December 17, 2009
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:
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.
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.
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.
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.
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.
January 12, 2011
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.
February 5, 2010
Drying is an important part of the process for any product made of hygroscopic (meaning affinity for moisture) thermoplastic. For medical implants made of bioabsorbable polymers, dryness is particularly critical. Inadequate drying can produce a variety of problematic results. These include: lack of tensile properties and impact resistance, as well as varying flow characteristics.
Bioresins, much like other hygroscopic thermoplastic resins, can suffer three types (or a combination of these three types) of degradation: thermal, mechanical or hydrolytic. In most thermoplastics these types of degradation occur chiefly during the molding process. With bioresins such as the PLA, PLG , and PGA families, hydrolytic degradation also occurs before and after the molding process.
An implantable device must decay or degrade in the body as part of the absorption process. Different materials and part designs have different rates of degradation in the body (where it is in a moist environment). The rate of degradation and retention of mechanical properties is affected in no small degree by the way the resin was dried and how the dried resin and finished part were handled.
If a bioresin grocery bag degrades quicker than it was designed to, the results can be the bottom falling out and groceries on the ground. If an implantable device degrades quicker (or slower) than designed to, the results can be harmful to the patient. The degradation process of the implant is key to resorption in the body.
Run of the mill dryers are generally not sufficient to control the moisture as well or reach the super-low moisture levels desired for absorbable implants. Many implant molders opt for vacuum dryers or compressed air with membrane dryers. Since most implants are small, vacuum ovens designed for lab use is another option for resin drying. In any case, the drying schedule and temperatures provided by the resin manufacturer must be strictly followed.
In many cases, the resins must be dried to less than 0.02% (200 ppm) and the resin and finished product must be maintained dry. This requirement mandates an inert gas such as nitrogen atmosphere in any non-vacuum dryer hopper, humidity controlled atmosphere in the cleanroom, vacuum packing with a desiccant and nitrogen, and refrigerated storage of the resin prior to drying.
It is not enough to strictly follow the drying and handling procedure, the resin dryness must be well tested, documented, and controlled. The dryness data is so important because it must be correlated with part degradation data to be able to predict implant device performance and absorption in the body. Lost weight or halogen type moisture analyzers are relatively economical devices but should be equipped with data acquisition and logging technology.
Drying bioabsorbable resins requires specialized knowledge, methods, and equipment, but is key to successful bioresin implant molding.