Tips for Plastic Injection Molders

plastic mold structure

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…


injection molding machine
1 – After installing a mold, check the alignment of the machine nozzle to the mold sprue bushing with a thin piece of cardboard (shirt cardboard) placed between the nozzle and sprue bushing.

Bring the nozzle to position and take a shot right through the cardboard.

Pull back the nozzle, and you will see an impression in the cardboard that displays exactly how the nozzle is aligned with the sprue bushing hole.

Make necessary adjustments to bring the 2 together exactly.

2 – Ideally, the injection unit should be sized so that it contains two full cycles’ worth of material.

In other words, 50% of the capacity of the injection cylinder should be emptied every time a cycle is completed.

This is referred to as the amount of “shot” a machine takes for each cycle because all of this material “shoots” into the mold during the injection phase.

In fact, you are using a half-barrel to make one shot while the other half is preparing for the next shot. This gives you the greatest degree of consistency.

The 50% rule is ideal, but a shot size should never be outside the range of a minimum of 20% or a maximum of 80%.

For example, if the total amount of material that is used for one complete cycle is 2 ounces, the ideal injection unit for that application would exist on a machine with a 4-ounce cylinder because 50% of 4 ounces equals 2 ounces, which is the initial requirement.

And, using the 20% to 80% minimum/maximum limits, the 2-ounce shot could be produced on as small a machine as one with a 2-1/2 ounce cylinder (80%) and on as large a machine as one with a 10-ounce cylinder (20%).

What determines this is the heat sensitivity of the specific material being molded.

Some materials are very heat sensitive and burn easily, while others are much less heat sensitive and are able to withstand longer exposures to elevated temperatures.

Heat sensitivity of a specific material is critical because it determines the amount of residence time that the material is allowed to stay within the heated injection cylinder before it begins to degrade.

Degraded material will not produce quality products.

The 50% rule-of-thumb noted earlier ensures that no material will degrade while being molded regardless of its allowed residence time.

The 20% rule-of-thumb applies to materials with low heat sensitivity, such as polyethylene.

And the 80% applies to materials that extremely heat sensitive such as PVC.

3 – Injection units are universally rated by the amount of polystyrene they can hold.

A conversion is required to determine how much of any other plastic they can hold, and this is done by comparing specific gravity values.

The specific gravity value of the other selected material (for example, polycarbonate) is divided by the specific gravity value of polystyrene to determine how much of the other material can be held in the cylinder.

For our example, the poly-carbonate s.g. of 1.20 is divided by the polystyrene s.g. of 1.04, giving a value of 1.15.

That 1.15 value is multiplied by the machine’s rating in ounces. Let’s say our example machine is rated as an 8-ounce machine.

That means it’s capable of injecting a maximum of 8 ounces of polystyrene. If we multiply that 8-ounce rating by the value we found earlier of 1.15; we find that it may also inject up to 9.2 ounces of polycarbonate.

4 – The injection temperature control indicator settings do NOT display the actual temperature of the plastic melted material.

It should only be used as a reference point. The actual melt temperature can only be measured by a probe that checks the material as it leaves the nozzle of the molding machine and enters the mold.

There may be as much as 30 degrees or more difference between those 2 readings, but the one that determines process conditions is the one that is taken by the probe.

5 – In most cases, the injection barrel heats should be established such that the plastic is heated lower at the rear zone and increased to be hottest at the front zone, with the nozzle set to be approximately 10 degrees (F) higher than the front zone of the barrel.

6 – If the injection screw turns while it is injecting material during the molding process, it indicates that the check ring is worn or cracked and must be replaced to maintain pressure on the plastic material.

7 – The Clamp Unit of an injection molding machine is rated by the maximum amount of clamp force that the machine is capable of producing.

This force is required in order to keep the mold closed during the injection process.

You should only use enough clamps to overcome the injection pressure as the excessive clamp will damage the mold.

For example, if you need 10,000 psi of injection pressure (5 tons), you would need only about 12,000 psi (6 tons) of clamp pressure to keep the mold closed.

8 – Use a machinist’s level (not a carpenter’s level) on tie bars when installing the machine.

Check for levelness every 6 months. An out-of-level machine will disrupt the process and material flow and could indicate machine frame warpage or floor settling.

9 – When laying out a floor plan, leave a minimum of 3 feet clearance around the entire molding machine footprint (including hoses and overhangs) to allow for maintenance, product flow, auxiliary equipment such as mold temperature control units and walkway for movement between machines and walls.

10 – Machine heat exchangers should be descaled at least once a month to make sure no scale deposits interfere with the unit’s effectiveness.

A 1/64” of buildup in the heat exchanger lines will result in a 40% loss of the cooling ability of the hydraulic oil used to operate the molding machine.

11 – For accuracy, measure actual melt temp with a probe as it leaves the nozzle instead of relying on temperature control settings.

12 – Make sure your screw rotation speed is set properly. If not, you may find burn marks, flash, trapped air pockets, and a variety of other unexplained defects.

13 – Backpressure should never be less than 50 psi and never more than 300 psi.

The lower settings should be used for heat-sensitive materials like PVC, while the higher settings can be used for less sensitive materials like polypropylene.

If in doubt, set the backpressure for 50 psi and only increase if necessary.

14 – When opening the mold, make sure the machine opens SLOWLY for the first 1/8” or so to allow time for the vacuum created by plastic entering the cavities in the mold to be eliminated.

This vacuum will keep the mold halves from separating and may pull the mold right off the platens if you open too quickly.

15 – When closing the mold, do so in 2 steps, the first being fast, and the second switching to very slow in the last ¼” or so.

Quickly slamming the mold halves together will shock the steel and cause cracking of the mold.

16 – The heating cylinder is wrapped with a series of circular electric heater bands to provide the required melt temperature within the barrel.

These will eventually burn out, but if one does burn out, the others stay active to ensure the heat is still being applied.

Of course, the others must work harder to provide the same level of heat, so it is cost-effective to replace the burned-out bands as soon as possible.

A quick way of determining if a band is burned out is to use a plastic sprue created by the mold running in the machine at the time, or even a plastic part that was defective.

Rub the plastic against the heater band when it is supposed to be activated. If the plastic melts, the band is working. If the plastic does not melt, the band is bad and needs replacement.

When replacing a heater band, you must make sure you replace it with an exact duplicate. Check for dimensional size as well as voltage and wattage.

17 – When transporting it to be mounted in the machine, make sure that the mold has a connecting strap installed.

This strap should connect the two halves of the mold and keep them from coming apart during transportation.

Normally this is a metal strap mounted across the “A” and “B” plate parting line. It is not safe or proper practice to install the mold as two separated halves.

18 – When shutting down a machine, the injection barrel should be purged clean by running scrap polyethylene through it (acrylic can be used first if the color was being molded).

As the polyethylene is purged through the barrel, which can reduce heat.

After the material comes out clean, the injection screw should be left in the forward position, which basically empties the barrel. Only then can the heat be turned off.

Upon restarting the machine, let the barrel come up to whatever heat is required for the next run. The polyethylene will not degrade under higher heat.

19 – For the greatest degree of efficiency and the highest level of productivity, the flow of materials through a molding facility should be as close to straight-line as possible.

This is easily accomplished with a side-by-side machine layout but more difficult with any other format.

The raw materials should enter one end of the building and travel through required processes to exit as a finished product at the other end of the building. This includes packaging and preparing for shipment.

  • MOLD

plastic mold
plastic mold

1 – An injection mold should have a minimum of 6 parting line vents, more for larger molds.

There should be a vent at each inch interval around the perimeter of the molding cavity, and each vent should extend to the outer edge of the mold into the atmosphere.

These should be cleaned out at least once every 24 hours and more if needed.
There cannot be too many vents or too much venting. As long as the vents are of the proper thickness and length, they can be any width, and they can be any number.

A good rule of thumb is to allow at least 30% of the parting line perimeter for venting. The following drawing demonstrates this.

2 – If you believe a mold needs a vent somewhere that is not vented, you can create a temporary vent by placing two pieces of ½” wide masking tape on the shutoff land in the area in question, leaving a ½” gap between them.

That will act as a vent and allow you to mold 1 or 2 cycles to see what difference it makes in the molded part.

3 – Never use a single mold temperature control unit to maintain a water temperature between 2 mold halves.

Each mold half should have its own temperature control unit for complete effectiveness.

If only one unit is used for both halves, the water leaving the first half may not be at the right temperature for the second half.

Each half should be maintained separately to accommodate the specific needs of each half.

And both halves should never be at the same temperature, or you will not be able to ensure that the molded part stays on the half with an ejection system in place.

4 – Waterlines should be hooked to the mold after it is mounted in the press so that water enters the mold near the bottom and exits the mold near the top.

That ensures that any air in the mold lines will be purged out immediately and never cause hot spots in the mold during the production run.

5 – it should check mold temperatures periodically by using a flat probed pyrometer.

Each mold half should be checked separately, and the probe should be touched to 5 or 6 points on each mold half.

There should not be more than a 10 degree (F) difference between any 2 of those points or between the two mold halves themselves.

If greater differences occur, it indicates improper cooling conditions, and this should be rectified by cleaning lines, adding cooling channels, inserting baffles to the cooling lines, etc.

6 – A molded part will always try to stay on the hottest half of a mold. In most cases, we want that to be the Clamp Unit side of the mold because that is where the ejection unit is to push the final part out of the mold.

Warpage and sticking might be controlled by keeping this thought in mind.

7 – The ejection half of the mold should be 5 to 10 degrees (F) hotter than the injection half to ensure the molded part will stay on the ejection half.

Be careful, though, because too much heat difference will cause a “lockup” of the 2 mold halves or galling of some metal components.

8 – A hot mold will produce a part with a finish that has more gloss than a part molded on a cold mold. A hot mold will also produce a darker part than a cold mold.

9 – Use of commercial insulation sheets placed between the mold and the platens of the machine will result in less energy used for maintaining mold temperatures and create greater consistency of temperature throughout the mold.

These are available in ¼” or ½” thick sheets and can be permanently mounted directly on the clamping faces of the injection mold halves.

10 – To check for proper water flow from and to a mold temperature control unit, hold your hand on the outgoing and returning hoses.

If the unit is properly maintaining a set temperature, you should feel NO difference between the two hoses.

If the unit is not cooling enough, you will find that the return line is much hotter than the outgoing line.

That is because there is still too much heat in the mold, and the unit is not cooling it fast enough or efficiently.

11 – Mold waterlines should be descaled at least once a month to make sure no scale deposits interfere with the lines’ effectiveness. A 1/64” scale buildup in a ¼” waterline will result in a 40% loss of cooling ability in the mold.

12 – Place a 1/8” (approx.) shim (metal washer) under mold mounting clamp heels to ensure downward force on mold at clamp toe.

This means the toe clamp should have its heel adjusted to point the toe slightly (1/8″ is fine) towards the platen, as shown in the following sketch.

This must be done because it is impossible to maintain exact parallelism of the clamp to the platen as is desired for maximum clamping force.

Expansion and contraction of the mold and machine result in clamps slipping loose when adjusted to be perfectly parallel.

If the clamps are adjusted so the toe is pointing away from the platen, the clamping force is also pointing away from the platen, and the mold may fall out due to insufficient clamp force.

Therefore, the toe should be adjusted to point in towards the platen to ensure that the clamping forces are directed towards the platen.

13 – Replacing straight waterline fittings on the mold with right-angled fittings will increase the turbulence of flow for water traveling through those lines.

That will ensure good overall temperature control and mimics the “Reynolds Number” approach to mold temperature control.

14 – When checking mold surface temperature, take at least 3 readings in different areas where the plastic will touch the mold.

There should be no more than 5 degrees (F) difference between the 3 readings.

15 – Keep in mind that it doesn’t really matter what the settings are on the mold temperature control unit.

What matters is only the temperature found on the mold itself and then only in the areas that will be touched by plastic.

If someone asks you, “What is the mold temperature?”

DO NOT give them the readings that are set on the controller.

Rather, give them the readings found by the pyrometer on the mold itself.

16 – Steel objects should never be used for removing stuck plastic from a mold or from a sprue bushing hole.

The steel will scratch the mold and sprue bushing, which will require expensive repairs.

Instead, use a wooden dowel, plastic putty knife, or brass tools.

17 – For removing a broken-off or sticking sprue from a sprue bushing, heat up a brass wood screw, push it into the stuck plastic and allow the plastic to harden.

Then clamp the screw head in a pair of brass pliers and tug the stuck sprue out of the sprue bushing.

The screw may easily be unscrewed from the plastic for future use.

18 – A copper or brass hacksaw-type blade can be heated and used for extracting stuck plastic from deep wall sections of a mold.

DO NOT USE STEEL blades as they will scratch the mold.

19 – Ideally, the injection gate should be placed so that the molten plastic enters the cavity image at the thickest portion of the part to be molded.

Then the material will be forced to fill the cavity and will get squeezed as it flows into the thinner section.

That causes resistance to build-up, which in turn creates a pressure buildup that helps finish the filling action.

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.


injection molding  process

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.

Identifying Characteristics

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.

Legal Requirements

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.