, production, assembly, marketing, and distribution as well as the expected price point for sales). Build in a conservative margin.
  1. Design Considerations
    1. Part Design: You want to design the part from day one with injection molding in mind. Simplifying geometry and minimizing the number of parts early on will pay dividends down the road.
    2. Tool Design: Make sure to design the mold tool to prevent defects during production. For a list of 10 common injection molding defects and how to fix or prevent them read here. Consider gate locations and run simulations using moldflow software like Solidworks Plastics.
  1. Production Considerations
    1. Cycle Time: Minimize cycle time in as much as it is possible. Using machines with hot runner technology will help as will well-thought-out tooling. Small changes can make a big difference and cutting a few seconds from your cycle time can translate into big savings when you’re producing millions of parts.
    2. Assembly: Design your part to minimize assembly. Much of the reason injection molding is done in southeast Asia is the cost of assembling simple parts during an injection molding run. To the extent that you can design assembly out of the process you will save significant money on the cost of labor.

An Example (Designing For Injection Molding)

Designing a part that’s suitable for injection molding versus one that’s suitable for machining, thermal forming, or 3D printing means taking into consideration some of the differences between the various fabrication techniques and recognizing when your project is better suited to one or the other. Typical parts you might want to injection mold include joints, brackets, or housings. For example, most consumer electronic tools are made with a plastic shell (housing) that’s injection molded and used for the body of the tool.

Consider the housing for an electric drill produced by Panasonic (see below):

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Picture courtesy of Panasonic

 

One of the most obvious advantages to injection molding is that the housing serves multiple purposes. First, it serves as a handle for the end user to interact with. It also acts as a receptacle for the battery and motor as well the location of various screw bosses that will be used to fasten the device together once the internal parts are assembled. In other words, injection molding is extremely effective when you need to organize a lot of internal parts within a housing. As a consequence, it’s a fantastic way to reduce the number of total parts (“piece count”). Of note, this part is also an overmolded part. For more on this process read here.

Some of the other reasons that injection molding is a good fit for this example include the fact that the drill is being produced in large volume. That is, Panasonic is creating a large number of copies of the same drill handle. Injection molding is wonderful for this kind of high volume production because the high initial costs pay the manufacturer back over time with low per unit costs. For this same reason injection molding can be a poor choice for low volume production. Additionally of note, there are some design constraints if using injection molding. For example, the part has nearly uniform wall thickness (which is important in order to avoid defects), and the part is made with a thermoplastic material (allowing for solid plastic stock to be repeatedly melted for the procedure). If you were designing a part with a thermoset material then injection molding would be more nuanced. You can injection mold a thermoset material but you can only do it once. Trying to melt a thermoset plastic a second time will result in burning the material. Similarly, a part with varied wall thickness would require more attention in the mold tool design to ensure uniform cooling and to avoid defects during production.

Conclusion

Injection molding is a great technology for finished production on a massive scale. It is also useful for finalized prototypes that are used for consumer and/or product testing. Prior to this late stage in production, however, 3D printing is much more affordable and flexible for products in the early stages of design.

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How Much Do Plastic Injection Molds Cost?

At least to some degree, asking what plastic injection molds cost is a lot like asking “How much is a car?”. Will your mold be a Pinto or a Porsche?

The determining factors in the cost of your mold are a combination of the part size, complexity, material, and anticipated quantities. For instance, if you wanted 1,000 washers per year, we would recommend a single cavity mold, meaning it makes one washer per machine cycle. In that case the mold would probably be $1,000-2,000.

On the other hand, if you are going to need 100,000 xbox controllers every month, we would build a 12 cavity hardened “family” mold which made four Fronts, four Backs, and four Button Trees every cycle, and you’d better have $60-$80,000 or more to invest.

Plastic Injection Mold

Of course these examples are extreme, but it illustrates the range of costs to anticipate. Your particular part will most likely fall somewhere in between that, and the molds that Rex Plastics builds average around $12,000.

Material selection is another consideration in determining what plastic injection molds cost. If your product requires a fiberglass filled material for instance, it will most likely need a mold made from hardened tool steel due to the wear those materials cause when being injected under high pressure.

If you have a target price established for your parts, it will be helpful to let your mold builder know that, because they can design the mold with that in mind, and plan for the right number of cavities to achieve that price. The more cavities (or parts made per cycle), the less expensive the parts will be, also allowing for higher output.

While you don’t need to know every detail about the types of molds available, it’s always a good idea to come prepared. Supplying the basics to your mold builder will help them build the best tool for your project. It’s also recommended that you find a mold maker who also has production capabilities. The company running the production won’t want to build an inadequate mold because they know they have to use it!

Whatever your project requires, one thing Rex Plastics won’t do is suggest you invest in a Ferrari when all you need is a Fiat!

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How to Install an Injection Mold

Injection molding is used in automotive, aerospace, ship-making and even jewelry fabrication. The process involves an injection machine that clamps down a mold. It then heats the mold and the material to be cast. The material is then injected into the mold under high pressure. If the mold is installed incorrectly the final product can be ruined. Often it will not inject all the way, leaving a shortened product or it will inject too much, and plastic can shoot out of the mold.

Instructions
1
Check to make sure the mold is clean and free of chips after it has been machined.

2
Open any safety guards on the injection molding machine and put the mold into the injection molding machine. The injection nozzle on the machine must be aligned with the mold cavity.

3
Adjust the injection pressure, clamp pressure and the volume of material to be injected (if this feature is available). The pressures and volume should have been determined during the design phase of the part and the mold.

4
Turn on the heater to melt the injection material and to heat the mold. If the mold is heated, the material will flow through more easily without freezing and making a short part.

5
Clamp the mold with the injection molder’s hydraulics.

6
Initiate the injection process with an injection button or other command (machines differ). If the part comes out full, the injection machine is set up. If the part is short or material leaks out of the mold, the pressures, volume and the air escape vents on the mold should be checked. Repeat this step until the part comes out full and as desired.

Tips & Warnings
Carefully read the manual and the injection molding process specific to your machine.

Work with the engineer who designed the mold so you know the pressures, volume and any other details about the part.

Dress accordingly. Machine shops can be dangerous, and safety attire should be worn.

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How to Make a Plastic Injection Mold

Plastic injection molding is used in many industrial applications, from toys and model parts to furniture and building materials. You can practice injection molding on a small scale at home by creating your own plastic objects. A homemade mold will not be as durable as a professional one, but will give you a clear understanding of how the process works.

Instructions
1
Sculpt a figure in oil-based clay. It is available in several hardnesses at art supply stores, a medium-hard clay is good for this project. Your figure can be a toy, mechanical part, random shape or any other object that you like.

2
Lay the figure down on a block of water-based clay and press it slowly and carefully halfway in. You want the edges of the water clay to be as close to a 90-degree angle against the figure as possible to minimize seam lines in the finished product. If needed add more clay and sculpt a good angle with a sculpting tool.

3
Press a few bamboo skewers halfway in the clay. The skewers should each lead away from the figure, with the blunt end of the skewer touching it. These will eventually form channels that allow air and excess plastic to escape the mold during injection, leaving behind plastic sprues to be trimmed off of the finished copy.

4
Press a stick the same diameter as the injection syringe into the clay in the same fashion as the bamboo skewers. This should be positioned in an out-of-the-way area on the figure as it will leave a larger sprue to be trimmed off.

5
Press your thumb a quarter inch into the water clay in a few areas. This will form keys, which will fit into holes on the other half of the mold to help it line up properly.

6
Press pieces of corrugated cardboard into the clay, forming a box around the figure. It should be at least an inch away from the figure on all sides. Notches should be cut for the sticks, which must extend past the cardboard. Tape the joints in the cardboard to prevent leakage.

7
Spray two thin coats of acrylic enamel spray into the mold, then paint the cardboard with a thin coat of petroleum jelly as a release agent.

8
Mix a batch of cement according to the instructions on the cement package.

9
Pour the cement into the mold until it is an inch higher than the highest point on the figure. Allow the cement to harden for a few hours.

10
Remove the cardboard and water clay from the mold, leaving the figure and sticks embedded halfway in the cement.

11
Brush the cement surface with a thin coat of petroleum jelly to prevent the second half of the mold from bonding with the first.

12
Build another cardboard box for the second half of the mold, extending a few inches above the surface of the first half. Glue the cardboard to the sides of the first half with hot glue and seal the joints.

13
Prepare the second mold with acrylic spray and petroleum jelly.

14
Mix a second batch of cement and pour it into the cardboard box, forming the second half of the mold.

15
Allow the cement to dry.

16
Open the two halves of the mold and remove the sticks and figure. If any of the channels leading away from the figure have become blocked with cement, scrape them clear with a sharp sculpting tool.

17
Spray both halves of the mold with a few coats of acrylic spray to prevent the plastic from seeping into the stone.

Tips & Warnings
To cast a plastic part out of the mold, paint a thin coat of hand soap into it as a release agent and duct tape it shut. Mix a liquid plastic compound and pour it into the injection syringe. Inject the plastic into the mold, plugging the air escape holes with wads of clay as they begin to leak plastic.

The air escape channels must always be clear every time you use the mold. If they are not you will have air bubbles and deformities in the finished plastic part.

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How To Find the Best Plastic Injection Molding Companies

In business, one of the crucial decisions that managers have to make is to find the best suppliers. This decision can be difficult to make especially if you do not have access to a long list of suppliers at hand. Yet, there are some proven and time tested ways to find the best plastic injection molding companies to partner with. Here are some of those ways.

Do an online search. The Internet is a treasure trove of information. Just type in the phrase “plastic injection molding companies” in your browser and it will pull up thousands of such companies for you in a matter of seconds. You can check out the web sites of these companies. On the other hand, you can go directly to more reputable web sites and conduct your search from there. Below are some online resources that you can use in your quest to find the best plastic molding injection company.

PlasticsToday.com — The official web site of the magazine Plastics Today, this site allows users to search for companies in the plastic industry including plastic injection molding companies. From here you can access the Supplier Directory of “Injection Molding” magazine called IMM Almanac. You can now search for possible suppliers here.
American Mold Builders Association — The association’s web site has a search feature that you can use for this particular task. All you need to do is fill out the search form at its web site.
Ask your local chamber of commerce. If there are plastic injection molding companies based in your city, there is a high probability that its executives are members of your local chamber of commerce. Contact your group’s secretary and ask for such information. If you are lucky, there will at least be one in your city.

Check out directories. Companies spend a considerable amount of money getting themselves listed in telephone directories. So take those dusty telephone directories out of the shelves and start scanning the pages for plastic injection molding companies. This should take about just a few minutes of your time.

Aside from printed telephone directories, check out online business directories and yellow pages. YellowPages.com and Manta.com are very popular directories that will suit your purpose. You can easily search for plastic injection molding companies by city and state using these two resources.

Ask for referrals. As a manager, you are probably connected with many people in your particular line of business. Take out your roll-o-deck and give some of your colleagues a call. Ask them if they know of any plastic injection molding company that you can get in touch with. If they do, ask them for feedback so you can get a good idea of how that company treats its customers.

Finding plastic injection molding companies or suppliers in general becomes an easier task using the Internet and business networks. Gladly, these tools are very much on hand. Use these tools and resources to find the best plastic injection molding company that you can do business with.

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What Is Plastic Compounding

Plastic compounding is a process for adding additional materials into a molten plastic base to produce a material with desired qualities. Additives and modifiers may result in plastic with a particular color, texture, strength, and so on. A manufacturer may incorporate one or more additives into the base material in the process of plastic compounding.

While the process is different in each facility depending on the product being produced, plastic compounding typically involves several basic steps. Additives in the form of pellets, flakes, or powders are conveyed to a container of a molten plastic base material. The mixture goes through a number of blending and dispersal steps to incorporate these additives into the base material and achieve a homogeneous final product. Processing may also include steps to reduce the chemical volatility of the material. Once all processing steps are complete, the material is cooled and extruded into pellets, which are then packaged for distribution or sale.

Polyethylene and polypropylene are the two most common base polymers used in the plastic compounding process. Modifiers may be added to these base polymers in the form of powder or small pellets. Sometimes recycled material is added in the form of chips or shavings produced in the recycling process.

Filler material may be classified as either inert or active. Inert filler material typically increases the volume of the material inexpensively without adding any beneficial features. Its primary purpose is to reduce the cost of the material. Active filler, on the other hand, is added to improve the physical properties of the material. If a filler increases the tensile strength of the base material, it may be referred to as a reinforcement.

Manufacturers must take into account a number of factors when incorporating additives. Physical properties such as particle size and shape of the additive must be compatible with the base material. Even if it improves performance, an expensive additive may drive the price of the final product up too much for its target market. Suitability of an additive in the manufacturing environment must also be considered. For example, abrasive filler materials can degrade plastic compounding equipment, and dust from an additive in powder form may contaminate the manufacturing facility.

Modifiers used in plastic compounding serve a number of purposes when added to base polymers. They may reduce the cost of the final material substantially, thereby providing an economic advantage in the marketplace. Use of recycled material as additives can reduce consumer or industrial waste in landfills and save on waste disposal expenses.

Additionally, additives may improve the quality of the final product in a number of ways. Flame retardants and antioxidants may improve the safety of the material or extend its useful lifetime. Antacids may be added to a material to reduce the impact it has on the equipment used for processing. Glass or carbon fibers can increase the strength of a base polymer when incorporated into it.

A wide variety of products are made with materials developed through plastic compounding. Consumer products that incorporate these materials include toys, furniture, appliances, and more. Industrial applications include use in automotive components, pipes, construction, and others. The diverse array of materials that can be created with plastic compounding ensure widespread use of this process in product manufacturing well into the future.

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What Is a Parting Line

A parting line is the place on a product or component where two or more molds met during the casting process. A number of considerations go into the placement of the parting line, with the goal of maintaining the overall integrity of the piece. Typically, as part of the finishing process, machinery will grind and smooth the parting line so it is no longer visible, if any extra material leaked into the space when the object was cast.

In a simple mold, there will be two halves that press together to create a cavity to fill with plastic, metal, or other materials. As an operator pours material into the mold, air can escape around the parting line, preventing bubble formation. When the material sets and people pull the molds apart, the finished object will drop out. It can be treated with finishing processes like sanding, painting, and so forth. Other molds may be more complex, with multiple components to address special shapes and design considerations.

When people design a mold for mass production, they want to place the parting line with care. Even operating under the assumption that it will not be visible after finishing, they need to think about the best position in terms of pulling the mold apart without damaging the product, and providing support while the molded material sets. If the parting line is too close to a fragile component, for instance, that part may deform during molding or break off when the operator removes the mold. Likewise, bad placement may prevent air bubbles from escaping, causing problems with the finished product.

If molds do not meet exactly, material will leach into the space between them, creating a situation called molding flash. The operator can plane, sand, scrape, or cut off this excess material and then smooth the underlying surface to make it match the rest of the object. This problem is more common with inexpensive molds, and in some cases, manufacturers will not bother to address it; cheap plastic toys, for example, may have a visible line around the middle, showing where the molds came apart.

In specifications for products made with molding, the technical drawings will include illustrations of the molds and a discussion about the location of the parting line. The mold maker will confirm that the designs are appropriate for the application and may make suggestions for changes to address concerns. For example, there could be worries that it will be difficult to file away any molding flash because the parting line is tucked into a corner of the mold, and thus will leave the product with a rough, unfinished appearance.

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What is High-Density Polyethylene

High-density polyethylene (HDPE) is a type of plastic made from petroleum. Since this material can be remolded by subsequent melting and shaping, it is classified as a polyethylene thermoplastic. It can also be joined in segments when welded or machined. However, it does not accept adhesives very well. Also known as polyethylene high-density (PEHD), products made of high-density polyethylene are marked by the imprint of the number “2” surrounded by the Möbius strip recognized as the universal recycling symbol.

As the name implies, high-density polyethylene is denser than most other polymer plastics, namely low-density polyethylene. This is due to its crystallization structure occurring in a linear fashion rather than branching out to form long chains of polyethylene. Instead, the lack of branching results in its carbon molecules bonding with more hydrogen molecules. This allows the final product to possess greater tensile strength, even though it is lighter than water. It also makes high-density polyethylene highly resistant to acids and solvents.

The production of high-density polyethylene does not happen by accident or natural event, however. In fact, the lack of branching during the polymerization process is deliberately induced by the addition of a type of reagent known as a Ziegler-Natta catalyst. Usually, these catalysts are derived from titanium compounds.

Since high-density polyethylene is so durable and chemically non-reactive, it has numerous applications in various industries. It is used in many different types of packaging containers, such as milk and laundry detergent bottles, as well as plastic grocery bags. It is also found in storage systems designed to store chemicals and fuels. In fact, high-density polyethylene is used to produce materials to act as chemical barriers, such as liners that are placed under landfills to help prevent soil and groundwater contamination. One of the most common uses of this material is in the manufacturing of wood plastic composites to make furniture, flooring, fencing, and landscaping materials.

In terms of environmental impact, products made of high-density polyethylene do not readily biodegrade in landfills. Such products can be recycled, though, albeit at the risk of losing some of its original tensile strength. Since this material is constructed of hydrogen and carbon, being subjected to high heat merely results in the release of water and carbon dioxide. However, additives, such as fire retardants, UV-stabilizers, and dyes, can produce other toxins. In addition, some environmental groups express concern over the potential hazard from the leeching of phthalates used in producing some children’s toys made from high-density polyethylene, such as teething rings.

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What Is Low-Density Polyethylene

Low-density polyethylene (LDPE) is a type of thermoplastic, a synthetic polymer that softens to a liquid when heated and freezes when cooled. It is made from petroleum. LDPE has a wide variety of applications because of its toughness and low reactivity at room temperature.

As a primary component of plastic bags, food and drink containers, trays and computer equipment such as disk drives, low-density polyethylene is an important plastic. It is resilient, easy to weld and shape and flexible to the point of being almost unbreakable. This makes it a popular choice for parts that need to be flexible in order to function correctly.

The resilience of low-density polyethylene is because of its chemical structure. Like other polymers, LDPE consists of repeating units of carbon and hydrogen atoms that form bonded chains. LDPE exhibits branching on about 2 percent of its carbon atoms, meaning that in some places, a hydrogen atom is replaced by another carbon-hydrogen chain. This makes LDPE’s tensile strength and intermolecular forces weaker, resulting in lower density and greater flexibility.

Linear low-density polyethylene (LLDPE) is a variety of low-density polyethylene that is widely used in commercial and industrial applications. It is composed of shorter branching structures than LDPE, which gives it a lower viscosity and the ability to elongate when stretched. LLDPE is used in plastic wrap and plastic bags where a thinner, stretchier material than LDPE is required.

LDPE is widely used in laboratory equipment. Its flexibility and translucence make it useful for wash bottles and tubing, and its chemical resistance allows it to be used in conjunction with chemicals that might corrode other materials. For example, LDPE has good resistance to acids, bases, alcohols, aldehydes and vegetable oils.

Laboratory equipment manufacturers state that LDPE can be used in temperatures as high as 176 degrees Fahrenheit (about 80 degrees Celsius) and as low as minus-58 degrees Fahrenheit (about minus-50 Celsius). It is recommended that special care be observed in maintaining LDPE equipment, because the material can be weakened by oxidizing agents and might soften and swell over time.

LDPE was initially developed as a variation on high-density polyethylene (HDPE). HDPE exhibits less branching in its hydrocarbon chains and is therefore a harder material than LDPE. It is used in some of the same products as LDPE, such as plastic bags, but it also can be found in more rigid materials such as milk jugs and bottle caps.

The global market for LDPE and LLDPE has grown rapidly since its inception during the mid-20th century. Although polymer science has continued to develop new materials to meet the challenges of packaging and manufacturing, LDPE has remained a popular material because of its versatility and durability. LDPE can also be recycled, which gives the material staying power in an increasingly environmentally conscious society.

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