New eBook "An Introduction to plastic injection molding"

14,635 views

Published on

A resource guide to plastic manufacturing for designers, engineers and purchasing agents.

Published in: Engineering, Business

New eBook "An Introduction to plastic injection molding"

  1. 1. An Introduction to Plastic Injection Molding A resource to help designers, engineers and purchasing professionals navigate the world of plastic injection molding A publication of
  2. 2. Table of Contents Introduction Chapter 1 – Plastics and society Chapter 2 – Types of plastic molding Chapter 3 – Key ingredients to achieving perfect parts Chapter 4 – The basics of an injection molding machine Chapter 5 – Cold runners versus hot runner systems Chapter 6 – Determining the cost of an injection mold Chapter 7 – Common part defects Glossary of terms Learn more about us 2www.rodongroup.com
  3. 3. INTRODUCTION We developed this eBook with designers, engineers and purchasing specialists in mind. It is written to provide a basic understanding of plastic injection molding presses, processes and costs. Our goal is to make our customers more knowledgeable about what goes into making a plastic part. We hope you find this eBook informative and useful. Please feel free to share it with your colleagues. 3www.rodongroup.com
  4. 4. CHAPTER ONE PLASTICS AND SOCIETY 4www.rodongroup.com
  5. 5. Plastics in modern society Many products that are a part of everyday life go unnoticed, either because they are components of larger items or they are so commonly used that little thought is given to their existence. Items manufactured by plastic injection molding often fall into this category. Looking around a home or business, you will find many products that exist because of the injection molding process. From toys to cars, plastics play an important role. Plastic injection molding is a process of forming this durable, resinous material into just about any form of fashion imaginable. The first injection molding machine was invented and patented by brothers John and Isaiah Hyatt in 1872. It resembled a large hypodermic needle, with a heated cylinder through which a large plunger forced the gooey mass into a mold. Today, the process is more complicated although, the basic principle of plastic being injected into a waiting mold is still the same. One of the biggest advancements has come by way of the materials used, and there are now thousands of different formulations available for making ‘plastic.’ Raw materials used in the plastic injection molding process include thermoplastics, thermosets, and elastomers. Also called polymers or resins, there are more than 20,000 unique formulations that can be injected into molds to produce parts with specific properties to be utilized for specific purposes. Examples of common thermosetting plastics include polymers such as epoxy and phenolic. Common thermoplastics are nylon, polyethylene and polystyrene. 5www.rodongroup.com
  6. 6. Injection molding machines are fairly simple and straightforward, consisting of a hopper where raw material is placed, a heating cylinder and an injection plunger. Molds are typically made from steel or aluminum. Major advantages to using plastic injection molding for the manufacture of parts include: • Ability to complete high-production rates • Repeatability of high tolerances • Labor costs • Minimal material loss • Minimal finishing • Wide range of materials available for specific applications Injection molding is the most common plastic molding process and is used to create a huge variety of complex parts of different size and shape. Whether it’s a snowboard or a vinyl window part being produced, injection molding is efficient and economical, especially if large numbers of items are being made. Highly complex parts can be produced at a low cost. The only real disadvantage is the initial start-up costs. “When you think about it, plastics play a very important role in our lives and the products we use every day.” 6www.rodongroup.com
  7. 7. CHAPTER TWO TYPES OF PLASTIC MOLDING 7www.rodongroup.com
  8. 8. Types of plastic molding In today’s manufacturing environment, plastics are being used to make everything from automotive body parts to human body parts. Each application requires a special manufacturing process that can mold the part based on specifications. This article provides a brief overview of the different types of molding and their advantages and applications. Blow Molding – Well suited for hollow objects, like bottles The process follows the basic steps found in glass blowing. A parison (heated plastic mass, generally a tube) is inflated by air. The air pushes the plastic against the mold to form the desired shape. Once cooled, the plastic is ejected. The blow molding process is designed to manufacture high volume, one-piece hollow objects. If you need to make lots of bottles, this is the process for you. Blow molding creates very uniform, thin walled containers. And, it can do so very economically. Compression Molding – Well suited for larger objects like auto parts The name of this molding method says everything. A heated plastic material is placed in a heated mold and is then compressed into shape. The plastic can be in bulk but often comes in sheets. The heating process, called curing, insures the final part will maintain its integrity. As with other molding methods, once the part has been shaped, it is then removed from the mold. If sheeting plastic material is used, the material is first trimmed in the mold before the part is removed. This method of molding is very suitable to high-strength compounds like thermosetting resins as well as fiberglass and reinforced plastics. The superior strength properties of the materials used in compression molding make it an invaluable process for the automotive industry. 8www.rodongroup.com
  9. 9. Extrusion Molding – Well suited for long hollow formed applications like tubing, pipes and straws While other forms of molding uses extrusion to get the plastic resins into a mold, this process extrudes the melted plastic directly into a die. The die shape, not a mold, determines the shape of the final product. The extruded “tubing” is cooled and can be cut or rolled for shipment. Injection molding - Well suited for high-quality, high-volume part manufacturing Injection molding is by far the most versatile of all injection molding techniques. The presses used in this process vary in size and are rated based on pressure or tonnage. Larger machines can injection mold car parts. Smaller machines can produce very precise plastic parts for surgical applications. In addition, there are many types of plastic resins and additives that can be used in the injection molding process, increasing its flexibility for designers and engineers. The process itself is fairly straightforward; however, there are many enhancements and customization techniques that can be used to produce the desired finish and structure. Injection molds, which are usually made from steel, contain cavities that will form the parts. Melted plastic is injected into the mold, filling the cavities. The mold is cooled, and the parts are ejected by pins. This process is similar to a jello mold which is filled then cooled to create the final product. The mold making costs in this method are relatively high; however, the cost per part is very economical. Low part cost along with resin and finish options have all contributed to injection molding’s popularity in today’s manufacturing landscape. 9www.rodongroup.com
  10. 10. Rotational Molding (Rotomolding) – Well suited for large, hollow, one-piece parts This process uses high temperatures and rotational movement to coat the inside of the mold and form the part. The constant rotation of the mold creates centrifugal force forming even-walled products. Because it is ideally suited to large hollow containers, such as tanks, it is not a fast moving process. However, it is a very economical process for particular applications and can be cheaper than other types of molding. Very little material is wasted using this process, and excess material can often be re-used, making it an environmentally viable manufacturing process. Conclusion Each type of molding has its strengths and weaknesses. Designers and engineers need to understand these differences and the production options available. There are always several approaches to a final manufacturing solution. The molding company who consults on a specific project should be able to provide additional insights into the applications and materials that are best suited to an individual project. “There are always several approaches to a final manufacturing solution.” 10www.rodongroup.com
  11. 11. FIND OUT HOW THE RODON GROUP CAN HELP YOU WITH YOUR NEXT PROJECT Answer some simple questions about your project and we will call you to schedule a consultation. CONTACT US TODAY 11www.rodongroup.com
  12. 12. CHAPTER THREE Key ingredients to achieving perfect plastic parts 12www.rodongroup.com
  13. 13. Key Ingredients Achieving perfect plastic parts The adage “If it can go wrong, it will go wrong” should never be true in the world of injection molding. In fact, problems can be easily avoided from the very beginning as long you are working with a turnkey precision molding manufacturer. Some companies opt to use a separate firm to design the mold, then contract another vendor to build the mold (often these are offshore mold builders) and another to run the parts. By separating these responsibilities, they often sacrifice control, accountability and quality. Mold Design Often problems arise at the very beginning of a project. You may have drawings and even a prototype, but without the expert advice of design engineers who understand how to optimize an injection mold, you may experience costly defects and delays. Many designers begin their careers as tool makers before gaining the knowledge and experience needed in CAD/CAM systems to become engineers. This expertise helps to develop molds that will perform at the highest production levels. The design team you choose to work with should conduct a Design for Manufacturability (DFM) analysis to ensure your parts meet the highest quality standards. 13www.rodongroup.com
  14. 14. Resin selection Choosing the right material for a project is one of the most important factors in creating perfect parts. The advances in polymer science have helped create a wide variety of resins to choose from based on the final application of the part. It is important to work with an injection molder that has experience with a wide range of resins and applications including resins that are compliant with FDA, RoHS, REACH and NSF. Reputable companies will have established strategic relationships with the best resin suppliers in the country. They should have a great deal of experience using certified commodity and engineering resins that adhere to stringent manufacturing standards. Mold Building and Testing Without careful attention to mold design, the end product may be non- conforming. It is important to create molds that accommodate enough draft for the selected resin and finish, for example. Plastic injection molders should create pre-production molds. These molds offer several benefits to the design and manufacture process. They are single cavity molds that are created using the same 3D software and tools as production molds; however they are made with less durable metal and steel. Pre-production molds can be modified to help determine the best production solution for the project including finishes and coloration. Various resins can also be “tested” in this environment. Alternatively, SLA models can be created using 3-D printers, though these parts cannot be used as pre-production samples. Once the pre-production sample parts have passed rigorous quality inspections and have satisfied the client’s expectations, the manufacturer can move onto the final stage of tooling, creating the production mold. 14www.rodongroup.com
  15. 15. Production and Quality Once the multi-cavity production molds are completed, a full cycle of samples are produced and checked for the quality standards outlined by the client. Adjustments are made as required before full production begins. Quality checks continue throughout the part production process. Working with a “Just in Time” manufacturer, they can monitor and adjust quantities. At The Rodon Group, we maintain our client’s inventory in- house until they require the parts. Our production molds are built to last, and we guarantee them for as long as we manufacture the parts. Our clients return year after year because our injection molds maintain their integrity. We use 420 stainless steel on all of our molds, and our highly- trained operators insure each tool is properly maintained to maximize quality and output. The Rodon Group is ISO 9001:2008 certified and we are very proud of our 99.8% part acceptance rate. “At Rodon, our production molds are built to last and we guarantee them for as long as We manufacture the parts.” 15www.rodongroup.com
  16. 16. CHAPTER FOUR BASICS OF A PLASTIC INJECTION MACHINE 16www.rodongroup.com
  17. 17. Basics of an injection molding machine While plastic injection molders will help you determine the size of the machine needed to get the best results, a project designer or engineer can get a good estimate based on some basic information. By knowing approximately what size machine will be required, you can better source a plastic injection molder that will meet your needs. First, let’s take a quick look at how plastic injection molding presses are rated or classified. Often plastic injection companies will provide a molding equipment list on their website. It may look something like this: 3- 68 Ton Injection Molding Presses 5- 123 Ton Injection Molding Presses 5- 154 Ton Injection Molding Presses 5- 202 Ton Injection Molding Presses 5- 233 Ton Injection Molding Presses 4- 400 Ton Injection Molding Presses So, what does this mean? Plastic injection molding presses are classified or rated based on tonnage, or more specifically, the clamping pressure or force. Presses can run in size from less than 5 tons of clamping pressure to over 4000. The higher the press ton rating, the larger the machine. 17www.rodongroup.com
  18. 18. A machine rated for 68 tons can deliver 68 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues. Too much or too little pressure can also cause flashing, where excess material appears on the part edge. Pressure also impacts the viscosity of the plastic being used in the project. Melt Flow Index or MFI is a measure of the ease of flow of the melt of a thermoplastic polymer. Plastic compounds react differently to pressure based on their MFI. The higher the MFI, the higher the pressure needed. Second, let’s figure out how much clamping force or pressure is required. There are many factors that are taken into consideration when determining the size of the press. The size of the part, the polymer being used and something called the safety factor. The safety factor is an additional numerical percentage buffer that is added to the calculation to help avoid defects in the final part. Some recommend adding 10% to allow for the safety factor. As mentioned earlier, the MFI (Melt Flow Index) of the plastic compound will also impact the pressure needed to produce the part. Many calculations include the platen size as well as the mold and part size, however, to get an estimate of the press size your project will need, we have simplified it even further. Many plastic injection professionals use a general rule of thumb of 2.5 times the surface square inches of the part to be produced. So, if you have a part with 42 square inches than you would need a press size with 105 tons of pressure. If you add 10% for a safety factor, you will need to use a press with a minimum of 115 tons of clamping force. A press size of 120 tons would be able to accommodate your plastic injection molded product. 18www.rodongroup.com
  19. 19. Lastly, let’s look at how you can identify a plastic injection molder that is right for your project. Once you have an estimate of the press size you will need, you can identify plastic injection molding companies that will meet your requirements. In general, molders with a greater number and wider selection of press sizes will be able to accommodate the needs of your project. If you are not working with a completed mold, look for a plastic injection company who can design and build the mold. They will have a better understanding of how to maximize the manufacturing process and will often offer tooling allowances. This, in turn, will minimize the overall cost of your project. In the end, your plastic injection molder will determine which machine would be best suited for your project. Larger presses can accommodate larger molds and multi-cavity molds often reducing the cost per part. However, larger molds are more expensive. Choosing the right press size can balance the upfront tooling expenditures with long-term manufacturing costs. 19www.rodongroup.com
  20. 20. CHAPTER FIVE COLD RUNNER VS. HOT RUNNER SYSTEMS 20www.rodongroup.com
  21. 21. Molding systems Cold runner vs. hot runner Every plastic part starts in a mold. Molds are classified into two main types, cold runner and hot runner. Each has its advantages and disadvantages. Your plastic injection molder will be able to give you the costs and benefits of using these different systems. However, by understanding the key differences of these technologies, you can have a more educated discussion on the type of mold that would best fit your project. First, let’s discuss cold runner molding systems Cold runner molds usually consist of 2 or 3 plates that are held within the mold base. The plastic is injected into the mold via the sprue and fills the runners which lead to the parts in the cavity. In 2 plate molds, the runner system and parts are attached, and an ejection system is used to separate the pair from the mold. For those of you who assembled a model car at some point in your youth, the runners and the parts were not separated. The child assembling the model was responsible for that final part of the process. In 3 plate molds, the runner is contained on a separate plate, leaving the parts to be ejected alone. In both systems, the runner is recycled and reground, reducing plastic waste. However, these processes can increase cycle time. 21www.rodongroup.com
  22. 22. Hot runner molding systems Hot runner molds consist of 2 plates that are heated with a manifold system. The manifold sends the melted plastic to nozzles which fill the part cavities. There are several types of hot runner systems, however, in general, they fall into two main categories; externally heated and internally heated. The externally heated systems are well suited to polymers that are sensitive to thermal variations. Internally heated systems offer better flow control. The hot runner process eliminates runners entirely, so recycling and regrind (which can only be done with virgin plastics) do not impact cycle times. A variation of this system is called an insulated runner. The insulation, rather than heat, keeps the plastic in a molten state. This system can only accommodate a few types of plastics, specifically semi-crystalline polymers which have a low thermal conductivity. Let’s look at a list of advantages and disadvantages of each type of injection molding system. Cold runner systems Advantages -Comparatively cheaper to produce and maintain -Accommodate a wide variety of polymers, both commodity and engineered -Color changes can be made quickly -Fast cycle times if the systems include robotic assist in removing runners Disadvantages -Cycle times are slower than hot runner systems -Plastic waste from runners (if they cannot be reground and recycled) 22www.rodongroup.com
  23. 23. Advantages and disadvantages continued Hot runner systems Advantages -Potential faster cycle times -Eliminates runners and potential waste -No need for robotics to remove runners -Can accommodate larger parts Disadvantages -More expensive molds to produce -Color cannot be easily changed -Higher maintenance costs and potential downtime -May not be suited to certain thermally sensitive materials Professionals in the field of plastic injection molding should be your primary resource for determining the best injection molding system for your project. Look for injection molders who are familiar with all types of plastics processing. They will be able to provide you with a cost/benefit analysis of the various systems available based on the part and the material used. 23www.rodongroup.com
  24. 24. Injection molding machine at work at The Rodon Group “It pays to work with injection molders who are familiar with all types of plastics processing.” 24www.rodongroup.com
  25. 25. FIND OUT HOW THE RODON GROUP CAN HELP YOU WITH YOUR NEXT PROJECT Answer some simple questions about your project and we will call you to schedule a consultation. CONTACT US TODAY 25www.rodongroup.com
  26. 26. CHAPTER SIX DETERMINING THE COST OF AN INJECTION MOLD 26www.rodongroup.com
  27. 27. Cost of an injection mold Key determining factors A common question for designers and engineers is “How much will a plastic injection mold cost?” It makes sense. Injection molds represent the greatest expense in upfront production costs. And, there are many factors that go into determining the cost. With any custom injection molding project, your injection molder will be able to give you the final price tag. In this article, we will review the variables that can impact the cost so that you can be better informed in making a mold purchasing decision. Not all quotes are created equal Procurement and purchasing managers have the unenviable task of obtaining quotes from a few mold makers for each project. Depending on the input (in terms of drawings, prototypes or sample parts), the cost quotes can vary greatly. Designers should look at all of these inputs and determine the best molding solution. They may re-design the part to maximize manufacturing efficiency and increase the number of parts that can be made with each molding cycle. Generally, molds made with tighter tolerances, more cavities and a longer production life will take longer to build and will cost more upfront. The savings with a high-quality mold are long-term. These molds require less maintenance and last longer than lower quality molds. 27www.rodongroup.com
  28. 28. Here are some variables that impact the cost of a plastic injection mold. The core metal. For shorter production runs, some mold makers will use molds made from aluminum. This is a perfectly reasonable choice if you will not need the mold to perform long-term. However, if a project requires that a mold lasts for several years, an aluminum mold may cost more in the long- run. The number of cavities. It is pretty intuitive when you think about it. Fewer cavities in a mold require less tooling work and time and ultimately less cost. A reputable experienced molder will be able to maximize cavitations in the mold to maintain the highest level of productivity. In general, most molders recommend creating one mold per part versus creating a family mold. Family molds are created with various cavities for assorted parts. They tend to produce inferior products and have more downtime due to maintenance issues. Mold base. Think of the mold base as a case that holds all of the mold cavities, inserts and components together. The cost of the base is estimated based on the size of the mold and the type of steel used to make the base as well as the customization required. Most mold bases come in standard sizes and are further machined to meet the requirements of a specific project. Core/Cavity machining. All molds must also be customized. Customization includes the placement of cores, cavities, ejectors, cooling lines, etc. The steel used in the tool also impacts cost. Hardened steel molds lasts the longest and are more expensive to machine. Once done, however, they have a long production life. Part complexity. Just as the number of cavities plays a role in determining the cost of the mold, so does part complexity. This includes the surface finish of the final part as well as the number of undercuts required. Parts, which demand tight tolerances, also contribute to the mold complexity. 28www.rodongroup.com
  29. 29. Turnkey or vertically integrated injection molders. Some mold builders also manufacture the parts. This type of integration can help defray the mold building cost. Often full service molding manufacturers will subsidize a portion or all of the cost of the mold based on the full term and value of the manufacturing contract. They will amortize the cost of the mold so they can maintain profit margins while providing the lowest possible per piece cost to their clients. The cost of a quality injection mold is certainly a major expense. However, tight- tolerance, precision molds that are made from the best steel available should last for years to come. The upfront cost must be calculated or amortized into the lifetime value of the project. Will these parts be in production for several years or several months? Does the project require a high-volume of parts? Are faster cycle times required? If you answered yes to these questions, then the initial investment in a quality mold will lower the per part cost and will end up saving money in the long-run. “Injection molds represent the greatest expense in upfront production costs.” 29www.rodongroup.com
  30. 30. CHAPTER SEVEN COMMON PART DEFECTS 30www.rodongroup.com
  31. 31. Common parts defects When purchasing injection molded parts, it is important to understand some of the common problems and defects that impact product quality. Being familiar with these imperfections and their causes can help you work with injection molders to insure the highest quality part production is achieved. Most defects in plastic parts can be traced back to three sources: 1. The material being used to make the part 2. The processing of the material in the mold 3. The mold itself We have grouped the defects together by their most common cause; however one or more factors may contribute to a defect. 31www.rodongroup.com
  32. 32. Common defects linked to the plastic resins or additives being used to manufacture a part include: • Color streaks – Just like the name implies, color streaks are random areas of color change that are often attributed to the non-uniform mixing of resins and colorants. • Delamination – This defect appears as a flaky surface layer on the part and is often caused by contamination or moisture in the resin pellets. • Discoloration - This can occur when the hopper and feed zone have not been flushed properly to remove any residual color. • Embedded contaminates – Particles or flecks of residual foreign material that can originate in the barrel of the press. • Splay marks or silver streaks – Circular marks appearing where the molten plastic enters the mold cavity. This is often caused by excessive moisture in the resin. Common defects linked to the processing of the plastic resin in the mold include: • Blistering – Raised imperfections that are generally caused improper heating and/or cooling or by gas/air bubbles. • Burn marks – Black or brown blemishes (which are carbon deposits) that are caused by improper ventilation or prolonged heating in the mold. • Cold slugs – A small non-uniform area on the part caused by an improperly heated piece of plastic becoming attached to the part. • Flow marks – A wavy pattern or discoloration caused by a slow injection speed which allows the material to cool too quickly. • Sink marks or shrinkage voids – Depressions or hollows in a part that can be attributed to excessive press pressure, non-uniform heating, inadequate cooling time or part design. • Stress cracking or stress crazing – This defect usually occurs as a result of over exposure to a high temperature. • Stringing – A thin strand of material attached to a part generally caused by a nozzle that is too hot. 32 www.rodongroup.com
  33. 33. Common defects linked to improper mold design and/or maintenance include: • Drag marks – Scratches that occur when the part is ejected from the mold. This is usually due to an improperly designed ejector system or one that is out of alignment. • Flash or burrs – A thin lip or protrusion beyond the body of the part that is generally caused by poor clamping force, improper mold design and/or mold damage. • Jetting –A snakelike line of material that cools independently of the material around it. This defect is generally due to poor tool design often relating to incorrect gate size and length or placement. • Short shot – An incomplete part due to lack of a filled mold. This problem is often attributed to a blockage or improper injection pressure. • Warping – A part with a distorted shape can be due to a poor cooling system in the mold. When the plastic material is cooled unevenly, the result is a bowing effect. Most of the defects listed here can be addressed by making changes to the processing, the material or the mold itself. The best way to avoid these part defects is to work with a plastic injection molder that has a great deal of experience with various resins and their applications. Using a turn-key manufacturer, who also builds and maintains the molds can help avoid any costly machining charges or mold replacements. “When purchasing injection molded parts, it is important to understand the common problems and defects that impact product quality.” 33www.rodongroup.com
  34. 34. GLOSSARY OF TERMS 34www.rodongroup.com
  35. 35. GLOSSARY Plastic injection molding may not be rocket science, but it comes pretty close. There are hundreds of terms used in the industry. We have chosen to highlight the most common nomenclature used when discussing mold parts, materials and problems. Additives – These compounds are added to resins to improve the overall performance and appearance of finished products. Alloy – A plastic alloy is a physical modification of an existing plastic to achieve higher performance and or functionality. These alloys are often used in the automobile industry and to replace metal parts. Annealing - Annealing is the heating and slow cooling of a plastic part which allows the polymer chains to recoil and relieve internal stresses. Assembly – A secondary manufacturing function of joining finished parts together. Backing plate – A plate, which supports the mold, pins and bushings in the injection machine. Back Pressure – The resistance of a melted plastic to move forward. This impacts not only the temperature but the cycle time as well. Blister – As the name says, this is a part defect which appears as a small bubble or blister on the surface of a part and it generally created by improper heating or cooling of by gas or air bubbles. Blow molding – The process follows the basic steps found in glass blowing. A parison (heated plastic mass, generally a tube) is inflated by air. The air pushes the plastic against the mold cavity to form the desired shape. Once cooled, the plastic is ejected. This method is used to make plastic bottles. Bridge tool – An injection mold that makes parts until the final tool is completed. These molds or tools are not meant to be production tools. Bubbles – Similar to blisters, gas pockets, or voids that have formed inside the plastic. 35www.rodongroup.com
  36. 36. GLOSSARY 36www.rodongroup.com Cavity - The machined shape within a mold which creates the form of the plastic part. Check valve – A device located at the end of the injection screw. The check valve makes sure that resin does not flow back into the machine after it is pumped into the mold. Clamp – The mechanism that holds the mold in location during the molding process. Cold slug – A defect characterized by a small non-uniform area on the part caused by an improperly heated piece of plastic becoming attached to the part. Colorant – A pigment system, usually in pelletized form, powder or liquid, which is mixed with resin to produce the desired color. Compression molding - The name of this molding method says everything. A heated plastic material is placed in a heated mold and is then compressed into shape. The plastic can be in bulk but often comes in sheets. The heating process, called curing, insures the final part will maintain its integrity. This molding method is often used to make large objects such as automobile components. Copolymer - A polymer derived from more than one type of monomer. Core - A protrusion or set of matching protrusions, which form the inner surface of a plastic part. Core pull – A device that retracts a core in a direction that is not parallel to the opening direction of the mold. Often referred to as a side action, this functionality assists in the manufacture of more complex parts. Crazing – A defect that causes small cracks often caused by over-stressing the plastic material. Creep – The “set” that a molded part takes under stress, and does not return to its original shape. Also known as “memory”. Crush Ring – A contact ring on the inner surface of the sprue bushing used to eliminate nozzle leakage. Cure – The process of allowing a plastic to harden or stabilize. Cushion – A space between the screw and the nozzle that provides a pressure pathway to enabling packing out the part.
  37. 37. Cycle – The time it takes for the plastic injection process to complete a finished part. Degassing – Opening and closing of a mold to allow gas to escape. Trapped gas and/or air can cause parts defects such as blistering. Delamination - This defect appears as a flaky surface layer on the part and is often caused by contamination or moisture in the resin pellets. Density – Mass per unit volume of a substance. Dimensional stability - Ability of a plastic part to retain the precise shape in which it was molded. Draft – The angle or degree of taper in a side wall to help facilitate removal of the parts from the mold. Ejection pin – Metal rods in the mold which push the parts from the mold. Ejector return pins – Pins that push the ejectors back into position once the parts have been released. Ejector rod – A bar that engages the ejector assembly and pins when the mold opens. Elasticity – The ability of a material to return to its original state when stretched. Elastomer – A rubber-like material, which is highly elastic. Extrusion – The process of forming tubes or continuous shapes by pushing melted material through a die aperture. Fabricating – The process of manufacturing plastic products through various molding and forming methods. Family mold – A mold which contains cavities for various parts. Fan gate – A gate with a wider width that helps reduce warping through stress. Fill – The packing of material into the mold. Fill Imbalance – Generally occurs in multi-cavity molds, when the plastic material fails to fill all of the cavitations. Filler – An inert additive that adds strength or hardness to a part. GLOSSARY 37 www.rodongroup.com
  38. 38. Finish – The surface texture to a part. Flash gate – An alternative to a fan gate, which conveys the melted resins into a thinner gate section creating a linear melt flow into the cavity. Flash or burrs – A thin lip or protrusion beyond the body of the part that is generally caused by poor clamping force, improper mold design and/or mold damage. Flow marks - A wavy pattern or discoloration caused by a slow injection speed which allows the material to cool too quickly. Flow rate – The volume of material passing a fixed point per unit time. Gate – The channel into which melted plastic flows into a mold. Gate seal or freeze – The holding time and pressure needed to insure the plastic material in the mold is set. Hardness – The resistance of a material to compression, indentation and scratching. Hot-runner mold – Hot runner molds consist of 2 plates that are heated with a manifold system. The manifold sends the melted plastic to nozzles which fill the part cavities. Hot to cold mold - Using a hot runner system to gate into a cold runner which in turn feeds each cavity. This configuration can be used when full cold runner molds aren't viable and full hot runner molds are cost prohibitive. Injection blow molding - A blow molding process in which the parison to be blown is formed by injection molding. Injection molding – A manufacturing process in which melted plastic is injected into a mold to form a part. Insert – An object, such as a magnet or screw, which is inserted into the molded part. Intensification ratio – A calculation used to convert hydraulic pressure to plastics pressure Line of draw - The parallel direction of the moving platen. GLOSSARY 38www.rodongroup.com
  39. 39. Machine shot capacity – The maximum volume of resin which can be injected in a single stroke. Masterbatch – A solid or liquid additive for plastic used for coloring plastics or imparting other properties to plastics. Melt temperature – Melt temperature is the actual temperature of the polymer as it exits the nozzle and enters the mold Memory – The action of plastic returning to its previous size and form. Mold – A hollow form that plastic is injected or inserted into to manufacture a plastic part. Mold release – A surface preparation used to aid in the ejection of the part from the mold. Molecular orientation - The manner in which polymer chains position themselves in the mold cavity. Polymers near the wall of the mold orient themselves by straightening out, while polymers near the center tend to stay coiled. Multi-shot molding – A process where two or more plastic substances are injected into the mold to form a part. Toothbrushes are often manufactured using this technique. Non-return valve – Also called a check ring, a valve that allows rapid material shut off for part weight consistency; and a smooth, high-flow profile to prevent material degradation. Nozzle - The hollow-cored metal nose screwed into the injection end of the barrel which forms a seal under pressure. Orange peel – A patchy rough surface defect caused by moisture in the mold cavity, or by incomplete pack-out. Over molding – A two-shot process, in which two plastic substances, are injected into a mold sequentially, usually a harder base material with a coating of softer material. Parting line – A line on a part formed when the two sides of the mold come together. Pinpoint gate – A very small gate, used in hot runner molds, to control the flow of material. Plastic – A polymeric substance of large molecular weight. Plasticity - The quality of being easily shaped or molded. GLOSSARY 39www.rodongroup.com
  40. 40. Platens – Steel plates in the molding machine onto which the mold is fastened. Polymer - A substance that has a molecular structure consisting chiefly or entirely of a large number of similar units bonded together, e.g., many synthetic organic materials used as plastics and resins. Projected area – The area of the mold that will be filled with plastic at the mold’s parting line. Prototype tool – Also called a soft tool, a preliminary mold built to produce prototype parts and used to make adjustments to the final production tool. Purging – The process of cleaning the injection machine of remnant color or materials prior to running a new part. Ram – A plunger-like part which pushes the melted material into the mold. Reciprocating screw – An injection molding machine mechanism which compresses melts, and conveys the material to the sprue and mold. Regrind – In thermoplastic resins, scrap material that is ground and recycled back into finished parts. Relative viscosity - Peak hydraulic pressure fill time Release agent – A compound, which is sprayed on the mold, or as an additive, molded into the part to help facilitate the release of the part. Retainer plate – A plate onto which the removable parts of the mold are mounted. Runner system – The channel system that allows the flow of the melted material to fill the part cavities. Screw decompression – Also called “Suck Back” the action of the screw return toward the hopper to eliminate drooling of the melted plastic from the nozzle. Short shot – A defect where the material does not fully fill the part cavity. Shot – A complete cycle of the injection machine. Shrinkage – The amount of volume reduction that takes place when a plastic material cools. Side-action, slide or cam - a mechanical, pneumatic or hydraulic action within the mold that forms a plastic part detail that is not in the line of draw of the mold. GLOSSARY 40www.rodongroup.com
  41. 41. Sprue – The opening feed that conveys material from the nozzle to runner system in the mold. Thermoplastic - A material that can be heated and cooled repeatedly without changing the material structure. Highly recyclable. Sprue bushing – This part seals tightly against the nozzle of the injection barrel of the molding machine to allow molten plastic to flow from the barrel into the mold. Thermoset – A material, which when heated, is pressed or molded into a shape. The heating process changes the structure of these materials, and for this reason they cannot by re-heated. Tie bars - Bars which provide structural support to the mold in the press. The spacing between the tie bars dictates the size of the mold that can be placed into the injection machine. The mold opens and closes riding on the tie bars. Toggle – A mechanism that is used to mechanically close the mold, as opposed to hydraulic clamping. Tool – The mold used to form plastic parts in an injection machine. Undercut – Can be a design flaw that results in an indentation or protrusion that inhibits the ejection of the part from the mold. Other times undercuts are designed into a mold to ensure a part holds onto the correct side of the mold. Velocity – The speed at which the plastic enters the cavity of the mold. Vent – A channel from the mold cavity that allows gas and air to escape as resin is being injected into the cavity to prevent many types of defects from occurring. Viscosity – The resistance to liquid flow. Weld line - Also called a knit line, the juncture where two flow fronts meet and are unable to join together during the molding process. These lines usually occur around holes or obstructions and cause localized weak areas in the molded part. GLOSSARY 41www.rodongroup.com
  42. 42. Thanks for reading! See how The Rodon Group can help you with your next plastic injection molding project. CONTACT US TODAY Subscribe to our blog and get the inside scoop on injection molding 42www.rodongroup.com
  43. 43. The Rodon Group is an ISO 9001:2008 certified, high volume (we're talking millions!) plastic injection molder. In business since 1956, we make billions of parts each year in our 125,000 square foot facility. We offer a turnkey manufacturing solution including mold design, mold building and high volume parts manufacturing. Our globally competitive prices eliminate the risks of sourcing offshore. “At K’NEX and Rodon, we’ve learned that manufacturing in America can actually be more cost effective than manufacturing overseas” - Michael Araten, President and CEO of K’NEX LP and The Rodon Group About Us 43www.rodongroup.com

×