Albright Technologies, Inc
>
> > From: David Comeau <albright@ici.net>
> > To: rp-ml@bart.lpt.fi
> > Subject: RETEC Article
> > Date: Saturday, November 23, 1996 10:27 AM
> >
> > Dear RP-ML members,
> >
> > For your information I have attached a copy of a paper we presented at
> an
> > SPE RETEC earlier this year, regarding our technology for prototype
> > thermoplastic injection molding.
> >
> >
> > Please feel free to contact us if you would like more information
> > regarding any relevant issues.
> >
> > David Comeau
> > Albright Technologies Inc.
> > albright@ici.net
> >
> > XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Attached Paper XXXXXXXXXXXXXXXXXXXXXXXXX
> >
> > A SOFT SURFACE TOOLING METHOD FOR RAPID PROTOTYPING
> >
> > David Comeau and Scott Dobson
> > Albright Technologies Inc.
> > Sterling, MA 01564
> >
> >
> > Abstract
> > A new technology for the rapid manufacture of injection molds is
> > presented. The method combines machining and casting processes to
yield
> a
> > composite tool that offers advantages to current conventional prototype
> > tools. The build process is outlined, mold features are listed, and
> > examples of parts produced using this technology are described.
> >
> > Introduction
> > The increasing demands within industry to shorten the
> design-to-market
> > cycle of new products, motivates the desire for faster methods of tool
> > manufacturing. The following is an overview of a unique and
> patent-pending
> > rapid-tooling process for injection molding of plastic products. The
> > method is currently being applied (but is not limited) to the
> manufacturing
> > of prototype and low volume (1000 parts or less) runs.
> > In development since Spring 1994, this tooling method has yielded
> over
> > 50 different experimental tools, approximately a dozen of which have
been
> > developed specifically for use in various industrial applications. The
> > Soft-Surface Tooling (SST) approach is a fast and cost-effective means
> for
> > replicating standard computer generated models in commercial injection
> > molded resins. The method offers a viable and often advantageous
> > alternative to the use of aluminum prototype tooling, or urethane
> casting.
> > The SST is a composite, utilizing varying materials for the purposes of
> > structural integrity and surface/geometrical replication. Depending
upon
> > the part geometry, it is possible to construct the tool in a single
> > man-day, with average tools being completed in a 40 man-hour work week.
> >
> > Soft-Surface Tooling Approach
> > Creating a mold is a combination of traditional machining and
> casting
> > processes. The fundamental task is to fulfill strength and rigidity
> > requirements for the mold through very crude, high tolerance machining
of
> > aluminum; with the surface qualities and fine details being replicated
> off
> > the master (SLA, SLS, LOM or machined) by a soft, resilient casting
> > material. Using a model with the desired surface qualities and
> appropriate
> > industry standards for molded resin shrinkage, the desired parting line
> is
> > designed and the two mold halves are machined. Machining is performed
> > without regard for surface finish and the molds are left oversized, by
a
> > generally uniform standoff distance from the model. This offset allows
> for
> > the introduction of the casting material, which maybe poured into the
> > cavity after suspending the model in its desired position and
> orientation.
> > Any difficult part geometry, which require precision tolerances (<
> 0.002")
> > should be addressed prior to casting, by fitting a machined insert to
the
> > mold in the specific local area. Upon casting of both mold halves, the
> > tool is complete, ready for molding, and requires no additional finish
> work
> > such as hand polishing. The SST maybe used in conventional molding
> > machines, without any additional special equipment.
> >
> > Features of SST
> > * Speed: The foremost advantage to this type of tooling is the
reduction
> > in build time as opposed to conventional machining. Build time is
> > influenced primarily by geometric and functional complexity of the
part,
> > with many examples having a one week lead time. For solid shapes such
as
> > decorative figurines, ultra-rapid tools have been built in several
> > man-hours. The quick turn-around for prototype parts translates
directly
> > into reduced cost, shorter product development cycles, and the
> opportunity
> > for pre-production debugging of designs.
> > * Tolerances: The SST approach allows a flexibility, whereby build
time
> > maybe compromised for more precise machining and dimensional
> > stability/accuracy. In general, tools built within the time frame as
> > stated above, yield parts with tolerances within ( 0.005" . Again, if
> > lower tolerances are required in specific dimensions, the local region
> > maybe more accurately machined. Also, tools maybe brought within
tighter
> > tolerance by recasting a new model which maybe modified after the
initial
> > tool is molded.
> >
> >
> > * Surface Finish: The casting material transfers the surface
> > characteristics of the model identically, within the broad spectrum of
> > highly polished to textured surfaces.
> > * Part Release/Ejection: By using a material that is considerably
softer
> > than the molded part, exceptional release properties are witnessed.
Most
> > molds are used without automated ejection, and even allow for the
release
> > of minor undercuts.
> > * Thermoplastic Resin Compatibility: SST has been used to mold a
variety
> > of commercial resins, including nylon, polycarbonate, polypropylene,
> > polystyrene, ABS, glass-reinforced resins and thermoplastic elastomers.
> >
> > Current Limitations
> > At present, the method is apparently not limited by choice of
resin.
>
> > However, the physical part dimensions of preference are within a 6 in
> cubic
> > envelope. The most influential factor in deciding the applicability of
> > this tooling approach is dimensional tolerances. Parts that have
either
> > numerous or complex core functional geometry such as fits and
> connections
> > are not likely candidates for the approach, simply because the required
> > precision machining would justify a straightforward conventional
> approach.
> > The use of hydraulic or pneumatic side actions has also not been
> > integrated into this tooling approach, at the present time; however,
> > irregular parting or multiple parting lines can be accomplished
> > effectively.
> >
> > Case Studies
> > Table 1 provides information regarding a selected number of parts
> made
> > by Soft-Surface Tooling.
> >
> > Acknowledgments
> > The authors would like to thank Cliff Basque of Basque Plastics
Inc.
> > and Dr. Francis Lai of the Institute for Plastics Innovation,
University
> of
> > Lowell, for their ongoing support and technical input. The assistance
of
> > the professionals at Apple Pattern Co., Sterling MA and Brookfield
Rapid
> > Solutions, Hudson, NH is also greatly appreciated.
> >
> >
> >
> >
> >
> >
> >
> > Part Name
> > Resin
> > Number of Parts
> > Part Features
> > Tongs
> > Nylon 6-6
> > 300 pairs
> > Two parts connected by press-fit, thick sections (>.500") without sink
> > Lever
> > Polypropylene
> > 250
> > 20% glass-filled, two parts with male/female mating surfaces
> > Latch
> > ABS
> > 50
> > 3 different molds built for part design evaluation within 3 week period
> > Light Pipe
> > Clear Polycarbonate
> > 100
> > Clear and void-free for light transmission, tolerances held to +0.002"
> for
> > slide-fit
> > Lens
> > Crystal Polystyrene
> > 50
> > Highly polished surface finish, thin sections (< .050")
> > Coin
> > High Impact Polystyrene
> > 1000
> > Mold life exceeds 1000 parts without deterioration
> >
> >
> >
> >
> >
> >
> >
> >
> > Original Presented at Society of Plastic Engineers RETEC, Pioneer
> Valley,
> > March 1996: updated November 1996
> >
> >
> >
> >
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