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| Design Basics |
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The "V" molds used in "Wasp" Mini-Jectors are adaptable to standard mold construction technology, but they also
have some requirements of their own.
The unique feature of "Wasp" machines is the self-clamping "V" mold that is removed from the machine each cycle.
Items commonly associated with injection molds, such as heating and cooling passages and part ejection systems, are
not applicable to these compact tools. The unique requirements are dictated by the fact that the mold comes in contact
with three surfaces every inject cycle:
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1) The two mold clamping shoes:
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The mold clamping shoes are made from #80-60-03 ductile iron. This material is compatible with the wedging and
ejecting action of the machine when using molds made of bronze, low- or high-carbon steel. Aluminum can also be
used for mold construction (see alternative mold materials), but it will
abraid after prolonged use.
The standard Mini-Jector mold blanks (part no. 121230 - 2 5/8" x 6" and part no. 420170 - 3" x 7") and the cavity
plates of the insert mold base (part no. 420180) are built from SAE #4150 annealed medium-carbon steel. This
material is free-machining; it may be heat-treated up to a hardness of Rockwell HRC 55-60 and polished to a smooth
finish. The surface contacting the mold clamping shoes (the 7.5° angle surface) does not require heat-treating to
work well against the shoes; however, if the mold is to be heat-treated anyway, this process will cause the angled
sides to last longer.
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2) The injection nozzle:
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The injection nozzle, either straight bore or non-drool, is very hard. The bearing surface where the nozzle
contacts the top of the mold is a 9/16" diameter round, which means that a force of nearly 25,000 PSI is pressing
on the top mold surface at the sprue opening. This force will indent or "coin" the top of the mold.
The sprue plate can be replaced if necessary without having to regrind the rest of the mold. The 9/16"
diameter is an optimum size selected because greater areas would not seal the sprue-nozzle interface.
Under no circumstances should any recess be machined into the mold to receive the nozzle. A flat surface
is required.
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3) The mold ejector bar:
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The dimensions of the mold ejector bar are 1" x 7", so adequate contact area is available for the ejection
force. Since the bar contacts both mold halves, be sure that the bottom surface of the mold is evenly machined
to ensure uniform contact on each half.
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| Tooling Sources |
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Many customers come to us to secure their tooling. We maintain a list that we can provide to you of specialists
in short-run prototype tooling and ASTM test specimen molds. Most molds require small modifications when initially
run. We recommend that you use a tooling source near your facility to speed the de-bugging of the tool.
We are available for consultation with your toolmaker at any time to ensure your satisfaction with our equipment.
We are also more than willing to examine your tooling drawings and suggest possible changes, if necessary.
If you use any of the mold blanks supplied by Miniature Plastic Molding, the side angles are precisely ground to
the correct taper. If you or your toolmaker build your own mold base, the 7.5° angle must be held to ± .05°
(or 3 minutes of angle). Spread over a 3" length, a .05° variance would cause an out-of-parallelism of .0024".
Any greater error would prevent uniform surface contact during clamping.
If you heat-treat the mold after completion, it may be necessary to re-grind both the "V" and the parting-line
surfaces of the mold due to the warpage that could occur during the heat-treating process.
Molds that are to be used for long periods of time, or molds that are to be used for molding materials such as
PVC, can be hard chrome-plated for resistance to wear and corrosion.
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| Various Types of Mold Construction |
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1) Conventional mold construction with cavities cut directly into the blank mold:
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This type of mold construction is recommended if the mold is for a product or procedure of an on-going nature,
e.g. molding color step chips. A permanent mold would be desirable since there would be no need for interchangeable
cavities. A high quality mold for color step chips would be fully heat-treated, polished, chrome-plated and fitted
with a replaceable sprue plate. (The sprue plate can be replaced without resurfacing the top of the mold).
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2) Insert construction:
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3) Insert-type construction with interchangeable cavities:
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Many times you may wish to produce a large variety of parts that are similar in size and/or configuration.
In this situation, it is possible to reduce tooling costs by interchanging just the cavities and utilizing the
same mold base for the different parts. The cavities can be held in position by small screws. Precision machining
of the pocket holding the cavities is necessary to ensure proper matching.
The interchangeable cavity design could also be used if the cavities are of a fragile nature, e.g. an epoxy
cavity for a very short production run (see alternative mold materials).
The cavity must be surrounded by the parent metal to prevent breakage. The support of the metal wall produces a
mold almost as sturdy as the permanent cavity system.
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| Heating and Cooling "V" Molds |
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Cartridge heaters in the shoes will heat the mold by contact. If additional mold heating is required, an optional
auxiliary control package is available. The package includes two heat controllers, two 200 Watt cartridge heaters to
insert into your mold, and two thermocouples. Pre-heating the mold in an oven will also allow the mold to reach a
stabilized temperature if cycle times are kept consistent. If mold temperatures over 300°F are used, the shoes must
be insulated from the machine frame.
Due to the small size of the "V" mold, it is impractical to run cooling lines to the mold itself. Molds run for
a period of time will soon become fairly hot, but they will seldom reach temperatures above 105°F.
Some customers use a pair of molds to allow each mold a cool-down period between cycles. A fan can be used to
cool the extra mold as a cycle is being run. It is also possible to use small cooling hoses to run cold tap water
into the mold, but remember these must be removable every cycle along with the mold.
Cooling passages in the "V" mold clamping shoes do not effectively remove mold heat.
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| Part Removal and Mold Handling |
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The "V" molds do not provide for any type of ejection of the finished part, so other techniques must be used
to get the finished part out of the mold.
When the mold is removed from the machine, the two halves must be separated to remove the part. Two
diagonally-placed dowel pins are used in the mold base. These need to engage only 0.125" into the hole. Wiggling
the two mold halves usually allows you to pry them apart. Adequate draft or taper on the molded part perpendicular
to the parting line will assist the separation.
Note the handles at the end of the mold facing the machine operator. These, along with the
short dowel at the opposite end, can be used as pry devices if mold separation is difficult. The handles can be
bolts, long dowel pins or a wooden file handle. Notice they are placed adjacent to the mold-locating dowel pins.
The pairs of protruding handles (or dowels) can be pried upon with hand tools or pushed onto an auxiliary fixture
with two wedges that pass between the pairs of pins. The handles also aid the operator greatly in handling the
mold, as the mold can become very warm after prolonged operation.
Screwdriver slots can be placed at the ends of the mold as an alternative to the protruding dowels; however,
this does not provide the mold handling convenience of the other method.
When designing your mold, remember that large, flat parts are more difficult to remove from molds. The gate
into the mold should be large enough to allow you to remove the entire shot, with runners and sprue, without
having the gate break off. Proper mold release compounds greatly assist in removal of the parts.
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| Alternative Mold Materials |
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1) Aluminum:
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Aluminum is very popular for prototype tooling due to its free machining and the potential of casting
complex cavities from models. Aluminum is recommended for prototype cavities plus short production runs (up
to several thousand pieces). Plastics are somewhat abrasive, and gate and cavity wear will occur when using
an aluminum mold. If the cavity walls are thin, they should be fully supported by the parent steel mold frame.
Mold bases constructed entirely of aluminum are feasible; however, the "V" sides of the mold will wear more
quickly than those constructed of steel.
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2) Bronze:
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Bronze is another popular material with many of the same benefits and drawbacks as aluminum. It can be highly
polished where prototype optic parts are being molded. Bronze cavities can also be cast.
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3) Non-Metallic Materials:
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Non-metallic materials such as epoxies, or combinations of epoxy and powdered metal fillers, can make
surprisingly durable short-run cavities. These materials would most commonly be cast, although they can be
machined as well. These cavities can only produce up to one thousand parts, but an inserted cast-epoxy cavity
creates an instant mold when time is crucial.
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