Molds and Tooling: The first step towards plastic injection profits

Many processors have experienced the frustration of a poorly performing plastic injection mold. Poor process control due to tooling failures can be one of the most aggravating situations a molder has to face, especially when the contributing factors have been identified, yet the true nature of the problem is ignored. Mold design, maintenance and modifications are a plastic company’s first line of defense in a successful molding strategy, and one of the most common failures in an active response molding ideology.

This article outlines specific tooling problems, which, when addressed successfully, can not only improve but eliminate processing issues leading to high scrap and low efficiency situations in which tooling is directly responsible for control failures.

Mold design
One of the first areas of concern is moldability. A part needs to be designed with the intention to be molded successfully. John Bozzelli points out, “Many tooling failures are often blamed on poor steel or design but you have bring in that often the plastic part was not designed for moldability.” It is absolutely paramount that when the time comes to design a mold based on your product, the design team includes the designers, toolmakers and processors, as well to assure that the end part design is feasible in terms of process control. The more diverse your design team is in terms of technical expertise, the better the company is able to identify potential failures long before the mold is being built. The layout of your design team is every bit as crucial in tool design as the mold design itself.

In addition, MoldFlow is a key design tool that should be used in your part design long before cutting steel. MoldFlow software will help to establish how the flowfront will react as it moves through the runners and cavities. In addition, it is a great tool for identifying cooling-related failures that may arise from the cooling configuration of the mold. Failing to recognize and utilize simulation software prior to building a mold is like running through a room with the lights off.. dangerous and more prone to failure than a concentrated effort to eliminate potential problems long before a mold is sampled and the initial process is determined.

Ed Faubert, the manufacturing technical lead at CTS Corp. points out, “Failure to shut off properly around terminals, inserts, etc. is a constant issue with regards to insert molding. Sometimes this is related to tool design, or it can be a failure of the insert supplier to maintain consistent dimensions.”

“Primary tooling failures can also be attributed to designers who have not yet been able to understand how plastic thinks. No draft, failing to radius wall intersections, and building the tool to shrinkage call outs on data sheets rather than keeping the tool steel safe can be reasons for plastic injection tools to perform poorly.”

Material choice and use are more key considerations in tool design... the materials you choose to provide a specific part functionality (such as strength, flexibility, weight, aesthetics, etc.) must be addressed for their potential behavioral benefits and limitations. For example, nylon and polycarbonate both boast superior strength capabilities, yet each is unique in terms of molding approach and potential processing limitations due to tool design. A company’s material supplier should be knowledgeable in the materials it produces and sells, and it must be willing to work with its customers when material-related issues arise. In addition, it is important to recognize that switching material suppliers midstream for the purpose of cost-savings can be self-defeating in terms of processing consistency, and vendor service/cooperation. 

Design your mold based on a specific material's processing properties and handling, and if you make the  decision to incorporate a new material, approach it as if you have built a brand new tool and are sampling it for the first time. Assuming that one material is the same as another because it is the same base resin can lead to disaster in a mid-production changeover so do so by sampling the material first prior to incorporating it into your production system.

Mold fabrication
The process of building your mold is a crucial component in the overall feasibility of using the tool in a full production setting.

It is important to understand that tooling can be costly in terms of repairs and unexpected breakdowns. A company that considers all potential failures prior to full use has the ability to engineer success into their mold, rather than frantically reacting to tooling repairs or replacement largely due to poor construction or design.

It is important to recognize that tool steel choice is critical in terms of mold engineering. Moving components should be built using an awareness that wear is expected and addressed as such. Trying to save money by using inadequate tool steel with a low Rockwell hardness not only leads to unnecessary mold repairs, but contributes to unexpected downtime and increased costs. When moving components are prone to failure, build the mold with the goal of in-press repair (when possible) and keep the necessary replacement components on hand to improve your ability put the mold back into a state of production readiness quickly.

Take into consideration the complexity of the tooling as you build it. Vent areas properly where gassing or stalling flowfronts are likely.

Examine and address the potential of knitting failures on the lee-side of obstructions the flowfront must flow around as it moves through a cavity. Remember that ribs, although they add to the rigidity of a part, are potential failures in terms of part ejection. When ribbing it is necessary be sure to add draft to these areas to help with part extraction. Be generous in terms of the number of ejector pins and pin diameter in rib areas to reduce pulling molds in the middle of a production run to replace a broken pin.

Support pillars can be another neglected concern. Insufficient plate support can lead to poor deflection when cavitation is regularly placed under high pressure, leading to increased parting line repair. Build your mold with the understanding that pillars do not add to cost; they promote savings by reducing your repairs and improving overall part integrity.

Lastly, understand the importance of monitoring moving mold components. Take advantage of sensor systems available for ejector plates, inserts and cores. These switches will help to not only prevent mold repairs due to component failure, but also help identify potential problems before they become costly in terms of downtime and the maintenance costs associated with major tool damage scenarios.

Mold maintenance
It is very important to understand that how the tool is handled on the shop floor minimizes possible tooling failures. A mold's life expectancy is only as strong as the preventative maintenance system a company develops and enforces. Insufficient mold maintenance and setup procedures promote tooling failure and breakdowns. Utilize the strength of your tool room personnel to train processors in all the steps needed in terms of mold preventative maintenance, so when the molds are in their care, they are maintained properly and with the proper safeguards in place to prevent unnecessary repairs and promote tool life.

Be certain that both your set-up and processing crew understand the importance of the press protections provided by the machine manufacturer.

Setting the high pressure close position based on mold touch is extremely important. In addition, using the lowest required close-slow pressure greatly minimizes the potential of mold damage due to stuck parts, or failing moving components.

When cleaning, use an approved mold cleaner. Avoid using rags and mold cleaner on polished surfaces, instead using a tissue and a mild metal polish. Do not apply mold cleaner directly to the surface being cleaned... it is a better practice to apply to the cleaning rag or tissue. The purpose of mold cleaner is to break down surface grime, oil or build up. By spraying mold cleaners directly on tooling,  you can cause moving parts to lose lubrication (a primary cause for galling), or create a scrap condition due to “bleeding” oil lubricants.

Pay close attention to vented areas. If a vent exists, it is important to recognize that it was are installed to allow gas to escape from the cavity. Clean vents liberally and inspect them for wear/ damage. If damage or wear exists, notify tooling personnel of the issue.

While cleaning, inspect tool for damage and wear-causing conditions, including lack of lubrication, galling, broken components, etc. Check clamp set-up for potential errors that could cause future problems as the mold is running.

At press shutdown, spray the mold liberally with a rust preventative. Again, inspect for damage or potential failures. NEVER put a clamp under high pressure at shutdown, unless you are in the process of changing the mold. Instead, touch the mold halves together to help protect the mold from moisture, and shut down all water sources to and from the mold.

In the case of a hot runner mold, it is important to remember to turn off the hot runner to prevent material degradation in the mold. Purging through the hot runner with an inert and low viscosity material is good practice prior to full shut down. It is also good practice to shut the hot runner temps off as the press cycles, if possible. When the mold begins to run short shots, the press can be removed from cycle. This helps to reduce the amount of time that material sits idle in the hot runner. It also reduces the potential of degradation, and the need for unneeded mold teardown/ servicing.

By addressing the needs for these primary tooling concerns, a company can reduce the costs of tooling substantially and improve its overall approach in process control. Tooling is the first step towards a successful production approach. Failing to recognize the importance of tooling in any active response facility is destined to not only override a facility's goal of low-to-zero scrap and high efficiencies. Instead, it will likely lead to manufacturing chaos and emergency responses in the middle of production runs. A company is only as strong as the tooling systems it identifies, implements and enforces. Strategic design, solid builds and a long-term preventative maintenance system not only extend the life of your mold, they will increase the overall profitability of your molding operation. 

About the author:
Garrett MacKenzie is the owner and editor of www.plastic411.com. Mackenzie started in plastics at the age of 19 as an operator, eventually moving up through the ranks to engineering over a 27-year timeframe. The positions he has held include: material handler, warehouse, QS9000 auditor, maintenance, warehouse coordinator, tool & die apprentice, set-up, process technician (1993) and robotics set-up/ programming