Mold Texturing Pitfalls and Solutions

The phone rings and your caller ID shows it’s your contact over at the engraving house. You answer, knowing it might be important and soon learn they have found some severe segregation in your tool cavity. You know this is going to be a problem because time is running short, and your customer is demanding grained parts for the next build. Don’t you wish there was a way to avoid this?

Tool steel inclusions can cause delays during engraving. Image courtesy of Standex Engraving Mold-Tech.

Even if you can’t always avoid segregation in your tool steel, there is a way to find out about it long before you’re at the point when options are limited. With some careful consideration ahead of the graining process, you won’t need to worry about these challenges at the last minute. Here are several preventive solutions to give you peace of mind.

Inaccurate scribes can be a showstopper

Inclusions within your steel are not the only issues that can cause delays during the engraving process. Another more common concern is the scribing needed on flat seal-off, also known as blind shutoffs. Without scribes, your engraver may be unable to accurately define some areas of the parting line. This can be a showstopper. Without accurate scribes, there are risks of an unsightly border, or worse, over grain.

Unlike steel inclusions and scribing, which can be addressed in a pre-grain time frame, there are other issues that can only be planned for, but not eliminated entirely, until after engraving has been completed and you are running post-grained shots in a mold press. One of these concerns is the dreaded scuff that occurs with plastic parts that are not cleanly ejected from the tool. This happens for numerous reasons. A few common causes are:

Related:The Troubleshooter: How to Fix Part and Runner Sticking in Injection Molding Processes

Defining expectations

The relationship between texture depth, structure, and draft is the hardest to anticipate. Image courtesy of Standex Engraving Mold-Tech.

While a slightly moving part or overpacked part may be correctable at the press, a part’s physical characteristics should be noticed during early feasibility reviews and a plan established that defines expectations. However, the relationship between texture depth, structure, and draft is by far the most reviewed, discussed . . . and hardest to anticipate. There are truly too many variables that contribute to scuffing, such as grain, material, gloss, temps, pack, hold times, etc. As a result, most OEMs have established their own parameters or rules that they use when participating in pre-grain mark-up events. These rules cover all situations regardless of material type, texture type, or gloss level.

The most common draft rules range from requiring 1.5 to 2.5 degrees of draft per every 0.001 in. of pattern depth. These are used in today’s market on shrink away conditions. When a part is shrinking toward steel, a common practice is to double the requirement. At Standex Engraving Mold-Tech, a global supplier of mold texturing and surface finishing services where I work, we offer the following solution.

Shrink away conditions

While we never attempt to present an engineering rule that would eliminate the possibility of scuff, we do believe strongly that the above solution would greatly reduce post-grain scuff in more than 90% of situations. Ultimately, we can recommend but do not have the ability to force this practice on the market. 

Before assembling the tool and mold first shots, best practices are as follows:

The number of post-grain issues can be significantly reduced when this pre-grain work is performed on each tool, especially on those larger molds, and if the previously mentioned draft rules were considered during mark-up events.   

Part gloss issues

Why do we have issues with part gloss? Your engraver finishes a mold using some mixture of blast media. This process not only provides the final cleaning, but the media is one of the major contributors to part gloss. This media can push both steel and part gloss toward a dull or glossy appearance. However, this mixture is not the only major contributor to part gloss: Blast media, material type, steel type, texture type, and the molding process all contribute to what level of gloss will be achieved.

Like baking a cake, you cannot know what it will taste like until all the ingredients have been added and baked. Well, when it comes to part gloss, you cannot know what level you will achieve until after the tool has been engraved and the production intent material is used at a home line press using a production-approved process. Most of the time your engraver will help you achieve a proper part gloss on a first attempt. This is because your engraver probably will use a mixture based on historical data. What was successful yesterday should be successful today. However, this is not always the case, and from time to time a tuning loop is necessary. 

To help reduce the need for a tuning loop, ask your engraver about testing “the ingredients” to determine what mixtures will provide the greatest chance for 100% success. If your project has multiple tools, it will be worth it.

Early bird tools

Also, having an “early bird” tool is a great practice. If you are working with a package of tools that will all have similar grain, material, and gloss requirements, get one moving early. Get this tool completed and molded, then present it to your OEM for a courtesy review to determine if it meets requirements or will need an adjustment. By doing so, you might have to make a change on the early bird, but knowing what the issues are will determine what is done to the many more tools to follow. Avoid the scenario whereby 15 tools have been completed before your customer ever provides any feedback on your gloss, resulting in a lot of re-work.