How ordinary items are made using plastic injection molding
Increasing use of thermoplastics in manufacturing took place during the middle of the past century, coming on especially strong during the war years, 1940-45, where many applications substituted the use of plastics for metal. Even in the sixties it was a booming and yet still infant, emerging technology. The 1967 movie, “The Graduate,” starring Dustin Hoffman and Anne Bancroft, made a powerful reference to the promise that plastics held in the American economy when, upon college graduation, the character Benjamin Braddock received a one-word piece of advice from a family friend. “Plastics,” he was told, as if this material was destined to be the next great breakthrough, which it was.
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. In a previous article on our blog, we break down the various types of plastic molding and their advantages.
Plastic injection molding is a process of forming this durable, resinous material into just about any form or 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.’
The first plastic injection molding process was used to produce hair combs, buttons and collar stays. Starting in the 1940s, when high demand existed for mass-produced products that were inexpensive to manufacture, the industry expanded exponentially. In 1946, at war’s end, came the invention of the first screw-type injection machine, which provided for more precise control of a consistent shot, thereby improving the quality of the products being made. Molds were being made for any variety of applications and what was once used to make buttons and combs was now being employed to produce a wide range of items for many industries, including:
Aerospace; Automotive; Consumer products;Construction;Medical;Packaging;Plumbing;Toys;
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.
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; Low labor costs; Minimal scrap loss; Little need for finishing; Wide range of materials available for specific applications
Whether it’s a snowboard or a vinyl window being produced, injection molding is efficient and economical, especially if large numbers of items are being made. The only real disadvantage is initial start-up costs incidental to obtaining the necessary equipment and the time, expertise and resources required for mold design.
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. Products can be produced of a highly complex geometry that other processes are unable to duplicate, and at a fraction of the cost. When one stops to think about the role plastics play in our everyday lives, it’s difficult to imagine a life without this ubiquitous substance. Plastics are now available that are actually stronger than steel, more durable than almost any substance on earth and relatively inexpensive to produce and reproduce a million times over and again.
How to Avoid Injection Molding Defects
When working with any manufacturing process, a number of defects unique to that process commonly occur. This is true across many processes and industries, including plastic injection molding and high volume injection molding.
Resin- and Additive-Caused Defects
Two common defects caused by issues with the resin or resin additives used during injection molding are delamination and discoloration.
Two common defects caused by issues with the resin or resin additives used during injection molding are delamination and discoloration.
Delamination:Delamination, when a finished part has a layer of flaky material at the surface, hurts both aesthetic of your part and its strength. Caused by moisture contamination of the resin pellets or by other contamination of the melted resin with a dissimilar resin, or by release agents in the mold, delamination is the result of the resin being prevented from bonding.
A number of methods, both simple and more complex, can be used to prevent delamination. If moisture is the issue, pre-drying the resin pellets or increasing mold temperature will help. If mold release agents are the cause, a mold redesign that places more focus on the ejection mechanism will help to eliminate mold release. If it is caused by cross-contaminated resins, that will need to be replaced with virgin material
Discoloration:Discoloration is simply when a finished part is a color different than intended. Caused most commonly by leftover pellets in the hopper, too hot of a barrel temperature or leftover resin in the feed zone, the problem can be addressed by thoroughly flushing the hopper and feed zone of a machine in between processes, thus preventing discoloration as a matter of course. Purging compound can also be effective to remove unwanted color or resin.
Process-Caused Defects
Despite continual advances in injection molding technology, process-derived injection molding defects still occur. Two of the most common are burn marks and flow marks.
Burn Marks:Burn marks are surface marks, sometimes advancing to degraded plastic, that are caused by either trapped air which becomes overheated or actual resin that overheats. There are three ways to avoid burn marks: decrease resin injection speeds, which will lower the probability of air becoming trapped; include or optimize venting and degassing systems; or reduce the mold and/or melt temperature.
Flow Marks:Flow marks are lined patterns, often wavy, or discoloration on a part surface. They are most commonly caused by resin cooling too quickly or improper gate location. In the best case scenario, flow marks can be eliminated by increasing injection speed and pressure, which will help to ensure uniform filling and cooling. In a worst case scenario, a mold redesign with an emphasis on avoiding sudden flow direction changes and gate location may be necessary.
Mold-Caused Defects
Flash and short shots are two of the more common injection molding defects caused by mold design or maintenance issues.
Flash:Sometimes known as burrs, flash is the occurrence of thin, wafer-like protrusions on a finished part caused when melted resin escapes the mold cavity. Most common along the parting line or up an ejector pin, flash can be caused by excessive injection speed or pressure, in which case the fix is a simple reduction. More often flash is due to poorly designed or severely degraded molds, in which case a redesign or retooling is required. Flash can also be caused by too high of a mold temperature and excessive barrel heat.
Short Shot:A short shot is literally when a shot of resin falls short of filling the mold. It can be caused by attempting to use the wrong resin type or by poor process settings, but is most commonly caused by gate blockages or too small of a gate diameter, a common problem due to too low pressure or not enough heat. If a higher melt index resin or increased melt temperature doesn’t solve a short shot problem, you may need to redesign the runner system to optimize flow.
Plastic Injection Molding Terms
Parting line – A line on a part formed when the two sides of the mold come together.
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.
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.
Runner system – The channel system that allows the flow of the melted material to fill the part cavities.
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.
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.
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.
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.
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.
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.
Additives – These compounds are added to resins to improve the overall performance and appearance of finished products. A key trend in this area today is using additives that are made from organic materials such as eggshells, wood pulp, rice hulls or materials that improve the biodegradability of the plastic.
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 gas or air bubbles.
Cavity – The machined shape within a mold which created the form of the plastic part.
Colorant – A pigment system, usually in pelletized form, powder or liquid, which is mixed with resin to produce the desired color.
Core – A protrusion or set of matching protrusions, which form the inner surface of a plastic part. They are often considered they “male” side of the part.
Cycle – The overall 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 and bubbles.
Delamination – This defect appears as a flaky surface layer on the part and is often caused by contamination or moisture in the resin pellets.
EDM or electric discharge machining – A manufacturing process used to create molds, where the shape of the mold cavity is obtained by removing metal material using electrical discharges.
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.
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.
Injection molding – A manufacturing process in which melted plastic is injected into a mold to form a part.
Masterbatch – A solid or liquid additive for plastic used for coloring plastics or imparting other properties to plastics.
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.
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.
Questions to Consider Before Getting an Injection Molding Quote
Injection molding is the most common plastic molding process and is used to create a wide variety of complex parts of different size and shape. Whether it’s an overcap for a water bottle or a vinyl window part being manufactured, injection molding is efficient and economical, especially if high volumes of parts are being produced.
Injection molds represent the greatest expense in upfront production costs. With any custom injection molding project, your injection molder should be able to provide you with a quote that breaks down the costs. Procurement and purchasing managers have the unenviable task of obtaining quotes for each project. Depending on the input (in terms of drawings, prototypes or sample parts), the quotes can vary greatly.
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.
Getting an injection molding quote is the first step in determining feasibility; however, there are many questions that should be answered before an accurate quote can be supplied.
Here are the top six questions you should able to provide initial answers to:
1. Are there CAD drawings, prints or samples of the part to be quoted?
To begin to form an accurate quote, the molder needs to know what you are asking them to make. Detailed dimensional drawings provide information on the size and complexity of the part. A sample or prototype can especially help the molder begin to determine how to maximize the design for manufacturability.
To begin to form an accurate quote, the molder needs to know what you are asking them to make. Detailed dimensional drawings provide information on the size and complexity of the part. A sample or prototype can especially help the molder begin to determine how to maximize the design for manufacturability.
2. Are you running the parts from an existing mold?
If so, it would be up to the molder to decide if they can run your parts from the existing mold. At Rodon, we look for a mold that can run in an automated fashion, unattended and one that is made from a quality stainless steel material. If we can run the mold the way it was originally designed and built then we will certainly consider it.
If so, it would be up to the molder to decide if they can run your parts from the existing mold. At Rodon, we look for a mold that can run in an automated fashion, unattended and one that is made from a quality stainless steel material. If we can run the mold the way it was originally designed and built then we will certainly consider it.
3. What is the part application? Are there chemical or environmental issues the part will be exposed to?
The injection molder you are working with needs to understand the end-use application of the part. This explanation will help the manufacturer determine how sturdy the part needs to be and what the wear and tear will be over time. The information you provide will help your molder make recommendations on the resins and/or additives required for your project.
The injection molder you are working with needs to understand the end-use application of the part. This explanation will help the manufacturer determine how sturdy the part needs to be and what the wear and tear will be over time. The information you provide will help your molder make recommendations on the resins and/or additives required for your project.
4. What quantities are needed?
All injection molds are not made alike. If you are interested in smaller quantities or a shorter production run, an aluminum mold might be the best option. If your project requires large quantities over a longer time span, then a hardened stainless steel mold would be the best choice. The upfront cost of the latter option is higher; however, it pays for itself over the life of the tool. High volume, precision molders like The Rodon Group specialize in building tools with hardened steel.
All injection molds are not made alike. If you are interested in smaller quantities or a shorter production run, an aluminum mold might be the best option. If your project requires large quantities over a longer time span, then a hardened stainless steel mold would be the best choice. The upfront cost of the latter option is higher; however, it pays for itself over the life of the tool. High volume, precision molders like The Rodon Group specialize in building tools with hardened steel.
5. What is the size and complexity of the part?
While many plastic parts are made through injection molding, other molding processes can be used to produce a part. Briefly, smaller parts that are more complex are ideally suited to the injection molding process. Larger parts may be produced by injection molding or compression molding. Oversize parts lend themselves to rotational molding while hollow objects, like bottles, are made with blow molding. You can learn more about each in this related post.
While many plastic parts are made through injection molding, other molding processes can be used to produce a part. Briefly, smaller parts that are more complex are ideally suited to the injection molding process. Larger parts may be produced by injection molding or compression molding. Oversize parts lend themselves to rotational molding while hollow objects, like bottles, are made with blow molding. You can learn more about each in this related post.
6. What types of polymers or resins are required for the part?
You may need to do some initial fact-finding, but having an understanding of the type of plastic material you feel best suits your project gives the molder a starting reference point. In the long-run, a qualified molder will recommend the resin and additives they think will provide the best result.
You may need to do some initial fact-finding, but having an understanding of the type of plastic material you feel best suits your project gives the molder a starting reference point. In the long-run, a qualified molder will recommend the resin and additives they think will provide the best result.
Blow vs Compression vs Extrusion vs Injection Molding Machine
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 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.
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.
Extrusion molding – well suited for long hollow formed applications like tubing, pipes and strawsWhile 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.
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.
Where to Source Plastic Molding Machine
Automatic molding, a semi-automated form of compression molding, is the process Sunkoo uses for the manufacture of medium-volume to large-volume net molded and near net molded PTFE components. The automatic molding process requires dedicated tooling, typically requires minimal operator intervention and plastic parts can be fabricated relatively rapidly.
With the use of precise tooling and processing, parts manufactured using the automatic molding process can be produced to relatively close tolerances, but not to the accuracy of a fully machined part. Components are economically molded by utilizing a process of automated filling of the die cavity, applying pressure, and part ejection.
The Sunkoo molding department has the capability to produce compression molded PTFE rod and tube stock to meet and exceed your exact specifications and tolerances. Sunkoo has over 1,000 mold sizes to manufacture stock shapes from 1” OD to 26” OD in Virgin PTFE, as well as all industry standard and custom blended materials. One of the benefits of compression molded rod or tube is the relatively low cost of this method. It typically does not require special tooling beyond what Sunkoo already possesses.
With an extensive array of in-house equipment, Sunkoo is your one-stop source to handle all aspects of your compression molding component requirements. Call the specialists at Sunkoo for immediate assistance.
Plastic Machinery Manufacturer
Sunkoo’s enormous strength comes from a wonderful combination of its highly efficient and professional management and a dedicated workforce of qualified and experienced engineers and technicians and marketing personnel, which in turn, leads to world-class manufacturing, testing and R & D , besides meeting requirements of clients’ specifications globally.
We have invested in advanced training to promoted knowledge in the area of technical development, research and production, creating preconditions for innovations. Based on market research and feedback from clients, our R&D department works to engineer products that are innovative and unique for PTFE & UHMWPE machinery.
The objective of our business has been dedicated to plastic extrusion industry with diversified & innovative engineering solutions in the application design & supply of wide range of Injection Molding plastic extruders, recycling units & extrusion ancillaries. we are proud of the leadership position we have earned for our Company.
We constantly aim to accept future challenges in the plastic extrusion technology with emphasis on our manufacturing potential. We have a wide vision for developing special purpose machines extending from light to heavy duty which can easily fulfill the needs of plastic industry and make us a leader.
We have created a wide market for our products with our honesty and integrity. Our clients have complete faith on us and we strive to upkeep the same. With our foresightedness and skills, we have supplied our products to various domestic and overseas markets. We strive to cater to new markets as well as retain our old clients with our expert quality and time bound delivery schedules.
