Mold Making and Resin Casting Starter Kit

Our starter kit has everything you need to make your own molds and castings.

We have assembled the Experts-Choice? Mold Making and Resin Casting Starter Kit with the beginner in mind. If you have never made a mold or casting before, you can now get started with all the products you need in one convenient package.

The Prop Builders Molding and Casting Handbook is a very informative reference book. There are interesting hints and tips on Mold Making and casting all kinds of objects. The book is fully illustrated and each section is well explained and thought out. There are even instructions on how to build a vacuform machine.

The kit supplies you with enough mold material to make a good-sized mold or several small ones. Experts-Choice? Mold Material LV is a fast curing 1:1 by volume soft rubber that is easy to mix and pour. Our molding compound is very low in viscosity so it will virtually de-air itself for a bubble free mold.

We also include 16 ounces of Por-A-Kast? resin. Por-A-Kast? is a 1:1 by volume casting resin that pours almost like water. Por-A-Kast? will pick up detail down to a fingerprint. This resin mixes easily and has a 1-? minute pot life and a 5-7 minute cure time. Your castings can be de-molded in 10-15 minutes.

Also with the starter kit is one pound of Klean Klay, a non-hardening re-useable clay, stirring sticks, and mixing cups. We also provide you with our own instructions that will help you understand how each product works. And should you find yourself in need of any help, Bare-l is always happy to answer your Mold Making questions; just give us a call.

Starter Kit Includes:

• One 16 oz. Trial size kit of Por-A-Kast? polyurethane resin

• One pound of Klean Klay re-usable non-drying clay

• The Prop Builders Molding and Casting Handbook

• Stirring sticks and Mixing cups

• Complete instructions

 

Cobbled together from German V2 missiles captured at the end of World War II and an early US sounding rocket, the Bumper WAC project was the first two-stage liquid propulsion rocket. First flown at the White Sands Missile Range in 1948, Bumper WAC became the first rocket to fly from Florida's Cape Canaveral on July 24, 1950.

This page will document the construction of a 1/26.5 scale model of the Bumper WAC rocket in preparation for the FAI 15th World Spacemodeling Championships, to be held at the Polish Air Force Academy in Deblin, Poland, in September, 2004. The model will be flown in the S5C Scale Altitude event, in which Tony Reynolds and I will both be flying Bumper WACs as we attempt to bring home a Gold Medal for the USA.

The current S5 rules greatly favor the Bumper WAC prototype, at least in theory, as it has a booster-to-sustainer diameter ratio of almost 6-1. This allows the modeler to meet the minimum diameter requirement of 50mm easily, while still maintaining a reasonably small sustainer diameter. The rules also specify a minimum overall length of 650mm, and the 1/26.5 scale model conveniently comes in at 651mm. (Download Excel Bumper WAC Dimensions.) Our rockets will lift off using a standard Estes B6 motor in the V2 booster, and will stage to a Delta B2 in the WAC sustainer. Or so we hope...

A major challenge will be weight management, as the rules specify a maximum weight of 150 grams, or about 5.3 ounces.

The fin mold was created by Tom Campbell, a friend and very gifted modeler. Tom engineered the masters and molds for the Bumper WAC models flown by the U.S. S5C team (Tom, Dr. Bob Kreutz, and myself) at the 2002 World Championships. The Bumper WAC has also been flown in FAI competition by Slovakian spacemodeler Jan Kohuta.

This pour was made with an experimental mixture of casting resin and micro-balloons, in an effort to keep weight down. Before trimming the excess resin, each fin weighs in at ~16 grams. That's too stinkin' heavy, unfortunately, so they'll need to be recast as a hollow piece.

A beautiful mandrel for the V2 air was machined for the project by Michael Herndon, a fellow member of the Austin Area Rocketry Group. Had Michael not stepped up to machine the mandrel for me, I would have been forced to go to a commercial supplier. I'm told that this would have been a VERY expensive component to have machined out in the "real world." Rocket people are great, huh?

Students of the V2 will note that no effort has been made to model the distinctive hammered appearance of the missile. Why? Because FAI Spacemodeling judges (a fairly joyless bunch) don't seem to like the look, and apparently want V2 models to appear "clean." Accuracy be dammed, then...

The mold box is a cut-down $5.99 plastic mailbox, courtesy of Home Depot, and the mold material is Synair 333, sourced from Bare l Foil. Given the volume of mold material needed, this turned out to be one of the more expensive parts of the project. So far, about three gallons of Synair 333 have been consumed, at the rate of $70 for each two-gallon set. This unpleasant expense is offset by the fact that beer in Poland is cheap, about .50￠ US.

Here's the finished mold for the forward V2/Bumper section. The pennies help align the mold halves during the casting process. My lovely wife, Sarah, would like to point out that I'm playing with this noxious mold goo atop her $1200 kitchen island. Ain't love grand?

Here's another view of the mold-making process, with a shot of the aft V2/Bumper mold still in the mold form.

Fresh out of the mold box, the aft V2/Bumper mold halves are shown with the mandrel.

Plater of Paris was poured into the silicone molds to create positive vacuum form mold halves.

The plaster mold halves had a few air bubbles, which were quickly fixed with a bit of filler putty. The vacuum form table is hooked up to a shop vac. A sheet of styrene plastic (.015" thick in this case) is secured in a styrene , then heated in a 375o oven. When the plastic gets droopy, the is placed on the vacuum table and the shop vac is turned on.

The warm plastic is sucked around the part by the vacuum pressure. Here's a view of a finished vacuformed part on the vacuum table. For a short video of the vacuform process, click here (1.8MB).

...and another view of a finished part. After the parts are trimmed, the two halves will be joined to create a finished part. Unlike fiberglass, the styrene parts are almost immediately ready to be painted.

Here are the finished forward V2 halves, ready to be trimmed and joined. Two complete sets of parts are being produced.

Plastic Injection Mold Making

Omnitech is a work corporative made up of three principal administrators: John C. McCoy (President of S.C.C. Inc.), Don Cawthorne and Earle Lenning.

The combined experience of us in plastic injection Mold Making is over 120 years. This vast experience assures any plastic part the best possible processing economy.

Shor Lok-Blok l Mold Making System: makes l molds that are the same dimensions as standard rubber molds. The process is simple (though somewhat time consuming) and requires no skills.

Why use l molds? l molds give a sharpness of detail that is absolutely impossible to achieve with rubber molds (even when the rubber molds are injected by a vacuum wax injector). In addition, if you are thinking of upgrading to plastic injection, only l molds can be used. l molds are most commonly used for detailed charms, class rings, and wherever razor sharp detail and consistent weight are desirable. Mold to the right is an example of a multi-piece l mold made over 50 years ago with an old fashioned cylindrical mold . Although used for many years, this mold is still as perfect as it was when it was first made.

Mold Making and Tooling Manufacturing Services

Johnson Plastics’ mold process is unique to other foam makers in the industry. Through much research and developments, we have fine-tuned our process enabling us to produce hard molds with different grains in the face. This gives us much higher consistency in every aspect of the process.

We are able to produce parts with high tolerance issues where traditional silicone molds cannot. The two piece mold also offers the ability to apply l or plastic inserts and plastic composites for strength directly in the molding process. This leads to another advantage of the hard molds... durability. Silicone molds only last, at best, a couple hundred good pours. The advances in our mold process has given us the ability to guarantee our molds for the job life. The updates of our release coatings plus the temperature resistance of our interior skins give us clean molds every pour, which enables the mold to have such a long life. Average mold costs run between $1000 to $2000 dollars which is minimal for a mold guaranteed for the life of the job.

HOW - TO: Large scale Mold Making - Making the negative "tool"

trebuchet03 sent in his lastest video on large scale Mold Making...

Making tools from the tools you already own... This almost sounds incestuous - almost.

This is the next phase of our fairing construction for the University of Central Florida's 2007 Human Powered Vehicle. Here we will take a positive male mold and begin making a female negative "tool." We call it a tool because it can be used to make many duplicate copies. This tool is in fact a negative female mold.

To reiterate, this whole project is daunting. However, each individual step is not too scary and within the capability of most people handy with a few simple power tools. On the other hand, the price tag is scary. For this half alone, we used 5 gallons of polyester resin (circa $130).

21 minute Video - Lots of Time Lapse (now with captioned instructions).

HOW TO - Large scale Mold Making

This is the first part of what should be a great howto series on large scale Mold Making.

We're making an HPV! That's human powered vehicle ;) We are required to have at least 1/3 of our frontal area covered by a fairing (a fairing is just an aerodynamic shell -- like your car's outer skin). We're going for a fully enclosed (aka "sealed") fairing.

To tackle this manufacturing problem, we will be making a "female" negative mold. Similar to how casts of fish are made. To make this mold, we first need to make a positive "male" plug.

Advantages of Plastic Injection Molding

Low cost and good repeatability. Extremely cost efficient in larger quantities.

Notes

You can arrange multiple pieces in one mold by connecting them with small bars ~0.1", however the sub-components must not have widely varying volumes. When the design does not allow for additional structures to improve strength, consider using a stronger material, such as glass fiber filled plastic. Consider specifying a fire retardant material when necessary. Contoured parts warp less than flat parts.

If desired specify the following:

• Where to place the gate - the location where plastic is injected - a small rough spot will appear at this location.

• Where to place the parting line - the location where the two mold halves meet - a thin line will appear at this location.

• What surface finish to use - polished, matte, textured.

A small slightly rough spot appears at the Gate. A thin line appears at the Parting Line. A circular mark appears at the ejector pin locations.

Living Hinges - A Living Hinge is a thin connection provided between two sections of a molded part so that it can be used as a hinge, e.g. a box, with a lid, molded as one piece. You can use living hinges in your Injection Molded parts.

Multiple parts in one design - In some cases you can design several different parts in one tool to avoid the cost of making separate molds for each part. Generally the limit is about four if the parts are fairly different and up to 24 if the parts are nearly identical. To have multiple parts in one tool, join the parts together with a narrow connecting channel. If desired, you can "Remove connecting links". Combining parts in one mold does not always decrease the cost - check pricing both ways.

Box seams - Since a seam between two halves of a box is difficult to fully hide, most designers make the seam pronounced - more visible - to make it look like it is decorative. Look at some molded products to see some styles.

Insert Molding - A screw driver with a plastic handle is an example of insert molding. Injection Molding is performed around a l (or another plastic) part. Additional examples include threaded l inserts and electrical plugs.

Material Selection

Material	Colors	Stiffness	Dimensional tolerance	Intricate design	Dark colors	Max wall thickness, mm
ABS	Many	High	Good	Good	Fair	5
Nylon	Many	High	Good	Fair	Fair	4
Delrin	Usually white	Med	Good	Good	Good	4
Polyethylene	Many	Low	Poor	Good	Good	4
Polypropylene	Many	Low	Fair	Good	Good	2
Polystyrene	Many (translucent, opaque, transparent, tinted)	Low	Good	Good	Good	5
Polycarbonate	Many (translucent, opaque, transparent, tinted)	Med	Good	Good	Fair	4

Plastic Injection Molding Design Guidelines

• Use an approximately uniform wall thickness throughout your design.

• Keep walls thin - typically between 1/32" and 1/10". This allows for proper cooling and reduces cost by minimizing use of material. Thin walls also reduce problems with material shrinkage. Although some unevenness will occur due to shrinkage, walls as thick as 1/5" can be used. Keep wall thickness at least wall length / 50. Keep 90 deg walls under 0.25" high. Keep thickness of ejection pin surface wall at least .07".

• To strengthen parts, instead of using thicker walls, use additional structures such as ribs. When using a rib, make the rib about half the main wall thickness. Use fillets at the base of ribs.

• When using a rib make it about half the main wall thickness.

• Round corners and edges wherever possible.

• For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) - especially for textured walls and walls higher than 0.25".

Avoid shapes that are impossible to remove from the mold. Lighter colors hide flow patterns better than dark colors. Choose the right material from the table on the right. Drawing dimensions should be of the final part - material shrinkage will automatically be considered in the design of the mold. Use raised text instead of recessed text when possible. Where walls meet at a 90 angle, round inside and outside to at least .05" radius - sharper outside corners can create molding problems and sharper inside corners will increase tooling cost. Keep holes at least .015" from edges. It should not be possible to fully hide a 0.3" diameter ball anywhere inside the material.

Injection Mold Example parts

Injection Molded parts are widely used in: aerospace, automotive, engineering prototypes, hydraulics and pneumatics, packaging, architecture, appliances, fiber optics, medical and dental, power tools, agriculture, electronics, geophysics, measuring instruments, telecommunication, caps, enclosures, valves, toys, levers, cams, etc.

Advantages of Plastic Injection Molding

Low cost and good repeatability. Extremely cost efficient in larger quantities.

Plastic Injection Molding

Injection Molding produces plastic parts by forcing molten material into a mold where it cools and hardens. The molded shape produced is a reverse image of the mold tool. Injection Molding is low cost for simple and complex parts. Tooling adds to the initial cost but is quickly amortized.

With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heating chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity. The mold remains cold so the plastic solidifies almost as soon as the mold is filled.

Injection Molding is an extremely versatile process for producing a wide range of simple or complex plastic parts - economically and with a good finish. Injection Molding's efficiency varies by the number of parts you plan to produce. For small quantities it is usually less expensive to simply machine the desired parts.

Syncro offers a range of contract manufacturing capabilities including:

l Stamping
Our stamping capabilities range from small, open-back inclinable bench presses to 100-ton, straight-side, double-crank lamination presses. Our machines are fitted with high-tech roll feeds, levelers and electronic “lock-outs” to prevent damage to tooling.

Injection Molding
Our Injection Molding capabilities are used to provide a range of products from simple knobs and housings to complex multiple-insert molding for fine wire coils, magnets, cable grommets and terminals.

Coil Winding
Our computer-controlled, multi-spindle precision winding machines deliver high volume quality production.

Engine Electrical Systems
We supply complete electrical systems to meet the requirements of the engine manufacturing industry. These components include custom-designed stators, voltage regulators, ignition, magnet assemblies and control modules utilized in an integrated system meeting our customer’s performance, delivery and cost criteria.

Printed Circuit Board Assembly
We have a business unit focused on electronics manufacturing services which provides cost effective printed circuit board assembly solutions for a wide variety of customers. For complete information on our electronic assembly capabilities visit: www.syncroems.com.

Production Integration
Our diverse manufacturing capabilities make us the ideal partner to do your final product integration. We can Injection Mold your covers, stamp your mounting bracket, assemble and test your PCB, integrate them all together and then distribute products directly to your end market. This can streamline your operation, improve financial performance and allow you to focus additional resources on product development and marketing.

Injection Molding Introduction

In the competitive market, organizations have been using technologies that aid the achievement of rapid new product development with desirable quality and costs (Jacobs, 1992 & 1996). Considering complex geometric parts, two technologies deserve attention from the production sectors: Rapid Prototyping (RP) and Injection Molding. Rapid prototyping techniques are important due to their ability to reduce development time, to identify early errors in the project, to achieve a better communication within the project development team and to evaluate the ality of the product along with many other benefits using physical prototypes/models. Others applications and evolutions from rapid prototyping are Rapid Tooling (RT) and Rapid Manufacturing (RM). In the case of rapid tooling, it is possible to build tools, such as molds, to produce pre-series of components by Injection Molding in a period shorter than one week. Stereolithography (SL) is one of the most versatile rapid prototyping techniques. This technology is capable of delivering fast and accurate 3D objects with a wide range of resins for many different uses (Wholers, 2001).

Powder Injection Molding (PIM) competes well with technologies such as casting, machining and forming when the components produced are small, complex and have tolerances within a narrow range. However, PIM has a high investment cost, because it needs an infrastructure with ovens, Injection Molding machines, a relatively expensive feedstock and a high accuracy mold, the latter two also influencing directly the final production cost of each component.1

Rapid tooling might possibly be used with powder Injection Molding to provide low-cost prototype molds. However, Hemrick et al (2001) and Gomide (2000) have described a particular behavior of PIM parts molded in SL tools. These authors affirm that irregularities in the mold surface have caused a strong attachment of the molded part to the mold. As a result the part broke in the mold opening and ejecting stages. Nevertheless, the Injection Molding parameters and mold design influence over the failures in the Injection Molding of mixtures of powder and binders were not completely described.

This paper describes case studies performed to evaluate the Injection Molding of stainless steel 316L l powder in rapid molds produced by stereolithography. To clarify, a review about stereolithography with its rapid molds and powder Injection Molding is presented.

Stereolithography Tools for Injection Molding

Stereolithography is used to manufacture three-dimensional objects by means of photo polymerization of a resin by the energy delivered through an ultraviolet laser beam. A basic sketch of the process is shown in Fig. 1.

Basically, every rapid prototyping process starts with a CAD (Computer Aided Design) system. In the CAD, a three-dimensional object that represents the product is designed. The model is translated into a common language known as STL which is a triangular shell mesh of the object. This STL file is imported into the CAM (Computer Aided Manufacturing) of each process and is sliced into thin layers. Each layer represents one step of the process in the rapid prototyping machine. The CAM system also generates paths and manufacturing parameters according to the material and machine that is going to be used to build the prototype. Later, in the stereolithography machine, data from the CAM is loaded to commence the manufacturing process. The ultraviolet laser beam, as shown in the Fig. 1, is driven by galvanometric mirrors that scan the vat surface which contains a liquid photosensitive resin. The laser radiation activates a polymerization process and the resin hardens forming a solid layer of the three-dimensional object. After finishing one layer the platform dives and the liquid resin spreads over the solid layer. As the resin is too viscous, a blade passes through the surface of the liquid resin leaving a gap between the solid layer and the blade. The scanner starts to solidify a new layer attaching the new layer to the previously made layer. Layer-by-layer the object is manufactured by scanning selectively the laser beam over the resin surface. This short deion is the original conception from 1988 developed by 3D Systems who developed the first RP commercial process. Furthermore, there are many variations in other stereolithography processes that use the photo-chemical principle to make and attach layers (Jacobs, 1996; 3D Systems, 1998).

This technology presents as an advantage its speed in manufacturing complex parts directly from the computer and it may be used in many different applications such as: prototypes for al testing, bio-models for surgery planning, models for aerodynamic tests and rapid molds for Injection Molding. Using rapid molds, pre-series of parts are obtained in record time with the Injection Molding material chosen to manufacture the final parts. So, tests may be extended to a wide range of applications before expending time and effort on a definitive hard tool. This technology can be used to evaluate the mold analyzing its injection gates, ejection pins, split-lines, etc.

On the other hand, according to Dickens (1999), SL molds can mold from 20 to 500 parts of polypropylene which is considered very low compared to the worst l molds that easily mold over 100.000 shots. The average life of the mold depends basically on the part complexity, Injection Molding material and procedures to set up the Injection Molding parameters in the Injection Molding machine. Most of the resins that are used to build SL molds have low mechanical properties above 70oC. Also the resins have low thermal conductivity (for example the SL resin DSM Somos 7110 k=0,2W/m.k). Thus, they are weak after they receive the thermal energy from the Injection Molded material that is injected usually above 180oC. When the mold is open and the part is ejected some features of the mold can break due to the ejection forces caused by the contraction of the part material over the mold cavity. To decrease early failures of the mold the Injection Molding parameters must be different from those used in l molds. Figure 2 compares graphically the differences in their main parameters. Notice that the Injection Molding cycles are longer for SL molds due to their low thermal conductivity. As a result the injected material can show different mechanical properties caused mainly by different degrees of crystallization achieved (Segal & Campbell, 2001).

Even though they present low mechanical properties, stereolithography molds can reproduce Injection Molding parts with fine details in less than 4 days without the high costs of a definitive tool (Gomide, 2000). Consequently it may aid engineering teams to evaluate their projects, avoiding error detection in later phases of new product development.

Powder Injection Molding

PIM technology is a combination of powder llurgy with Injection Molding of thermoplastics. As a result it is possible to manufacture complex parts with ls, ceramics & composites. The hard particles of ceramics or ls are mixed with a binder system that covers the particles. This binder system is made usually of thermoplastic, wax and additives that allow the mixture to be molten and injected inside a mold in a way similar to that performed for thermoplastics alone. A basic sequence of the powder Injection Molding process is presented in Fig. 3.

The process starts from choosing a combination of powder and binder system. The powder will be responsible for giving the proper mechanical, thermal, electrical and chemical properties to the manufactured part. The appropriate binder system will transport the particles to the inside of the mold and hold the shape, the part impression. After mixing, homogenizing and granulating the mixture it is placed in the feed system of the Injection Molding machine. The material is heated in the barrel by heater bands and shear caused by the rotation of the screw. After the material becomes molten it is injected inside the closed mold, applying a holding pressure to compensate the material contraction after it cools down. When the part is strong enough to be ejected the mold is opened and ejection pins extract the part from the mold obtaining a "green" part. The green part undergoes thermal and/or chemical processes of debinding to extract the binder system before the final thermal treatment to sinter the part. Sintering is responsible for achieving the optimal physical/chemical properties of the part material. The debinding and sintering treatments cause a 15-25% contraction in the part depending on the powder; binder, proportions and their applications. The process presented in Fig. 3 is a basic process but there are many variations in each step.

The use of powder Injection Molding may be employed for economical or technical reasons. In Figure 4 many examples of parts obtained by l powder Injection Molding, with simple and complex geometries are presented. There is a wide range of applications for this technology and it may be used to produce parts for industries such as automotive, aerospace, consumer products, medical implants, computers, armaments and etc.

German & Bose (1997) mention as main advantages of this technology well designed and highly complex parts, low costs for large scale production, high precision and repeatability, a high diversity of materials with excellent properties (mechanical, chemical, etc).

Powder Injection Molding has appeared due to limitations in the conventional uniaxial compaction which does not allow efficient compaction of complex parts. With PIM it is possible to obtain parts with densities higher than 95% of the theoretical value homogenously distributed in the part (German & Bose, 1997).

Methodology

To evaluate the use of stereolithography tools to mold PIM parts two case studies were performed. The first case study was carried out to identify the process difficulties. A second case study, using a more complex geometry considered the results obtained from the first. Conclusions were then drawn to establish the positive and negative points of this application.

Design and Manufacturing of the SL Molds

For the first case study a geometry that can be considered simple in relation to the Injection Molding process was designed. The geometry was easy to mold and eject with constant thickness. For the second case study a more complex geometry was designed with a welding line caused by injection material flow around a core that made the ejection difficult. These geometries with the injection gate position and welding line are presented in Fig. 5.

The dimensions of the case study 1 specimen were chosen without any concern for the final dimension of the parts obtained. The percentages of material contraction after the Injection Molding, after the debinding and after the sintering were therefore not incorporated into the CAD design. On the other hand, for the second case study the specimen was designed considering a volumetric contraction of 21,5% which is considered standard for the chosen material.

To design the mold for each case study, design guidelines for stereolithography molds (Gomide, 2000; Cedorge et al, 1999) and design rules for powder Injection Molding (German & Bose, 1997) were considered. To aid the ejection of the part from the mold ejector pins were distributed along the cavity surfaces. It is very important to homogeneously apply the ejection forces of the pins to the part surface because the green part is very weak even to handle. Also a draft angle of 1,5o was used in the first case study and 1o in the second. Both CAD mold designs can be observed in Figs. 6 and 7.

The molds were built in a stereolithography machine model SLA-250/30A with the resin DSM Somos 7110 using a standard building strategy with a layer thickness of 150mm. After building the molds they were cleaned with isopropyl alcohol and post cured inside an ultraviolet chamber for 1 hour. This procedure is standard for parts using this resin but it is possible to obtain extra cure with heat treatments. To avoid dimensional distortions no post finishing process was applied to the mold surfaces. As a result the staircase effect reported by Ahrens et al (2001) which is caused by the layer-by-layer manufacturing was reproduced in the molded parts. For economical reasons the molds as seen in Figs. 6 and 7 were designed in shell format. This approach saved resin and machine use hours. For this reason the molds needed a backfilling procedure to increase the mold strength. In case study 1 a polyester based resin (Massa Plástica Anjo) in-house filled with iron powder to diminish contraction was used. For the second case study an epoxy based resin with aluminum powder was used (Vantico Renshape RP 4036 resin & RP 1500 hardener - heat resistant casting system). To monitor the temperature, a K type thermocouple was placed in the back of the SL mold shell in this case study (Fig. 7). Figure 8 shows the mold of study 2 placed and adjusted in the bolsters ready for Injection Molding.

The Injection Molding Procedures

Stainless steel 316L was used as the powder and the binder chosen to be used in the experiments was that presented in Table 1. The 316L was selected due to its economical importance indicated by German & Bose (1997).

After assembling the bolster in the Injection Molding machine (Arburg Allrounder 320S 50T), procedures to adjust the Injection Molding parameters were taken.

For case study 1, the Injection Molding procedure started using safe parameters to avoid early failures to the stereolithography mold. This meant that low pressures, low clamping forces, low injection speeds, no holding pressures and lowest temperature to decrease viscosity of the feedstock were used. These parameters were changed gradually until successfully injection and extraction of parts from the mold without perceptible defects. Between each shot the impression was analyzed and a demolder (PVA, poly vinyl alcohol) was applied to the surface to reduce the ejection forces.

The parameters obtained from the first case study were used to indicate those for the second. Gradually the values were changed to make it possible to mold and eject parts considered good quality. After the 10th part obtained no demolder was used. To help the mold to cool down an air stream was used between each shot.

Debinding and Sintering Treatments

The best 19 parts obtained from case study 1 and 30 from case study 2 were debinded and sintered. The treatments were performed in the production line of Steelinject (Lupatech Industries Group). A deion of the debinding and sintering is shown in Table 2.

Measurement Procedures

Concerning the dimensional control, the dimensions from the molds before and after the Injection Molding were taken. The parts were measured after the molding and sintering. Figure 9 shows the dimensions designed in CAD for case study 1 where contraction was not considered. The same dimensions were measured on obtained parts.

Figure 10 presents the target dimensions for sintered parts and the dimensions measured in the mold for the case study 2. The parts chosen for measure were taken in cycles when the Injection Molding process was considered stable (after the initial adjustments) with constant time, speed and pressures. Also in case study 2, the mass of each measured part was taken after the molding and after the sintering treatment.

Furthermore, as previously described, the temperature of the cavity was monitored using a K type thermocouple connected to a monitoring system (Picolog TC08, Picoteck Technology) with readings taken every 5 seconds.

Results

The parts obtained presented a good superficial quality after molding and sintering. Nevertheless, in case study 1 the incorrect adjustment of the ejection pins caused marks on the part that can be seen in Fig. 11. The parts obtained from case study 2 presented another defect. An excessive flash (thickness of 0,4mm) occurred due to the imprecise adjustment of the cavity closing. Later adjustments of the molds reduced the flash thickness to below 0,15mm. Figure 12 shows green and sintered parts obtained in case study 2 without flash.

Some of the most important Injection Molding parameters used to mold the parts in the case studies are presented in Table 3. The main divergences between the case studies are the holding pressures that were applied with more efficiency in case 2..

The cooling time measured in case study 2 was longer compared to case study 1. One of the main reasons for this was the stability of the Injection Molding cycles and the control of the temperature between each shot. Using the thermocouple it was possible to estimate better the right time to open the mold and to eject the part. The minimum time necessary to cool down the mold before starting another injection cycle was also precise. Figure 13 presents the monitoring results from the Injection Molding cycles for case study 2. The graph shows temperature variation over time for a thermocouple position of 0,5mm from cavity surface, pointing out the maximum temperature peak being 49oC. Additionally, it was possible to obtain the total time for each cycle which ranged between 60 and 360 seconds due to the cleaning and cooling of the cavity.

The dimensions from case study 1 reveal that the percentage contraction of the parts after the sintering was 21,56%. This is a value close to the theoretical value, considering the measurement errors, for the mixture of powder and binder system used (refer to Table 1). In Table 4 the dimensional percentage deviations compared with the CAD target dimensions for the final parts of case study 2 are presented. It can be observed that the mold dimensions change after the injection cycles. This occurs because the stereolithography resin is not completely cured after the standard building and post-processing procedures. During the injection cycles the combination of pressures and temperatures help to cure the resin but unfortunately change its shape. The final average results for the sintered parts present a discrepancy in dimension "C" which is related to the part thickness. This error is caused by the excessive flash mentioned previously which added an extra thickness to the parts. However, the results indicated that it is possible to optimize the mold design and finishing processes in order to overcome this problem.

Analyzing the mass variation of the parts it was possible to evaluate the pores percentage. Considering the CAD model for 100% dense stainless steel 316L and the mass average of the sintered parts the pore percentage was almost the same as those for parts molded in steel molds (3,65%).

Conclusions

Gomide (2000) & Hemrick et al (2001) affirm that the powder and binder system mixture sticks to the stereolithography molds causing difficulties in ejecting parts from the mold. Nevertheless, the adherence of the mixture to the cavity surface can not be completely explained by a single phenomenon. As in llic molds, the Injection Molding process in SL molds presents many similarities although the values differ.

As in llic molds, too high a holding pressure or too long a cooling period can make the ejection of parts difficult. They may cause the part to break or to crack. A well designed ejection system can diminish these problems. The holding pressures applied to Injection Molds made with the stereolithography resin are low due to the low mechanical properties of DSM Somos 7110 resin at high temperatures (Hopkinson et al, 2000). Ascertaining the correct cooling periods before opening and ejecting the part is therefore very crucial to the success of the process. A long cooling period leads to high contraction of the mixture (~1%) and it becomes too fragile to eject from the mold. If the mixture is not solid enough the part will deform in the mold opening. The injection and holding pressures are vital when injecting powder parts using stereolithography molds. Finding the correct pressure adjustments is a complex process because there is a minimum value to mold the part without defects and a maximum value that will not cause the part to excessively adhere to the mold cavities. It is also necessary to avoid pressures which are too high, in order to prevent early failure of the mold.

The superficial quality of the mold also plays an important role in the ejection of the part. In both cases studies no kind of surface treatment was applied. For the designed specimens this was not a problem because they were planar. However, for more complex parts it may be necessary to use techniques such as sanding, polishing and electroplating to achieve better surface qualities. It is also important to note that the machine used had a building resolution of 150mm and in new models it is possible to build 20mm layers giving a better surface quality and higher precision.

The injection speed used in the evaluated geometries was equal to those recommended for llic molds (German & Bose, 1997 e Haupt & Walcher, 1998). The speed can not be too high otherwise it will cause flow jetting. Low speeds would not cause problems because the resin works as an insulator and it is unlikely that flow freezing would occur. When injection pressures are too low it is necessary to compensate with higher injection speeds.

The results obtained from the dimensional analysis, especially from the second case study, have proven satisfactory. Excluding the dimension affected by the excessive flash the dimensional errors were under 1.7% when comparing the target dimension to the final sintered part. Many authors (Kulkarni, 1996; German & Bose, 1997) affirm that tolerances for the powder Injection Molding process must be close to ±0,3%. Nevertheless, David (1998) affirms that for industrial applications tolerances of ±1% are more realistic. Considering that the use of stereolithography molds is mainly for design evaluation purposes inaccuracies around 2% are acceptable. Moreover, there are new resins for stereolithography with excellent mechanical properties even at high temperatures. Also, there is a new ceramic filled resin specifically for building Injection Molds.

The time to obtain a mold for stereolithography can be considered the greatest advantage of its application. Table 5 shows the time expended to obtain 100 green parts in case study 2. It is important to note that to obtain the final sintered parts more than 68 hours are necessary to debind and to sinter them. This is inherent to the powder Injection Molding process.

The time to build the molds was 16 hours, using a laser power of 13mW (over time HeCd lasers units lose their power). With a new laser power of 40mW it is possible to build the molds in 9 hours, in the same machine. Using new machines with a laser power of 200mW and a faster recoating system the time can be reduced to 3 hours.

The period necessary to cure the backfilling material is also relative because as with the rapid prototyping machine it can be performed at the end of the day to get the objects ready for the next morning.

This work proves that it is possible to obtain powder Injection Molded parts from stereolithography molds. Despite the low complexity of the specimens it is possible to gain important information relevant to performing Injection Molding of more complex parts.

Acknowledgements

The authors would like to thank to the post-graduate program of Departamento de Engenharia Mecanica from Universidade Federal de Santa Catarina, CAPES, CNPq, Finep and to Steelinject (Lupatech Group - Caxias do Sul, RS) who kindly donated feedstock material and space in its production line.

ABSTRACT

The utilization of stereolithography molds in the manufacture pre-series for Injection Molded plastic parts aims to reduce costs throughout the product life-time, but mainly during design and manufacturing phases. The use of this Rapid Tooling technique in powder l Injection Molding is evaluated in this work. One of the greatest differences between traditional and stereolithography tools is related to the heat conductivity of the materials employed. For example, steel molds have a heat conductivity coefficient 300 times higher than molds made with the photosensitive resin used in the stereolithography process. The discrepancy regarding the cooling rate of the molded parts during the injection cycle must be compensated with adjustments in the Injection Molding parameters, such as temperature, pressure and speed. The optimization of these parameters made it possible to eject green parts from the mold without causing defects which would become evident in debinding and sintering stages. The dimensional analysis performed at the end of each case study showed that the shrinking factor of the component after the sintering had the same value obtained for components using traditional llic molds. Moreover, the dimensional error remains under 2% which can be considered low for a pre-series of components (or prototype series).

Keywords: Rapid prototyping, rapid tooling, powder llurgy, Injection Molding, stereolithography

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Using Rapid Injection Molding to Improve Product Design

Speed adds time to design process

—edited by Richard Mandel

Innovative technology is rapidly changing the way one medical-device manufacturer upgrades its products to better meet customer needs and maintain a competitive edge in the market. San Diego-based Tensys Medical Inc. often relies on prototyping to identify potential improvements in the design of its medical devices. But tight turnaround times and limited budgets have historically restricted the ability to build al prototypes. Through the use of rapid Injection Molding, the company’s engineers beat their internal deadline, allowing them additional time for preliminary testing and further design improvements.

Sensing in real time
Tensys manufactures the T-Line Tensymeter, a noninvasive device designed to measure arterial blood pressure in real time, on a beat-to-beat basis. Unlike the standard method used to monitor a patient’s blood pressure during surgery — a cuff-based system that provides intermittent measurements every three to five minutes — the T-Line extracts radial arterial pressure using a transdermal pressure sensor. Continuous blood pressure status allows the surgical staff to constantly monitor a patient’s response to anesthetic agents and surgical intervention, thereby helping anesthesiologists quickly recognize and treat rapid changes in blood pressure to prevent potential patient outcomes such as stroke, postoperative heart attack or even death.

Introduced in 2002, the T-Line has proved popular with clinicians and anesthesiologists nationwide. As the product gained acceptance, Tensys worked to gather feedback from its core users, focusing particularly on ways to improve and enhance the device. Users said they wanted the T-Line to be easier to operate, so Tensys simplified the product by redesigning its key components.

“Although the application process for using the T-Line is relatively simple, some of our users rotate between different hospitals and may use the device only once every few weeks, so they may have to re-acclimate themselves to the process,” says Russ Hempstead, senior engineer, Tensys. “To help reduce the learning curve, we removed a few steps from the application and redesigned certain components.”

Specifically, Tensys engineers redesigned the T-Line’s plastic sensor to allow medical staff to place the sensor over a patient’s radial artery. With the previous sensor , the sensor itself was attached to the via polyethylene tape, which was manually applied during manufacturing. The new design integrates a serpentine “arm” that is fabricated as part of the original sensor , eliminating the manual labor and costs associated with the prior attachment method. It also re-centers the sensor after any shifting due to patient movement, helping users more easily maintain proper placement of the device.

Addressing prototyping, production challenges
Although the redesigned sensor improved the product’s ality, the design’s particular geometry posed a challenge for Tensys’ current prototyping and production techniques. Company engineers first tried using stereolithography to create a master part to create urethane castings of the T-Line components. Although the castings provided a conceptual design check, the limited material selection and short tool life posed testing and design verification constraints. The geometry also jeopardized their production plans.

“With huge wall thickness variations and sharp transitions, the sensor design is an extremely challenging one for a molder to accommodate,” Hempstead says. “When I presented the design to a number of different molders, they seriously thought I was joking because, at first glance, it looks like it will never fill properly.”

With a rough prototype in hand, the medical engineers contacted their existing production molder in hopes of keeping production on track for the approaching deadline. Because of the design’s unconventional geometries, the company’s production tool vendor wasn’t able to construct the tool properly. As a result, the new tool created a tremendous amount of flash — excess material caused when plastic leaks from a mold cavity and sticks out from the edge of the part.

“When flash occurs, you need to manually trim each part,” Hempstead says. “This usually isn’t a big deal for us if we’re dealing with prototypes. However, in a production situation, it’s not acceptable because of the quality and expense.”

Tensys engineers and the production tooling vendor were struggling to get production tooling underway. In the interim, a design engineer came across information about a prototyping and low-volume production process that seemed well suited for the redesigned T-Line — rapid Injection Molding. With a deadline approaching, engineers decided to take an unconventional step back in the process and check out rapid Injection Molding’s abilities for prototyping and pilot production.

Exploring new methods
Tensys was attracted particularly to rapid Injection Molding because of its fast turnaround time and low costs. Developed by The Protomold Company, based in Maple Plain, MN, rapid Injection Molding uses proprietary software technology and high-speed CNC machining to produce injection-molded parts from 3D CAD models in as little as three days.

Tensys design engineers accessed Protomold’s Internet site and submitted their 3D CAD file for the redesigned T-Line. Within 24 hours, the molder sent back an interactive ProtoQuote — a web-based price quotation illustrating the effect of using different materials, with comparisons of lead-time options and a list of final price points based on quantity. It also included suggestions for potential design improvements.

“The fact that I can simply upload a CAD file directly to the site, add a few detailed notes and just walk away, easily saves me 50% of the time I typically would spend on the logistics of a quote,” Hempstead says. “If I had gone to a different molder, I might have been forced to deal with incompatibility issues, while trying to meet another vendor’s CAD file format or 2-D drawing requirements. Not having to translate my CAD files or correlate software versions allows me time to conduct other work.”

ProtoQuote also gave Tensys engineers valuable design guidance on how to work within the rapid Injection Molding process specifications, highlighting areas where wall thickness was significantly greater or less than nominal and areas where draft was less than three degrees, prohibiting texture additions. Based on the ProtoQuote suggestions, Tensys design engineers revised their CAD file and submitted an order for 25 sensor prototypes. Within six days, the molding company delivered the completed parts.

“We subjected Protomold to a difficult trial by submitting our toughest part as a test, and the company rose to the challenge,” Hempstead says. “Even better, the company amazed us by delivering a final product while our production vendor was still struggling to produce a workable tool. Overall, rapid Injection Molding delivers in a time 90% faster than other molders we’ve worked with, which poses huge benefits for us.”

Pleased with the rapid Injection Molding prototype results, Tensys Medical engineers immediately stopped production tooling to shift efforts toward further improving the T-Line component designs, while keeping on target with the original timing specifications.

Squeezing more from budget, schedule
Originally, Tensys sought to place a single order for a prototype to meet its internal deadline. But the molding company’s quick turnaround allowed the design engineers to use the additional timesavings to conduct preliminary validation tests and further improve the product design through successive iterations.

“Before we discovered rapid Injection Molding, we were struggling to make a schedule that would let us create production parts we needed,” Hempstead says. “But once it became apparent we could meet this need using rapid Injection Molding, we began asking ourselves, ‘What else can we improve before we get to production?’”

The rapid Injection Molding process allowed the design engineers to shorten the product design and development program time cycle. “We used to have significant lag time between design and production, sometimes waiting as much as 14 weeks while our tools were being built,” Hempstead says. “Now we’re able to do more design work and testing because everyone here knows we can use Protomold’s rapid Injection Molding process to incorporate our design changes in a week or less.”

Rapid Injection Molding also helps the medical device engineers reduce the costs of production tooling and final parts. “The tooling costs from Protomold are 50 to 60% lower than prototype tooling quotes from other molders,” Hempstead notes.

Tensys now relies on Protomold to supply its prototypes and, in cases where the needed quantities are prohibitively high for standard rapid prototyping methods, for pilot production parts. In fact, the company currently is developing 30 tools in conjunction with the molder. And the two companies are cooperating on the fifth design iteration of the T-Line redesign.

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