21 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 –For smaller injection molds, a minimum of 6 parting line vents is needed, while larger molds should have many more. Throughout the perimeter of the mold cavity, there should be multiple vents extending from the perimeter to the outer edge of the mold cavity. Each vent should extend into the surrounding atmosphere.

It is a good idea to clean them at least every 24 hours.

Venting cannot be overdone. Any width and any number of vents are okay, as long as they are of the proper thickness and length.


As a rule of thumb, venting should take up at least 30% of the perimeter of the parting line.

2 –The case might arise that a mold needs a vent somewhere that does not have one, in this case, you can make one by securing two pieces of masking tape, 1 2″ wide, on the shutoff in question, leaving a 1 2″ gap between each.

In this way, you will be able to mold one or two cycles to determine what the difference is in the part after molding.

3 – Maintaining the temperature of water between mold halves using a single mold temperature control unit is never advised. Mold halves should be controlled separately to ensure complete effectiveness. One unit may not be able to heat both halves of the water at the right temperature if only one unit is used. To meet the individual needs of each half, separate units should be used.

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 – Once the mold has been mounted in the press, water lines need to be connected to the mold, so that the water enters the mold near the bottom and leaves near the top of the mold.The advantage of this procedure is that any air that may have been trapped within the mold lines will be purged from the mold immediately, protecting the mold from hot spots during production.

5 – You should periodically check the mold temperature with a flat probed pyrometer. On each mold half, the probe should be inserted to five or six different spots on the mold half, and after checking each mold half the probe should be reinserted.

Between any two points in the mold or between the two mold halves themselves, there shouldn’t be more than a 10 degree (F) difference between them. Variations with higher numbers indicate that improper cooling conditions exist. To correct this, clean the cooling lines, add cooling channels to the cooling lines, insert baffles within the cooling lines, etc.

6 – Parts molded in a mold have an inherent tendency to remain on the hottest half of the mold. This is generally the side of the mold which has the clamp unit since the ejection unit is used to push the final part out of the mold when the clamp unit is located on that side. In this regard, you may be able to prevent warpage and sticking from occurring.

plastic mold structure

7 – In order to ensure the molded part will stay on the ejection half of the mold, it is important to heat the ejection half of the mold by 5 to 12 degrees (F) more than the injection half. There is, however, a danger of too much heat difference causing a “lock-up” of the mold halves or a galling of some metal components.

8 – With a hot mold, you will get a more glossy part than you would with a cold mold. In addition to producing a darker product, a hot mold also produces a darker part.

9 – Using commercial insulation between the mold and the platens of the machine will keep mold temperatures higher and more even. These sheets can be directly clamped to injection mold halves, which are available in 14″ and 12″ thicknesses.

10 -Hold your hand over the outgoing and returning hoses of a mold temperature control unit to ensure proper water flow. It should feel the same between the two hoses if the heater is keeping the set temperature.

There will be a significant difference in temperature between the return line and the outgoing line if the unit is not cooled properly. This is due to the fact that there is still too much heat in the mold and the unit is not able to cool it fast or effectively.

11 – To ensure that scaling does not interfere with the efficiency of mold waterlines, they should be descaled at least once a month. When 1/64″ of scale builds up in a 14″ waterline, the mold will lose 40% of its cooling ability.

12 – In order to ensure downward force on mold at clamp toe, please place under the clamp heels a 1/8″ (approx.) shim (metal washer). To this end, the toe clamp’s heel should be adjusted so that its toe is pointing toward the platen slightly (1/8″ is okay). The reason this must be done is that no exact parallelism can be maintained between the clamp and platen in order to obtain maximum clamping force.

The expansion and contraction of molds and machines cause clamps to slip when they are arranged so that they are perfectly parallel.

By adjusting the clamps so that the toe of the clamp points away from the platen, the clamping force is also being directed away from the platen and the mold may fall out if there is insufficient clamp force.

Thus, the toe should be adjusted so it points toward the platen so that the clamping force is directed towards the platen as the platen itself is the clamping surface.

injection molding

13 – When right-angled fittings are replaced on the mold, water that travels along straight water lines will be turbulent.

In areas where the plastic will come into contact with the mold, take a minimum of three measurements of the surface temperature. Comparing this method to that of Reynolds Number, it ensures good temperature control. Differentiation between the three readings should not exceed five degrees.

14 – In most cases, the mold temperature control unit won’t matter what the settings are.

A mold’s temperature only matters when it is found on the mold itself, and then only in the areas that will have contact with the plastic.

It is not a good idea to give someone the temperature readings from the controller when they ask “What’s the mold temperature?”. It is better to show them the pyrometer measurements.

15- The use of steel objects to remove plastic from a mold or bushing hole is never safe.

The steel will scratch the molds and sprue bushings, requiring costly repairs. In its place, you can use brass tools, wood dowels, plastic putty knives, or plastic putty knives.

16 – You can remove a broken sprue from a sprue bushing by heating and inserting a brass wood screw into it. Once the plastic has hardened, you can remove it.

Pull out the sprue stuck in the bushing by clamping the screw head with brass pliers. Simply unscrew the screw from the plastic to reuse it in the future.

17 – Heat a copper or brass hacksaw blade to remove stuck plastic from deep wall sections of a mold. Avoid using steel blades on metal molds, since they will scratch the surface.

18 – When the injection gate is placed properly, the molten plastic should be directed into the cavity at its thickest point. If this is the case, the material will become forced into the cavity and squeezed as it moves through the thinner part of the cavity. This is a result of resistance building up, and as a result, pressure builds up in such a way that it assists with the completion of the filling process.

The only way to develop injection pressure is for the melt to encounter some form of resistance, as this is the only way to achieve it. Despite the fact that this may happen to some extent as the material passes through the heated barrel and is deposited in the mold, it is only during the filling process that the resulting high packing pressure is produced.

19 – Quite a few thousand dollars can go a long way when it comes to the price of molds; however, large, sophisticated molds can easily cost hundreds of thousands (even millions) of dollars.

The customer usually bears the cost of this investment, but the molder has the responsibility of maintaining the mold while it remains in their possession. Mold maintenance costs usually range between 5% and 7% of the initial cost of the mold over the course of one year.

As a moldmaker, it is the mold maker’s responsibility to ensure the quality of the products in the mold, whereas the mold designer is responsible for the design and the performance of the mold.

20 – If there is a production run that ends in a “last shot”, it is a good idea to collect that shot and store it with the mold in storage. The final shot should contain all the elements that were produced in the process, including the parts, runner, flash, and anything else produced.

It provides a visual representation of the steps that were involved in producing the parts for the mold maintenance area. In order to determine whether or not the part is in good shape, the condition of the cavity surface, or the position of the ejector pin, a repair person must inspect the part.

The last shot should be accompanied by a written statement of the problems as seen by the molding room personnel.

21- In order to minimize the possibility of damaging rust forming within the mold as soon as it is extracted from the molding machine, it should be thoroughly cleaned, inspected, and coated with a rust preventative material (primarily on the inside but lightly on the outside) to keep it protected from rust forming.

In order to store a coating for a long period of time (more than 30 days), it is recommended that it be extra heavy. The water lines must be cleaned and coated as well. We recommend rinsing waterlines with an acidic solution to prevent deposits from forming again.