The Benefits of Quality Control in Custom Injection Molding

Published on May 19, 2023

Quality control is imperative to modern day industry. Like other manufacturers, custom injection molding companies require programs that can manage quality in their facilities to identify and correct defects earlier in the fabrication process. This in turn streamlines production and enhances efficiency. By ensuring the quality of their products, these businesses build a positive reputation and ensure higher levels of customer satisfaction.

Quality control also applies to the service a manufacturer provides its customers. Injection molding companies, like any other manufacturer, must differentiate themselves from their competition. To properly commit to the quality of the products they produce, these businesses must invest in quality control as an ongoing process. Custom injection molding relies on a number of quality control measures to improve operations and meet customer needs, and understanding how best to improve quality will likely improve a company’s competitiveness.

Quality Control for Custom Injection Molding

A relatively simple process, injection molding has been around since the nineteenth century. Used today to mass produce complex components in a consistent manner, the injection molding sector is increasingly tasked with making parts that are also aesthetically pleasing. To make products that are more appealing to consumers, the industry seeks to advance custom injection molding techniques and technologies through research and development.

The custom injection molding process is often the most efficient way in which to fabricate these complex components, as it allows for the making of hundreds or thousands of parts from a single mold. This means costs can be driven to just a few dollars, or less, per unit. Since the process involves using the same molding tool throughout, it also provides consistent quality. Yet there can be issues with parts made via the custom injection molding process, depending on the material used, how long it sets, temperature and other factors.

Part Design and Software

Quality in custom injection molding starts with good part and mold design.  Quality systems and Scientific Injection Molding processes can only be effectively used when combined with good part design that considers potential defect areas prior to production.  While hardware has advanced for the custom injection molding, so too has the software. Computer-aided design (CAD) and computer-aided manufacturing (CAM) software bring precision to injection molding processes, helping to ensure better results and keeping defective parts to a minimum. CAM software helps manufacturers develop processes that correspond to component designs, whereas CAD software is used in conjunction with 3D models for more detailed engineering design. Using these tools to analyze material and mold flow properties prior to finalizing part and tool design helps manufacturers build better molds more cost-effectively and with quicker lead times. Use of these tools can help identify potential trouble spots in the part and mold that can lead to common injection molding part defects.

Quality issues that manufacturers look for in custom injection molding include:

  • Flash: In custom injection molding, a flash is a thin layer where two halves of a mold meet and plastic flows outside the cavity. When two components fuse together to a point where they cannot be easily separated, this creates a flash. They occur along the mold’s parting line, appearing on the edge of a component, or with parts at the ejector pin or other points where the molded material meets molded material.
  • Knit lines: This is the point at which two different areas initially come together in a mold, and need not be visible. Knit lines may be a sign of a possible structural issue or simply a cosmetic blemish, though those that aren’t noticeable may pose no problem at all.
  • Short shots: An effect from not using enough material in a mold, it results in an incomplete component due to a mold cavity not being filled completely with molten material, often resulting in soft corners or chipping. This problem may result from the material being overly viscous, unsatisfactory discharge of material into the mold, hard-to-reach areas within the mold cavity, blockage of flow channels that prevents filling the mold cavity or injection pressure and speed being too low.
  • Sink marks: The most common defect in custom injection molding, sink marks refer to a localized indentation that occurs on the outer skin of a plastic component as it cools and hardens. Generally sink marks appear on thicker areas of the molded object due to variability in cooling rates or inadequate cooling, though high gate temperatures or low pressure in the cavity can cause this issue as well.
  • Weld lines: These refer to color changes, lines or notches on the molded component resulting from the point where two separate flows of molten material converged. Where these flows of molten material meet and rejoin are known as “welds,” and they frequently form seemingly insignificant lines on the component. However, this point of interfusing material where flows rejoin can reduce a component’s strength.
  • Burn marks: These marks appear on the surface as discolored areas and are typically deep red or black in color. Also referred to as gas marks, these occur when plastics are left in the molding cavity too long, or when compressed, heated air within the mold isn’t able to escape and causes the air to build up within the mold, scorching the plastic. The most common causes of burn marks include temperatures that are too high, overly fast injection speeds, gating that is too small to allow sufficient gas venting or material leftover from a previous piece that’s overheated.

Parts made via custom injection molding should have a uniform appearance from one part to the other. Yet no manufacturing process is free of error, so a system must be in place to inspect components prior to their shipment to customers.

Advantages of Quality Control in Custom Injection Molding

From a manufacturer’s perspective, maintaining a high level of quality leads to happier and more satisfied customers. Yet quality control measures also help encourage factory personnel to ensure the quality of their own work. Teaching the importance of quality control to workers will incorporate the idea of quality throughout the fabrication process, encouraging employees to better manage the quality of their work.

In custom injection molding, as with other industries, a well-managed quality control program reduces production costs by decreasing defects during fabrication. It also reduces material waste and other inefficiencies, as fewer parts will be rejected with effective quality control measures in place. Though the main aim of quality control in injection molding looks to prevent defects, these will still occur, so manufacturers must have measures set up to handle them.

Developments in Custom Injection Molding Quality Control

Though the first injection molding machine was developed in the late 1800s, the 20th century saw injection molding process increasingly used in the manufacturing of precision components. Multiple factors, however, can affect the performance of these molded parts, and advances in quality control methods have helped manufacturers improve their own processes. To ensure consistency throughout the molding injection presses, it’s important to understand the factors that can cause issues during production.

Factors influencing the custom injection molding process include:

  • Barrel temperatures: These directly affect how material flows throughout the mold, which in turn affects the mold’s temperature. For this reason, barrel temperatures need to be monitored very carefully so that they’re between the material’s thermal decomposition temperature and melting point. When temperatures are set too high, overflow and flash result. When they’re set too low, it results in unfilled molds, slowed flows, melt lines and even rippling.
  • Material cooling rates and times: The differences between the material melt temperatures and mold temperatures directly affect cooling rates. For this reason, a range of mold temperatures became established for each material used by manufacturers, with consideration given to customer recommendations for certain components. Cooling times are also established based on mold and component designs, along with the materials used, to assure that components are properly sized, have the desired properties and have an aesthetically pleasing appearance.
  • Material flow rates: When it comes to custom injection molding flow rates, especially for heated plastics, the material needs to be injected as quickly as possible until the molds are nearly full. Flowrate will determine how viscous the material will be as it flows into the mold cavity, with the pressurization affecting the size and aesthetics of the part. When pressures are too high, this results in flash or overflow, whereas when pressures are too low there will be voiding problems.
  • Material melt temperatures: To accurately predict a material’s melt temperature, it’s important to consider the temperatures of the barrel, nozzle and the mold itself.
  • Material pressure: Also called “screw back pressure”, they’re controlled via relief valves within the hydraulics of the system. During the modern custom injection molding process, pressure applied to the molten material occurs when the screw head moves forward, a process now controlled by automated systems. Viscosity, or the ability for a fluid to flow,  increases as injection pressure rises, which can result in flash or overflowing if it gets too high. Pressures that are too low result in voids or bubbles.
  • Nozzle temperatures: These should always be lower than that of the barrel; if they’re too high it will result in material decomposition caused by the molten material dribbling into the nozzle. This can sometimes cause material to block the nozzle, resulting in defective parts. Mold temperatures impact the cooling speed and flowability of the material, as well as the performance of the completed component.

Along with these physical factors that manufacturers can directly control come other dynamics that can affect the quality of a molded part during the custom injection molding process. These include humidity in the environment and seasonal changes, along with material resistance in flow from the mold’s exhaust system, changes in material between batches, human error and a manufacturer’s quality management system. All these can impact product quality significantly.

ISO Quality Control

The International Organization for Standardization (ISO) provides guidelines that support the quality of a manufacturer’s products. For custom injection molding, ISO 9001 established guidelines are used to certify that a company has certain controls in place regarding continuity, efficiency and integrity, along with a culture that seeks to constantly improve its methods and products.

The current ISO standard revolves around these seven fundamental tenets:

  • Customer focus: This concerns identifying and meeting customers’ requirements, along with ensuring their satisfaction.
  • Engagement with people: Through engagement with all stakeholders regarding objectives, businesses can enhance collaborative efforts, developmental involvement, satisfaction and other activities that advance their objectives.
  • Evidence-based decision-making: To be successful, businesses must base decisions on reliable and pertinent data, so should establish consistent processes to gather, evaluate and explain this data.
  • Improvement: This looks at what actions a business takes to improve upon their products and services, along with the products they make.
  • Leadership: Motivating employees to work towards a common goal is a skill that involves planning and strategizing to meet the needs of a business.
  • Process approach: To function well, businesses must manage numerous activities that are linked to one another in order to transform resources into processes that result in viable products.
  • Relationship management: Augmenting company performance by responding to commercial opportunities and checks on business activities by establishing relationships, especially regarding supply chain management.

Being ISO 9001 certified shows potential customers that a company’s products and services meet certain requirements. Though it’s not necessarily mandated in every instance or industry, the ISO certification process confirms that a custom injection molding provider maintains a certain level of quality control.

Founded in the 19th century, Spaulding Composites has a solid reputation, along with a wide range of experience in custom injection molding going back nearly half a century. To learn more about quality control in injection molding and how we can help, contact the experts at Spaulding today.