You’ve probably seen or heard the word ‘quality’ used in slogans and advertising over and over again. “Quality is Job 1” is one that comes to mind pretty quickly. The question isn’t if quality control is important, but really what does using the term ‘quality control’ mean? It starts with the specifications, and then the judging criteria against the intent. Dimensional stability, color, gloss, and molding defects define how well the product matches the intent, and if your consumer agrees your product is built with quality in mind.
What is Plastic Injection Molding?
Injection molding is a process that utilizes a molten material that is forced into a mold to form a part. The molten material could be metal, glass, rubber, or plastic. A mold is filled under pressure with the molten material, then cooled to form a solid part before being removed from the mold. It uses one of thousands of resins available to form solid parts from an engineered resin.
Injection molding has been around since the early 1870s when the first plastic injection molding machine was developed to produce billiard balls. For the next six decades, plastic injection molding was used to develop new products with newly formed plastic resins. In the 1930s plastic injection molding took off with great options for products using thermoplastics, such as polyolefins, polystyrene, and polyvinyl chloride (PVC).
What Products Are Made by Injection Molding?
With the technology and engineered resins currently available, it may be a fair question to ask what isn’t made by injection molding. Injection molding can utilize metal, plastic, rubber, and glass as the molten material, which means almost anything can be made by injection molding At SEA-LECT Plastics we utilize plastic injection molding to make drink tumblers, outdoor adventure gear, military and automotive products, and even musical instruments.
How Does Dimensional Stability Affect Quality?
Most of the time dimensions translate into fit between components. If your product is a single piece that has no other parts, nor does it connect to anything else, having the correct dimensions doesn’t matter. It won’t affect sales, customer opinion, or your manufacturing process. If your components fit together to form an assembled product, having the correct dimensions is critical for product success.
Each individual component has dimensions that it must stay within in order to fit with the other components. That includes being too large, too small, and being the correct shape to correctly fit with the other components in an assembly. Imagine trying to put together blocks that don’t fit. The complete assembly doesn’t work together, and that can stop a manufacturing line or your customer from using your product all together.
What Can Affect the Color in Injection Molding?
Injection molding is meant to be an economical process to produce parts and products, but the never-ending challenge of faster and cheaper products can affect details like color. Changing process parameters, such as increasing or decreasing temperature and pressure, can affect the end color of your product. There are also different ways to color the plastic resin, which may be a pre-colored resin from a resin manufacturer or a blended resin you complete at the molding facility. Both can achieve the same color with a slightly different way to create the resin.
Color harmony is needed to ensure each component meets the specification. That means each component meets the intended color within a few shades, and it also is the correct shade compared to the other components in an assembly. Slightly varying color shades may not be a critical criteria for an economical product like a small household appliance, but it may stop a high-end product from shipping to the customer. Your intended product should show uniformity and consistency of color between all components, and overall meet the design intent.
How Does Gloss Affect Product Quality?
The gloss of a product is a small detail that can make a huge impact on the perceived quality of the product. High gloss used in the wrong place can indicate a cheap product to some buyers, while a dull finish may not accentuate your product features in the correct way. Having the right gloss in the right place can influence your consumer, and make your product look good visually.
The process parameters during molding can influence gloss to a certain degree. Too high of temperature may increase gloss, while the time in the mold could decrease the gloss level of your product. Materials used in the resin can also influence whether the gloss level is too high or too low compared to the intended specification.
Another key item that can influence the gloss of your product is the surface finish used in the injection mold. Understanding the intention and function of your product, and how the customer will use it, may help to determine what surface finish you should be choose for a particular gloss level. Multiple surface options can offer a texture and finish that will be durable and visually appealing, and still meet the gloss level requirements needed to serve the customer demands.
What Molding Defects Could Affect Product Quality?
Your consumer may or may not pay attention to molding defects, but they can affect the quality of your product. You may not know what defects can occur in injection molding, so here are the five most common defects that may occur:
- Flow / Weld Lines are lines or streaks in the final molded part. They can be off-color or discolored material compared to the intended color of the part. Most of the time they are caused by plastic resin cooling at different rates as it flows through the mold. They can occur where two sections meet in the mold, when the injection speed is too slow and allows the resin to cool too fast, or the wall thickness varies slightly between the sections meeting inside the mold.
- Sink marks are small indentions or depressions that can happen in thicker areas of an injection molded part. It can occur due to shrinkage of the material when cooling time is insufficient for the material to fully cool before being removed from the molding dies. Sinks marks can also be caused by low pressure in the molding cavities or too high of a molding temperature at the plastic gate.
- Short shots occur when the injection molded part isn’t complete after the molding cycle. The molten plastic does not fill the mold cavities, which results in a portion where there is no plastic. The final product becomes defective because it is incomplete.
- Burn marks are labeled as a discoloration that appears on the visible surface of the injection molded part. They may be a dark, a black, or a dark red / rusty color on the surface when the material is overheated and burns against the surface of the injection mold.
- Flashing is a term used for molten plastic that escapes from the mold cavity during injection molding. Typically it escapes through the parting line between the cavities halves or ejector pin locations. The flash occurs when the molding cavity halves are not held together with enough pressure to prevent plastic from escaping the mold and cooling.
Each of these defects can cause the quality of the overall product to be diminished. Your customer may not want to own a product that features extraneous lines, sink marks, missing plastic, or extra material on an edge. But that doesn’t mean that the defects are an automatic reason to discard product. These defects may be acceptable on an internal component that can’t be seen after assembly. The product specification should outline if a defect is acceptable, where it can be located, and how defective it can be.
Quality is a word that is thrown around a lot, used as a buzz word, added to brand slogans, but can mean a variety of things to different people and organizations. With quality in mind, a company needs to have a reference of measure to define what their quality is and how it is measured and managed. If you are looking for a new strategic partner, you may want to seek one that has earned an ISO9001:2015 certification. This is important as it shows a structured focus to a business or organization with defined metrics based around customer demands and regulation with examples of the defined structure in use in the organization. They can define what their quality is and show how it is measured.
SEA-LECT Plastics excels at injection mold design, tooling creation, manufacturing, assembly, and global logistics. We are a turnkey manufacturer that can organize a new product launch from concept to consumer. We have a complete turn-key option for your product, and we are ISO9001:2015 certified. When you’re ready to discuss your current or next project, or want to know more about plastic injection molding, contact us at email@example.com or give us a call at (425) 339-0288.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.