Plastic Mold Building Tips
I’m sure you’ll agree with me when I say:
Building a perfect plastic mold is really complicated.
Or is it?
Well, it turns out that finish this job may not be as hard as you’d have thought. All you have to do is to check the following building tips and follow them.
In this article, I’m going to show you exactly how a good moldmaker to do.
If you want to know more, all you have to do is read on…
- Place a thin piece of cardboard (shirt cardboard) between the nozzle and sprue bushing after installing a mold to ensure the alignment of the machine nozzle.
Shoot right through the cardboard with the nozzle in position.
A mark in the cardboard demonstrates exactly where the nozzle aligns with the sprue bushing hole when you pull the nozzle back.
Ensure the two are perfectly aligned.
- Injection units should have enough material for two full cycles of injection.
To put it another way, every cycle should empty 50% of the injection cylinder’s capacity.
A machine takes this amount of “shots” for each cycle since all of this material is injected into the mold.
While one half of the barrel prepares for the next shot, the other half makes one shot. Consistency is achieved in this way.
It is ideal to use a 50% rule, but shots should never be larger than 20% or smaller than 80%.
In this example, the ideal injection unit would be one with a 4-ounce cylinder if the total material usage is 2 ounces for one complete cycle, since 50% of 4 ounces equals 2 ounces.
Accordingly, a machine with a 2-1/2 ounce cylinder (80%) and a 10-ounce cylinder (20%) could produce a 2-ounce shot according to the 20% to 80% limits.
It is determined by the material’s heat sensitivity.
Heat-sensitive materials easily burn, while heat-resistant materials can withstand longer temperatures and are less susceptible to overheating.
It is critical to determine the heat sensitivity of a specific material in order to find out how long it can stay in the heated injection cylinder before degrading.
It is impossible to produce quality products with degraded materials.
It is important to note that the 50% rule-of-thumb mentioned earlier ensures that no material will degrade while being molded regardless of the time allowed for it to be in contact with the mold.
In the case of low heat-sensitive materials, such as polyethylene, the 20% rule-of-thumb applies.
As for materials that are particularly heat sensitive, such as PVC, 80% of the percentage applies.
- The amount of polystyrene that an injection unit can hold is universally rated.
In order to determine how much of another plastic they can hold, specific gravity values must be compared.
The amount of other material that can be contained in the cylinder is determined by dividing the specific gravity value of that other material by the specific gravity value of polystyrene.
A machine’s rating in ounces is multiplied by the 1.15 value. Suppose our example machine weighs 8 ounces.
So it can inject eight ounces of polystyrene at a time. This 8-ounce rating was calculated by multiplying it by 1.15, which yields a 9.2 ounce polycarbonate injection capacity.
- There is no indication of the actual temperature of the plastic melt when the injection temperature control indicator is set.
Using it as a reference only makes sense. In order to measure the melt temperature, a probe must be used that detects material as it exits the mold from the molding machine’s nozzle.
Even though there may be a 30 degree difference between those two readings, the probe reading is the one that describes the process conditions.
- The nozzle should be set approximately 10 degrees (F) higher than the front zone so that the plastic will be at its hottest at the front zone and at its coldest at the rear zone.
- To maintain pressure on the plastic material, the check ring must be replaced if the injection screw turns while injecting material during the molding process.
- Injection molding machines are rated based on their clamp force capability.
The mold must be kept closed during injection with this force.
Injection pressure should be overcome with only enough clamps, otherwise the mold will be damaged.
The clamp pressure for a mold, for instance, would only need to be 12,000 psi (6 tons) if the injection pressure is 10,000 psi (5 tons).
- Using a machinist’s level (not an ordinary carpenter’s level) while installing the machine will ensure that the tie bars are level.
Levelness should be checked every six months. A machine that is out of level disrupts the process and material flow and could indicate warpage or settling of the frame.
- To allow for maintenance, product flow, auxiliary equipment such as mold temperature control units, and a walkway between machines and walls, make sure there is at least 3 feet clearance around the entire footprint of the molding machine (including hoses and overhangs).
- At least once a month, machine heat exchangers should be descaled to ensure they remain effective.
The hydraulic oil used to operate the molding machine loses 40% of its cooling power for every 1/64″ buildup in the heat exchanger lines.
- Rather than relying on temperature settings, measure melt temperature as the melt leaves the nozzle.
- It is important to set the screw rotation speed correctly. Other unexplained defects include burn marks, flash, trapped air pockets, etc.
- Never let your backpressure go below 50 psi or go above 300 psi.
Heat-sensitive materials such as PVC should be used at the lower settings, and less sensitive materials such as polypropylene can be used at the higher settings.
Backpressure should be set at 50 psi if in doubt, and only increased if needed.
- For the first 1/8″ or so after opening the mold, ensure that you open the machine slowly to eliminate the vacuum created by plastic entering the mold cavities.
Mold halves will be prevented from separating with the vacuum, and if the mold is opened too quickly, it may be pulled off the platens.
- Closing the mold should be done in two steps; the first should be fast, and the second should be very slow in the last 14 inches.
The steel in the mold will be shocked if the mold halves are slammed together quickly.
- For the barrel to melt at the desired temperature, circular electric heater bands are wrapped around the heating cylinder.
If one burns out, the others continue to deliver heat if the first does.
It is cost-effective to replace the burned-out bands as soon as possible since the others must work harder for the same level of heat.
A plastic sprue that was created by the mold that was running at the time can be used to determine if a band is burned out, or even a defective plastic part.
The heater band should be activated by rubbing the plastic against it. A band that melts indicates that it is working. Plastic bands that don’t melt are bad and should be replaced.
It is essential that you replace a heater band with an exact duplicate. The wattage and voltage should be checked, along with its dimensional size.
- Ensure that the mold is fitted with a connecting strap before it is transported to be mounted in the machine.
In order to prevent the molds from coming apart during transportation, this strap should connect the two halves of the mold.
An “A” and “B” plate parting line is normally marked by a metal strap. Molds should not be installed in two halves as it is not safe or proper.
- To clear an injection barrel when shutting down a machine, scrap polyethylene (or acrylic if the color was being molded) should be run through it.
Heat can be reduced as the polyethylene is purged through the barrel.
Injection screws should be left in the forward position after the material has come out clean, essentially emptying the barrel. It is only then possible to turn off the heat.
Allow the barrel to come up to the appropriate temperature after restarting the machine. When the polyethylene is heated to a higher temperature, it will not degrade.
- Materials should flow through molding facilities in a straight-line fashion in order to maximize efficiency and productivity.
In a side-by-side layout, this is easy, but in any other layout it is more difficult.
At one end of the building, raw materials should enter, travel through necessary processes, and exit as finished products. As part of this process, packaging and preparation for shipping are required
There should be a minimum of six parting line vents on an injection mold, and more on larger molds.
Around the perimeter of the molding cavity, vents should be located every inch, and each vent should extend into the atmosphere.
Every 24 hours or more if necessary, these should be cleaned out.
Vents and venting cannot be overdone. The vents can be any width and number as long as they are thick and long enough.
Venting should be allowed on at least 30% of the parting line perimeter.
You can temporarily vent an area not vented by placing two pieces of masking tape 112″ wide on the shutoff land and leaving 112″ between them, if you believe a mold needs a vent.
To see what difference it makes, mold one or two cycles with that vent in place.
Maintaining a water temperature between two mold halves shouldn’t be done with a single mold temperature control unit.
Temperature control units should be installed in each half of the mold for maximum effectiveness.
It is possible that the temperature of the water leaving the first half may not be appropriate for the water leaving the second half if only one unit is used for both halves.
In order to accommodate the specific needs of each half, each half should be maintained separately.
With an ejection system installed, the molded part cannot stay on the half if both halves are at the same temperature
Molds should be mounted in the press with waterlines connected to the bottom and top to allow water to enter and exit.
During production, air will be purged immediately from the mold lines, ensuring that no hot spots happen.
Using a flat probed pyrometer, mold temperatures should be checked periodically.
Probes should be touched to five or six points on each mold half when checking each half separately.
In addition, each of those points or the mold halves themselves shouldn’t differ by more than 10 degrees (F).
The greater the difference, the more likely it is that the cooling system has improper conditions, which need to be corrected with a cleaning of the lines, the addition of cooling channels, the addition of baffles to the cooling lines, etc.
Whenever it can, a molded part stays on the hottest part of a mold. For most molds, this will be the side of the clamp unit, because that’s where the ejection unit will push the final assembly from.
Keeping this in mind might help control warpage and sticking.
It is recommended that the ejection half of the mold be heated 5 to 10 degrees (F) colder than the injection half in order to ensure that the molded part will remain on this half.
There is a risk of locking up the mold halves or galling in some metal parts if there is too much heat difference.
In contrast to a cold mold, a hot mold produces parts with more gloss. The part produced by a hot mold will also be lighter than the part produced by a cold mold.
In addition to conserving energy, commercial insulation sheets placed between the mold and the machine’s platens help maintain a consistent temperature throughout the mold.
A sheet with a thickness of 14″ or 12″ can be permanently mounted directly onto an injection mold’s clamping faces.
Hold the hoses that return and send water from a mold temperature control unit in your hand to check for proper flow.
There should be no difference between the two hoses if the unit maintains a set temperature properly.
Return lines are much hotter than outgoing lines if the unit is not cooling well.
As a result, the mold is still too hot, and the unit is not cool it quickly enough.
At least once a month, mold waterlines should be descaled to prevent scale deposits from interfering with their effectiveness. Mold cooling ability can be lost by 40% if 1/64″ of scale forms on a 14″ waterline.
The mold mounting clamp toes should be forced downward by a 1/8″ shim (metal washer).
As shown in the following sketch, the toe clamp’s heel needs to be adjusted so that the toe points slightly towards the platen (1/8 inch is fine).
The clamp cannot be parallel to the platen at the maximum clamping force if this step is not taken.
When clamps are adjusted perfectly parallel, they slip loose due to expansion and contraction of the mold and machine.
Due to insufficient clamp force, the mold may fall out if the clamps are adjusted so that the toe points away from the platen.
Thus, the toe should be oriented towards the platen in order to direct clamping forces there.
When straight waterlines are replaced with right-angled fittings on the mold, water will travel through those lines with more turbulence.
As a result, mold temperature control will be more efficient and mimics the “Reynolds Number” concept.
Check the mold surface temperature at least three times in different places where plastic will contact the mold.
A difference of no more than five degrees (F) should exist between the three readings.
Temperature control settings on the mold don’t really matter.
In the areas where plastic will be touched, only the mold temperature matters.
The question is, “What is the mold temperature? ”
It is not a good idea to give them the controller’s readings.
Instead, provide them with the mold’s pyrometer readings.
Molds and sprue bushing holes should never be cleaned with steel objects.
Mold and sprue bushings will be scratched by the steel, requiring expensive repairs.
Make use of wooden dowels, plastic putty knives, or brass tools instead.
Using a brass wood screw, heat up a broken-off sprue bushing and push the screw into the hardened plastic to remove the broken-off sprue.
Pull the stuck sprue out of the sprue bushing using brass pliers clamped over the screw head.
Future use of the screw is easily accomplished by unscrewing it from the plastic.
Molds with deep walls can be extracted from stuck plastic using a hacksaw-type blade made of copper or brass.
The mold will be scratched if you use steel blades.
A good injection gate placement is so that the plastic melts at the thickest part of the mold cavity.
Flowing into the thinner section, the material will become squeezed as it fills the cavity.
By building up resistance, pressure builds up, enabling the filling process to be completed.
The melt must meet some type of resistance in order to develop injection pressure.
In fact, the material builds up some high packing pressure when it fills the cavity after passing through the heated barrel and entering the mold.
In some cases, molds can cost as much as millions of dollars for large, complex molds, while for smaller, simpler molds they can cost just a few thousand dollars.
While the customer usually bears the cost of this investment, the molder is responsible for maintaining the mold while it is in his or her possession.
The average annual maintenance cost for molds is 5% to 7% of the initial cost.
Construction of the mold is the responsibility of the moldmaker, and mold design is the responsibility of the mold designer.
If you are going to make a production run, save the “last shot” and put it in storage along with the mold.
It should include everything produced in a single cycle, including parts runner, flash, etc.
The mold maintenance area can be seen in this picture, which shows how parts are produced.
By inspecting the parts, a repair person can determine whether the parting line is fit, the cavity surface is in good condition, and the ejector pin is positioned correctly.
Molding room personnel should accompany this final shot with a written explanation of the problems they see.
Molds should be thoroughly cleaned, inspected, and coated with a rust preventive material (primarily inside, but lightly outside) as soon as they are removed from the molding machine.
When storing something for a long period of time (more than 30 days), the coating should be especially heavy.
Cleaning out the waterlines and coating them is also important.
In order to prevent deposits from returning, waterlines should be rinsed with an acid solution.
You can only develop injection pressure by having the melt meet up with a resistance of some type.
While that does happen slightly while the material is moving through the heated barrel and into the mold, the high packing pressure buildup occurs only when the material fills the cavity.
20 – Molds are expensive, ranging from a few thousand dollars for small and simple molds to hundreds of thousands (even millions) of dollars for large, sophisticated molds.
The customer usually bears this investment, but the molder takes on the responsibility to maintain that mold while in the molders possession.
Maintenance costs for molds average around 5% to 7% of the initial cost of the mold per year.
The moldmaker is responsible for the quality of construction in the mold, and the mold designer takes responsibility for the performance of the mold design.
21 – It is a good idea to save the “last shot” from any production run and keep it with the mold in storage.
The last shot should be complete and includes the parts runner, flash, and anything else produced in a single cycle.
This provides a visual example of how the parts were being produced for the mold maintenance area.
A repair person can inspect the parts to determine the fitness of the parting line, cavity surface condition, ejector pin position, and other pertinent information.
A written statement of problems as seen by the molding room personnel should accompany this last shot.
22 – When the mold is pulled from the molding machine, it should be thoroughly cleaned, inspected, and coated (primarily inside but lightly outside) with a rust preventive material to minimize the possibility of damaging rust forming.
The coating should be especially heavy for long-term (over 30 days) storage.
It is important to clean out the waterlines and coat them also.
An acid rinse of waterlines is recommended to remove deposits and protect against their return.
1 – Any successful method of controlling the quality and the cost of a product depends heavily on the consistency of the process used to manufacture that product.
Consistency can only be achieved by tightly controlling as many parameters as possible used during the manufacturing process.
2 – The environment surrounding a molding machine location has an immediate effect on the molding process.
For example, the process parameter settings for a machine molding parts on a sunny day with hot, dry outside temperatures may be totally different for that same job running during a rainy, cold night.
You can anticipate having to adjust the process to meet the environmental conditions.
Some very fortunate molders work in controlled environments with air conditioning and humidity controls in order to maintain consistent process conditions 24 hours a day.
3 – Use a mold release spray only for the first few shots coming off of a brand-new mold or at the start of a production run after the mold was placed in storage.
Mold releases will keep plastic molecules from bonding and create a molded part that will be susceptible to cracking or breaking.
If a part tends to stick, make a determination as to why it is sticking, and remedy the cause. Undercuts, rough cavity surfaces, inadequate draft angles, contaminated resin, and improper process conditions are the most common causes of plastic sticking in molds.
4 – Any actual dimensional inspections should only be made on a molded part once it has cooled to room temperature, which takes approximately 3 hours.
Although molded thermoplastic products appear to be stable immediately, they will continue to cool and shrink for up to 30 days after being ejected from the mold.
Most (95%) of the total shrinkage will occur during the time the plastic is cooling in the mold.
The remaining 5% will take place over the next 30 days, but most of that will happen within the first few hours after being ejected from the mold.
5 – Holding pressure settings can normally be set at half the initial main injection pressure, as a rule of thumb.
Adjustments may need to be made to that rule based on a variety of conditions, but it is a good place to start.
6 – Hold Time is used to hold pressure against the plastic as it cools enough to start to solidify in the mold.
Once the material in the gate has “frozen,” the Hold Time can be stopped, but not until then.
If the Hold Time is removed too soon, the still-molten material in the mold cavity will actually be sucked back out through the gate, thus causing inconsistent part weight.
7 – Round bubbles within a transparent part means there is too much moisture in part.
Oval or elongated bubbles mean there is excessive shrinkage in that area of the part.
8 – There are three major rules of thumb to follow when making adjustments to molding parameters:
- a – only one single change should be made at any time;
- b – a machine must be allowed to stabilize for a period of 10 to 20 cycles after any single change is made to the process:
- c – If a change does not solve the problem, change it back again and wait for 10-20 more cycles before making a different change.
9 – The overall mold cycle time is heavily influenced by the opening and closing distances of the molding machine.
Each mold should be tailored to open and close only as far as necessary to get the finished parts out of the mold.
Cycle times are costly, and for every second you can trim off the overall cycle, you will save approximately $10,000 annually if the mold were to run continuously during that time.
10 – Proper mold temperatures are critical to quality molding. Many molders believe that a colder mold means faster cycles and higher profits than a warmer mold.
In fact, especially when molding crystalline materials, the plastic may require a slow cool down to attain maximum physical and visual properties in the molded part.
Actually, there are very few situations where “cold” molds should be utilized. It is best to follow the material supplier’s recommendations for mold temperature settings for any specific plastic.
11 – Water flow through a mold is described as being in one of 2 methods; laminar or turbulent. Laminar means the water flows in “stacked layers” as it moves through the lines and only the outer layers actually pull heat from the mold steel.
Turbulent means the water is constantly being tumbled and mixed as it moves through the lines, resulting in the water coming in contact with the mold steel.
Turbulent flow is desired because it is 5 to 6 times more efficient and less costly than the laminar method.
12 – To determine if you have turbulent flow in your water lines, you can feel the entry and exit hoses coming from the mold itself.
The exit hose should be no more than 10 degrees (F) hotter than the entry hose. While that may sound incorrect, remember that we are trying to maintain a mold temperature at a certain value.
The ideal result would be that the ingoing water and the outgoing water would be identical.
That would mean we are maintaining it. However, in the real world, we can expect up to a 10 degree (F) difference in the hoses to indicate proper turbulence.
13 – Creating turbulence in a mold waterline can be done using a scientific principle and formula called Reynold’s Number.
This defines actual mold waterline diameters, water flow rates, water temperature, and water viscosity values.
Creating turbulence can also be accomplished easily by making sure there are flow obstructions within the waterlines.
An example of a flow obstruction is using a right-angled connector fitting where the water hose attaches to the mold and another at the exit site.
You can also install a “baffle” device (available from mold suppliers such as DME) in the line that disperses the water through a series of baffle plates.
14 – Too often, a technician, engineer, or operator will be presented with a molding defect and will start turning dials, flipping switches, and adjusting timers without understanding what they are doing or knowing what results to expect.
Due to schedule requirements, a quick fix is often desired, and the technician is pushed into a management-directed panic mode.
The result is pandemonium, as attempts to correct defects only seem to make matters worse, and the entire molding process quickly goes out of control.
While this is a standard scenario in most molding companies (but not highly publicized nor recognized), it does not have to be that way.
The situation should be such that the troubleshooting individual (regardless of title) can objectively analyze a molding defect and determine a probable solution before making any changes.
The solution should be attempted, followed by another decision. Each solution should be determined independently and rationally.
There should be no guesswork, and, when necessary, assistance from outside sources should be solicited and welcomed.
15 – There are over 200 different parameters that must be established and controlled to achieve proper injection molding of a plastic part.
These parameters fall within four major areas: pressure, temperature, time, and distance, as shown below. Notice that the circles intertwine.
That shows that changing a parameter in any one area may also affect parameters in other areas.
16 – In life, pressure causes stress. In the injection molding process, injection pressure will create molded-in stress to the molded product.
The higher the pressure, the greater the stress. And that stress will be released at some time.
There is no question as to its being released, only as to when it will be released.
The greater the stress, the greater the impact on the molded part when it is released.
Usually, the stress releases in the form of cracking or shattering, but it can also manifest as warpage or discoloration.
17 – To minimize molded-in stress in a molded part(which later releases in the form of cracking, shattering, breaking, discoloration, or warping), we need to attempt to process the molten plastic at the lowest possible heat and the lowest possible pressure, and in the lowest amount of time.
Our goal is to allow the plastic to enter the mold cavity as quickly as possible with the least amount of stress built up.
Increased heat and pressure will increase stress. Increased time will increase the cost. Using a material supplier’s datasheet will ensure these parameters are met.
1 – “Auxiliary Equipment” is considered any additional equipment required for assisting the primary equipment (molding machine) in producing the final production of the molded parts. It includes items such as granulators, mold temperature controllers, hopper loaders, and the like.
These should not be confused with “Secondary Equipment,” which is considered and additional equipment required to manipulate the plastic parts after being molded. This would include drilling machines, paint systems, packaging equipment, and the like.
If robots are used for removing molded parts from the molding machine, they are considered auxiliary equipment. If they are used to pick molded parts from a container and place them in shipping boxes, they are considered secondary equipment.
2 – Always keep your hopper cover in place. One major source of material contamination is ceiling debris such as dust, water condensation on overhead pipes, and other air-borne trash.
3 – Before filling an empty hopper, make sure your hopper is clean. Blow it out with air to remove any dust that may be present. Then use a clean shop rag sprayed lightly with vegetable oil (such as PAM) and wipe down the walls inside the hopper to catch any fines that might be left from the last run.
Do not use a paper towel as this will leave paper dust which will cause defective parts.
4 – Make sure the screen in your granulator (grinder) has the proper hole and space settings for making regrind from the material you are grinding. If the holes are too large, the regrind material will take much longer to melt than the virgin it is mixed with. That can result in an improperly structured “melt” and will cause defective molded products.
5 – Mold temperature controllers are designed to “maintain” a predetermined temperature of the injection mold by circulating water (or, in some cases, oil for mold temperatures above 200 degrees F) through the mold using hoses connected to metal fittings or pipes on the mold.
The controller compares the average temperature of the circulated water to the preset desired temperature and either add cold water or adds more heat (using electric heater coils) to make sure the mold keeps a stable temperature for molding.
Understanding this process is necessary for knowing how to control it. There is an indicator on the unit that displays the temperature setting you have set for the mold you are running. BUT, that is NOT actually the temperature of the mold.
So if someone were to ask you at what temperature are you running your mold, you should NOT tell them the setting on the water control unit. You should actually take a pyrometer and check a few points on the molding surfaces of the mold in an open position. You will find there may be a big difference between those two values.
6 – Vacuum loaders are commonly used for transporting material from a container to a hopper on the molding machine. Due to friction, a buildup of dust will accumulate inside the clear plastic hose used for that transporting.
When changing material types or colors, removing that fine dust caused by the previous material is critical. You can use a clean shop rag to clean the dust away by simply placing it in the hose and vacuuming it through to the hopper.
Repeat as necessary, and on the final trip, lightly spray the rag with PAM (or you can use a very light coat of mold release spray). This will aid in getting the last few particles of dust and will also help keep dust from forming during the next molding run.
7 – A machine’s hopper is designed as a basic unit that holds approximately 2 hours’ worth of gen, eric polystyrene. The bigger the machine, the larger the hopper, but it is still designed for 2 hours’ worth of plastic.
The reason is that the machine manufacturers know that it must dry plastic material prior to molding but that it only stays dry from 2 to 4 hours after the initial drying activity. Hopper extensions are available but should only be used if hopper dryer units are also installed to keep the plastic dry while it resides in the hopper.
1 – Keep containers of virgin and regrind materials well covered and well-identified.It is human nature to consider large cardboard uncovered barrel to be nothing more than a trash barrel when located in a manufacturing facility.
Therefore, keep material barrels tightly covered and post a large, bold letter sign on each one stating what is in it. Contaminated material in an uncovered barrel container
2 – It is well-known that certain materials (such as nylon, ABS, polycarbonate) are hygroscopic by nature and absorb moisture directly from their surroundings. We are told that they must be dried before molding because the moisture turns to steam in the injection barrel and causes defective parts.
But, it is beneficial to pre-dry ALL materials prior to molding. The non-hygroscopic materials (such as polypropylene and polyethylene) may not absorb moisture. However, moisture can still be present in the form of condensation on the pellet surfaces, especially during humid summer months.
Because drying involves heat, there is also the extra benefit of preheating the plastic in preparation for molding. Once you dry a load of material, it must be used within 2-4 hours, or it will require more drying.
3 – A change of each 10 degrees (F) in a material’s melt temperature will require 10 cycles before the barrel temperature has fully stabilized.
Thus a 30-degree increase or decrease will require a minimum of 30 cycles to stabilize.
4 – Material additives usually fall into two basic categories; reinforcements and fillers. While it is true that reinforcement may be considered a filler, it is not necessarily true that a filler can be considered reinforcement.
Reinforcements are those additives that are used to enhance physical strength properties. Fillers are those additives that are used to enhance properties other than strength. So, although it has become common practice to identify a certain material as being “glass-filled,” it is in reality simply “glass-reinforced.”
5 – Understand the actual temperature requirement for any specific plastic material. The material suppliers will list a certain temperature range within which that material should be molded, and they refer to it as the “Melt Temperature.”
For instance, the melt temperature range for polycarbonate is usually listed at between 500 F and 600 F.But, the ideal temperature should be considered the midpoint of that range, 550 F.
You should attempt to optimize your process to utilize the polycarbonate running at 550 F as it leaves the nozzle and enters the mold. You can then adjust up or down from that value to accommodate actual circumstances.
6 – You may want to keep a copy of specific plastic material datasheets with your notes. I find reading them to be very eye-opening. And, find as much data as you can find, even if it may seem extraneous to your project.
The material people know their resin better than anyone else, and you need to take advantage of their expertise as much as possible. In fact, don’t hesitate to pick up the phone and call them. They’ll be thrilled.
7 – Once a material has been properly dried, it must be molded within 2-4 hours or need drying again. That applies to regrind, too, and means if you are using a granulator on a specific molding job, you must dry that regrind before people can use it again. You cannot simply add it back to the hopper unless you are doing so within the 2-4 hour time margin.
8 – Regrind will require a higher temperature to melt than virgin. If too much regrind is being used compared to a virgin, the regrind may require so much heat to melt it that the virgin may degrade.
It may be wiser to use all regrind than to use a regrind/virgin blend that contains over 50% regrind.
Examples: Virgin and Regrind pellet sizes
9 – A change in the material may require a rebuild for the mold. Every mold is built to accommodate a specific plastic material. That material is chosen for specific characteristics and values it will instill in a properly designed and processed molded product.
The moldmaker must consider such items as shrink factors, gloss requirements, dimensional stability, and a host of other parameters when building the mold that will produce a product made from a certain plastic.
Therefore, the final mold is designed and built to run only 1 plastic (of the over 50,000 available). If any other plastic is injected into the mold, the resultant product may not even resemble the original one.
10 – Most molding processes and materials can utilize the addition of regrind to the virgin pellets without affecting the required molded product properties. The normally accepted level of regrind use is a maximum of 15%. If a runner system scrap weighs up to 15% of the total shot size, you can use that generated regrind to help reduce operating costs.
If it is more than 15%, you may have to store the amount over 15% and use it elsewhere or sell it to other molders or material brokers who specialize in regrind purchase.
11 – If your molding operation tends to generate a lot of regrinding in the form of scrap runners and sprues and defective parts, and you have no source you can sell it to, you may still be able to use it.
Consider designing a “give-away” item such as a plastic letter opener or drinking cup, or key tabs. These can carry your advertising information and be handed out to potential and existing customers.
Or you may be able to invent a molded product you could actually sell and use your regrind or extra virgin to produce it.
12 – If you find that the molding process suddenly goes out of control and parts are being molded at different weights and fill patterns from shot to shot, you may have a material issue. Your material may be out of spec and incapable of producing good parts.
One quick way to check for an inconsistent material batch is to use the Melt Flow Index value. Each batch of incoming material should have the MFI run and the value recorded. Then, if problems arise, you can check the MFI for a given batch to see what the MFI value was.
This can be compared with any batch you ran where the parts were all good. If the MFI values are noticeably different, you can be assured that you have a materials issue.
Melt Flow System Melt Flow Concept
The Melt Flow Index is measured on a machine that injects a specially prepared sample of plastic through a heated plunger device, similar to an injection needle. The value is a number that reflects how much plastic is extruded through the barrel in a given amount of time, and this can be used to determine the flowability of a specific batch of plastic.
A series of tests done on a single batch will show if that batch is consistent within itself and other batches of the same material.
13 – Moisture is the number “1” cause of molded defects.
The moisture level of resins must be in the area of 1/10th of 1 percent by weight. If the moisture level is higher than that, the moisture turns to steam as it travels through the heating cylinder of the molding machine.
The steam prevents plastic molecules from bonding together properly, and weak parts will be produced. In addition, the visual evidence of this steam (splay) is usually not acceptable from an aesthetic standpoint.
1 – To attain consistency in operator-controlled cycles, ask the operator to “be ready” for gate opening by anticipating the event. The operator can count, hum a tune, recite a poem, sing a song, tap a foot, listen for valves switching, or any number of things to accomplish this feat. Explain the importance of consistency in the cycle, and the operator will probably come up with some ingenious methods him/her self.
2 – The timing of the gate closing must be controlled as consistently as possible. The operators should be trained and informed that any slight change in the pace at which they close the gate may greatly affect the overall machine cycle.
In fact, an increase of 1 second in the average cycle time of 30 seconds can result in a loss of approximately $10,000 annually depending on the number of cavities, hourly wages, and cost of utilities. The molder must pay the additional cost because the customer is not responsible for the increased cycle time.
3 – The most common cause of defects in molded parts is the molding machine which is 60% of the time. This is followed by 20% caused by the mold, 10% caused by the material, and only 10% caused by the operator.
However, it has been common for us to first consider the operator (assuming one is present) as being the major cause of our defect problems. Based on the figures cited, we now know the operator is the last place to look for defect causes.
4 – Of all the various components that come together to make up the injection molding process, the machine OPERATOR is by far the most important. All of the equipment, including the machine and all the auxiliaries, and the mold, can be fine-tuned to run flawlessly from cycle to cycle. But the operator is the only component that actually can think, and therefore can adjust his/her own activities at once to whatever is needed from cycle to cycle.
This attribute can be extremely beneficial to an employer because the operator can make on-the-spot observations regarding how well (or poorly) a job is running. This ability can keep a machine from producing dozens, or even hundreds or thousands, of reject parts.
In addition, the operator is the only person in the constant vicinity of the molding process and soon can identify every noise, odor, visual image, and timing of the entire process.He/she can quickly notice any unusual changes in these actions and can immediately inform a supervisor, or if allowed. Can make changes to the process to bring everything back to normal.
5 – When the quality level of the molded part demands the highest degree of consistency during the molding cycle, it is time to reconsider the use of an operator to open and close the safety gate, thus controlling the cycle time of the operation.
To attain the required consistency, it will be necessary to automate the entire molding process. This decision will demand the use of robots and computer-controlled conveyance systems at a minimum. The investment will need to be scrutinized to determine proper financial payoffs, and the existing operator can be utilized in an inspection role to ensure the process is working properly.
6 – If a molding machine, mold, and accessory equipment, along with product design, are analyzed and prepared in advance, an automated molding process can be very successful and profitable. The initial investment may be substantial but, in the long run, will surely pay for itself in lower defects, higher quality, faster cycles, and lower wages.
7 – Because good employees with proper training are difficult to find, the automation concept should not be considered as an effort to simply get rid of employee costs. But it can be used as a tool to fill in for attrition over time.
1 – You can usually tell if a machine’s hydraulic oil is going bad by looking, touching, and smelling. If the oil feels like it contains microscopic lumps of soapy particles, some additives have come out of suspension, and it is degraded.
If the oil is dark rather than light-colored, it is thermally degraded and worn out. And, if it smells burnt, it is. Replace it. On average, the oil needs to be replaced at least once a year and more often if conditions warrant.
2 – In an injection molding machine utilizing a screw-design injection unit, the rotation of the screw bringing new material into the barrel, will generate a lot of heat. This helps the heater bands on the outside of the barrel to maintain the proper heat for the plastic being melted.
Once a machine has been stabilized for a production run, the heater bands will only need to activate approximately 30% of the time as a result of the screw rotation generating its portion of the heat to the melt.
3 – A 40 degree (F) change in melt temperature can result in a change in dimensions of the final molded part by ½%.Therefore, if you run a melt temperature at 400 F and increase it to 440 F, your product dimensions will increase by ½% and probably go out of specification. Hotter materials will produce larger parts, and cooler materials will produce smaller parts.
4 – Although the fact in #3 above is true, you should never try to control part dimensions by adjusting melt or mold temperatures.
You should always attempt to produce a part using ideal, efficient, nominal processing. If the parts produced do not meet specifications, you should remove the mold and have it revised under those circumstances.
5 – How it all began – the start of the plastic injection molding industry.
In 1868, an enterprising young gentleman named John Wesley Hyatt developed a plastic material called Celluloid and entered it in a contest created by a billiard ball manufacturer that was held to find a substitute for ivory which was becoming expensive and difficult to obtain.
Celluloid was actually invented in 1851 by Alexander Parkes, but Hyatt perfected it to the point of being able to process it into a finished form. He used it to replace the billiard ball ivory and won the contest’s grand prize of $10,000, a rich man’s sum in those days.
Unfortunately, after the prize was won, some billiard balls exploded on impact during a demonstration (due to the instability and high flammability of Celluloid). Further perfection was required to use it in commercial ventures.
But, the Plastics Industry was born. It would start to flourish when John Wesley Hyatt and his brother Isaiah patented the first injection molding machine (1872). They were able to injection mold Celluloid plastic. The industry of plastic injection molding was begun.
6 – Plastic “troubleshooting” can be defined as an activity to determine the cause of and solution for defects found in a molded part.
This activity usually occurs while parts are being molded and occur when the normal production of acceptable parts is interrupted by the unexpected production of one or more defective, unacceptable parts.
In some cases, troubleshooting occurs when analyzing parts previously molded, such as when parts are returned from the field because they did not properly perform their intended function. Usually, this situation is analyzed using failure analysis activities, but troubleshooting may also be called upon.
7 – Outside of basic product design issues, plastic injection molded defects can be traced to problems with one or more of the following four items: the molding machine, the mold; the plastic material; and the molding machine operator.
The most interesting thing is what percentage of each of these items contributes to the defects’ cause. The most common cause of defects in molded parts is the molding machine itself, 60% of the time.
This is followed by 20% of the defects caused by the mold, 10% caused by the material, and only 10% caused by the operator. However, it has been common for us to first consider the operator (assuming one is present) as the major cause of our defect problems. Based on the figures shown here, the operator is the last place to look for defect causes.
8 – It is important to understand one major economic fact. Someone will pay for every part molded, whether it is good or bad. The customer will pay for every GOOD part molded, and an injection molding company will pay for every BAD part that is molded. So, bad parts must be discovered quickly so they can make adjustments in the machine settings and/or material use and mold conditions.
9 – How Many Plastics Are There? There are approximately 50,000 different plastic materials (polymers) now being manufactured, including the alloys and blends, and each year there are at least 500 more being introduced.
10 – For the purpose of injection molding, the word PLASTIC may be defined as Any complex, organic, polymerized compound capable of being shaped or formed.
11 – Because of the additional cost to perform secondary operations, it must be considered that secondary operations can be eliminated by proper part design and proper mold design. This can be stated as follows:
“ALL secondary operations can be eliminated through part design and mold design IF cost and time are not a consideration.”
12 – An injection molding plant can be designed to a variety of layouts depending on what will be molded there, the number of machines, degree of automation, and a host of other issues. Normally the molding facility will include a number of supporting areas such as front offices, quality control, maintenance, etc.
While any layout may be acceptable, the following bird’s eye sketch depicts some concepts that should be considered.
Note that the supporting areas surround the upper and left sides of the main molding area. Also, the flow of operations is direct from top to bottom in the sketch, and expansion is allowed without upsetting existing areas.
Can “Quality” be defined?
Dictionaries list many definitions for “quality,” but those that might directly apply to injection molded products include “…superiority of kind” and “…degree or level of excellence”. While the actual definition may be debatable, the amount of time and money spent each year by our industry to achieve “high quality” standards demonstrates the importance of being able to provide a customer (or potential customer) a product exactly to the customers’ expectations and/or specifications.
Is this possible? In a word, yes, but it can be costly if not reasonably defined. The actual definition of quality must be created for each and every product being molded and must be as detailed and quantitative as possible without going to extremes. It is not reasonable, for instance, for a customer to state “part must be 6 inches in length”. The 6″ dimension must have a tolerance placed on it to show the highest and lowest dimensions, which still allow the part to be used.
For normal purposes, this might be plus or minus 1/8 inch. But for extremely critical fit or function, the tolerance might be plus or minus 0.001 inches. This tighter tolerance reduces the processing window for the molder to a point at which constant “tweaking” must be performed to mold parts that fall within the acceptable range.
During the tweaking process, many defective parts will be reduced, and these will have to be scrapped or reworked. Of course, every part must be paid for, so the cost of the scrapped parts must be absorbed by the cost for the accepted parts, which results in a much higher piece price than if the looser tolerance had been specified.
Any dimension tolerance or characteristic note regarding the expected quality level of a molded part must be defined in proper detail. Saying that a part must be “blue” in color is too vague because there are thousands of shades of blue available to the molder.
The vague statement allows parts to be molded in anywhere from light, pale blue to a dark, midnight blue. A standard color code number should be matched from one of the many industry color palettes that are available. Of course, if the shade or hue is NOT important, the vague statement is acceptable.
Then, as long as the parts are molded in ANY blue, they can be determined to have an extremely high-quality level, as least as far as the color match goes. The cost for this low-detail quality is much less than for a high-detail quality.
Quality requirements are not difficult to determine once it is understood that they must be well defined. In most cases, the function of the finished part (or assembly of parts) determines whether or not it is acceptable. If functional parts are produced and acceptable, the non-conforming dimensions can be changed on the print to reflect the acceptance.
This can become a major problem with many customers, but the molder should make the request because the part print is considered the only legal document concerning part quality. Letterheads with formal exceptions are only temporary and cannot be used as a permanent override of the print callouts.
Phone calls and scraps of paper with scribbled notes will not stand up in court. So the molder must be able to make parts to the legally accepted part print. But, the part print must be defined properly to allow the molder to do that. It is usually easier, and less expensive, and time-consuming to change the part print to match the parts rather than change the tooling and process to match the print.
The Quality Manual
Every injection molding company needs documents that explain standard procedures to perform for specific conditions. None of these documents is more important than the quality manual. This document must be created to delineate individual and organizational responsibilities in connection with process control, product test requirements, and audit responsibilities for the total manufacturing of plastic parts and assemblies.
The quality manual should be designed to mandate that certain procedures and methods be used to ensure that the highest possible level of excellence (as negotiated with the customer) is achieved in the production, inspection, testing, and shipping (including special packaging) of the finished product. It should direct the manufacturing management to ensure that manufacturing personnel utilizes the manufacturing equipment prescribed in an accompanying manufacturing certification manual.
With the cooperation of processing departments, the manual should also dictate the listing and monitoring of critical parameter settings, times, and temperatures related to the molding operation on a daily basis. It should also outline the testing procedures and methods required to be performed (at specified frequencies) by quality or manufacturing personnel for critical dimensions as determined by the customer. And, periodic auditing (regular and random) of finished products should be outlined and implemented through the direction of the quality manual.
Other items that are typically included in a quality manual are detailed inspection procedures for specific parts, material vendor certification requirements, material handling and tracing procedures, and specifications and procedures for special testing (such as melt flow index or moisture content) to ensure molding uniformity.