plastic injection molded components

(Updated May 2019)

Flash is a common injection molding defect, and is a thin layer of plastic that flows outside of the cavity, typically where the two halves of the injection mold meet. Flash can appear on the part’s edge along the parting line of the mold. It can also occur at ejector pin locations and anyplace where the mold has metal meeting metal to form the boundary of the part.

Flash cools and remains attached to the final product. Injection molders must remove unwanted flash by a process called deflashing which may involve operations personnel or robots trimming the excess material in between cycles. The presence of flash usually results in a longer, more labor-intensive process, lower product quality, and ultimately damage to the mold – all leading to higher costs! For these reasons, the root cause of flash issues needs to be identified and resolved.   

Flashing on a part can occur for several reasons - from variations in the process to material to tooling trouble. Flash during the injection molding of a part can occur during the filling (first injection stage) or during the pack/hold (second stage). It can occur with conventional parts - and with thin-walled parts. Below are some causes – and remedies – for flash.

 
Preventing Parting Line Mismatch With The Tool

The best way to prevent parting line mismatch in the tool is to clean off any plastic, dirt or contaminants that could keep the mold from closing properly, especially behind slides and inside the leader pin bushings.

Solving potential flash issues means ensuring correctly fitting mold plates by making sure the mold is sealing properly and cleaning the molding press from any obstructions.  Since molding pressure can deform mold plates during injection and packing, sometimes pillar supports need to be added and mold plates need to be thickened to reduce or eliminate flash.

The mold plates should have adequate surface area that seal off at the parting line, evenly distributing the clamp force during molding.  This is especially important in the area around the part cavity, where a sufficient "land" of seal-off area should be provided.  Any situation where the seal-off area is uneven or inadequate should be avoided. 

Flash Can Be Caused By Improper Venting 

If flash is present, the mold should be checked for adequate vents. Check with the supplier of the plastic resin being used for their recommendations of vent size.  On older tools that are showing wear, vents may have to be re-machined.

The viscosity of the plastic being molded determines the depth of the vent.  Stiff materials can utilize deeper vents - but fluid materials require thinner vents. In either case - the concept is to remove air from the mold as fast as possible with as deep a vent as the material viscosity will allow.

Clamping Pressure Challenges

If the part passes the first stage short shot test without flash, the pack/hold phase may be pushing the parting line apart.  Increasing the clamp tonnage may be the solution – the clamp pressure needs to be strong enough to withstand the forces generated by the plastic material flowing through the mold. Size the mold to run in the proper machine to ensure that adequate clamp pressure will be used.

Sprue Bushing Length

The sprue bushing in the mold can also create flash.  When the press nozzle seals on the sprue bushing, it can create forces from 5 to 15 tons.  If there is inadequate support in the mold for the sprue bushing, this pressure could transfer to the parting line.  Flash could also be caused by thermal expansion in the sprue busing.  If the face of the sprue bushing seals off on the parting line, nozzle pressure or thermal expansion will hold the mold open, and flash will result.

Adjusting Process Variables to Avoid Flash

If the material viscosity is too low, flashing can be the result.  There are several reasons for this; too high of a melt temperature, excessive residence time which causes degradation, the presence of moisture from improper drying, or an excessive amount of colorant that contains a lubricating vehicle. 

Over-packing, which should be avoided, causes increased localized pressure in the molded part. Sub-optimal molding conditions, due to material viscosity, injection rate and the runner system can reduce pressures and shot size to the minimum required. Ideally, a shot size would equal 50% of the capacity of the barrel.  However, this depends on the material being used.

As this ratio drops the time of residence for the material in the barrel, this pressure increases and the material gets hotter. It will flow much easier and enter areas where it could not at its normal viscosity. 

Reduce the injection and packing pressure to reduce the clamp force requirement.  If too much pressure is used, the press may not be able to hold the mold closed. By reducing the injection pressure, the tendency for the material to flash is reduced.  Also, reducing the area where the two mold halves are touching will focus the clamp-force and allow less clamp tonnage to be used. 

Decrease the barrel temperature and nozzle temperature – high barrel temperatures increase the temperature of the material, making it more fluid than it should be.  It can then enter small openings and crevices that it could not normally enter at the correct viscosity. By reducing the barrel temperature to the recommended material temperatures, the material can be heated properly through the different stages of the barrel.

Reduce the feed setting (stroke length) to reduce metering (over-fill).
If the total cycle time is too long there is a good change the material is overheating in the barrel. This means you should reduce the cycle time. Reductions to the cycle time can be made during the cooling phase. 

Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick–walled products can use a material that has a stiffer consistency. Be sure to utilize a material that has the stiffest flow possible without causing non-fill.

Crescent has been manufacturing injection molded components for over 70+ years and has a rigorous process for designing tooling to meet our customers’ specific needs, and optimizing operations to avoid product defects such as flash. 

For additional information, please click below to get our white paper “How to Overcome Common Design Concerns for Injection Molded Components”. This white paper addresses some of the key concerns in the design step that are a 'must' to consider.

How to Overcome Design Concerns for Injection Molded Components White Paper

Topics: custom injection molding