Injection Molding Challenges

Feb 13, 2024

Injection Molding Challenges

Plastic Injection Molding-Solutions to Common Challenges

Plastic injection molding is among the most dependable, versatile and economical methods of mass producing parts and products out of plastic. That said, the process is not without its challenges. As a prominent resource for injection-molded plastic products to a wide swath of industries, Darter Plastics addresses these challenges every day. In this article, we examine some of the more common complications that can arise during the injection molding process, along with the strategies we employ to prevent and correct them.

Flashing

Perhaps the most vexing issue with plastic injection molding occurs when molten plastic escapes the confines of the mold during the injection process. Upon cooling, the extraneous material remaining attached to the molded part is called flash, or flashing. At its worst, flashing can render a part unusable and lead to the degradation or destruction of the mold. Because removing even minimal flash from a part can be painstaking and time-consuming, it is far preferable to prevent flash from happening in the first place.

Strategies for Flash Prevention

Plastic can be forced from the mold for a number of reasons. It may be too viscous or injected with too much force. The mold itself may be improperly designed, in poor condition, or not clamped correctly. The best way to prevent flash is to get all the variables–clamping, viscosity, velocity, pressure, temperature, etc.– adjusted properly, monitor them constantly, and readjust them as needed throughout the run. Still, some flash is inevitable and being able to effectively and efficiently remove it is critical to a successful operation.

Methods of Flash Removal

Removing flash can be done multiple ways. The best option depends on specifics such as the degree and location of the flashing on the part, the amount of parts affected, the importance of aesthetics, and other factors. The most common methods of flash removal include:

  • Manual Removal: While labor-intensive, removing flash by hand is the most versatile option and generally yields the best results. In most cases, however, it is not the most cost-effective option. 
  • Heat: Heating the flash can melt it back into the surface of the part. This works best with fine flashing, usually on the parting line. 
  • Cold: Cryogenic freezing makes flashing brittle and easy to remove. Though very effective, this method requires dedicated equipment that can be prohibitively expensive.

Jetting

A snaking pattern of ripples across the surface of an injection molded plastic part indicates a problem called jetting. Jetting occurs when the plastic passes through a gate in the mold with too much pressure behind it. Rather than flowing smoothly into the chamber, it is forced through as a jet of plastic that snakes back and forth, cooling immediately into the snaking pattern as it hits the cooler metal of the mold.

Preventing Jetting

Jetting can be rectified by moving or enlarging or repositioning gates, increasing the temperature of the melt and/or the die, or adjusting the injection speed.

Shrinkage & Warpage

When a molded part has cooled and is ejected from the mold, it may display warping. Generally, warping happens when the part cools unevenly, shrinking more in some areas than in others.

Preventing Shrinkage & Warpage

Shrinkage, and therefore, warpage, can be prevented by controlling processing conditions. The ultimate degree of shrinkage can be controlled by managing the pressure and rate of the injection, as well as the proper placement and size of the ejector pins.

Weld Lines

When plastic is injected into the mold, it may flow in different directions, forming multiple flow fronts. As the void fills, those fronts inevitably join. If the material is too thick, flowing too slowly, or already cooling, the point at which they meet may form a visible mark (a weld line) on the surface of the finished part.

Solutions for Weld Lines

Weld lines can be avoided by lowering the viscosity of the molten plastic, increasing the injection velocity, or raising the temperature.

Gas Traps and Bubbles

As the molten plastic flows throughout the mold, air or gas is displaced. If that air has nowhere to go, the plastic cannot displace it, and bubbles form within the part, rendering it unusable.

Preventing Bubbles

Proper venting of the mold will allow air to escape as plastic flows in to fill the void completely.

Uneven Dimensions & Quality

When supposedly identical parts are emerging from the molds with inconsistent quality and dimensional discrepancies, there is a problem with the flow of material.

Achieving Consistent Quality

The distribution of the material through the mold is determined largely by the gate design and positioning. To achieve thorough and even material distribution to all parts of the mold, injection pressure and speed must be properly adjusted.

Short Shorts

When parts are only partially formed when ejected from the molds, it is called a short shot.

Solutions to Short Shorts

When molds aren’t filling completely, it must be determined why. Either an air pocket is forming in the cavity, keeping the plastic from entering, or the flow itself is too weak, too slow, or obstructed. If the flow checks out, move to add ventilation to the mold.

Clearly, most of these and other common problems encountered in plastic injection molding can be addressed by adjusting the parameters of the process. Pressure, velocity, viscosity, ventilation, gate placement, etc. Other factors come into play as well, such as the make-up of the material being injected, the design and condition of the molds, and more. Finally, most of these issues can be avoided by following the tenets of Design for Manufacturing (DFM), a practice whereby products are designed from the outset with the aim of simplifying and streamlining their production.

Darter Plastics offers acknowledged excellence in all aspects of plastic injection molding, including  mold design and production, prototyping, and more. Find out more at Darter.com.

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