Topics related to plastic injection mold design: important factors to consider and challenging dilemmas to resolve when designing cost-effective, quality tooling. Contributed by various employees of Matrix Tooling, Inc. & Matrix Plastic Products.
3D Printer Provides Multiple Benefits
Recently, Matrix purchased a 3D printer for our mold design & engineering department. One main reason we did this was to offer in-house rapid prototyping services to our customers, 70% of whom are medical device OEM’s who rely on us to help bring their cutting-edge products from concept to reality. Being able to take their data model and provide them with physical part samples they can hold in their hand is a value-added service that is especially helpful during the R&D phase of a new project.
The majority of our work is in developing and producing complex components, so we needed a unit that would be capable of printing the high-quality, finely detailed models we work with every day. We selected the Objet30 which features a range of 5 printing materials, varying in physical and mechanical properties (strength and flexibility), and available in a choice of 4 colors. With this new capability, our designers can test out customers’ concepts before time and money are invested in production tooling.
But there is another significant benefit to having this additive technology under our roof.
Matrix is very fortunate to have a group of very creative people working here, in many areas and levels of the company. Those people will use a tool like this to experiment and develop unique ways of making things. With their talent and this technology, the possibilities are endless and exciting!
As an ISO 13485 certified manufacturer, managing risk is always a priority. And the most basic concept of that management process is first understanding what those risks are.
Recently, the question was raised: "Can any trace mold corrosion be transferred to the final molded product?" This opened a spirited discussion because, while we've been building injection molds for over thirty years, nobody here had a definitive answer.
After consulting with others in the industry, as well as a metallurgist at our steel supplier, we came to the conclusion that while it's theoretically possible to transfer any contaminant from one surface to another, having a problem with bio-burden testing or introducing a contaminant into a product during plastic injection molding production is highly unlikely.
We are involved in a program that overmolds stainless steel with plastic. A stamped steel piece and a metal injected piece are both loaded into a tool steel mold, then overmolded with a high temperature nylon. The stamped piece is passivated; the MIM piece is as-molded and sintered. In speaking with our metallurgist, we learned that the main sources of corrosion on the mold cavities would be degraded resin coupled with high mold temperatures. Fortunately, as we are the molder on this program, we have control over both of these issues. Our processing and mold temperatures are both within manufacturers' specifications, so that helps control a major source of any corrosion. The metallurgist also explained that any corrosion sufficient enough to create a transfer problem would be visible to the naked eye.
The material we chose for this program was a high hardness, general purpose tool steel. We did this due to the fact that two metal inserts are being inserted into the mold cavity for overmolding . There are other grades with similar hardness and stainless properties available, but based on what we learned we do not feel it is necessary to change steel grades at this time. However, we have decided to add a visual check to our regular preventive maintenance plan for this job, a solution we now feel is sufficient to catch a problem before it occurs.
Product designers/inventors can spend countless hours developing their concepts without fully understanding the manufacturing process that goes into mass producing their parts in the most cost-effective manner. Matrix Tooling, Inc. & Matrix Plastic Products excels at consulting with our customers, from Fortune 100 companies to individuals with breakthrough ideas, to provide manufacturing insight during their product design stages. Sometimes we begin with nothing more than a simple sketch concept of a part. Often, there can be multiple ways of getting from that initial concept to the finished product; our job is to explain those options to our customer and help them select the one best suited to their application.
Several factors can impact the ability to design & build a functioning tool that will efficiently produce a plastic part. Something that might seem like a simple feature might actually involve complicated tooling mechanics to achieve in the molding process. As the custom manufacturer, it is our responsibility to point out the most economical way to accomplish this. So when customers share the mechanical requirements and intended use of their product with us before the part design is frozen, it allows for some discussion of which features are set in stone and which may offer some flexibility. This makes it possible for us to pinpoint suggestions that may significantly impact tooling and/or production costs.
Take snaps for example. Snaps can be achieved by various means. Coring an opening through the part, allowing for the removal of the trapped plastic, is the least expensive option. Depending upon the application, this may be a perfectly acceptable choice. However, if this is not desirable from an aesthetic standpoint, snaps can also be achieved mechanically using lifters or slides, which add more expense and require more labor (as well as more mold maintenance down the road in production.) Both tooling methods will create the snap, but ultimately the customer’s needs and budget will determine which route we take in designing the mold.
During this critical stage – before significant investments of time and money have been made - is the time to consider the many options that affect the function, lead time and cost of tooling: gating location, parting line location, draft, processing behavior of the selected resin , just to name a few. Involving Matrix at this point is a win-win because it ensures the most efficient use of both parties’ available resources: we help confirm that the part design is within an acceptable range of manufacturability, which helps our customers avoid misunderstandings and costly re-designs down the road.
The basic process of creating drawings to represent objects has been around since the days of the caveman. In the world of manufacturing, this process began with basic 2D drawings known as “blueprints” that showed three basic views of an object: Plan, Front and Side. If additional views were needed (inside, outside, isometric, etc.), each of these views had to be created separately. The designer had to first be able to visualize the whole entity in order to project each of the necessary views onto a blueprint. Others could then read the blueprint to view and understand the whole entity.
Machine operators studied blueprints and extracted the information they needed to ultimately produce a physical object that matched the views shown. They entered coordinates and determined cutter types & sizes, drill bits, taps, etc., and then began machining.
The time required to create a 2D drawing was relative to the complexity of the part or the assembly of parts. A very simple part could be four lines that create a square, and if you need some through holes in the corners you could add four circles. Notes and dimensions were added as required. Creating an assembly required multiple sketches or drawings. This could either be done by sketching on paper or using a drafting board. When multiple copies were needed for distribution, paper sketches were copied by hand using see-through paper whereas drafting board drawings could be duplicated more quickly by a blueprint machine.
Then came electronic 2D design with CAD software. This enhancement allowed a company with computer monitors throughout its facility to give everyone involved access to the drawings, making hard copy paper drawings unnecessary! Any revisions to the original CAD drawing were automatically viewed by everyone opening it on their computer. Electronic design software presented a distinct advantage because there everyone had access to the one and only “master” CAD model. When changes were made to this master model, everyone instantly had access to the update. This saved the time and expense of having to manually revise multiple copies of paper drawings.
From a manufacturing standpoint, 2D electronic data can be used to generate manufacturing programs that drive a machine tool to follow a given outline. Since it is limited to XY vectors, 2D data is sufficient for Wire EDM (Electrical Discharge Machining) and the majority of all through machining applications, holes, slots, window pockets, spline shapes.
The introduction of 3D electronic data made it possible to represent a solid object in 3 dimensions. It includes XY and Z vectors (or IJK vectors). The benefits of creating a 3D model are numerous.
Once the 3D model is created, it can be viewed by multiple people as if they were holding the physical object in their hands. It is a solid body with volume, mass, internal and external features, and it can be rotated to any viewpoint allowing you to extract the information you need. At Matrix Tooling, Inc. our designers use NX software (formerly Unigraphics).
The time required to create a 3D solid model is dependent upon its complexity, and 3D solid assemblies of multiple parts can also be created; an automobile assembly, for instance, might be used for display, sale, mechanical function, or aesthetic purposes.
The availability of 3D data has virtually eliminated the need for any 2D drawings in manufacturing, although some customers will still ask for them. It takes considerably less time to create views on a drawing using 3D data; it’s just a matter of placing canned or custom views on a drawing that are linked to the 3D solid model or assembly. The views are always to size and if a revision is made to the model, the drawing views are updated automatically.
Manufacturing using 3D data allows a machine operator or a CAM specialist to generate any type of machine path, limited only to the machine tool’s axis – be it 2D or multiple axis.
While 3D software packages are certainly more expensive than 2D systems, we have found that the benefits far outweigh the costs. In our business of designing & building complex plastic injection molds, 3D design has not only helped us become a leader but will also play a critical role in maintaining our advantage.
Since we've added web conferencing several years ago, it becomes more and more evident how this tool significantly improves the design / build process as costs are scrutinized and deliveries compressed. One recent program stands out, a stapling device with numerous metal and plastic parts that were activated by a series of gears and pulleys. Our initial design review with the customer using our web conferencing program allowed us to review the entire assembly get an overview of the device with a diverse group of Matrix personnel. Representatives from our design, manufacturing and quality areas all reviewed the device from their own point of view. And with the convenience of a voip phone call, our marketing manager attended the meeting remotely. During the review, suggestions were made to the customer that allowed them to eliminate several parts by redesign of the current assembly. Parts were combined, reducing the part count in the assembly. Slightly more complicated tooling, but far less costly in the long run. The customer immediately embraced those suggestions, as their COGS target for the device was going to be difficult to achieve. The savings our suggestions allowed gave them an immediate benefit. And, during the review, a fundamental design flaw was flushed out when this group of a dozen technical people got into a spirited discussion on the mechanics of the device, which was corrected within days. And as our mold design work was firming up, we held a concurrent review of both tool and product design, which saved significant time. Mold design (ours) and device design (theirs) were being toggled back and forth, with mods to both being made as the meeting continued. A very fast and productive use of time, for sure.
- Paul Ziegenhorn
A thermoplastic injection mold is like most anything you buy in life; you get what you pay for. If you want a throwaway mold with a limited life expectancy that produces simple parts and allows for generous dimensional and flash tolerances (and may require post-molding defect corrections like flash trimming), then by all means purchase inexpensive tooling from a low-cost supplier. But if factors like part consistency, uptime, conforming to quality standards, on-time delivery, low maintenance costs, long mold life, and fewer headaches are important to you, you’ll likely want to consider buying a quality mold upfront.
An injection mold is not a small purchase to be taken lightly, even for a tiny plastic part produced by a large corporation. It should be viewed as an investment, with each running cycle giving back a portion of your ROI.
For many of the molded parts of bygone years, an inexpensive mold might have been sufficient. Times have changed though and products have become more demanding. Their geometries and resins have demanded a more complex, precise and robust mold. An inexpensive mold won’t be able to give you these parts, at least not for long. What good is a cheap mold that breaks down in the middle of a production run, fails to make in-tolerance parts, or runs slower than the calculated cycle when the customer needs a steady stream of good parts promptly and consistently?
There will always be a place for simple and cheap molds in certain applications, but if there is any complexity to the part or tool, it would be foolish to build and design based on price alone. Overseas low-cost providers are an option, but that opens up potential issues with communication. Not only due to language problems, but time zones, local customs, and general business practices can add on top of that. Logistic issues and rising transportation costs should also be considered.
Reputable mold builders stake their reputations on every mold they build. They want a robust mold, built correctly with the best materials, that doesn’t come back for repair or adjustment. They want the customer to be there if at all possible for design reviews and samplings. All the teleconferencing in the world can’t take the place of personal meetings at times. These personal meetings are with the mold maker’s technical staff and design specialists, not some sales rep or consultant for a cheap offshore mold builder.
Often, time to market is critical, and control of the project timeline is not always possible with an offshore supplier. When a cheap mold is late, produces out of tolerance parts, or breaks down, its low purchase price suddenly becomes very expensive. Many times a cheap mold that doesn’t perform like it should can end up being more costly to correct than a more expensive North American mold would have been in the first place. Losses in time and productivity are often just as costly and are even harder to recoup.
When the whole picture is looked at, you can see that in the purchase of an injection mold the old adage of “you get what you pay for” holds so true.
Senior Process Engineer (Older Molder)
Of great interest to buyers, accountants, quality managers, toolmakers as well to, of course, molders, is the projected service life of an injection mold for thermoplastics. Many people in the injection mold industry use the SPI Mold Classifications as guides for estimating the expected life of a mold. The common classifications are:
- Class 101
For a life in excess of a million cycles, with a hardened mold base (minimum of 28 R/C), hard molding surfaces (minimum of 48 R/C) with other details of hardened steel. Guided ejection is mandated as are other features such as wear plates for slides. Parting line locks are mandated, and corrosion resistance is suggested for cooling channels. This is the highest quality of the SPI classifications, usually accompanied by the highest price.
- Class 102
This is specified for a lifetime not to exceed 1 million cycles. This features the mold base hardness of class 101, molding surfaces (cavities and cores) also feature the hardness specified in 101, and functional details are heat treated. Parting line locks are recommended. Guided ejection, wear plates, and corrosion resistance of water passages are not mandatory, but dependent on expected total production quantities. If expected cycles approach the maximum, then these features should be specified.
- Class 103
Aimed at molds intended for under 500,000 cycles. These are molds for low to medium production needs, and corresponding price ranges. Mold bases are at least 8 R/C and cavities and cores in excess of 28 R/C. Any extras must be agreed upon.
- Class 104
For less than 100,000 cycles and limited production. These are lower priced molds. The base can be aluminum or mild steel. Cavities and cores can be of the same or a metal agreed upon.
- Class 105
These are for cycles less than 500 (prototyping only) and are very inexpensive. They can be of cast metal or epoxy.
These SPI, or Society of the Plastics Industry ( http://www.plasticsindustry.org ), classifications should and do take much of the guesswork out of estimating the useful life of an injection mold, but not every class 101 mold is the same, and this is true in all the mold classifications. Classifications indicate, but don’t guarantee quality.
No matter the class of mold, how the molder treats the mold can determine the life of the tool. I have seen and heard of aluminum molds that have lasted for years, indeed decades, and conversely witnessed class 101 tools rapidly turned into junk. Much of what the molder does, or how he treats the tool will determine the life of the mold.
Never over-clamp (use more than required clamp force) the mold not only will you wear, stress, or deform the steel prematurely, you will peen closed the vents, leading to a viscous circle of more injection pressure being dictated and then even more clamp force.
Don’t neglect preventive maintenance on the tool, devise a schedule or consult generic schedules, or better yet consult a reputable mold builder. Taking the mold down for a day or two for PM can add years of life to a mold. If you don’t have in-house tooling capabilities for this you can contact a mold builder such as Matrix Tooling Inc. A great part of mold PM is disassembly and cleaning and replacing components such as springs, o-rings, and pins. Many molding shops designate a person for these relatively simple but extremely important tasks.
Don’t skimp on mold protection, sometimes called low clamp pressure. You want to be set “fat” enough to stop the mold from clamping well before a possible stuck part is crushed by the mold faces. Your press maker can train you in this if there are any doubts. Many mold protection settings can be defeated by closing the mold too fast. Never slam a mold closed. Where there are slides or other actions and angle pins, you should slow the movement before they engage. The possibility of saving a half second on the cycle here could cost days of lost production while repairing the damage that a defeated mold protection could produce.
Daily cleaning of mold faces and lubing components such as pins and slides will extend the life of any class mold. Use the right lube for the job: FDA and medical grease where required and high temp grease for hot running tools such as those running PEEK , PEI, PPS and PSU, where mold temps can exceed 400°F. Remember it is the film of grease a few thousands of an inch thick that does the job, so don’t goop the grease on. It is counterproductive and can attract dirt.
Again the SPI classifications can give the molder a good idea of the potential lifetime of an injection mold, but not all molds in any one classification are made equally. One should always have their molds designed and built by a reputable mold builder. A mold builder such as Matrix Tooling Inc. will stand behind and care for every mold the build over its extended lifetime.
Senior Process Engineer (Older Molder)