22 Experts Reveal How To Build A Reliable Plastic Mold

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 do is to check the following 22 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…

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 like 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 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 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 - Mold temperatures should be checked 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 gall 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 are interfering with the effectiveness of the lines.  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 travelling 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) different 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 a resistance to build up which in turn creates a pressure buildup which 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.

This investment is usually borne by the customer, 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.