The mill.
OK so you've mastered I-DEAS and g-code generation and want to SAFELY make an injection mold and take some moldings. First read these MANDATORY instructions to familiarise yourself with the operation of the MINIJECTOR
You are now ready to proceed to the Pictorial Tutorial. Do not proceed unless you have read the General Safety Information.1. Machine mold insert and mold
2. Machine flow
channels in mold
3. Turn on injection
molder
4. Empty and fill hopper
5. Purge injection molder
6. Running the minijector
7. Fine tuning molder
8. Other suggestions
NOTE: A machine operating guide including blueprints and example mold patterns is kept at the base of the mini-jector for reference. DO NOT REMOVE THIS FOLDER (white plastic). It has all the extra information you need in-case you run into problems including temperature settings and pressure changes.
1.Hot plastic is very dangerous, it is fluid as well as hot and shows no visible signs that it could burn you. It can drip like mollasses and should be treated very cautiously. Burns are not pleasant. If burnt immediately rinse affected area under cold water for an extended period until you get yourself to a medical professional.
2.Always wear safety glasses when operating mini-jector.
3.Molds by their nature absorb heat, be extremely careful handling molds and wear appropriate insulating gloves at all times.
4.Always use handles or dowels on the mold to lessen need of handling hot molds.
5.Only one person should operate the machine at any given time.
6.NEVER reach into machine - there are extremely hot parts as well as moving parts that will easily crush and pinch fingers.
7.Make sure the machine is at processing temperature before extruding or turning screw.
8.Use dry resins such that contained moisture will not cause pressure build-ups in mold.
9.Never leave heaters on for an extended period of time when machine is not being operated. - Material may degrade to the point of exploding or expelling dangerous gasses.
10.Shut off all power for any maintenance procedures and refer to appropriate maintenance manuals.
General Operating Principle of this machine.
A screw rotates inside a heated cylinder to create the high shear rates necessary for heating, mixing and fluidising the polymer. This is called the 'extrusion' part of the process and fills the barrel with enough material for a 'shot' to be injected into the mold.
The entire screw is then plunged vertically by a hydraulic ram during the injection process to inject this shot of molten polymer into your mold.
In larger industrial machines the screw mechanism is generally twin-screws counter rotating for higher shear rates that feed material into a shot reservoir whereupon hydraulic rams inject material into the mold.
- In order from top to bottom of machine. Familiarise yourself before beginning!
1.Large green start
button at top of machine switches on the hydraulic motor that drives the
hydraulic ram. Switch this on.
2.Illuminated ORANGE
power on/off switch turns machine on and acts as overload protection for
machine.
3.Heaters on/off toggle
- turns on 3 pyrometers that control heaters.
4.Timers on/off toggle
- activates timers for running in semi-automatic mode. For short
runs leave off and operate in manual mode.
5.De-compress on/off
toggle - Selects whether the screw 'de-compresses' - ie retracts 1/4' after
it has reached the extrusion limit switch.
6.Extrude - When pushed
this button allows screw motor to rotate - To use manual extrude,
inject, or retract, Carriage selector switch MUST be in.
7.Inject / Retract
- Switch permits screw to be turned Forward (INJECT) or Back (RETRACT).
8.Carriage Out-In
- Cycle Start - This switch raises and lowers entire injection assembly
to mold surface. Turning down to the "In-Start-Cycle" position lowers
injection nozzle onto mold. Turning lever up to "carriage out' raises
injection unit.
9.EMERGENCY STOP -
Cuts machine power - Get to know this switch!!!
First machine the mold insert. Currently we have an aluminium
die that is machined to take a 5" x 3" x 3/8" inserts in either half of
the mold. This gives (roughly) a maximum part dimension of 4" x 2"
x 1/2". For moldings larger than this it will be necessary to order
an Aluminum blank from Minijector.
The mill.
1. Using the facing tool for the mill, face the aluminum inserts
to give a flat, smooth, surface finish.
2. Cut pieces to 5" x 3"
3. Run your g-code to cut out piece to be molded.
Cutting the molded piece with a 3/8" flat bit.
2.
Machine flow channels in mold
Milling recess in opposite side of mold.
4. It is now time to mill your flow channels. Note that there is an over-run at the end of each line to accept the cold 'nose' of incoming polymer. This ensures that hot flowing polymer enters your mold.
5. For an extra good finish and to ensure alignment of your molds it is a good idea to place alignment pins. Two in opposite corners should be enough. A press fit stud in one side of the mold (see below) and a neat fitting hole on the other side are perfect.
6. Note that the surface finish of your mould determines the surface finish of your part. If you desire a smooth finish hand polishing of the surfaces is possible. Sandblasting and other techniques can be used to give textured finishes.
Press fitted alignment stud.
The two finished halves of the mold.
6. You are now ready to use the injection molding machine.
The feed hopper can be seen from above when looking down at the machine.
This is where your polymer pellets are placed.
Green start button switches on hydraulic units.
Red power button switched on heating units and control panel.
Big green start button turns on hydraulic pumps for ram and screw.
Big red power button turns on heating elements, timers and control mechanisms.
NOTE: HUGE RED EMERGENCY BUTTON AT THE BOTTOM OF THE MACHINE. PLEASE BE AWARE OF THIS AS THE METHOD FOR TURNING THE MACHINE OFF IF ANYTHING SEEMS TO BE GOING WRONG. REMEMBER, EMERGENCY STOP AND ASK QUESTIONS LATER.
The feed hopper.
Note that the feed hopper can be rotated to prevent or allow flow of
pellets into machine. To empty the feed hopper rotate the hopper
to the purge hole and pour material into a bucket under the purge hole.
Feed hopper re-aligned.
7. It is now time to purge the machine. Place purging fixture into machine as shown. Be sure to align nozzle with hole in the purging fixture.
Purging fixture.
IN - START CYCLE switch
Switch to IN - START CYCLE. This engages nozzle to purging fixture.
Nozzle engaged to purging fixture
Pushing EXTRUDE button.
Pushing extrude button turns the injection screw and loads polymer from the hopper into the heaters and then through the nozzle into the fixture. Note this only gives a low pressure in the melt and is not suitable for injecting into actual molds. Keep extruding until polymer stops flowing (if you are intending to clear the injection molder for a different colour or polymer). Switch IN - START CYCLE button to CARRIAGE OUT and remove purging fixture and purged polymer.
Removing purging fixture and purged polymer.
Firsty blend your polymers and place them in the feed hopper.
Mixing polymers. - HDPE with blue polymer dye.
Filling hopper
Placing mold inserts in minijector blank die.
It always helps to smile and be happy when injection moulding!
Inserts in blank die
Insert die into v-shaped cavity such that entrance hole in mold is
alligned with the injection nozzle. - nb. it is a good idea
to make allignment marks on the die for repeated moldings.
The shot may have been incomplete because the mold was cold (ie. the polymer freezes before reaching the extremities of the mold. One way to aid this is to pre-heat the mold. A hot air gun can be used to achieve this. In some commercial molds heating elements are included in the mold.
Preheating the mold with a hot air gun.
Slightly better with a pre-heated mold but not perfect - Note
leather safety gloves!!!
Another option is to increase the molding pressure. The minijector has a fine tuning adjustment for increasing injection pressure at the rear of the machine. If you are at all unsure of operating this refer to the manual and/or a proficient minijector user.
The guage which shows injection pressure is located on the right hand side rear of the machine (see below). The adjustment valve is behind this ar the very rear of the machine.
The injection pressure gauge is pictured above.
The small black wheel at the rear is rotated to adjust the injection
pressure.
The injection pressure valve (Wheel with hex-key centre).
Voila! Raising the injection pressure has successfully
solved the problem.
Now you can experiment with different colours!!
Please add suggestions and lessons learnt from injection molding to this page as they happen. As we are all learning (particularly in reference to mold design) lets share our knowledge and experience.